External Skeletal Fixation

Introduction

The external skeletal fixation (ESF) system integrates the use of transfixation pins, an external frame, and sometimes an intramedullary pin connected to the frame for definitive fixation. Clinical use of the ESF system may include supplemental inter-fragmentary fixation techniques such as lag screws, K-wires, and cerclage wires when appropriate. Bone plate and screw fixation and the interlocking nail are examples of other fixation systems. All three of these major fixation systems are used for similar indications including fracture management, arthrodesis, and corrective osteotomy repair. Each fixation system has its unique advantages and disadvantages, and no single fixation system is preferred in all instances. They all provide suitably rigid fixation of fractures. The two internal fixation systems provide the advantage of more straight forward postoperative care compared to the ESF system. ESF, however, provides better opportunity to maximize the biologic potential for healing within the fracture zone. Specific advantages and disadvantages of the ESF system are summarized in Table 53-1.
 

This section will cover terminology and basic principles of external skeletal fixation applicable to all of the different ESF devices commonly used in North America. Subsequent sections will cover specific application techniques for the acrylic and pin external fixator (APEF), Securos external fixator, IMEX-SK external fixator, circular external fixator, and hybrid external fixator. All of these devices provide stronger, more reliable fixation that is easier to apply compared to earlier experience with the Kirschner-Ehmer (KE) external fixator. For these reasons, the second generation veterinary external fixation devices mentioned above have for the most part replaced the KE splint in current clinical usage.

Nomenclature of ESF

An external fixator has two fundamental elements regardless of the device being used. These are the fixation pins and the connecting column (fixation frame). Fixation pins are percutaneous devices that engage both the near cortex and far cortex of major bone segments for attachment of the fixator to the bone. Originally, fixation pins were smooth Steinmann pins with trocar points that were passed at convergent or divergent angles relative to the long axis of the bone. Angling of this type of fixation pin was necessary to limit pin migration attributable to poor security of the pin-bone interface with smooth implants. One of the most important improvements in veterinary ESF was the development of affordable, properly-sized, threaded fixation pins with a raised thread profile (positive profile thread). These modern fixation pins have significantly reduced the morbidity formerly experienced with ESF due to the fact that they provide reliable, long-term pin-bone interface security. The most recent development in veterinary fixation pin technology is an intelligently designed negative profile threaded pin with a taper run out junction to alleviate the stress concentration point normally found at the junction between the threads and the shaft of the pin (Duraface pins – IMEX Veterinary Inc.). Owing to a larger pin shaft diameter, Duraface pins have been shown to be mechanically superior to positive profile pins of the same thread diameter.

Fixation pins are classified as either half-pins or full-pins. Half-pins penetrate the near side soft tissues to transfix the bone with the end-threaded portion of the pin, and they are attached to a single connecting column (Figure 53-1). Full-pins go through the near side soft tissues to transfix the bone with centrally placed threads, proceed through the far side soft tissues, and are attached to two different connecting columns, one on the lateral side of the limb and one on the medial side (Figure 53-1).
 

Connecting Columns are fastened to and interconnect the fixation pins, thus providing support for the fixation pins and the fractured bone. It is in the design of the connecting columns that the different ESF devices find their uniqueness. Similar to the KE splint, the Securos fixator and the IMEX-SK fixator use clamps and rods to form the connecting column or fixation frame. However, both of the newer devices are superior to the KE splint in terms of strength and versatility. The APEF uses acrylic cement to both grip and interconnect the fixation pins. Low cost and greater freedom in terms of the shape of the frame are the inherent advantages of the APEF frame. The clamp and rod devices offer the advantages of reusable components and greater ability to easily make adjustments of the frame in terms of fracture alignment and fixation rigidity. Once the acrylic frame has cured into a rigid solid, frame adjustments are laborious and messy.

Classification of different external fixator frame configurations is useful in that it evokes a mental picture of what a given construct looks like, and furthermore, the classification predicts the mechanical performance of one construct versus others. The most commonly used classification system initially considers whether a fixator is unilateral (connecting column on one side of the limb) or bilateral (connecting columns on both sides of the limb), and then further considers whether it is uniplanar (all fixation pins placed in more or less a single plane) or biplanar (fixation pins placed in two distinctly different planes). This consideration results in four different classification types (Figure 53-2). In order of weakest to strongest they are: Type I-a (a one column construct that is unilateral and uniplanar); Type I-b (a two column construct that is unilateral and biplanar); Type II (a two column construct that is bilateral and uniplanar); and Type III (a three column construct that is bilateral and biplanar). At each step forward in this progression, construct rigidity increases (i.e Type I-a is the weakest and Type III is the strongest).
 

Type I-a configurations (Figure 53-2A) may be appropriate for straight forward fractures in patients that are likely to heal quickly (i.e not comminuted fractures in patients that are elderly or have other medical problems that will delay bone healing). Type I-a frames are generally applied to the medial aspect of the tibia, the lateral aspect of the femur and humerus, and the craniomedial aspect of the radius. They may be supplemented with a small intra-medullary pin (one that fills approximately 40% of the medullary cavity) and there are mechanical advantages to attaching this pin to the external fixation frame. This is called an intramedullary (IM) pin tie-in configuration (Figure 53-3). This strategy is most often employed with external fixators applied to the femur or humerus. The increased distance of the frame from the central axis of these bones (because of thick overlying soft tissue) makes the external fixator extremely vulnerable to disruptive bending forces without the additional strength provided by the centrally placed IM pin. The IM pin tie-in strategy is seldom used for the tibia, and is contraindicated for the radius. An acceptable alternative for the bones of the antebrachium involves placing a small IM pin in the ulna and applying an external fixator to the radius.

A Type I-b configuration (Figure 53-2B) is basically the combination of two Type I-a frames placed on different aspects of the bone. Mechanical performance is optimized when the second frame is placed in an orthogonal position relative to the first (i.e. the plane of the fixation pins of one frame is 90° different from the plane of the fixation pins of the second frame). On the tibia this would translate as a medial frame and a cranial frame. The two pin planes on the radius are usually craniomedial and craniolateral (coming in on either side of the extensor muscles) and the degree of separation between the pin planes is often less than 90°.

Anatomical restrictions necessitate the construction of modified Type I-b frames for the femur and humerus (Figure 53-4). The major safe corridor for pin placement is found laterally, although a reasonably safe craniolateral corridor may also be used in the proximal 25 to 30% of these bones. Two different modifications are shown: a two-frame construct supplemented with an IM pin tie-in (Figure 53-4A); and a three-frame construct (Figure 53-4B). The major frame has been placed laterally in both cases. Fixation pins placed in a craniolateral plane enable the construction of a second craniolateral frame. If a full-pin is placed through the distal metaphysis, the medial aspect of this pin provides the opportunity for construction of a third (craniomedial) frame. The three-frame construct can also be supplemented with an IM pin tie-in (not shown in Figure 53-4B). Modified Type I-b frames are sometimes used on other bones as well (Figure 53-5).

In order for a fixation frame to qualify for Type II status, it must have a minimum of two full pins, one in the proximal segment and one in the distal segment. In a minimal Type II frame, all of the rest of the fixation pins in the construct are half-pins (Figure 53-2C). If full-pins are used at all positions within the construct, this constitutes a maximal Type II frame (Figure 53-2D). The creative APEF construct shown in Figure 53-5 falls just short of being a Type II configuration due to lack of a proximal full-pin. Although it has two connecting columns and multiple full-pins, all of the fixation pins applied to the proximal segment are half-pins. This is another example of a modified Type I-b construct. It has two connecting columns (medial and craniomedial) but it is not bilateral proximally, and it has groups of fixation pins placed in distinctly different planes (i.e. a two column configuration that is unilateral and biplanar).
 

General Strategies for External Fixator Application

The following general principles are important for the proper application of an external fixator regardless of the specific device being used. A complete work-up including good quality, properly positioned radiographs (two orthogonal projections including the injured bone and the joint above and below it), thoughtful pre-operative planning, and aseptic surgical technique are required similar to other orthopedic procedures.

The Hanging Limb Technique

The hanging limb technique (Figure 53-6) and four-corner patient draping are performed so that the surgeon has access to the entire circumference of the limb. If a cancellous bone graft is needed, the patient must be clipped, prepped, and draped to accommodate this as well. 
 

Important aspects of the hanging limb technique are as follows. The equipment needed to hang the limb is a sturdy hook positioned in the ceiling directly over the surgery table and a surgical table that can be raised and lowered. Adhesive tape is secured to the paw of the injured limb leaving very long ends to form a stirrup. The tape should be placed on the paw securely so that it will not fall off with tension, but should not be so tight that it will constrict blood supply to the foot. Elevation of the surgical table should position the injured segment of the limb at a convenient working height for the surgeon when the limb is suspended from the hook with the tape stirrup. The surgical table is then lowered until the injured hindquarter or forequater is suspended about 1cm above the surface of the table. The hook, tape stirrup, and limb should form a straight vertical line when viewed cranially or caudally and laterally. The paw and the proximal limb should be palpated through the drapes to ensure that there is no rotational malalignment.

Suspension of the injured leg is often maintained throughout surgery when an external fixator is being applied to either the radius or the tibia. This provides a linear traction force that provides approximate alignment of the fracture and proper positioning of the overlying soft tissues. The distal portion of the leg and a portion of the tape used to suspend it are covered with sterile drape material to prevent contamination of the surgery site. If the surgeon needs to move the joints above and below the fractured bone during surgery to check for proper rotational alignment, the surgery table can be temporarily raised thus relieving traction on the limb. After proper alignment is verified, the table is lowered to restore traction on the limb. This provides a much more stable working environment greatly facilitating external fixator application.

Keeping the limb suspended throughout surgery is not recommended for application of an external fixator to the femur or humerus. The larger muscle mass surrounding these bones effectively resists the ability of traction to restore proper alignment of the fractured bone. Additionally, a small IM pin is often used to supplement the external fixator, and placement of this pin is extremely difficult with the limb suspended. For fractrures of the femur or humerus, the hanging limb technique is used to prepare the leg for surgery, but after the limb is draped, the paw is grasped with a piece of sterile drape material and the tape suspending the limb is cut. The distal portion of the limb is then covered by wrapping it with the sterile piece of drape material.

Open Versus Closed Repair Techniques

The choice of various open versus closed repair techniques should be based upon the specific bone involved, the type of fracture, and what can be accomplished in terms of restoring meaningful load-sharing.

Invasive open technique involves a panoramic surgical approach to the fractured bone and no restrictions in terms of handling intermediate fragments within the fracture zone. This method results in a variable degree of disruption of blood supply to the injured tissues. The goal of invasive open technique is anatomic reconstruction of fracture segments and fragments to restore a load-sharing bony column. Invasive open technique should not be used unless achievement of this goal is relatively certain.

Open but do not touch technique (OBDNT) is a relatively atrumatic method with a goal of restoring normal alignment of the major proximal and distal fracture segments. No attempt is made to reduce intermediate fracture fragments. A panoramic approach is made to the fractured bone to facilitate proper alignment of the fracture. The major proximal and distal segments can be grasped at a safe distance away from the fracture zone and manipulated to restore axial alignment, normal length of the injured limb segment, and proper rotational alignment. The surgeon accepts a “hands off” (do not touch) philosophy with regard to the fracture zone and the intermediate fragments and fracture hematoma that are found within it. Intermediate fracture fragments are left in situ to act as a living bone graft. Liberal application of cancellous bone graft over the fracture region is usually done as well. The OBDNT technique is extremely useful for treatment of comminuted shaft fractures, especially those involving the femur or humerus where thick overlying soft tissues often complicate the process of restoring normal fracture alignment.

Miniexposure technique is more invasive than OBDNT but less invasive than the open repair technique. This method involves making a small incision over the fracture region to enable manipulation of the proximal and distal segments with the goal of improving alignment or achieving anatomic reduction. A two-piece oblique midshaft fracture of the tibia is a clinical example of where the miniexposure technique would be helpful. In this case a limited incision would be made over the medial aspect of the bone. The incision should be of sufficient length to allow for reduction of the fracture and application of several lag screws to maintain it. The bone would then be spanned with an external skeletal fixator for definitive stabilization. Similar to other open repair techniques, application of a cancellous bone graft should be considered to offset the negative biologic effects of the surgical approach when the miniexposure technique is used.

Closed Technique preserves the biological environment of the hard and soft tissues in that no surgical approach is made to the fracture region. Functional alignment of the fractured bone rather than anatomic reduction is the goal of this technique. Closed repair technique is most applicable to comminuted shaft fractures of the tibia and radius/ulna. It is seldom a useful technique for dealing with similar fractures of the femur or humerus due to the large muscle mass surrounding these bones. Approximate alignment of the fracture is obtained by use of the hanging limb technique. Fixation pins of the external fixator are placed through separate 1cm long access incisions over the bone. Alignment of the fracture is adjusted, if necessary, as a spanning external fixator is placed to stabilize the fractured bone.

Principles of Fixation Pin Selection

Threaded pins with a raised (positive) thread profile and Duraface negative profile threaded pins provide for optimal pin-bone interface security and longevity. Other negative profile threaded pins and smooth (nonthreaded) fixation pins are notorious for loosening prematurely and should be avoided. There are two basic types of positive profile threaded pins: end-threaded half-pins; and centrally-threaded full-pins. Further details about the fixation pins available from different manufacturers can be found in the later sections of this chapter on specific ESF devices (APEF, Securos Fixator, and IMEX-SK Fixator).

Fixation pins must be appropriate in size relative to the bone in which they are placed. The threaded diameter of the fixation pin selected should be approximately 25% of the bone diameter. The ability of a pin to tolerate disruptive forces increases exponentially with increasing diameter, but using too large a pin weakens the bone and increases the risk of secondary fracture through the pin-bone interface when it is confronted with postoperative weight-bearing loads.

Use of fixation pins with deeper and broader threads (cancellous thread) in areas of soft metaphyseal bone will prolong the pin-bone interface at these locations. In hard diaphyseal bone, fixation pins with a standard (cortical) thread pattern should be used. The difference between cancellous thread and cortical thread is shown in Figure 53-7. Hardness of the bone can be gauged during pre-drilling, but when in doubt the surgeon should select fixation pins with cortical thread. Placement of cancellous thread pins in hard cortical bone results in microfractures that compromise pin-bone interface security. The most appropriate locations for the use of cancellous thread fixation pins are the proximal metaphysis of the humerus and tibia, and the distal metaphysis of the femur.
 

Principles of Fixation Pin Insertion

Optimal pin-bone interface security depends upon fixation pins being placed in mechanically intact bone. This requires pin application a safe distance away from fracture lines and fissure lines. The safe distance has been stated to be 1 cm away from the fracture zone. A more useful guideline suggests that the safe distance is equal to one bone diameter. This guideline adjusts up and down according to patient size which is entirely appropriate, as 1 cm can be a dauntingly long distance in the bone of a tiny kitten, yet a negligible distance in the bone of a Great Dane.

Pin-bone interface security also depends upon fixation pins being properly centered within the bone. When there is maximal distance between where the pin penetrates the near cortex and the far cortex, the pin is optimally centered within the bone (Figure 53-8A). Pre-drilling of the bone should precede fixation pin insertion. The surgeon must pay careful attention while pre-drilling to make sure that the hole is correctly centered in the bone. With a properly centered hole, there is initial resistance as the bit cuts through the near cortex, followed by no resistance as the bit falls some distance across the medullary cavity. A second point of resistance is encountered as the bit cuts through the far cortex. If these separate resistance points are not felt during pre-drilling, the hole is probably through the side of the bone (Figure 53-8B). When this problem is encountered, the surgeon should abandon that hole and drill another one that is properly centered a safe distance away from the poorly positioned hole.

When a bone is approximately round in cross-section, what feels like the center when walking the drill sleeve across it is the proper location for the pre-drilled hole. The proximal tibia is triangular in cross-section. At this location a properly centered hole should be placed slightly caudal to what feels like the center of the bone in order to increase the distance between the points of near cortical and far cortical engagement.
 

Fixation pins are typically placed through separate release incisions at least 1 cm in length that are made over the center of the bone. With minimally invasive technique, the surgeon is frequently unable to see the bone. To determine the location of the bone, the surgeon can probe through the overlying soft tissues with sterile hypodermic needles and mark the edges with strategically placed needles. An incision centered over the bone is then made through the skin and subcutaneous tissues with a scalpel blade. The release incision through deeper tissues is made by blunt dissection down to the bone using a mosquito hemostat or Metzenbaum scissors. Muscle tissue should be divided parallel to the direction of its fibers (usually parallel to the long axis of the bone). Exposure can be maintained by placement of mini Gelpi retractors in the release incision.

A drill sleeve is passed through the incision down to the level of the bone and walked along its surface to locate the edges. Once the drill sleeve is centered over the bone, a drill bit is inserted through it and a hole is pre-drilled in the bone. The diameter of the drill bit should be equal to or slightly smaller than the diameter of the smooth shaft of the pin to be applied. The drill must be spinning in a clockwise direction for the drill bit to cut through the bone. Operation of the drill at high speed during pre-drilling is safe because the flutes of the bit allow an escape channel for debris, thus avoiding thermal necrosis of the bone. In contrast, the drill must be spinning clockwise at a much slower speed when the threaded fixation pin is applied to the bone. This is due to the lack of an escape channel for debris as the threads of the pin cut corresponding threads into the bone. If the pin is allowed to spin too quickly, thermal necrosis of bone immediately surrounding the pin will occur, thus jeopardizing the security of the pin-bone interface.

Once the threads of a positive profile pin cut the initial threads in the near cortex of the bone, the slow clockwise rotation of the pin will advance it through the bone by gear effect. Any attempt of the surgeon to speed this up (e.g. applying greater pressure on the drill or using it a higher speed) is detrimental to the pin-bone interface and should be avoided. The full threaded diameter of the pin should engage the far side of the far cortex in order to obtain a mechanically optimal pin-bone interface. This means that the trocar tip of the pin will extend into the soft tissues on the far side of the bone when a half-pin is applied. The surgeon can usually palpate the tip of the pin exiting the far cortex through the overlying soft tissues in order to judge proper length. Ideally, only the trocar tip of the pin exits the far side. If a longer portion of the pin protrudes and there are no vital anatomic structures near it, it is better to leave the pin “too long” instead of switching the drill to reverse (counterclockwise spin) and backing it up. Two-way insertion of a fixation pin (going in too far and then partially backing out) has been shown to weaken the pin-bone interface.

The greater the amount of soft tissue that a fixation pin must traverse before reaching the bone, the greater the likelihood that it will cause postoperative morbidity. Cross-sectional anatomy of the limb at various levels proximal to distal on the injured bone should be carefully considered in order to select the safest soft tissue corridors to be used for fixation pin placement sites. It is important to avoid large muscle bellies, tendons, blood vessels, and nerves. If penetration of a muscle belly cannot be avoided, an ample release incision to prevent soft tissue tension on the fixation pin is necessary in order to keep morbidity low. Preferred pin placement corridors in different bones are summarized in Table 53-2. The importance of ample release incisions at every pin placement site cannot be overemphasized. When placement of the external fixator is complete, the surgeon should release traction on the leg and move the joints above and below the injured bone through full ranges of motion. If there is soft tissue tension detected at a pin placement site during movement of the joints, the release incision should be enlarged to relieve it.
 

Principles of Frame Construction

Significant mechanical gains occur with the application of additional fixation pins in a fracture segment up to and including the 4th pin. As a general rule, the surgeon should strive to place a minimum of three fixation pins proximal to the fracture region and three fixation pins distal to it. Fixation pins in different planes can be summed to achieve this goal (i.e. a Type I-b fixator with two medially placed pins and one cranially placed pin in the proximal fracture segment would provide the recommended minimum of three fixation pins per segment).

The working lengths of the fixation pins and the fixation frame (Figure 53-9) should be kept as short as possible to optimize the mechanical performance of the external fixator. The concept of working length can be appreciated by taking a 1/8” (3.2 mm) Steinmann pin and applying a controlled amount of bending force to it. When the pin is grasped with both hands, one at each end of the pin (long working length) and force is applied, the pin feels quite flexible. When the pin is grasped more toward the middle portion (i.e. short working length) and the same amount of force is applied, it feels more rigid. Fixation pin working length is the distance between where the pin attaches to the frame (where it exits the bolt of the pin-gripping clamp) and where it enters the near cortex of the bone. Soft tissue thickness over the bone basically dictates the fixation pin working length. Some degree of postoperative swelling should be anticipated and the fixation frame should be positioned far enough away from the skin to accommodate this. It is recommended that the nearest portion of the fixation frame (usually the inner aspect of the fixation clamps) should be positioned about 1cm away from the skin. This keeps fixation pin working length relatively short, but allows a small amount of space for postoperative swelling. Fixation frame working length is the distance between the fixation pins placed immediately proximal to and immediately distal to the fracture zone. These implants should be placed as close to the fracture as possible while respecting the guideline of safe distance, which is one bone diameter away from the fracture region. Frame working length is mainly determined by the length of the fracture zone.
 

Clamps should be positioned on the connecting rod such that fixation pin working length is kept as short as possible. When the clamp is positioned such that the pin-gripping bolt is between the connecting rod and the skin surface (Figure 53-10A), this is referred to as the “clamp-in” position. This is the preferred position because it shortens fixation pin working length. When the clamp is positioned such that the pin-gripping bolt is toward the outer aspect of the connecting rod (Figure 53-10B), this is referred to as the “clamp-out” position”. This unnecessarily increases fixation pin working length. The clamp-out position should only be used when it provides a unique angle required to place the pin in a safe region of the bone that cannot be obtained with the clamp-in position.

Fixation pins should be placed in a specific order during construction of the frame (Figure 53-9). Pins are initially placed at the proximal and distal ends of the bone. A connecting rod is attached to the proximal and distal pins with clamps. The surgeon should check at this point to make sure that acceptable alignment of the bone has been achieved before proceeding. The proximal and distal clamps can be loosened to permit adjustment of fracture alignment if needed. Adjustments are easy to make at this early phase of frame construction, but become increasingly difficult as additional fixation pins are added to each segment of the bone. The fixation pins closest to the fracture zone are applied next. Empty clamps are placed on the connecting rod and pre-drilling is done via a drill sleeve passed through the pin bolts of these clamps. This far-far-near-near strategy of pin placement relative to the fracture zone provides for optimal mechanical performance of the fixation frame. Additional fixation pins are placed in the middle portion of each fracture segment until sufficient stability is obtained.
 

The simplest frame configuration that will provide adequate stability for a given fracture should be used. For relatively straight forward two-piece midshaft fractures, a Type I-a frame is often sufficient on the tibia or the radius. With this type of fracture in the femur or the humerus, use of a Type I-a frame with an IM pin tie-in is recommended. If intraoperative evaluation by palpation of the fracture reveals that a Type I-a frame is allowing too much deflection of the fracture, it is easy to add a second frame in another plane thus converting it to a stronger Type I-b construct. This strategy is applicable to all four bones mentioned. For challenging comminuted shaft fractures or the tibia or radius, the surgeon should plan for a stronger frame and start with either a Type I-b or a Type II construct. Challenging fractures of the femur and humerus can often be more reliably managed with internal fixation techniques such as interlocking nail (see Chapter 50), bone plate and screw fixation (see Chapter 51), or plate-rod fixation (see Chapter 52).

When application of the external fixator is complete, do not trim the fixation pins short until acceptable fracture alignment has been verified with postoperative radiographs. The ability to make adjustments is often compromised once the fixation pins have been cut short. Once acceptable alignment has been obtained, all fixation pins should be trimmed such that the cut edge of the pin does not extend beyond the outer edge of the clamp. Even shorter than this is preferable, when possible (the size and style of the pin cutter often determines the degree to which pins can be trimmed).

Postoperative Care

Effective postoperative management of an external fixator is defined by the following goals: 1) a healthy patient that walks comfortably on the limb throughout the healing period; 2) clinical union of the fracture and removal of the fixator as quickly as possible; and 3) avoidance of fixator induced injuries to the patient, owner, and veterinarian. Achieving these goals depends upon a carefully structured program of controlled physical activity, soft tissue care, pin tract hygiene, bandaging of the fixator, and appropriately timed staged disassembly of multiplanar frames. Because the fixator is external to the limb and has many edges (some of which are sharp), it can potentially injure the patient or owner if it is not properly bandaged. Worse yet, if the fixator becomes entangled in elements of the animal’s environment (i.e. chain-link fence, etc.) and the animal struggles to free itself, the repair may be torn apart. Careful bandaging of the fixator allows it to bounce off of environmental objects rather than being caught up in them and protects the owner and the patient from being injured by the sharp edges of the fixator. Postoperative care of the soft tissues surrounding the fixation pins is equally critical to patient comfort during the early stages of healing. Excessive pin tract inflammation will increase patient morbidity and decrease use of the limb. This inflammatory response is attributed to the presence of a contaminated foreign object (the fixation pin), inadequate drainage, and too much soft tissue motion around the fixation pins.

Careful attention to wound management during the first five to seven days after ESF application is critical in order to control of pin tract contamination and soft tissue inflammation. The pin tracts are vulnerable to infection until the proliferative stage of healing (fibroblasts and neocapillaries) leads to development of a bacteriostatic lining of granulation tissue. The more contaminated the early pin tract wound becomes, the longer it remains in the debridement stage of wound healing (polymorphonuclear leukocytes and macrophages). The longer the pin tract remains in the debridement stage the greater the likelihood of infection. This is because the microorganism load will begin to overwhelm regional defense mechanisms. Infection will further prolong the debridement stage, creating even more inflammation. This negative cycle of events leads to high patient morbidity, eventual disruption of the pin-bone interface, and finally to loosening of the fixator. To avoid this vicious cycle, the microorganism load of the pin tracts must be kept as low as possible to enable a brief debridement stage, rapid onset of the proliferative stage, and development of healthy granulation tissue around the fixation pins.

Reduction of soft tissue motion can be attained by packing the area around the pins and between the skin surface and the fixator frame with a bulky wad of gauze as part of the standard postoperative bandaging regime. It makes little difference to the pin tract microflora whether the fixation pin is moving in the soft tissues or the soft tissue is sliding along the pin. The effect is the same, that being increased pin tract inflammation, and pain. All pins will cause some degree of inflammation and drainage. This drainage will inevitably contain bacteria. When the fluid can drain freely, secondary infection is rare unless the pin is loose in the bone or the soft tissues are moving excessively on the pin. If this drainage is blocked, secondary infection of the pin tract is likely.

Adequate release incisions facilitate drainage and regularly changed gauze packing acts like a wick to pull it out from the wound. Clinical signs distinguish normal drainage (usually serous) from that associated with pin tract sepsis. Signs suggestive of an infected pin include excessive drainage (usually thick and foul-smelling), pain, lameness, induration or erythema of the soft tissues, and pin laxity. Failure to keep the pin tract clean and freely draining and failure to relieve soft tissue tension on the pin can promote infection, increase patient morbidity, and lead to pin loosening.

Early Postoperative Management

Systemic antibiotics are given throughout surgery and during recovery from general anesthesia and are usually discontinued thereafter. Immediate post-op pain management is generally achieved with morphine. The day after surgery, a 1 week course of carprofen (2.2 mg/kg per os BID) is started.

Pin tract wounds should be covered with a sterile dressing for the first five to seven days (or until healthy granulation tissue develops). Pin-skin junctions are cleaned with dilute hydrogen peroxide solution to remove blood clots, serum crusts, etc. A thin film of triple antibiotic ointment (polymyxin, neomycin, bacitracin) is applied to the skin around each pin placement site. Wads of “fluffed-up” gauze sponge are packed around the pins and between the skin surface and the fixator frame to immobilize the soft tissues, to keep the pin tracts clean, and to wick drainage away from the wounds. Gauze packing is held in place with an overwrap of Kling bandaging gauze. This sterile dressing is covered with a modified Robert Jones bandage for at least the first 36 to 48 hours to prevent swelling in the distal portion of the limb. The fixator dressing is changed at 36 to 48 hours and every other day thereafter until healthy granulation tissue develops. Application of a Robert Jones bandage over the dressing should be continued during the first week after surgery.

After about 1 week, the Robert Jones bandage is usually abandoned in favor of a simpler “bumper” bandage. This is intended to pad and cover the edges of the frame to reduce the likelihood of it causing injury or entanglement. At each bandage change the same methods of skin hygiene and gauze packing described above are used. Physical activity is limited to short walks outside on a leash for urination and defecation.

Care at Home

After granulation tissue develops the owner is instructed to change the bandage and packing on an as needed basis, usually every 3 to 5 days. If the bandage becomes wet or dirty, if wound drainage increases, if odor is detected, or if the animal is licking or biting the wrap, more frequent bandage changes may be necessary. Physical activity is restricted to leash walks. Running, jumping, and playing with other animals or children should be discouraged. Walking up and down stairs should be kept to a minimum. Good functional usage of the limb is expected throughout the healing period. If this suddenly declines the animal should be re-examined as soon as possible. If the owner is willing, rechecking these patients every other week even if they are doing well is recommended. Radiographic examination at about 6 weeks after surgery should be done to assess healing and to enable staged disassembly of the fixator.

Staged Disassembly of the External Fixator

It is biologically advantageous to reduce the stiffness of the fixator (via staged disassembly) during the later stages of fracture healing. This involves the sequential removal of fixation elements to allow the healing bone to be stimulated by carefully controlled increases in axial stress. During the early stages of bone healing, rigid fixation benefits revascularization of the fracture region, maintains tissue strain at a low enough level to enable the formation of bridging callus, and allows the patient to walk comfortably on the limb. During the later stages of bone healing, strategic reduction of fixation rigidity transfers a greater percentage of axial weight bearing forces to the injured bone while continuing to protect against disruptive bending and rotational forces, and stimulates bony remodeling according to Wolff’s Law.

Research has shown that there is an “optimum time window” for initiating staged disassembly. In mature patients this interval is generally felt to be at 6 to 8 weeks after surgery. In young growing patients, this window probably occurs several weeks earlier. The decision to begin staged disassembly is based upon radiographic appearance and palpation of the fracture. When there is scant evidence of bridging callus and palpable instability of the fracture, staged disassembly is delayed.

When disassembly is determined to be appropriate, the following guidelines are applied: 1) The external fixator is examined for any fixation pins that are showing signs such as excess drainage or inflammation. If there are “problem pins”, the disassembly strategy should include their removal. 2) Consider removing any pins that have the potential to cause morbidity. Examples would include a pin in the soft bone of the distal femur, or one that goes through the thick lateral soft tissues of the proximal tibia. 3) When possible, it is best to remove frames rather than just individual pins. 4) When a Type II or Type III configuration is present, conversion to a Type I-a or Type I-b to encourage axial loading of the bone is recommended. When a Type I-b is present, down-staging to a Type I-a is appropraite. 5) When an IM pin “tie-in” configuration is present, the IM pin is usually removed last in an attempt to encourage axial loading while protecting against bending stress. It is advisable to retain one proximal fixator pin to enable maintenance of the “tie-in” in order to prevent IM pin migration. However, if the IM pin is a significant source of morbidity, it may be the first element of the fixation to be removed. 6) Staged disassembly is different and individualistic for each and every case depending upon the progression of healing. With some cases, disassembly may be a one or two step process and for others more steps may be required. 7) Once the frame has been simplified to a Type I-a construct, removal of central pins will increase the working length of the frame and reduce fixation stiffness.

External Fixator Removal

Radiographic exams are scheduled based upon the expected healing time for a particular fracture and patient. When radiographic evidence of healing is sufficient, the frame is loosened and the limb segment is palpated to verify clinical union. If the bone has united, the remaining portion of the fixator is removed. Exercise restriction should continue for about 4 to 6 weeks after fixator removal while the empty holes in the bone begin to heal. These empty holes can act as stress raisers predisposing to fracture of the healed bone through a bony pin tract.

Suggested Readings

Aron DN, Palmer RH, Johnson AL: Biologic strategies and a balanced concept for repair of highly comminuted long bone fractures. Comp Contin Educ Pract Vet 17:35, 1995.

Johnson AL, Egger EL, Eurell JAC, Losonsky JM: Biomechanics and biology of fracture healing with external skeletal fixation. Comp Contin Educ Pract Vet 20:487, 1998.

Kraus KH, Toombs JP, Ness MG: External Fixation in Small Animal Practice. Oxford: Blackwell Publishing, 2003.

Griffin H, Toombs JP, Bronson DG, et al: Mechanical evaluation of a tapered thread-run-out half-pin designed for external skeletal fixation in small animals. Vet Comp Orthop Traumatol 24:257, 2011.
 

Introduction

Acrylic frame fixators are devices in which the pin-gripping clamps and connecting rods are replaced with acrylic columns (methyl methacrylate) to form the external fixation frame. A powder component (polymer) is added to a liquid component (monomer) to form a liquid or dough that can be poured or molded. The mixture undergoes an exothermic reaction and forms a rigid solid about 8 to 12 minutes after mixing. The resulting acrylic column grips and interconnects the fixation pins thus forming the fixation frame. Different sizes of fixation pins can easily be used in the same construct and frames can be built to any shape that the surgeon desires (i.e. fixation pins do not have to line up to connect with a linear rod as they do with the clamp and rod ESF devices). The use of curved acrylic columns, when needed, does not compromise the stiffness of the frame.

Acrylic frame fixators can be applied to most bones but they are particularly useful for mandibular fractures and transarticular applications because the acrylic connecting columns are easily contoured to the shape of the body and joint angles. The acrylic used is radiolucent, which does not interfere with radiographic assessment of initial reduction or fracture healing. The first reports of acrylic frame fixators involved the use of Steinmann pins or very long orthopedic screws as fixation pins. The screws were inserted in the bones leaving the heads extended externally where they were connected with a column of dental acrylic. Homemade acrylic-pin splints are similarly constructed using methyl methacrylate that is available as hoof repair or dental molding acrylic. “Plumber’s Epoxy” has also been described for similar applications. The APEF System^a utilizes acrylic and positive profile threaded fixation pins and provides all of the basic components required to facilitate the construction of an acrylic frame fixator.

^a Innovative Animal Products LLC, 5812 Highway 52 North, Rochester, MN 5590

Components of the APEF System

The acrylic frame is constructed with acrylic bi-packs, plastic sidebar tubes for molding liquid acrylic, and end caps to plug the molding tubes. A temporary frame alignment device is useful for maintaining fracture reduction/alignment while the applied acrylic frame is setting.

Acrylic Bi-Packs (Figure 53-11) offer pre-measured volumes of polymer and monomer packaged in separate compartments of a mixing bag. When the ends of the bag are pulled, a plastic divider strip pops off and the mixing bag becomes a single compartment. Acrylic is mixed for 2 to 3 minutes until a smooth consistency is achieved and then the corner of the bag is cut off. The acrylic is poured into plastic sidebar tubes that have been pushed onto the ends of the fixation pins thus providing an injection mold for the acrylic. The effect of acrylic column diameter has been studied. In general, bending strength increases proportionally with the diameter of the column until about 2.5 cm, at which point increasing diameter may result in heat-generated “vaporization” of the acrylic monomer creating voids in the column and strength loss. While an objective rule for optimal acrylic column diameter for every fracture is not possible, a convenient guideline is that the acrylic diameter should be the same size or larger than the outer diameter of the bone being stabilized. For more complex, unstable, or slower healing fractures, this relationship may be augmented by increasing the diameter of a single column or by using multiple columns. This bi-pack preparation and application technique minimizes the mess and odor associated with mixing acrylic (compared to Caulk Dental Acrylic and Technovit Hoof Acrylic used in “homemade” versions of the acrylic frame fixator), but the surgeon does pay an increased price for this convenience. Acrylic Bi-Packs are available in five different volumes: a triple pack contains 150 ml of mixed acrylic and will fill approximately 18 inches of the 21 mm tubing (enough to apply a Type III frame to a large dog); a double pack contains 100 ml of mixed acrylic and will fill 12 inches of 21 mm tubing (sufficient for a Type I-b frame or a Type II frame in a large dog); a single pack contains 50 ml of mixed acrylic and will fill 6 inches of 21 mm sidebar tube or 12 inches of 15 mm tubing (enough for a Type I-a frame in a large dog, or either a Type I-b or Type II frame in a small dog or a cat); a half pack contains 25 ml of mixed acrylic and will fill one small tube (sufficient for a Type I-a frame in a small dog or a cat); and a quarter pack contains 12.5 ml of mixed acrylic that is generally used with the 10 mm diameter sidebar tube (sufficient for an acrylic frame in a small bird or other small exotic pet).
 

Sidebar Tubes (Figure 53-11) are pushed onto fixation pins to provide a mold for liquid acrylic to form a cylindrically-shaped mass that acts as both a linkage device and a connector. The result is a neat, professional-looking frame (unlike some of the acrylic frames made by hand-molding dough stage acrylic onto fixation pins). Sidebar tubes are less prone to leak liquid acrylic than other types of tubes used for improvised versions of the acrylic frame external fixator. Stock sidebar tubing is sold as 48 or 60 inch long segments that are easily cut with scissors to the desired length. Sidebar tubes are available in three different diameters: standard sidebar tubes are 21 mm (appropriate for patients 8 to 10 kg or larger); small sidebar tubes are 15 mm (appropriate for small dogs, cats, and some avian patients); and mini sidebar tubes are 10 mm in diameter (appropriate for very small puppies and kittens, small birds, and other small exotic pets).

End caps are available in three sizes (21, 15, and 10 mm diameter) to plug the dependent ends of sidebar tubes. This prevents leakage when liquid acrylic is poured into the sidebar tube to form the frame.

Frame Alignment Device (Figure 53-12.) This is used to as a temporary mechanical splint to maintain fracture alignment or reduction until the primary splint (the acrylic frame) becomes rigid. This is referred to as a bi-phase technique. The frame alignment device consists of four universal clamps (that can be tightened without a wrench) and stainless steel connecting rods. The clamps can be applied close to the skin, just inside the sidebar tubes, and can be easily removed after the acrylic frame has become rigid. With bi-phase technique utilizing K-E components for the temporary splint, the clamps must be placed external to the sidebar tubes to enable their later removal. Because of their closer proximity to the bone, the frame alignment clamps have a mechanical advantage over traditional mechanical clamps in maintaining fracture alignment. Additionally, the position of the frame alignment clamp inside of the sidebar tube ensures that the frame will be at least 1 cm away from the skin. Maintaining this distance is important to avoid thermal injury to soft and bony tissue that can occur during the exothermic phase of the acrylic setting period.
 

Technique for APEF Application

The APEF system is usually applied using bi-phase technique (application of a temporary mechanical splint to maintain alignment while a definitive acrylic frame is applied and sets up). Key steps for application to a fracture involving the radius and ulna are illustrated in Figure 53-13. The injured limb is prepared for surgery and suspended using the hanging limb technique. Aseptic technique must be maintained throughout the pin placement and wound closure phases of the procedure.
 

Fixation pins are placed using appropriate insertion techniques (pre-drilling of the bone, proper centering of the pin, and slow-speed power insertion). Pin orientation and order of pin insertion are not restricted by frame or clamp design. Typically, at least 3 pins are applied proximal to the fracture region, and 3 more are applied distal to it (Figure 53-13A). Phase 1 reduction is obtained by applying a temporary clamp and rod device (alignment frame). The alignment frame is attached to pins, the fracture reduced, and the clamps tightened to maintain reduction (Figure 53-13B). Open reduction incisions are sutured, and pins are cut off one tube diameter away from the clamps. From this point on, aseptic technique is not required as some of the components used to build the frame are supplied from the manufacturer clean, but not sterile. Our current research is finding that either using knurled pins or placing at least five notches in the portion of the pin that will reside within the acrylic column will increase the strength of the pin-connecting column interface to approximately that of the pin-bone interface when positive profile threaded pins are used. Sidebar tubing is pushed onto the cut ends of the fixation pins such that the pins penetrate the inner wall of the tube and stop short of penetrating the outer wall. The dependent ends of tubes are plugged with end caps (Figure 53-13C). Acrylic is mixed for 2 to 3 minutes after removing the bi-pack divider. The corner of the acrylic bi-pack bag is cut off and acrylic is poured or injected into the open ends of the sidebar tubes (Figure 53-13D). Any acrylic that leaks out from the tube (at points of pin penetration) can be caught in a paper cup and poured back into the top of the tube. Studies of significantly curving the acrylic column (as usually occurs in a transarticular application) reveal a decreased resistance to axial compressive forces. Consequently, we often add a 1/8 to 3/16” diameter Steinmann pin link from one end of the curve to the other to restore overall construct strength. Similar linkages are also commonly placed to connect different columns in biplanar frames (Type I-b and Type III constructs).

After the acrylic hardens (approximately 10-12 minutes), the alignment frame and end caps are removed, and any excess tube length, if present, is cut away (Figure 53-13E).

Readjustment of a Completed Acrylic Frame

If unacceptable fracture alignment is evident in postoperative radiographs, adjustment of the fixation frame is more difficult than would be the case with clamp and rod ESF devices. Splint adjustment requires removing a short segment of the acrylic column with a saw or cast cutter. The plastic molding tube is peeled back from each end and several holes are drilled in the cut ends of the acrylic to provide a base for the patch. A small amount of new acrylic is mixed and hand molded to fill the gap and overlap the existing column ends. The fracture is then manipulated into correct alignment and is held in this position while the acrylic cures. More fixation pins can be added to either replace existing pins or increase overall frame stability. The plastic molding tubing is removed from the existing acrylic column and several 1/8” diameter holes are drilled in the acrylic adjacent to the proposed pin insertion site. After aseptically preparing the skin and acrylic column surface, the new fixation pin is inserted using appropriate technique. The free end of the pin is bent to contact or cross the column. A mini pack of APEF acrylic or other acrylic is mixed to dough consistency and molded around the new pin and existing column to incorporate it.

Staged Disassembly of an Acrylic Frame

Progressive staged disassembly of an acrylic frame is done by cutting fixation pins to disengage them from the frame and/or by removal of portions of acrylic column (Figure 53-14). Acrylic frames can be cut with a cast saw, Gigli wire, OB wire, or a hacksaw blade. Different options available for staged disassembly of a Type II acrylic frame are shown in Figure 53-14. These include: 1) Removal of central segment of the lateral connecting column converts the construct to a Type I-a configuration (i.e. the lateral portion of the frame is now irrelevant mechanically). This strategy simplifies protective bandaging of the fixator compared to the next option; 2) Conversion to a Type I-a frame can also be accomplished by cutting all of the fixation pins as they exit laterally. This strategy may increase morbidity and make safe bandaging of the fixator more complicated; and 3) Cutting the central fixation pins on the medial aspect of the limb can be done as a later staged disassembly. This increases working length of the medial frame which, in turn, decreases frame stiffness.

Acrylic Frame Removal

Frame removal is achieved by cutting each fixation pin between the skin and acrylic column. Each pin is then removed using a hand chuck or pliers. Alternatively, the acrylic connecting bar can be cut between pins and each pin removed using the small block of acrylic as a handle.
 

Suggested Readings

Martinez SA, Arnoczky SP, Flo GL, Brinker WO. Dissipation of heat during polymerization of acrylics used for external skeletal fixator connecting bars. Vet Surg 26:290, 1997.

Ness MG. The acrylic and pin external fixator system. In Kraus KH, Toombs JP, and Ness MG: External Fixation in Small Animal Practice. Oxford: Blackwell Science Ltd, a Blackwell Publishing Company, 2003, p. 60.

Okrasinski EB, Pardo AD, Graehler RA. Biomechanical evaluation of acrylic external skeletal fixation in dogs and cats. J Am Vet Med Assoc 199:1590, 1991.

Shahar R. Evaluation of stiffness and stress of external fixators with curved acrylic connecting bars. Vet Comp Orthop Traumatol 13:65, 2000.

Staumbaugh JE, Nunmaker DM: External skeletal fixation of comminuted maxillary fractures in dogs. Vet Surg 2:72,1982.

Tomlinson JL, Constantinescu GM: Acrylic external skeletal fixation of fractures. Comp Cont Educ 13:235,1991.

Willer RL, Egger EL, Histand MB: A comparison of stainless steel versus acrylic for the connecting bar of external skeletal fixators. J Am Anim Hosp Assoc 27:541-548,1991.

Herndon GD, Egger EL: The effect of contouring the connecting bar in an acrylic-pin external fixator. Vet Comp Orthop Traumatol 14:190, 2001.

Roe SC, Keo T: Epoxy putty for free-form external skeletal fixators. Vet Surg 26:472, 1997.

Amsellem PM, Egger EL, Wilson DL: Bending Characteristics of PMMA columns, connecting bars of carbon fiber, titanium, and stainless steel used in external skeletal fixation and an acrylic interface. Vet Surg 39: 631-637, 2010.

Case JB, Egger EL: Evaluation of Strength at the Acrylic-Pin Interface for Variably Treated Exteranl Skeletal Fixator Pins. Vet Surg 40:211-215, 2011.
 

Introduction

The Securos external fixation system was designed to be a simple and economical orthopedic device, which clinically enables state-of-the-art techniques to optimize mechanical and biologic requirements for bone healing with external fixators. These techniques include adding and subtracting fixator clamps transversely, stronger connecting frames, guide for pre-drilling pilot holes and placing full pins, radially preloaded positive profile fixation pins, and axial dynamization. The Securos and SK systems have supplanted the Kirschner-Ehmer fixator system in veterinary surgery.

Fixation Pins and Connecting Rods

Fixation pins are available in four sizes: 1.6 mm (1/16th inch), 2.4 mm (3/32nd inch), 3.2 mm (1/8th inch), and 4 mm (5/32nd inch) shaft diameters. Both end threaded and center threaded pins are available in each size. The pins are made of 316L stainless steel that has been hardened to 210,000 psi, far greater than the stiffness of Steinman pins, and act much like locking orthopedic screws. The thread profile is like an orthopedic screw called a buttress thread and is self-tapping (Figure 53-15). This thread profile results in less bone being removed during insertion therefore less damage to the bone. The diameter of the core of the pin in the area of threads is 2% larger than the pilot hole and shaft diameter of the pin. As the pin is inserted, the slightly larger diameter in the area of the threads that engages bone expands on the hole slightly. This effect, called radial preload, enhances the pin-bone interface. The connecting rods are 9.5 mm for large, 4.8 mm for medium and 3.2 mm for small fixators. The small and medium connecting rods are 308 stainless steel, which is stiffer and stronger than 316 stainless steel. The connecting rods for the large fixator are either carbon fiber, or titanium which are both stronger and lighter than 316 stainless steel.
 

Clamps

There are three sizes of clamps. Small and medium sized clamps are composed of three components, a U-shaped body, a pin-gripping head, and a bolt (Figure 53-16). Similar components comprise the large clamp except that the clamp body has two parts (Figure 53-16). The large clamp accommodates 3.2 mm and 4mm fixation pins, the medium clamp accommodates 3.2 mm and 2.4 mm inch fixation pins, and the smaller clamp accommodates 2.4 mm and 1.6 mm fixation pins. The U-Shaped part and the head can be placed together then slid over a fixation pin and snapped transversely on a connecting rod (Figure 53-17). A bolt screws into the head component. As the head part is drawn into the U-shaped part a bevel on the head part contacts the connecting rod. At this contact area there is a small deformation of the stainless steel that rigidly unites the clamp, pin, and connecting rod, much like a spot weld. The U-shaped component only bends elastically. Thereby during use it is acting like a lock washer preventing loosening. The clamps do not plastically deform with proper usage and therefore can easily be reused. Double connecting clamps are made by using two U–shaped components, a head component, a longer bolt, and small sleeve (Figure 53-18). Two new or used U-shaped components and one new or used head component can be used with the longer bolt and sleeve, obviating the need for separate complete double clamps.
 

Aiming Instrument

An aiming instrument is available for all three sizes and allows simple pre-drilling of pilot holes, and accurate placement of half-pins or full-pins (Figure 53-19). The handle contains a drill sleeve for drilling pilot holes for fixation pins. Once two pins are placed and connecting rods are installed, the handle connects to the connecting rod. The drill guide places a fixation pinhole in exact relationship to the connecting rod for application of a clamp. The pin can be angled proximally and distally up to 30 degrees, and can also be angled either cranially or caudally. With the drill sleeve removed, the handle directs the fixation pin to the pilot hole. If a full-pin is being installed, an arm on the aiming instrument is used to direct the fixation pin to the exact position on the opposite connecting rod to install a clamp. The pilot hole and fixation pin can be directed to either side of the opposite connecting rod and angled proximally and distally as much as 30 degrees.
 

A unique feature of the Securos system is a method of simply changing the fixation frame to allow weight bearing forces to go through the long axis of the bone (axial dynamization) without removing fixation pins. In bilateral fixators, the clamp bolt can be replaced with one that is slightly longer. This bolt has a square head instead of a hexagonal head for easy identification. This allows the clamps to slide along the connecting rod, but the pin is fixed to the clamp (Figure 53-20). Therefore weight bearing will cause pure axial loads to be exerted on a healing fracture while the bone is supported in torsion, translation and bending.
 

Application Technique

The fracture is reduced and a proximal fixation pin and distal fixation pin are placed near the ends of the long bone. Connecting rods are secured to the fixation pins with clamps and the clamps are tightened. Clamps are not pre-placed on the connecting rods. The aiming instrument is used to place additional fixation pins. In placing half-pins, only the handle of the aiming tool is used (Figure 53-21). It is placed on the connecting rod and a drill sleeve is inserted. An intramedullary pin is advanced to the desired location and used as a trochar to locate proper placement in bone. The aiming instrument is tightened to maintain its position on the connecting rod. A releasing incision is made and the drill sleeve is advanced to bone. The Steinmann pin is removed then a pilot hole is drilled with a twist drill bit. A pilot hole the same diameter of the shaft of the fixation pin is used (1.6 mm, 2.4 mm, 3.2 mm, 4 mm). There is a separate drill sleeve for each drill bit size.
 

After the pilot hole is drilled, the drill sleeve is removed and the fixation pin is inserted. The aiming instrument will guide the fixation pin to the pilot hole. The pin should be placed with a power drill capable of spinning a low speed while still providing high torque. The fixation pin is placed so that it penetrates both corticies such that only the trocar point can be felt protruding from the far cortex. The aiming instrument is then removed. A clamp is then applied by placing the U-shaped body component with the head shaped component and sliding it over the fixation pins. Together, they are snapped on the connecting rod. The bolt is then inserted and tightened. The larger clamp is placed somewhat differently in that the two body components are placed on the connecting rod, then the head component, then the bolt.

Full-pins in bilateral fixators are placed in similar manner, but the arm on the aiming instrument is used. The most proximal and distal fixation pins are placed with connecting rods on both medial and lateral aspects of the limb. The aiming instrument is placed on either connecting rod with the arm in place (Figure 53-22). There are two grooves on the far end of the arm. The arm is slid so that the opposite connecting rod rests in either one of these two grooves. A Steinman pin is inserted into the drill sleeve and through skin to see whether it will contact bone. A Steinman pin is also inserted in a hole between the two grooves on the arm and through skin, again to see whether it will contact bone. This assures that in this position a full-pin will have sufficient bone purchase. If in the first position there is not sufficient pin purchase, then the other groove in the arm of the aiming tool is used. If these two positions do not result in adequate pin purchase, the handle of the aiming instrument is flipped over so that the fixation pins starts from the opposite side of the connecting rod. This allows four possible positions to accomplish secure full-pin fixation. If none of these positions result in being able to place a properly-centered full-pin, then a half-pin is placed instead. Pilot holes for full-pins are drilled in similar manner to that described for half-pins, the drill sleeve is removed, then the full fixation pin is placed. It will advance through the hole on the arm of the aiming instrument. The instrument is then removed and clamps slid on the fixation pins then snapped on the connecting rod and tightened (Figure 53-23).
 

Suggested Readings

Kraus KH, Toombs JP, Ness MG. External Fixation in Small Animal Practice. Oxford: Blackwel Publishing, 2003, 43.

Kraus KH, Wotton HM: Effect of clamp type on four-pin type II external fixator stiffness. Vet Comp Orthop and Traumatology, 12:178, 1999.

Kraus KH, Wotton HM, Rand WM: Mechanical Comparison of Two External Fixator Clamp Designs. Vet Surg 27:224, 1998.

Kraus KH, Wotton HM, Schwartz LA, et. al. Type-II external fixation using new clamps and positive-profile threaded pins, for treatment of fractures of the radius and tibia in dogs. J Am Vet Med Assoc 212:1267, 1998.
 

Introduction

In order to improve the performance of external fixators in small animal patients, newer devices have addressed the following problems characteristic of the Kirschner-Ehmer (KE) splint: 1) weak frame components often necessitate the use of complex full-pin frames; 2) single clamps do not easily accommodate positive profile fixation pins; 3) fixation pin size is dictated by clamp size and the use of different pin diameters within a single construct is difficult; 4) clamps are susceptible to permanent deformation and loosening; and 5) clamps cannot be easily added to or subtracted from the middle portion of a construct. The IMEX^TM SK^TM external fixator was designed to overcome all of these problems.

Application of axial compression to a unilateral K-E splint in a fracture gap model reveals the connecting rod to be the weak link in the construct. With the K-E splint, this weakness is compensated for by using an aggressive Type II or Type III frame when dealing with an unstable comminuted fracture. Although use of multiple full-pins improves mechanical performance of the external fixator, it often does so at the expense of increased patient morbidity attributable to full-pins traversing a thick layer of soft tissue on one side of the limb.

Design of the SK fixator is based upon the use of larger connecting rods made of strong, light-weight material (carbon fiber composite or titanium). Increased connecting rod strength enables the use simpler, half-pin, Type I-a or Type I-b frames to successfully manage unstable comminuted fractures with the SK^TM device. This in turn reduces the amount of soft tissue that will be penetrated by the fixation pins, thus reducing patient morbidity.

Components of the SK External Fixator

Clamps

Both single clamps and double clamps are available (Figure 53-24). Single clamps are used for attaching fixation pins to a connecting rod and double clamps are used for making rod-to-rod connections between fixation frames that have been applied in different planes. SK clamps are available in 3 different sizes: mini, small and large (Figure 53-25).
 

The SK single clamp is comprised of B-1 and B-2 aluminum body parts, and stainless steel components including a primary pin-gripping bolt with a slotted washer, a nut to tighten the primary bolt, and a secondary bolt. Correct assembly of the clamp is shown in (Figure 53-26). The clamp is symmetrically tightened by a secondary bolt on one side of the clamp and by a primary bolt and a nut at the opposite end of the clamp. The slotted washer on the primary pin-gripping bolt enables the clamp to securely grip a wide variety of different fixation pin diameters. Fixation pin sizes, connecting rod materials and diameters, and the appropriate wrench size specific to each clamp size are summarized in Table 53-3.
 

Connecting Rods

Mini SK connecting rods are 3.2 mm in diameter, available in lengths ranging from 50 mm to 150 mm, and are made of stainless steel. Small SK connecting rods are 6.3 mm in diameter, available in lengths ranging from 50 mm to 250 mm, and are made from either carbon fiber composite or titanium. Small carbon fiber composite rods have similar bending stiffness to the 4.8 mm stainless steel connecting rods utilized by the size medium K-E splint, whereas small titanium connecting rods are twice as stiff. Large SK connecting rods are 9.5 mm in diameter, available in lengths ranging from 50 mm to 350 mm, and are made from either aluminum or carbon fiber composite. Large SK rods offer a four-fold increase in bending stiffness compared to small titanium connecting rods.

Fixation Pins

During the 1980s, small animal surgeons began to use positive profile threaded fixation pins in external fixator constructs. Early experience was gained with some of the smaller diameter implants designed for human patients such as the centrally-threaded skeletal traction pin (Synthes) and the end–threaded Turner hip pin (Zimmer). Although improved results were seen with these implants compared to the use of smooth fixation pins, many of the pins specifically designed for ESF in humans were too large to enable safe use in dogs and cats. The negative profile end-threaded fixation pins designed for small animal patients (Ellis^TM pin from Kirschner and SCAT^TM pin from IMEX) offered only modest improvement compared to results obtained with smooth pins. In the early 1990s positive profile threaded pins were developed specifically for use in small animal patients. These implants have greatly improved the success rate of ESF in challenging fracture cases.

Positive profile end-threaded half-pins (INTERFACE^TM pins) and centrally-threaded full-pins (CENTERFACE^TM pins) made for the SK fixator are summarized in Tables 53-4 and 53-5. These fixation pins are available with a standard or cortical thread profile for use in diaphyseal bone, and a cancellous thread profile for use in soft metaphyseal bone (Figure 53-27). Cancellous thread versions feature a greater thread diameter, deeper threads and a larger pitch than compared to pins with cortical thread. Use of cancellous pins should be confined to the proximal metaphysis of the tibia, the distal metaphysis of the femur, and the proximal metaphysis of the humerus. Their use in hard diaphyseal bone is contraindicated. Fixation pins with extended thread length are available and are occasionally required in order to fully purchase the increased diameter of the bone in some metaphyseal locations.The majority of pin sizes are available with either a trocar point or with an atraumatic NP (no point) tip (see Figure 53-27). Since NP pins to not have a cutting trocar point, the surgeon is forced to use proper pre-drilling technique to apply them. Compared to pins with a trocar point, NP pins require slightly greater insertional force until the initial threads engage and cut threads in the near cortex of the bone. After that, the gear effect of pin threads moving on bone threads allows the fixation pin to smoothly advance across the bone. The first version of the NP pin had a rounded tip, and its ability to accurately center itself in the pre-drilled hole was slightly less than that of a pin with a trocar tip. The tip of the NP pin was later revised to a blunted trocar tip to improve the ability of the pin to properly center within the pre-drilled bone hole.
 

The most recent development in fixation pin technology is the DURAFACE pin (Figure 53-27). It is a pin with a larger diameter smooth shaft and a taper run-out junction leading to a negative profile thread at the end of the pin. Unlike other negative profile pins, this implant has improved mechanical performance compared to other pins with the same thread diameter, but does not have a stress concentration point at the smooth-threaded junction that could predispose bending or breakage of the implant. DURAFACE pin options are summarized in Table 53-5.

Application Techniques

The slotted washer on the primary pin-gripping bolt enables the use of a wide range of different pin sizes for each SK clamp size (Tables 53-3 and 53-6). The curvature of the meniscus in the washer corresponds to the smallest pin shaft diameter that can be gripped by the primary bolt. The hole in the primary bolt is large enough to accommodate sleeved pre-drilling and application of a positive profile pin directly through the bolt. The diameter of the pin-gripping channel in the primary bolt determines the maximum diameter of a positive profile threaded pin that can be passed through it. When a larger threaded pin is desired, sleeved pre-drilling of the bone is done through the clamp, the clamp is temporarily removed, the pin is applied to the bone, the pin-gripping bolt is applied to the smooth shaft of the pin, and the clamp is re-assembled to attach the pin to the rod. This technique is applicable when a size medium cancellous INTERFACE half-pin is used with a small SK clamp at positions other than the most proximal and most distal ones within a construct. The shaft and thread diameters of this pin are 3.2 mm and 4.8 mm respectively, and the diameter of the pin-gripping channel in the primary bolt of a small clamp is 4.0 mm. Although the threaded diameter won’t pass through the clamp, the primary bolt is able to grip the shaft diameter of the pin.

The slotted washer of the primary bolt has a multi-toothed surface that engages the outer surface of the clamp body when the clamp bolt is tightened (Figure 53-28). This provides positive retention between the washer and the clamp body thus eliminating pin-bolt slippage in relation to the connecting rod. The circular shape of the serrated area on the washer makes its positive retention capability function at any desired angle using either half-pins or full-pins.

The split body design of the SK clamp allows for easy addition or subtraction of a clamp from a construct without taking the frame apart (as would be necessary with a KE splint). Primary and secondary bolts enable symmetrical tightening of the clamp to securely grip both the fixation pin and the rod. This is accomplished without deforming the clamp body.

In the early phase applying a linear fixator, disruptive torque forces produced by the tightening of the first several clamps may cause loss of fracture reduction or alignment. SK clamps have a feature that makes it easy to counter these forces. The flat surfaces on the end of the primary pin-gripping bolt and the flat surfaces on the assembled clamp body (Figure 53-29) are the same dimension as the wrench used to tighten the clamp. A second wrench can be applied to either of these surfaces to counter disruptive torque forces during clamp tightening.

While the secondary bolt allows for symmetrical tightening of the SK clamp, it also enables an empty clamp to serve as a targeting device. For example, when the surgeon wants to place a pin in the same plane as the pin adjacent to it, this is accomplished a follows. A drill sleeve is inserted through the pin-gripping channel of the primary bolt (Figure 53-30) of a loose clamp placed on the connecting rod. The clamp is rotated until the long axis of the drill sleeve is a plane identical to that of the fixation pin adjacent to it. The secondary bolt is tightened to maintain this orientation and the nut on the primary bolt is partially tightened to secure the drill sleeve (NOTE – over-tightening of the nut will crimp the wall of the drill sleeve which is to be avoided). The bone is pre-drilled through the sleeve, the primary bolt is loosened to remove the sleeve, and the threaded fixation pin is inserted through the clamp and into the bone. Regardless of the desired plane of pin insertion, the secondary bolt can be used to stabilize the position of the clamp/drill sleeve unit to facilitate accurate pre-drilling.
 

Application of a Type I-a Construct

The fracture is reduced (hanging limb technique is useful for accomplishing this in fractures of the radius / ulna or tibia) and a proximal fixation pin and a distal fixation pin are placed near the ends of the bone. The example shown in Figures 53-31 and 53-32 involves fixator application to the tibia, in which the fixation pins are passed in a mediolateral plane through the medial aspect of the bone (preferred anatomic corridors for fixation pins in other bones has been covered in the earlier chapter – Basic Principles for the Application of External Fixators). At each intended pin placement site, a liberal release incision at least 1 cm in length is made through the skin and soft tissues over the center of bone. Placement of a miniature Gelpi retractor in the incision is helpful for maintaining exposure. Pre-drilling is done through the release incision using a drill sleeve to protect the soft tissues and a drill bit that is equal to the core diameter of the fixation pin. Each fixation pin is applied to the pre-drilled hole using slow speed power insertion technique. A connecting rod is secured to the first two pins using SK single clamps. Considerable torque force occurs as these clamps are tightened. A second wrench should be used to neutralize forces that could disrupt fracture alignment as the clamps are tightened.
 

Empty clamps to accommodate the anticipated number of additional fixation pins required can be pre-placed onto the connecting rod or added later (Figure 53-31). A release incision at least 1 cm in length is made at the next pin placement site. An empty clamp is positioned over the release incision and a drill sleeve is inserted through the hole in the clamp bolt down to the level of the bone. After the clamp and drill sleeve are oriented to provide proper centering of the hole that will be pre-drilled through the bone, this position is maintained by tightening the secondary bolt and gently tightening the nut on the primary bolt to secure the drill sleeve. Pre-drilling of the near and far cortex is done with a twist drill bit. The nut on the primary bolt is loosened to enable removal of the drill sleeve and a fixation pin is applied through the hole in the primary clamp bolt and advanced into the pre-drilled hole in the bone using slow speed power insertion technique. It is important for the threads of the pin to fully engage the far side of the far cortex of the bone. In order to accomplish this, several millimeters of the tip of the pin must extend into the soft tissues beyond the far cortex. If vital anatomic structures are likely to be present in this location, a NP pin should be used. The clamp is secured by alternate tightening of the secondary bolt and the nut on the primary bolt. These steps are repeated at each pin placement site until at least three fixation pins have been placed both proximally and distally. The order of pin placement is generally as follows: the most-proximal and most-distal pins are placed first; the central pins immediately above and below the fracture region are placed next; and pins in intermediate locations are placed last (Figure 53-32). Fixation pins should not be trimmed until acceptable fracture alignment has been verified on post-operative radiographs. Each pin should then be trimmed such that the cut edge stops short of the outer surface of the clamp.

Application of a Type I-b Construct

For comminuted shaft fractures, a Type I-a construct may not provide sufficient stability. In these cases, a second Type I-a frame is applied in a different plane (orthogonal to the first frame is optimal mechanically). For the tibia this would involve application of fixation pins in a craniolateral plane through the cranial aspect of the bone (Figure 53-33).
 

Linkages are sometimes made between the lateral frame and the cranial frame to improve construct rigidity. These connections can be made proximally and distally (See Figure 53-33) or diagonally (Figures 53-34 and 53-35). Diagonal connections provide greater strength because they span the fracture region. Linkages can be built using double clamps (Figures 53-33 and 53-34) or by leaving selected fixation pins long and placing additional single clamps on the pins external to the frames (Figure 53-36) and connecting these “stacked” clamps with a rod.

Application of a Type II Construct

Some surgeons prefer to use a Type II frame (instead of Type I-b) for challenging shaft fractures. For the tibia, this entails application of at least two full-pins in a mediolateral plane through the medial aspect of the bone. The remainder of the frame is often built with medially applied half-pins resulting in a minimal or modified Type II construct (Figure 53-35).

A full-pin is applied using the same techniques described for the placement of a half-pin except that a second release incision must be made laterally to enable the full-pin to exit on the opposite side of the leg. After a full-pin has been placed in both the proximal and distal ends of the bone, these are connected medially and laterally with connecting rods and SK clamps (Figure 53-36). The remainder of the construct is completed by applying the required number of additional fixation pins from the medial side of the tibia. Half-pins or full-pins or a combination of these may be used to complete the fixator, however, a full-pin at the most proximal location on the tibia tends to cause higher postoperative morbidity than a medially placed half-pin at this position. This is due to the pin traversing a thick layer of soft tissue on the lateral aspect of the leg in a high motion area near the stifle joint.
 

Staged Disassembly of SK External Fixators

Rigid constructs benefit revascularization of the injured bone and other early fracture healing events, but high fixator stiffness may actually delay the later stages of bone healing and remodeling. Strategic reduction of external fixator rigidity to benefit the later stages of healing is accomplished by a process called staged disassembly. This can be done in several ways: 1) simplifying the frame configuration (e.g. conversion of a Type I-b to a Type I-a); 2) downsizing the frame by replacement of the connecting rods and clamps with smaller components (Table 53-7 and Figure 53-37); and 3) by removal of fixation pins from the central portion of a frame (strategy used for Type I-a fixators).

In skeletally mature patients, staged disassembly should be initiated at approximately 6 weeks after surgery. In adolescent patients, this process can often be started at 3 to 4 weeks post-op. Staged disassembly can usually be done with the dog or cat under heavy sedation, but some patients may require brief duration general anesthesia with propofol. The fixation frame(s) should be temporarily removed to enable critical palpation of the fracture for evidence of callus formation (Figure 53-38). If the fracture feels “sticky” due to the presence of soft callus, it is appropriate to begin staged disassembly. If any of the fixation pins are causing morbidity, strongly consider removal of these fixation elements as part of the staged disassembly strategy. An example of this would be a Type I-b fixator applied to the radius in which fixation pins have been applied craniomedially and craniolaterally. Fixation pins of the craniomedial frame generally traverse less soft tissue than those of the craniolateral frame. On examination at 6 weeks after surgery, the pin tracts of the craniolateral frame might appear to be slightly inflamed compared to those of the craniomedial frame. If this was the case and the surgeon planned to convert the Type I-b frame to a Type I-a frame as part of the staged disassembly strategy, it would be logical to remove the craniolateral frame and its fixation pins.
 

Suggested Readings

Bronson DG, Toombs JP, Welch RD. Influence of the connecting rod on the biomechanical properties of five external skeletal fixation configurations. Vet Comp Orthop & Traumatol 16:8, 2003.

Lewis DD, Cross AR, Carmichael S, Anderson MA. Recent advances in external skeletal fixation. J Sm Anim Pract 42:103, 2001.

Toombs JP, Bronson DG, Ross D, Welch RD. The SK external fixation system: Description of components, instrumentation, and application techniques. Vet Comp Orthop & Traumatol 16: 76, 2003.

White DT, Bronson DG, Welch RD. A mechanical comparison of veterinary linear external fixation systems. Vet Surg 32:507, 2003.

Griffin H, Toombs JP, Bronson DG, et al: Mechanical evaluation of a tapered thread-run-out half-pin designed for external skeletal fixation in small animals. Vet Comp Orthop Traumatol 24:257, 2011.
 

Since the writing of the topic, methodology and nomenclature adopted from Dror Paley’s Principles of Deformity Correction have been adapted and become accepted as the convention in small animal orthopedics.

Introduction

Circular external skeletal fixation (CESF) was pioneered by the Russian physician, Gavriil Ilizarov. These are modular systems which can be assembled in numerous configurations to stabilize fractures and arthrodeses, perform bone lengthening and transport as well as correct angular, translational and rotational deformities and are being used with increased frequency in dogs and cats. Circular fixator (CF) frames consist of a series of complete and/or incomplete external rings that are interconnected by multiple threaded rods. Rings are secured in position along these rods by placing nuts on opposing surfaces of each ring. Circular fixators are uniquely designed, allowing the frame to be elongated or shortened during or following surgery. Elongation of the frame during the convalescent period allows for distraction osteogenesis in which regenerate bone is formed within the osteotomy gap resulting from gradual separation of the secured bone segments.

Components, Implants and Instrumentation

The IMEX^TM CESF System (IMEX^TM Veterinary, Inc., Longview, TX) is the CF system used most commonly by North American veterinarians. This system was developed in conjunction with the Comparative Orthopedics Research Laboratory of the Texas Scottish Rite Hospital for Children in Dallas, Texas and is modeled after a device utilized in human patients. This system has several evolutionary advances which simplify frame construction, improve precision and decrease patient morbidity. The utilization of lighter metals and engineered plastics facilitated this process. Several new components have been developed which substantially decrease the total number of parts necessary for frame assembly, thereby reducing pre-operative frame preparation time.

Traditional CF constructs consist of supporting elements (complete rings, partial rings and arches), connecting elements (threaded rods, linear and angular motors and hinge assemblies), fixation elements (small diameter wires) and assembly elements (cannulated and/or slotted bolts, nuts, washers, plates and posts). The following section describes components of the IMEX^TM CESF System.

Supporting Elements

Rings in this system are manufactured from a high-strength tempered aluminum alloy which imparts strength to the supporting elements while keeping the fixators weight-appropriate for use in dogs and cats (Figure 53-39). The rings have holes located about their circumference in which connecting and assembly elements are secured. Ring components are available in 50 mm, 66 mm, 84 mm and 118 mm internal diameters. While it is biomechanically preferable to utilize complete rings, anatomic constraints prohibit their use proximal to the elbow and stifle and often adjacent to other joints. Traditional CFs are mainly applicable for managing conditions involving or distal to the elbow or stifle, while hybrid linear-CF constructs (see section on Hybrid Constructs, Chapter 55) are typically used to manage injuries and abnormalities involving the humerus or femur. Five-eighths partial rings are often used to secure the proximal radius and distal tibia, while stretch ring arches have been developed which facilitate CF application to the proximal tibia and ulna. Stretch ring arches also simplify construction of CFs for transarticular stabilization of the hock and stifle regions. One-third partial ring arches are also available.
 

Connecting Elements

Threaded rods (6 mm thread diameter x 1 mm thread pitch) are the most commonly used connecting elements. Threaded rods are available in 60 mm, 80 mm, 100 mm 150 mm and 225 lengths with a 3 mm hex drive fitting at their ends to accommodate a 3 mm angled or straight hex driver. This hex broach fitting allows rapid replacement or exchange of rods if necessary.

A unique design feature of the IMEX^TM CESF System is its zero tolerance, zero motion connecting elements (Figure 53-40). Adjustable components used for angular and linear distraction have nylon drive bushings or inserts between metal parts which prevent binding, allowing adjustments to be made without loosening and retightening nuts. This makes the distraction process simple and precise by eliminating frame instability which causes patient discomfort. Distraction or compression is performed simply with a wrench, facilitating client compliance and negating the need for prolonged hospitalizations.
 

Linear motors, available in 50 mm, 70 mm and 100 mm lengths are composed of a threaded rod encased in stainless steel housing and are used to perform linear distraction or compression. A nylon drive bushing is positioned between the stainless steel housing and threaded rod (6 mm diameter x 1 mm thread pitch). During distraction or compression, the drive bushing allows distraction or compression without loosening and retightening nuts, eliminating frame instability and thereby minimizing patient discomfort. Distraction or compression is performed simply by turning the clearly marked drive bushing with a 10 mm wrench. One complete revolution of the drive bushing produces 1 mm of linear movement.

Threaded rods can also be used for linear distraction/ compression if 10 mm (6 mm thread diameter) paired nylon nuts are used to secure a ring to the rod. Simultaneous rotation of the paired nylon nuts with a double jawed 10 mm wrench which can engage nuts positioned on both sides of a ring will accomplish linear distraction/compression. The use of paired nylon nuts instead of linear motors to achieve linear distraction is most beneficial when adjacent rings are in close proximity. One complete revolution of the paired nylon nuts results in 1 mm of linear movement of the secured ring.

Connecting elements used for angular correction include hinge assemblies and angular motor assemblies. Hinge assemblies are used in pairs to provide pivot points between two rings. Angular motor assemblies provide asymmetric distraction of two rings articulated using paired hinge assemblies. Both elements have nylon inserts which confer zero tolerance, zero motion properties. Like the linear motor units, the angular motor assembly is clearly marked to aid in daily distractions during the convalescent period. Hinges and angular motor assemblies have a hex drive fastener which can be tightened once distraction is complete to lock the fixator in place. It should be noted that the hex drive fastener elements should be loosened prior to steam sterilization to prevent damage to the component as a result of expansion of the nylon insert.

Fixation Elements

Unlike linear fixator systems, traditional CFs use small diameter (1.0 or 1.6 mm) wires, rather than larger diameter pins, as fixation elements. Two wires are generally placed on each ring with the wires secured to opposing surfaces of the ring. The fixation wires are typically tensioned to improve their stiffness characteristics. Although standard Kirschner wires can be used as fixation wires, use of wires with an efficient single lip cutting point is recommended. Fixation wires are also available with olives (or stoppers) to increase stability of the construct and/or to manipulate and secure bone segments. Olive (or stopper) wires have a raised bead (olive) fixed along their length. This olive is brought into contact with the cortex of the bone. The olive can also be used to pull a bone segment into alignment and prevents translation of a secured bone segment along the wire. Inexpensive calibrated tensioning devices are now available to tension wires. Although not a part of Ilizarov’s traditional armamentarium, positive and negative profile partially threaded (end threaded) half-pins and positive profile partially threaded (centrally threaded) full-pins can also be used as fixation elements.

Assembly Elements

All assembly elements have 10 mm wrench flats or 3 mm hexagonal recesses, thus keeping instrumentation to a minimum (Figure 53-41). Ten mm (6 mm thread diameter) stainless steel nuts are also used to secure connecting and assembly elements. Fixation wires are secured to the rings with 6 mm wire fixation bolts which are both slotted (for capture of wires that cross rings between holes) and cannulated (for capture of wires that cross rings over a hole). Slotted 6 mm washers are available to capture wires at sites occupied by connecting elements and 6 mm flat washers are available to be used as a spacer for capturing wires that are not inserted immediately adjacent to a ring. Pin fixation bolts are also available which allow the utilization of half-pins and full-pins. The pin fixation bolts accommodate fixation pins ranging from 2.3 mm to 5 mm in diameter and are similar in design to the pin-gripping bolt of IMEX^TM SK^TM fixation clamp.

Hemi-spherical washers and hemi-spherical nuts are also available. When used in combination the hemi-spherical washers and hemi-spherical nuts allow for angulation of connecting rods. Thus, rings can be secured to each other without being in exact parallel alignment. This permits minor adjustments in reduction of fracture segments and fine adjustments in correcting angular deformities. When utilized with two-hole plates, the hemi-spherical washers and hemi-spherical nuts can be used to connect adjacent rings without utilizing corresponding holes and are particularly useful in constructing complex or transarticular frames.
 

Two-hole plates are available to allow the use of different diameter rings within the same fixator frame. The plate is bolted to the ring extending away from its center. A connecting rod or motor can then be attached to the plate and linked directly to the next larger diameter ring. One- and two-hole posts are utilized to secure fixation wires and pins elevated remote to the surface of a ring, to create hinge assemblies and to secure connecting elements that are not positioned perpendicular to the surface of a ring. Plates and posts are extremely useful when constructing transarticular or other complex frames.

Biomechanics

Circular fixators possess biomechanical characteristics which purportedly enhance fracture healing as well as allow for distraction osteogenesis. The biomechanics of CFs differ primarily from linear fixators in that the tensioned wires stabilizing the bone segments adequately resist bending, shear, and torsional forces while maintaining some degree of axial elasticity. Load/deformation curves of CF constructs undergoing axial compression have a characteristic initial exponential increase in stiffness which is ascribed to tensioning of the wires when subjected to loading. Construct stiffness increases until the slope of the load/deformation curve becomes linear with continued loading, protecting the osteotomy or fracture gap from excessive strain during ambulation. The “axial micro-motion” occurring at physiological loads purportedly creates a mechanical environment conducive to bone formation.

Numerous extrinsic (apparatus-related) factors have been shown to affect the stability of the fixation including the number, type, angle of intersection, applied tension and diameter of the fixation wires, as well as the number, conformation, diameter and position of the rings and connecting elements. Intrinsic factors which theoretically contribute to stability of the bone-fixator construct include the area of contact and nature of the interlock between bone segments, the modulus of elasticity of tissue between bone segments, and the tension of the regional soft tissues.

The number of levels of fixation influences the mechanical properties of any fixator construct. Ilizarov found that four-ring CFs (two rings per bone segment) were more stable than two-ring CFs (one ring per bone segment). Additional studies have shown that if a four-ring construct is used to stabilize a fracture, the stability of the CF is increased if the central two rings are positioned in close proximity to the fracture or osteotomy and the proximal and distal rings are positioned adjacent to the joints at the end of the each major bone segment. This distributes the weight-bearing forces evenly over the involved limb segment in a “far-near-near-far” arrangement.

Ring diameter is the single most important parameter influencing the biomechanical profile of any CF constructs. While ring diameter affects stability in all modes of loading, ring diameter has its greatest effect on axial stability. Ring diameter is selected based on anatomic constraints: the smallest diameter rings which can be accommodated should be selected; however, a minimum 1 to 2 cm of clearance should be maintained between the ring and the circumference of the limb to allow for soft tissue swelling and daily management of the wire-skin interfaces.

Since the diameter of rings used in dogs and cats is much smaller than those used in human patients, even children, the biomechanics of CFs used in dogs and cats are markedly different from those used in human patients. Several biomechanical studies have been done evaluating IMEX^TM CF constructs and it appears that there is little need to tension wires when using the 50 mm and possibly the 66 mm rings (although wires are usually tensioned to 30 kg when using 66 mm rings). Tensioning of wires on larger diameter rings is warranted with the recommendation to apply 60 kg of tension when using 84 mm rings and 90 kg of tension when using 118 mm rings. Some surgeons advocate simultaneously tensioning wires secured to the same ring (Figure 53-42) and wires secured to partial rings or posts should not be tensioned beyond 30 kg to avoid deformation of the ring or posts.

Olive wires can enhance the stability of fixation. Placing two opposed olive wires to secure a bone segment can significantly improve bending stiffness and stability by minimizing translation of the secured bone segment along the wire. This is particularly important when wires are placed on the same ring with little divergency. Opposing interfragmentary olive wires can also be used to compress anatomically reduced long oblique or spiral fractures.

CF constructs utilizing tensioned wires in combination with half-pins or full-pins are being used with increasing frequency. The use of half-pins has been advocated in locations where divergent fixation wires would pass through prominent muscle masses, such as the proximal tibia, or near vital soft tissue structures. These constructs have been shown to have biomechanical characteristics intermediate between those of conventional linear fixators and traditional CFs. The combination of wire and half-pin fixation can be problematic. When used in combination with wires, a single or an inadequate number of half-pins may be subjected to excessive loading as the wires initially deform when subjected to loading. Thus, if the number of half-pins utilized is not sufficient, excessive stress occurs at the pin-bone interface. The use of three (or preferably more) evenly distributed, divergent half-pins per bone segment (depending on concurrent wire utilization) is advocated in these configurations to avoid problems associated with premature pin loosening and pin tract drainage.
 

Distraction Osteogenesis

Distraction osteogenesis describes the mechanical induction of new bone formation in the gap produced by the gradual separation of two bone segments. Much of what is known regarding the biology of distraction osteogenesis was elucidated by Ilizarov and his colleagues; however, recent investigations have focused on the cellular and molecular events of bone formation in both fracture healing and distraction osteogenesis. Distraction osteogenesis shares many morphologic and biomechanical similarities with early fracture healing. Bone retains the inherent capacity to remodel and repair and these processes are influenced by the local mechanical environment. The new bone which forms in the distraction gap during distraction osteogenesis is referred to as “regenerate” bone (Figure 53-43).
 

Cyclic axial loading is necessary for remodeling and maintaining bone mass and numerous experimental and clinical studies suggest that axial dynamization accelerates fracture healing. Traditional CFs allow some degree of axial micro-motion, while providing adequate bending and torsional resistance. Clinical studies evaluating the use of CFs to manage fractures in dogs and cats support the contention that CFs promote rapid fracture healing.

Ilizarov advocated performing a corticotomy, which preserved both periosteal and endosteal tissues, for optimal regenerate bone formation during distraction osteogenesis. Recent clinical and experimental studies, however, have shown that preservation of the periosteum has the most significant influence on regenerate bone formation: the method utilized to perform the osteotomy (Gigli wire, bone saw, drill holes-osteotome) has a nominal effect on regenerate formation as long as the periosteal envelope is preserved and most small animal surgeons perform subperiosteal osteotomies using a pneumatic oscillating saw.

Latency or delay refers to the time period following osteotomy before beginning distraction. The latency period used in human patients is typically 4 to 7 days. Several factors will influence the prescribed latency period: the patient’s age, the bone involved, the location of the osteotomy, soft tissue trauma present prior to or incurred during surgery, and the primary condition necessitating treatment. Metaphyseal lengthenings produce higher quality regenerate bone than diaphyseal lengthenings. The metaphyseal region has a greater blood supply and bone surface area in comparison to diaphyseal bone. Proximally located osteotomies produce higher quality regenerate bone than more distally located osteotomies. The latency period allows early vascularization and soft callus formation before lengthening commences. Poor regenerate formation and non-union can occur if distraction is initiated too early. Premature consolidation can occur if the latency period is too prolonged, particularly in young or skeletally immature animals.

The recommended latency period prior to initiating distraction is typically short in dogs undergoing lengthening or angular correction. In young dogs in which the periosteal sleeve was well preserved, a delay period may be unnecessary. Most small animal surgeons generally initiate distraction 1 to 3 days following surgery in dogs in this age group. It is prudent to observe a 3 to 5 day delay before initiating distraction with animals that are 3 to 8 years of age. Longer delay periods may be advisable in older dogs or if the periosteum had been damaged substantially prior to or during surgery.

Rate refers to the amount of distraction that will be performed over a 24 hour period. Experimental and clinical studies indicate that the amount of lengthening performed should be in the range of 0.5 to 2.0 mm/day to promote viable regenerate bone formation. The formation of regenerate bone can be monitored radiographically and the rate adjusted accordingly. Rates for skeletally immature patients undergoing metaphyseal osteotomies may be near the higher limit as these animals have a greater osteogenic potential.

Rhythm describes the frequency (number of fractionations) at which the distractions are performed during a 24 hour time period. Ilizarov had reported that increasing the rhythm from 1 or 2 times per day up to 60 times per day significantly increased regenerate formation and decreased consolidation times; however, studies evaluating rhythms of 1, 4, and 720 times per day in a caprine lengthening model found no significant effects of rhythm on radiographic, mechanical, or histomorphologic regenerate parameters. Increased rhythms, however, allow for superior accommodation of the regional soft tissues, decreasing morbidity during the distraction period. In our clinic we generally perform distractions at a rate of 1.0 to 1.5 mm/day using a rhythm of three or four distractions/day.

Clinical Applications in Dogs and Cats

Circular fixators have been used to manage a number of developmental and traumatic orthopedic conditions in dogs and cats. The most notable of these being limb deformity correction, most frequently antebrachial limb deformity correction. Pre-operative assessment and planning, a thorough knowledge of the instrumentation and its application and conscientious post-operative patient care are essential for a successful outcome. Traditional CFs have also been used to perform deformity corrections and lengthenings of the crus and pes. These systems are also useful for stabilizing complex fractures of the antebrachium and crus, as well as transarticular stabilization, particularly in performing arthrodeses. Frames utilizing hinges allow the surgeon the latitude to adjust the angle of arthrodesis during the early convalescent period and the use of these devices facilitates the removal of all implants following fusion. Finally, CFs can be used to perform bone transport to resolve large traumatic segmental bone defects and oncologic surgeons are now utilizing bone transport in limb salvage procedures in dogs with appendicular bone tumors (see section on Distraction Osteogenesis as an Alternative to Bone Grafting in Chapter 56).

Fracture Management

Circular external skeletal fixation has been utilized extensively for fracture management in human patients and there are recent reports describing the use of CFs for fracture management in dogs and cats. Traditional CFs are most applicable for the stabilization of non-articular antebrachial and crural fractures. Circular fixators are particularly useful for stabilizing fractures with short juxta-articular fracture segments as the divergent placement of small diameter wires provides multiplanar stability. With experience, a surgeon can achieve accurate closed reductions of both simple and complex fractures with relatively short operative times.

Frames are constructed prior to surgery based on preoperative radiographs of the fractured and contralateral intact (if applicable) limb segment. When constructing the fixator, complete rings are generally used to secure the middle and distal portions of the limb segment. Partial rings are used proximally to avoid soft tissue impingement or compromised joint mobility. Stretch rings are useful for securing the proximal ulna and tibia, while 5/8th rings can be used to secure the proximal radius or if the most distal ring interferes with carpal or hock motion. The smallest diameter rings that can be comfortably placed about the circumference of the limb, allowing for post-operative swelling without soft tissue impingement, should be selected. Pre-construction of a frame greatly reduces surgical time. Minor adjustment of the frame should be anticipated and performed as necessary at the time of surgery.

A standard frame configuration consists of three or four rings. A single ring or pair of rings that engage a fracture segment and which are secured together by connecting elements constitute a functional unit referred to as a ring block. While it is preferable to use two or more rings to construct a ring block, there may only be sufficient room to accommodate a single ring in fractures with a short proximal or distal segment. A typical CF construct that would be used to stabilize a crural fracture is composed of two independent ring blocks articulated by linear motors or threaded connecting rods which are secured only to the rings positioned adjacent to the fracture site. This arrangement allows simple adjustment of the distance between the two ring blocks, allowing the major fracture segments to be distracted or compressed. Thus, the frame can be used intra-operatively to distract the fractured limb segment to its normal length which greatly facilitates reduction.

When constructing a CF that will be used to distract a crural fracture out to length, the two ring blocks are constructed based on the length of the major fracture segments. Appropriate length of each ring block is confirmed by measuring each ring block against the fracture segment it will be used to stabilize on the lateral view radiograph. The articulating intermediate linear motors or connecting rods are then placed between the two ring blocks and the construct is placed over a lateral view radiograph of the contralateral intact limb segment (if available) to assess appropriate frame length (Figure 53-44A). The most proximal and distal rings should be placed at or near their respective metaphyses. The CF is then positioned so that the lateral radiographic image of the intact tibia is appropriately situated within the frame and each ring should be marked, both medially and laterally, along the tibia’s central longitudinal axis with a permanent marker. Thus reasonable reduction can be achieved at surgery by placing fixation wires through the tibia in a medial-to-lateral plane and attaching each wire to its corresponding ring at the marked location, if the limb segment has been distracted out to normal length. The frame is then placed over the lateral view radiograph of the fractured limb segment and the intermediate linear motors or connecting rods are compressed to account for shortening of the limb segment as the result of the fracture. The frame is then sterilized in preparation for surgery.

When applying the fixator at surgery, the dog is positioned in dorsal recumbency and the CF construct is slid over the limb and a wire is placed in each metaphysis, parallel to both the proximal and distal joint surfaces. These wires should be placed in the medial-to-lateral plane. The use of intra-operative fluoroscopy, if available, facilitates proper wire placement. These initial two wires are then attached to the abaxial surface of the most proximal and distal rings at the predetermined locations as marked on the frame prior to surgery (Figure 53-44B). The wires are tensioned if indicated depending on ring diameter. If the fracture is over-ridden, the distance between the proximal and distal ring blocks, which are now secured to the bones via the fixation wires, can be increased by turning the intermediate linear motors or the nuts securing the intermediate connecting rods to bring the limb segment out to length. Distraction will create tension in the regional soft tissues which will help reduce the fracture (Figure 53-43C). An attempt should be made to “over-distract” the fracture by a couple of millimeters. Alignment of the fracture can be assessed by palpation, or by fluoroscopy if available.

The next two wires should be placed in the medial-to-lateral plane through the longitudinal axis of the tibia adjacent to the intermediate rings (Figure 53-44D). Attaching (and, if necessary, tensioning) these wires at the predetermined locations as marked on the frame prior to surgery, should result in reasonable craniocaudal alignment of the fracture (Figure 53-44E). If one or both fracture segments need(s) to be transposed in a medial or lateral direction, the segment(s) can be translated along the initial fixation wire(s) by simply applying digital pressure to the bone segment(s) (Figures 53-44F and G). Alternatively, olive wires can be used to translate bone segments. An olive wire is placed on the appropriate, intermediate ring in the medial-to-lateral plane with the olive positioned adjacent to the cortex on the side of the bone which is to be pulled into place. By using the tensioner, which is placed on the exposed end of the wire opposite the olive, the olive wire along with the bone segment can be translated toward the tensioning device.

If a bone segment needs to be translated cranial or caudal, again an olive wire can be used, but this can cause unnecessary impingement of the regional soft tissues. Alternatively, reattaching one or potentially both of the wires on the intermediate ring at holes immediately cranial or caudal (direction opposite of the displacement) to its original position will result in bowing of the wire as it is reattached to the ring with fixation bolts. As the wire is retensioned, the bow in the wire will be eliminated and the bone segment will be translated in the desired direction.

If the fracture was slightly over-distracted, the distance between the ring blocks should be decreased, restoring normal length to the limb segment. Once reduction is acceptable, the remaining fixation wires are placed to complete the construct. Two additional wires should be placed on each ring. These wires should be oriented at 45° to 90°‚ to each other and olive wires should be used to minimize translation of bone segments. Fixation wires should be placed parallel to the surface of the rings. Wires that are not in immediate contact with the surface of the ring should be secured with flat washers placed subjacent to the wire when it is secured with a fixation bolt. If the wire is bowed as it is attached to the ring, displacement of the bone segment will occur. Proper tensioning of wires will also maximize stability. It is prudent not to cut the fixation wires too short or to bend the wires over until the fracture reduction is evaluated radiographically. This makes any necessary post-operative adjustments simpler to perform.

Isolated double ring block constructs are generally not used to stabilize radius and ulnar fractures as suspension of the limb can be used to facilitate reduction of antebrachial fractures. A typical CF construct that would be used to stabilize an antebrachial fracture consists of three or four rings, all of which are interconnected by long threaded connecting rods which span the entire length of the frame. The construct is assembled and laid on the lateral radiographic view of the contralateral intact limb segment (if available) to assess that the frame length is appropriate with the most proximal and distal rings positioned at or near their respective metaphyses. Position of the intermediate ring is confirmed by comparing its distance from the corresponding proximal or distal ring to the length of the fracture segment those two rings will secure. The frame is then repositioned over the lateral radiographic image of the intact antebrachium such that the radius is appropriately situated within the frame and each ring should be marked, both medially and laterally, along the radius’ central longitudinal axis with a permanent marker. Again reasonable reduction should be achieved at surgery by placing fixation wires through the radius in a medial-to-lateral plane and attaching each wire to its corresponding ring at the marked location, but in this case suspension of the limb will be used to distract the limb segment out to normal length. The frame is then sterilized in preparation for surgery.

The dog is positioned in dorsal recumbency for surgery and the CF construct is slid over the limb. The limb is then suspended from the ceiling to distract the limb segment out to length. Tension in the regional soft tissues should again help reduce the fracture (Figure 53-45A). Wires are placed in each metaphysis, parallel to both the proximal and distal joint surfaces. These wires should be placed in the medial-to-lateral plane (Figure 53-45B). The use of intra-operative fluoroscopy, if available, facilitates proper wire placement. These initial two wires are then attached to the abaxial surface of the most proximal and distal rings at the predetermined locations as marked on the frame prior to surgery. The wires are tensioned if indicated depending on ring diameter. If the distraction created by suspending the limb is not sufficient to produce an acceptable reduction, the distance between the proximal and distal rings, which are now secured to the bones via the fixation wires, can be increased by turning the nuts securing one of these rings. An attempt should be made to “over-distract” the fracture by a couple of millimeters. If fluoroscopy is available, alignment of the fracture can be visually assessed. If fluoroscopy is not available, alignment is assessed by palpation. The remainder of the process is similar to that described for reduction and stabilization of crural fractures (Figure 53-45C).
 

Limb Deformity Corrections

The most common limb deformity occurring in dogs results from premature closure of the distal ulnar physis. Premature distal ulnar physeal closure typically produces valgus and caudal angular deviation with external rotation and procurvatum of the distal radius. Concurrent proximal subluxation of radial head is often present in these dogs which can result in failure of the anconeal process to unite with the remainder of the ulna. Eccentric or complete closure of the distal radial physis can also be a sequella to premature distal ulnar physeal closure. Premature distal radial physeal closure is the second, but less common, limb deformity occurring in dogs. Affected dogs have a shortened radius, and often shortening of the entire antebrachium, with distal subluxation of the radial head. Angular and rotational deformities can be present in more severely affected dogs. Premature proximal radial physeal closure occurs infrequently, but will produce distal subluxation of the radial head.

Acute correction of limb deformities with bone plates or linear external fixators may be limited by tension in the regional soft tissues. The use of CFs and the methods of Ilizarov allow for acute or progressive correction of angular, rotational and translational deformity as well as length discrepancies. Circular fixators also allow the surgeon to make precise adjustments following surgery and throughout the convalescent period.

Limb Lengthening

Limb lengthening is warranted when length discrepancies produce a gait abnormality that impairs limb function. Lengthening may be done as an isolated procedure or in conjunction with angular, translational and/or rotational corrections. Since the radius and/or ulna are the bones which are most frequently lengthened, this discussion will focus on longitudinal antebrachial lengthenings. Craniocaudal and mediolateral view radiographs of both antebrachii, including the manus, should be obtained prior to surgery and length discrepancies between limbs measured. Premature closure of the distal radial physis can require lengthening of the entire antebrachium and is generally done using a three ring construct (Figure 53-46). The proximal ring is positioned near the radial head, the central ring is positioned over the mid-antebrachium and the distal ring at the distal metaphysis. If the radial head is subluxated distally (as an isolated abnormality or in conjunction with abnormalities of the distal antebrachium), a subperiosteal osteotomy is made at the proximal metaphysealdiaphyseal junction (distal to the position of the proximal ring) and the fixation wires on the proximal ring should only engage the radius. It is helpful to isolate the proximal radius and initiate, but not complete the osteotomy before placing the fixator on the limb. This limits the amount of surgery that must be performed within the frame, but allows the fixation wires to be placed into a stable bone segment. Once the frame and fixation wires are placed and the bone segments are stable, the osteotomy is completed. The proximal ring should be articulated with the central ring using linear motors or threaded rods secured with nylon nuts. This will allow distraction of the proximal radius to correct the existing elbow incongruency. To lengthen the entire distal antebrachium the wires attached to the distal ring should engage both the distal radius and ulna. Subperiosteal osteotomies are made at the distal radial and ulnar metaphyseal junction, proximal to the position of the distal ring. Performing the distal ulnar osteotomy and approaching and initiating the distal radial osteotomy prior to placing the frame over the limb, again simplifies the procedure. The distal ring should be articulated to the central ring using linear motors or threaded rods secured with nylon nuts. When applying the frame, the connecting elements should be positioned parallel to the longitudinal axis of the radius and ulna to produce the most functional lengthening.
 

Angular and Rotational Correction

The discussion will focus on correction of an antebrachial deformity resulting from premature closure of the distal ulnar physis as this abnormality constitutes the most common deformity correction performed in dogs. Pre-operative planning is critical to obtaining optimal results. Although trigonometric preoperative planning methods have been described, a simplified graphic method is preferred to define both the apex and plane of the angular deformity (Figure 53-47). Craniocaudal and mediolateral view radiographs of the entire limb including and distal to the elbow are obtained and tracings of these radiographs should be made in order to plan the procedure. Although the antebrachium is a paired bone system, the radius is the principle weight- bearing bone and the deformity is characterized according to conformational abnormalities of the radius. Straight lines are drawn through the axial plane of the proximal and distal radial segment. These lines are centered through the metaphysis and perpendicular to the adjacent articular surface. The intersection of these two lines denotes the apex of the deformity. It should be noted that in some animals, the apex of the deformity may not be isotopic in orthogonal planes.
 

To define the plane of the deformity, the mediolateral and craniocaudal components of the deformity, which are vectors and thus have both direction and magnitude, must be calculated. A line is drawn connecting the center of the proximal and distal articular surface of the radius on the tracings of both the craniocaudal and mediolateral radiographs: these lines represent the mechanical axis of the radius. Another line is drawn from the previously defined apex of the deformity perpendicular to the mechanical axis on the tracing on each radiographic projection. The measured length of the line on the craniocaudal radiograph constitutes the medial (varus) or lateral (valgus) component of the deformity. The measured length of the line on the mediolateral radiograph constitutes the cranial or caudal component of the deformity. These same measurements are obtained from line drawings developed from tracings of radiographs of the contralateral normal limb, and the component vectors measured on the normal limb are subtracted from those obtained from the abnormal limb. A tracing (or photocopy) of an appropriate diameter ring (Figure 53-48) which will be used to construct the fixator is made. An X (mediolateral)/Y (craniocaudal) grid is constructed with its origin centered in the ring. This drawing represents the proximal surface of the rings of the proximal ring blocks and should be marked correctly with respect to medial, lateral, cranial and caudal for the limb (left or right) that is being corrected. The plane of deformity is determined by plotting the two adjusted (abnormal minus normal) vector components of the deformity on the X/Y grid. The resultant vector defines the plane of deformity.

A drawing representing an outline of a transverse section of the radius, based on measurements of the craniocaudal and mediolateral dimensions of the radius obtained at the level of the apex of deformity on the pre-operative radiographs, is centered over the X-Y intersection. The circumferential position of the hinges can now be determined. A line drawn between the centers of the paired hinges constitutes the hinge axis. The plane of deformity is located along the concave surface of the radius and the hinges need to be located on the opposite side (convex surface) of the radius in order to correct the deformity. Thus the hinge axis should be positioned roughly perpendicular to the plane of the deformity and tangent to the outline of the radius opposite the plane of the deformity. The tangential location of the hinge axis will result in angular correction without additional lengthening. The two holes on the ring that are intersected by the hinge axis mark the position at which the hinges should be placed. A single angular motor is placed opposite the hinge axis, approximately equal distant from two hinges, which will be located on the concave surface of the deformity.
 

The fixator is assembled prior to surgery. A three ring construct is used in most dogs with two rings used to secure the proximal radial segment and a single ring used to secure the distal radial segment (Figure 53-49). It is advisable to mark the medial, lateral, cranial and caudal positions of the proximal surface of each ring appropriately for the limb that is to be corrected. The paired angular hinges and an angular motor are placed between the ring blocks at the appropriate holes as determined on the pre-operative drawing. The hinges can be bolted directly to the distal ring if the apex of deformity is located at or near the distal epiphysis, but the hinges are usually secured to both rings using short threaded rods at the holes as determined on the pre-operative drawing. Paired nylon nuts can be used to secure the rods to one of the rings if lengthening is anticipated. The longitudinal position of the hinges is located at the apex of the deformity. Construct dimensions, ring position and hinge position are determined by laying the frame directly over the lateral view radiograph of the deformed antebrachium. Frame angulation can be adjusted to conform to the deformity by adjusting the angular motor.
 

At surgery the entire forelimb is clipped and prepared for aseptic surgery and the dog is positioned in dorsal recumbency. A 2 to 4 cm subperiosteal segmental ostectomy of the distal ulna is performed at the level of the apex of the deformity. Following closure of the ulnar approach, subperiosteal isolation of the distal radius is performed exposing the location of the apex of the deformity. An osteotomy is initiated, but not completed, perpendicular to the longitudinal axis of the distal radial segment and parallel to the plane of the deformity. The longitudinal location of the radial osteotomy will influence the impact acute rotational correction will have on the plane of deformity. The plane of deformity should not be changed appreciably by acute rotational correction if the radial osteotomy is performed at or preferably slightly distal to the apex of the deformity. Performing the radial osteotomy proximal to the apex of the deformity is not advised if acute rotational correction is to be done, as the plane of deformity will be altered by rotational correction.

The frame is then placed on the limb and a fixation wire is placed in the proximal radius in the medial-to-lateral plane, perpendicular to the longitudinal axis of the radius. This wire is secured to the proximal surface of the proximal ring of the fixator and the longitudinal position of this wire should place the hinge axis at the apex of the deformity. Consideration should be given to placing this wire the day prior to surgery and then radiographing the limb, as constructing the fixator based on tracings of radiographs obtained with the first wire already in place simplifies placement of the hinge axis precisely at the apex of the deformity at surgery. The wire can then be bent over against the antebrachium and the limb coapted until surgery. At surgery the wire can be straightened out and tensioned or carefully replaced by inserting a new wire through the same hole in the radius. The connecting elements of the frame should be aligned parallel to the longitudinal axis of the radius and the hinges positioned at the apex of the deformity. The frame is rotated about the antebrachium until the hinge axis is positioned perpendicular to the plane of deformity and tangential to the convex cortex of the radius. It is important to center the radius, rather than the antebrachium, within the frame. A fixation wire is then placed parallel to the distal surface of the distal ring. This wire should be placed in the “true” mediolateral plane (from styloid process to styloid process) which will not be co-planar with the wire in the proximal radius if rotational deformity is present. Two divergent olive wires should then be placed on each of the rings of the proximal ring block to stabilize the proximal radial segment and the radial osteotomy is then completed.

Rotational deformity, if present, should be corrected before additional fixation wires are placed in the distal radial segment (Figure 53-50). Rotational deformity is estimated by comparing the planes of flexion and extension of the ipsilateral elbow and antebrachiocarpal joint. The plane of extension of the elbow (which is caudal) is marked on the distal surface of the distal ring. The antebrachiaocarpal joint is then flexed so that the paw is positioned parallel with the distal surface of the distal ring. The location of the division between metacarpal bones III and IV is marked on the distal surface of the distal ring. The number of holes between these two marks is counted and this represents the amount of rotation the wire securing the distal radial segment must be rotated about the surface of the distal ring to have the elbow and antebrachiocarpal joint flex and extend through the same plane. It is highly advisable to mark this wires’ position, and the position where the wire will be resecured at on the ring, before loosening and moving the fixation bolts.
 

Once rotational correction has been performed and the wire is secured to the distal ring, flexion and extension of the elbow and antebrachiocarpal joint should be compared. Adjustments can be made if deemed necessary. Two divergent olive wires should be placed to secure the distal radial segment to the distal ring. Following surgery in addition to obtaining standard craniocaudal and mediolateral view radiographs of the antebrachium, a radiograph centered through the hinge axis should also be obtained. The hinges should be superimposed over one another on this view and the entire frame should be visible on the film so that the distraction protocol can be calculated. Distraction is measured along the concave cortex of the radius and a rate of 0.75 to 1.50 mm/day fractionated into three or four incremental distractions is considered acceptable. The amount of distraction of the angular motor that will produce the appropriate amount of distraction at the osteotomy can be calculated using the method of similar triangles (Figure 53-51). Once the distraction period is completed and the deformity is corrected the hex drive fastener elements on the hinge assemblies and the angular motor should be tightened to lock the frame in position.
 

Bone Transport

Bone transport is a specific application of distraction osteogenesis used to resolve large segmental bone defects. With this technique an intercalary bone transport segment is created by performing a transverse osteotomy in the viable bone segment 1 to 2 cm adjacent to an osseous defect. Regenerate bone is produced in the distraction gap which develops as the transport segment is sequentially moved across the bone defect. Longitudinal bone transport is typically performed using a five ring construct, with two rings securing both the proximal and distal bone segments and the intermediate ring securing the transport segment. The transport ring is secured to the frame using paired nylon nuts which allow the ring to be moved precisely along the threaded rods at a rate of 0.5 to 2.0 mm per day. A delay period of 5 to 7 days may be warranted depending upon the age of the animal, the condition of the regional soft tissues and the location of the osteotomy. Radiographs should be obtained bi-weekly during the distraction process and the rate increased or decreased if necessitated by the appearance of the regenerate bone. Docking refers to the process of the transport segment contacting and eventually obtaining union with the bone at the opposite end of the osseous defect. Obtaining union at the docking site can be facilitated by placing a cancellous bone graft at the site several days prior to docking and constructing the fixator such that the transport ring can be moved several mm beyond the bone defect, thus facilitating in compression of the docking site.

Bone transport has been used in dogs to resolve large segmental defects resulting from highly comminuted fractures, following sequestrectomy in infected fractures and in performing limb salvage procedures in dogs with appendicular bone tumors. These large segmental defects have traditionally been managed with massive bone allo- or autografts or prostheses, which are prone to infection and implant failure. Regenerate bone is highly vascular and resistant to infection and all implants can be removed once the docking site has achieved union and the regenerate bone has consolidated (Figure 53-52).
 

Postoperative Management and Complications

Management of animals with CFs is similar to that of animals with traditional linear fixators. Following surgery, the CF is wrapped to limit postoperative swelling and to protect any incisions, open wounds and wire/pin insertion wounds. Sterile gauze is placed over any wounds and the insertion sites and the foam portion of recycled disposable surgical scrub brushes which are impregnated with chlorhexidine are packed between the skin and the frame to limit postoperative swelling. Care must be taken not to pack the sponges too tightly within the frame or edema and swelling of the distal limb may be aggravated. Cast padding and Vetwrap tape is then used to apply a bandage around the entire fixator. The CF is initially unwrapped and the limb and surgical sites assessed for swelling or complications on a daily basis. The wire/pin-skin interfaces are cleaned aggressively with a gauze or cotton tip applicator and diluted chlorhexidine solution. The CF is then re-wrapped. When the acute swelling and edema has subsided and the wire/pin-skin insertion sites heal sufficiently, packing sponges within the frame can be discontinued. The owners should be directed to construct a shroud or sleeve that fits securely over the entire fixator, but can be easily removed for daily cleaning of the wire/pin-skin interfaces.

Performing intensive, frequent (a minimum of three times a day) physical therapy is important during lengthening and correction of angular deformities to reduce the development of muscle (especially flexor muscles) contracture. Contracture is less of a problem with higher rhythms (more fractionated distractions). The administration of nonsteroidal anti-inflammatory drugs is also beneficial in encouraging weight-bearing, mitigating contracture during the distraction period.

The most common complication associated with the use of CFs is wire/pin tract drainage and bone lysis surrounding the fixation elements. Inflammation associated with wire/pin tract drainage typically develops several weeks after surgery and generally does not influence the final outcome even when the fixator must be maintained for an extended period of time. Minor wire/ pin tract drainage may resolve with broad spectrum antibiotic administration. If drainage is substantial and/or purulent and/ or there is substantial bone lysis and proliferation adjacent to a wire/pin, that fixation element should be removed and replaced if necessary. Proper insertion techniques and meticulous, appropriate daily care can greatly decrease the incidence of wire/pin tract complications.

Suggested Readings

Anderson GM, Lewis DD, Radasch RM, et al.: Circular external skeletal fixation stabilization of antebrachial and crural fractures in 25 dogs. J Am Anim Hosp Assoc 39:479, 2002.

ASAMI Group. Basic principles of operative technique. In: Bianchi-Maiocchi A, Aronson J, eds.: Operative Principals of Ilizarov. Milan, Italy, Medi Surgical Vido, 1991, p 65.

Bianchi-Maiocchi A: The Ilizarov compression-distraction apparatus. In: Bianchi-Maiocchi A, ed.: Advances in Ilizarov Apparatus Assembly. Milan, Italy, Medi Plastic Sri, 1994, p 5.

Bronson DG, Samchukov ML, Birch JG, et al.: Stability of external circular fixation: A multi-variable biomechanical analysis. Clin Biomech 13:441, 1998.

Catagni M: Fractures of the leg (tibia). In: Bianchi Maiocchi A, Aronson J, eds.: Operative Principles of Ilizarov. Milan, Italy, Medi Surgical Vido, 1991, p 91.

Collins KE, Lewis DD, Lanz OI, et al.: Use of a circular external skeletal fixator for stifle arthrodesis in a dog. J Sm Anim Pract 41:312, 2000.

Cross AR, Lewis DD, Murphy ST, et al.: Effect of ring diameter and wire tension on the axial biomechanics of four-ring circular external skeletal fixator constructs. Am J Vet Res 62:1025, 2001.

Cross AR, Lewis DD, Rigaud S, et al.: Effect of wire tension on the biomechanics of asymmetric four-ring circular external skeletal fixator constructs. Vet Comp Orthop Traumatol 15:44, 2002.

Egger EL, Histand MB, Norrdin RW, et al.: Canine osteotomy healing when stabilized with decreasingly rigid fixation compared to constantly rigid fixation. Vet Comp Orthop Traumatol 6:182, 1993.

Ehrhart N: Longitudinal bone transport for treatment of primary bone tumors in dogs: technique description and outcome in 9 dogs. Vet Surg 34:24, 2005.

Elkins AD, Morandi M, Zembo M: Distraction osteogenesis in the dog using the Ilizarov external ring fixator. J Am Anim Hosp Assoc 29:419, 1993.

Farese JP, Lewis DD, Cross AR, et al.: Use of IMEX SK-circular external fixator hybrid constructs for fracture stabilization in dogs and cats. J Am Anim Hosp Assoc 38:279, 2002.

Ferretti A: The application of the Ilizarov technique to veterinary medicine. In: Bianchi-Maiocchi A, Aronson J, eds.: Operative Principles of Ilizarov. Milan, Italy, Medi Surgical Vido, 1991, p 551.

Goodship AE, Kenwright J: The influence of induced micromotion upon the healing of experimental tibial fractures. J Bone Joint Surg 67[B]:650, 1985.

Green SA, Harris NL, Wall DM, et al.: The Rancho mounting technique for the Ilizarov method. Clin Orthop 280:104, 1992.

Halling KB, Lewis DD, Jones RW, et al.: Use of circular fixator constructs to stabilize intertarsal/tarsometarsal arthrodeses in three dogs. Vet Corp Ortho Traumatol 17:204, 2004.

Ilizarov GA: The apparatus: Components and biomechanical principles of application. In: Ilizarov GA ed.: Transosseous Osteosynthesis. Theoretical and Clinical Aspects of the Regeneration and Growth of Tissue. Berlin, Springer-Verlag, 1992, p 63.

Jerram RM, Walker AM, Sutherland-Smith J, et al.: Treating pedal shortening in a dog by metatarsal distraction osteogenesis. J Sm Anim Pract 26:191, 2005.

Kenwright J, Goodship AE: Controlled mechanical stimulation in the treatment of tibial fractures. Clin Orthop 241:36, 1989.

Kenwright J, Goodship AE, Kelly DJ, et al.: Effect of controlled axial micromotion on healing of tibial fractures. Lancet 8517(2):1185, 1986.

Kummer FJ: Biomechanics of the Ilizarov external fixator. Clin Orthop 280:11, 1992.

Langley-Hobbs SJ, Carmichael S, Pead MJ, et al.: Management of antebrachial deformity and shortening secondary to a synostosis in a dog. J Sm Anim Pract 37:359, 1996.

Leuno, CY, Ma RYP, Clark JA, et al.: Viscoelastic behavior of tissue in leg lengthening by distraction. Clin Orthop 139:102, 1979.

Lewis DD, Bronson DG, Cross AR, et al.: Axial characteristics of circular external skeletal fixator single ring constructs. Vet Surg 30:386, 2001.

Lewis DD, Bronson DG, Samchukov ML, et al.: Biomechanics of circular external skeletal fixation. Vet Surg 27:454, 1998.

Lewis DD, Radasch RM, Beale BS, et al.: Initial clinical experience with the IMEX Circular External Skeletal Fixation System. Part I: Use in fractures of arthrodeses. Vet Comp Orthop Traumatol 12:108, 1999.

Lewis DD, Radasch RM, Beale BS, et al.: Initial clinical experience with the IMEX Circular External Skeletal Fixation System. Part II: Use in bone lengthening and correction of angular and rotational deformities. Vet Comp Orthop Traumatol 12:118, 1999.

Marcellin-Little DJ: Treatment of bone deformities with circular external skeletal fixation. Comp Cont Ed Pract Vet 21:481, 1999.

Marcellin-Little DJ: Fracture treatment with circular external fixation. Vet Clin N Am: Sm Anim Pract 29:1153, 1999.

Marcellin-Little DJ, Ferretti A, Roe SC, et al.: Hinged Ilizarov external fixation for correction of antebrachial deformities. Vet Surg 27:231, 1998.

Orbay GO, Kummer FJ, Frankel VN: The effect of wire configuration on the stability of the Ilizarov external fixator. Clin Orthop 279:299, 1992.

Owen MA: Use of the Ilizarov method to manage a septic tibial fracture nonunion with a large cortical defect. J Sm Anim Pract 41:124-127, 2000.

Paley D: Biomechanics of the Ilizarov-external fixator. In: Bianchi-Maiocchi A, Aronson J, ed.: Operative Principals of Ilizarov. Milan, Italy, Medi Surgical Vido, 1991, p 31.

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Introduction

Hybrid circular external skeletal fixation (HCESF) combines the fixator components, methodology of application, and biomechanics of traditional linear and circular external fixation devices. The benefits of both systems are enhanced when a hybrid circular fixator (HCF) is properly applied in appropriate situations. Hybrid fixators have been used clinically to manage long bone and spinal fractures, as well as bone deformities. The circular components of a HCF allow adequate fixation of small juxta-articular bone segments using two to three narrow (1.0 -1.6 mm) tensioned transfixation wires secured to a ring. The linear components of the frame are then applied to the primary bone segment using traditional positive profile half-pins or full-pins. The basic components of a HCF are the supporting, connecting, fixation and assembly elements (Figure 53-53). All components, except the fixation elements (wires and pins) are reusable numerous times, making HCESF economically practical for veterinary practice. The only HCF currently available in veterinary orthopedics is manufactured by the IMEX Company (Longview, Texas). Therefore, this section will concentrate on the application of the IMEX HCESF system utilizing IMEX-SK components, pins, wires and rings.
 

Indications

Fractures involving the metaphyseal region of all long bones are relatively common in veterinary medicine. These fractures frequently have a short juxta-articular bone segment precluding the use of many implant systems (Figure 53-54), and are often comminuted, making anatomical reduction of the bone fragments impossible and of questionable mechanical benefit. A HCF can be applied using closed, or “mini-approach” techniques utilizing the principles and benefits of biological osteosynthesis, while still allowing manipulation of the main bone segments to achieve adequate fracture reduction or alignment. In addition, adjustments of the frame can be performed during the postoperative period to improve reduction or to stimulate the later stages of bone healing.
 

Hybrid Circular Fixator Components

The majority of components used to create a HCF frame have been thoroughly described in the proceeding sections. However, there are several unique components used almost exclusively with HCF frames. Successful management of juxta-articular fractures while minimizing complications depends upon a thorough knowledge of how each individual component affects the overall mechanical characteristics of the HCF. In addition, a working understanding of how each component can be used and integrated into the system will allow manipulation of the juxta-articular and primary bone segments to achieve proper fracture reduction or alignment.

Supporting Elements

A full ring is the most commonly used supporting element of a HCF. Partial or stretch rings and arches are often used instead of a full ring to prevent interference with joint motion (Figure 53-55). Partial rings and arches are very useful in managing fractures involving the proximal tibia, proximal radius, spine, distal femur, and humerus. Rings (partial or full) and arches are available with 35, 45, 50, 66, 84, and 118 mm internal diameters. The appropriate ring or arch size chosen will depend upon the animal’s weight as well as the diameter of the regional anatomy. The inner ring diameter should be approximately 2.0 cm larger than the regional anatomy at the level where the ring will be positioned. This diameter will allow for typical postoperative soft tissue swelling without the ring compressing or cutting into the underlying soft tissues.
 

Connecting Elements

The standard connecting element of the IMEX HCF is the small 6.3 mm hybrid rod, currently available in 50, 75, 100, 150, 200, and 250 mm lengths (Figure 53-56). Hybrid rods are also available in 9.5 mm diameter (lengths = 50, 75, 100, 150, 200, 250, 300, and 350 mm) and 3.2 mini (length = 125 mm). Large and small hybrid rods are made of titanium, and mini hybrid rods are made of stainless steel. The hybrid rod serves the same function as the connecting rod in a linear external fixator. IMEX-SK clamps attach along the smooth shaft of the hybrid rod to secure fixation pins to the frame. Large, small, and mini SK clamps are used with the large, small and mini hybrid connecting rods, respectively. In the author’s experience, the small hybrid rod (6.3 mm diameter) and SK clamps are used most commonly to create a HCF for both dogs and cats. The large hybrid rod and SK clamps would be used for giant breed dogs, while the mini system would be reserved for toy breed dogs and small cats.

One end of the hybrid rod is threaded, allowing it to be secured to the supporting element (ring, arch or partial ring). The large and small hybrid rods have 6.0 mm thread diameter and therefore are secured to the supporting element with two 6.0 mm nuts. The mini hybrid rod has a 4.0 mm thread diameter and is secured using two 4.0 mm nuts. The threaded section of the large and small hybrid rods can be placed into the holes of the 50, 66, 84, and 118 mm rings or arches. The mini hybrid rod can only be used with the mini 35 or 45 mm rings and is placed through either the slots or holes of the mini ring.

The hybrid rods can also be attached to the supporting ring or arch using paired 6.3 mm (used with small and large hybrid rods) HEMI-spherical washers and nuts, which allow the rod to be angulated approximately 7.5° in relationship to the ring surface (Figure 53-57). Advantages of angulating the hybrid rod include: 1) facilitating manipulation of bone segments to achieve fracture reduction; 2) allowing small angular corrections; and 3) enabling biomechanical improvement of frame stiffness by reducing fixation pin working length (distance between the hybrid connecting rod, pin-gripping bolt of the SK clamp, and the near cortex of primary bone segment). The hybrid rod can be positioned anywhere on the supporting ring or arch. However, the hybrid rod should be positioned so that the fixation pins, attached to the rod, will pass through safe soft tissue corridors (as described in previous sections), in order to minimize postoperative pin tract morbidity.

An alternative to a threaded hybrid rod, is the use of a 6.3 mm titanium or radiolucent carbon fiber SK^TM linear external fixation connecting rod, connected to the ring element using a Universal SK^TM Hybrid Adapter (Figure 53-58). The Universal SK^TM Hybrid Adapter allows the connecting rod to be angulated approximately 32.5°, in any direction to the ring surface. The Universal SK^TM Hybrid Adapter is secured to the ring element using two 6 mm bolts and nuts. The two 8 mm bolts of the rod-gripping element, when tightened, lock the connecting rod into the desired orientation, by compressing the broach surrounding the connecting rod. The Universal SK^TM Hybrid Adapter provides the same benefits of the hemispherical washer and nuts with the additional advantages of: 1) greater range of angulation of the connecting rod in relationship to the ring (32.5° vs. 7.5°) and 2) use of radiolucent carbon fiber connecting rods which can facilitate imaging the fracture. The Universal SK^TM Hybrid Adapter can only be used on 50-118 mm ring diameters.
 

Assembly Elements

Assembly elements unique to an IMEX HCF include pin fixation bolts, one and two hole posts, two hole plates, and threaded SK clamps. Pin fixation bolts allow 3.0 to 4.8 mm half-pin or full-pins to be placed directly on a 50, 60, 84 or 118 mm ring or arch (Figure 53-59). Juxta-articular fractures involving the femur, humerus or spine are best stabilized with half-pins or full-pins instead of divergent narrow wires. Divergent wires in these locations can penetrate large muscle masses, or result in impingement of normal flexion and extension of the associated joint. A pin fixation bolt is secured to a ring or arch with a 6.0 mm nut. The supporting element then serves as a platform for pin insertion into the juxta-articular bone segment. The basic anatomy and mechanics of the pin fixation bolt is identical to that of the pin-gripping bolt/washer assembly of the linear SK clamp. As the 6.0 mm nut attaching the fixation bolt to the ring is tightened, the fixation pin is trapped and secured between the hole in the head of the bolt and the meniscus of the sliding washer. The pin fixation bolt will accept the IMEX drill sleeve, which should be used to minimize soft tissue trauma during pre-drilling of holes prior to pin insertion.

One and two hole posts serve several important functions (Figure 53-60). The threaded portion of the post allows it to be secured to the ring using a nut. The hole(s) in the post will accept the threaded portion of a wire fixation bolt, a pin fixation bolt, or a hybrid rod. These components are secured to the post with appropriate size nuts. Two posts, on opposite sides of the ring, can be used to place an additional wire above or below the ring to improve fracture stability. The wire is secured to the post using wire fixation bolts and nuts. A wire attached to a post is referred to as a “drop wire”. Fixation pins can also be placed above or below a ring using a single post and a pin fixation bolt. However, there should never be a combination of wires and pins secured to the same bone segment. Axial micromotion occurs with bone segments secured to a ring with two or more narrow wires during weight bearing. Fixation pins rigidly secure bone segments and do not allow for micromotion in any plane. Therefore if a juxta-articular bone segment is secured to a ring with wires and a single fixation pin, the axial micromotion provided by the wires will result in cyclic bending forces at the fixation pin-bone interface. This may result in premature loosening of the fixation pin with subsequent pin tract drainage, sepsis and patient discomfort.
 

Two posts can also be connected to each other with a nut to create an adjustable articulation (Figure 53-61). Alternatively, a hybrid rod can be connected to a ring or arch with an angular hinge assembly to also create an adjustable articulation (Figure 53-62). Articulations will allow diagonal struts to be constructed using small or large hybrid rods. Diagonal struts improve the stiffness of a HCF frame allowing the use of a simple type I-a frame when appropriate. Furthermore, a diagonal can be removed 6 to 8 weeks after surgery to destabilize the frame and stimulate the later stages of bone healing. To construct a diagonal, the threaded end of a hybrid rod is placed through a hole in one of the posts of the articulation, and secured using two nuts. The threaded end of the other post is secured to the ring or arch with a nut. The smooth shaft of the hybrid diagonal rod can be secured utilizing the “stacked clamp” technique (as described in the previous section: Basic Principles for the Application of External Fixators) to a fixation pin using a SK single clamp. Alternatively, the diagonal strut can be attached to the hybrid connecting rod of the HCF frame using a SK double clamp.
 

A two-hole plate can be used to offset a small or large hybrid connecting rod from the ring (Figure 53-63). This may be necessary if the hybrid connecting rod and associated SK clamps are too close to the limb and could potentially result in pressure necrosis of the underlying soft tissues. One hole of the plate is attached to the ring using a 6.0 mm bolt and nut. The threaded portion of a large or small hybrid connecting rod is secured to the second hole in the plate using two 6.0 mm nuts. Alternatively, the hybrid rod can be attached to the plate using paired 6.0 mm spherical nuts and washers, to allow angulation of the rod up to 15° away from perpendicular relative to the plane of the ring. The design of the spherical nuts and washers allows for angulation of the rod in any direction that is desired.

A modified threaded small SK single clamp is available for placement of half pins from all-thread connecting rod elements (Figure 53-64). This 6 mm clamp is identical to the linear SK clamp, except that the portion secured to the connecting element is threaded instead of smooth. The thread pitch and size of the threaded SK clamp correlates to the 6.0 mm thread on the all-thread connecting rod. The threaded SK clamp is extremely useful when creating a hybrid spinal frame. Threaded SK clamps can be positioned anywhere along the all-thread connecting rod of a spinal frame. Half-pins can be angulated proximal, distal, dorsal, and ventral; in addition, the clamp can be moved forwards or backwards to facilitate placement of half pins into vertebral bodies.
 

Hybrid Circular Fixator Frame Design

The same nomenclature used to describe linear external fixator frames has been adopted for description of HCF frames. HCF frames can be uniplaner (type I-a, or type II); biplaner or modified multiplaner (type I-b) (Figure 53-65). The number and position of hybrid rod(s) utilized in fixator construction are the principle differences between the frames. Insertion of the fixation pins should always be through safe soft tissue corridors, thus dictating the possible positions of the hybrid rod(s) and the frame configurations available for a particular bone.

The most basic frame is the unilateral-uniplaner type I-a frame. This frame is most commonly used to manage fractures or simple bone deformities involving the radius/ulna or tibia in small to medium sized patients. Additional frame stiffness should be created by the addition of a diagnol strut to all type I-A frames, improving bending, torsional and axial stiffness. A type I-b frame utilizes two hybrid rods placed 60 to 90° apart. Hybrid I-b frames are often used to manage comminuted fractures of the radius/ ulna or tibia, especially in large and giant breeds. A type I-b frame can easily be converted to a type I-a frame, by removing a hybrid connecting rod, and the associated SK clamps and fixation pins. This is referred to as staged disassembly or destabilization of the frame, and may stimulate the later stages of bone healing.

Type II HCF frames can be used to manage fractures of the tibia. However application of type II frames to the radius is not recomended because full pins passed through the bone may result in iatrogenic pin tract fractures. Furthermore, full-pins passed in the proximal half of the antebrachium penetrate large muscle masses and generally cause substantial pin tract morbidity and patient discomfort. If a type II hybrid frame is necessary for the management of a fractured tibia, half-pins should be placed in the proximal-medial aspect of the bone and the full-pin(s) placed in the distal two-thirds of the bone. This pin orientation will help reduce pin tract morbidly by keeping fixation pins out of the large proximal-lateral musculature of the tibia. Therefore, to minimize pin tract complications without jeopardizing stability, hybrid 1-b frames are usually preferred over hybrid II frames for tibial fractures. The hybrid type I-b frame offers mechanical characteristics similar to a type II frame. Additionally, the type I-b frame allows more freedom of placement of the two hybrid connecting rod, so that half pins can be placed through safe soft tissue corridors with minimal soft tissue penetration.

Multiplaner type I-b frames can be applied to metaphyseal fractures of the femur and humerus. However, full rings cannot be used to create a HCF for fractures involving these bones due to the extensive musculature surrounding the elbow, shoulder, stifle, and hip joint. Arches and partial rings are easily positioned near these joints without impinging joint function or encroaching upon regional soft tissues. Articulations and diagonal struts are generally incorporated to create multiplaner type I-b frames in the humerus and femur, in order to improve frame stiffness.
 

Parabolic shaped arches with 140 and 220 mm internal diameters are available to create spinal HCF frames (Figure 53-66). Two to four spinal arches can be connected with three or four sections of 6.0 mm all-thread connecting rod. Half pins can then be placed into the vertebral bodies and secured to the arches using half pin fixation bolts. Modified threaded small SK clamps are positioned along the all-thread connecting rods of the frame. Half pins can be placed into vertebral bodies from the threaded SK clamps, as well as from the spinal arches using pin fixation bolts. In the author’s experience a simpler two arch spinal frame used in conjunction with threaded SK clamps is easier to position on the spine, and place fixation pins into vertebral bodies, than the multi arch frame.
 

Application of a HCF

The application of a HCF is relatively easy; however, close adherence to basic linear and circular fixator application principles should be followed to improve clinical results and reduce postoperative complications. One primary advantage of any external fixation device, is that it can be applied using the principles of biological osteosynthesis. Whenever possible, the HCF should be applied using a closed technique. If necessary, to adequately reduce or align the major bone segments, a mini-approach can be used. However, the local fracture hematoma should not be disrupted. If a mini-approach is used, addition of an autogenous cancellous bone graft at the fracture site is recommended. Fixation pins should always be placed into appropriately sized pre-drilled holes using low speed insertion to avoid mechanical and thermal bone damage. Fixation pins should either be positive profile or tapered Thread-Run-Out (TRO) and inserted perpendicular to the diaphysis. Cortical thread pin design is used in all locations except the proximal tibia, distal femur, and proximal humerus. A cancellous thread profile is preferred in these locations where the cortex is thin and the bone is relatively soft. The threaded diameter of the fixation pin should not exceed twenty-five percent of bone diameter. Pins should be placed a distance of at least 1-bone diameter from the fracture region or fissures. A minimum of three positive profile fixation pins should be inserted into the primary bone segment.

Fixation wires are inserted into the small juxta-articular bone segment at divergent angles. The wires should be placed through the juxta-articular bone segment in regions with minimal overlying soft tissues. Care should be taken to prevent placement of wires through extensor and flexor tendons and major muscles. One wire should be placed on each surface of the ring. If possible, a third “drop wire” should be integrated into the short bone segment to improve resistance to bending and torsional forces. The use of counter-opposed stopper wires (described in the section: Application of Ring Fixators) will also improve fracture stability by minimizing bone translation along the wires. Special 1.0 to 1.6 mm transfixation wires with a free cutting point that easily penetrates cortical bone are recommended instead of trocar pointed K-wires. Wires placed on 66, 84, and 118 mm rings should be tensioned to approximately 90 Kg. “Drop wires” and wires placed on 50 mm rings should not be tensioned. Full rings should be used whenever possible to improve fracture stability. Several lengths of sterile hybrid rods should always be available for intra-operative frame construction and modification if necessary. The hybrid rod should be at least the same length as the fractured bone once axial reduction is achieved. Slightly longer hybrid rods may be needed if diagonal struts will be created. Release incisions in the skin should always be created around each fixation wire and pin. The skin release incision should be 1 to 2 cm long for fixation pins and 0.5 cm for fixation wires. The skin release incisions should be deepened down to the level of the bone by blunt dissection using a hemostat.

The overall goal of fracture reduction and stabilization using a HCF is to, 1) restore the bone to its normal axial length, 2) position the proximal and distal joint surfaces in normal anatomical alignment to each other, and 3) prevent any translational malalignment of the primary and juxta-articular bone segments. Orthogonal radiographs of the normal limb can help determine the correct angular alignment of the proximal and distal joint surfaces, as well as the normal axial length expected of the fractured bone once reduction is complete. Intra-operative fluoroscopy, if available, can assist with wire and pin placement as well as for assessment of fracture reduction and joint alignment. If fluoroscopy is not available, joint surfaces can be “mapped-out” by inserting several 22 to 24 gauge needles into the corresponding joints in several different planes. The “hanging leg prep,” (as described in the Basic Principles for the Application of External Fixators), is very useful to reduce fractures involving the radius and ulna when applying a HCF. If a full ring is used to construct the frame, it must be placed over the limb prior to hanging the limb. The “hanging leg prep” in the author’s experience, often makes reduction of juxta-articular tibial fractures difficult and therefore is not recommended.

Application of a HCF to a Tibial Fracture

The following description applies to the placement of a type I-a or a type I-b HCF frame to a fractured tibia with a short juxta-articular component. Several factors need to be considered to determine if a hybrid type I-a or a type I-b frame should be used to manage the fracture. These factors include: 1) the fracture configuration (two or three piece fracture with load sharing versus a comminuted fracture with no load sharing); 2) patient weight; and 3) patient age. As a general rule, a type I-a HCF frame is applied to fractures with some degree of load sharing by the primary and juxta-articular bone segments, immature or young patients, and fractures in small to medium sized patients. To minimize bending forces acting on the fracture and frame components, a diagnol articulation (“strut”) is always recommended when using a type I-a HCF frame. A type I-b HCF frame is usually reserved for large and giant breeds, or comminuted juxta-articular, non-load sharing fractures configurations. If a type I-a HCF frame is applied to the tibia, the hybrid rod should be positioned over the medial aspect of the limb. If a type I-b HCF frame is used, hybrid rods are placed over the medial and anterior regions of the limb.

General Application Steps for a Fractured Tibia

1) The patient is positioned in dorsal recumbency, at the end of the table to facilitate traction on the tibia in order to achieve fracture reduction. If a full ring is used for frame construction, it should be placed over the limb prior to any wire or pin insertion. The hybrid rod does not need to be attached to the ring if it will interfere with reduction or initial wire placement.
2) A 1.0 to 1.6 mm transfixation wire, preferably with a stopper (olive wire), is passed through the juxta-articular bone segment. The wire should be parallel to the corresponding joint surface, perpendicular to the longitudinal axis of the tibia, and placed in the true medial-lateral plane of the juxta-articular bone segment (Figure 53-67). Positioning of this first wire is critical. The wire should be placed 5 to 10 mm from the joint surface.
3) The juxta-articular bone segment is centered within the ring. The wire is secured to the outside ring surface with two wire fixation bolts or a bolt and slotted washer. The wire should be tensioned. If a hybrid rod has not already been attached to the ring it should be positioned on the medial aspect of the ring and secured with paired nuts or spherical nuts and washers (Figure 53-68).
4) A pilot hole is pre-drilled into the primary bone segment after creating a release incision in the regional soft tissues, approximately 1 to 2 cm from the associated joint surface. A drill sleeve should be used while pre-drilling to protect the surrounding soft tissues. The pilot hole should be created parallel to the corresponding joint surface, perpendicular to the tibial diaphysis and placed in the true medial-lateral plane of the primary bone segment (Figure 53-69). Proper positioning of the pre-drilled pilot hole is critical.
5) A positive profile or a tapered Thread-Run-Out (TRO) half-pin is inserted, using low speed into the pilot hole until several threads penetrate the far cortex. Note: at this stage, a half-pin should be positioned in the primary bone segment, and a wire placed through the short juxta-articular segment. The pin and wire should be parallel to their corresponding joint surfaces, perpendicular to the longitudinal axis of the tibia and in the true medial-lateral plane of their corresponding bone segments (Figure 53-70).
6) Axial reduction is achieved by placing traction on the ring (Figure 53-71A). Rotational and angular corrections are accomplished by rotating and manipulating the ring until the wire and half pin are parallel and in the same sagittal plane to each other (Figure 53-71B). If necessary, a mini-approach to the fracture can be made to facilitate reduction.
 

7) The half-pin is attached to the hybrid connecting rod, using an SK clamp while maintaining axial, rotational and angular alignment. If necessary, the position of the hybrid rod can be changed on the ring to improve reduction or to allow subsequent fixation pins to be passed through safe soft tissue corridors. If spherical washers and nuts or a universal hybrid rod adapter were used in frame construction, the hybrid rod can be angulated to improve reduction. However, the orientation of the pin and wire to each other should not be altered, assuming they were correctly positioned initially.
8) If substantial medial-lateral translational malalignment of the juxta-articular and primary bone segment exist, the SK clamp can be loosened and the half pin translated medial or lateral, as needed to improve reduction (Figure 53-72). Likewise, the wire fixation bolts can be loosened and the juxta-articular bone segment translated medial or lateral. However, the orientation of the pin and wire to each other should not be altered. Note: at this stage: 1) the proximal and distal joint surfaces should be parallel; 2) the primary bone segment and the short juxta-articular segment aligned in the medial-lateral and anterior-posterior planes; 3) anatomical axial length re-established; and 4) rotational malalignment corrected.
9) A second wire, preferably with a stopper, is passed through the juxta-articular bone segment. This wire should be orientated approximately 40-60° to the first wire, parallel and flush to the inner ring surface. Before placing the second wire, it is imperative that the juxta-articular bone segment is not tilted in an anterior-posterior direction. Digital pressure placed on the malaligned bone segment will improve reduction. The wire should be passed through regions with minimal soft tissue coverage over the bone. Using counter-opposed stopper wires will help prevent translation of the short juxta-articular bone segment. The wire is then secured to the ring with wire fixation bolts and tensioned (Figure 53-73). 10) Place two or three additional SK clamps on the hybrid connecting rod. Position a clamp over the primary bone segment near the fracture site. Digitally correct any anterior-posterior tilting of the primary bone segment at the fracture site. Insert a positive profile half-pin into the primary bone segment at a distance of at least one times the diameter of the bone at the fracture, after creating a pre-drilled pilot hole. The SK clamp can be used as a drill guide with the aid of a drill sleeve. Secure the SK clamp and pin to the hybrid connecting rod. Place additional half-pins into the medial aspect of the primary bone segment. The half-pins should be equally spaced along the medial shaft of the tibia (Figure 53-74). Typically three to four positive profile fixation pins should be placed into the primary bone segment.
11) Add a “drop wire” to the ring using two posts if sufficient bone is available. Do not tension this wire. If possible, the drop wire should be placed in a different plane than the previous two wires placed on the ring, to improve bending and torsional stability.
12) A diagonal strut can be created if additional frame stiffness is necessary.
13) A type I-b HCF frame can be created, if necessary, by attaching a second hybrid rod to the anterior surface of the ring using two nuts. Position two to four SK clamps along its shaft and insert half-pins into the anterior surface of the tibia, as previously described. These pins should be passed between the half pins inserted from the medial hybrid rod (Figure 53-75).
 

Application of a HCF to a Radius/Ulna Fracture

A “hanging leg prep” will often facilitate re-establishment of axial limb length of radial/ulnar fractures. If a full ring is used, it must be placed over the antebrachium prior to hanging the limb. A more detailed description of the “hanging leg prep” can be found in the Basic Principles for Application of External Fixators section. The basic steps used to apply a HCF to the tibia can be followed for the radius with only several modifications. One primary difference is that the position of the hybrid connecting rod(s) in relationship to the limb will be altered to allow placement of half pins through safe soft tissue corridors. Due to the flat ovoid shape of the radius it is difficult to pass fixation pins in the medial-lateral plane. However, the bone is a relatively easy target to insert half pins in the cranial-medial and cranial-lateral planes. Fixation pins placed in these locations will pass through safe soft tissue corridors. Similar to the tibia, the decision to use a type I-a frame (with or without a diagonal), or a type I-b frame will be dependent upon the fracture configuration (load or non-load sharing), patient’s age and weight. Hybrid type II frames are not recommended on the radius. To place a hybrid type I-a frame on the radius, the hybrid connecting rod should be positioned over the cranial-medial aspect of the antebrachium. If a hybrid type I-b frame is used, hybrid connecting rods are positioned over the cranial-medial and cranial-lateral aspect of the antibrachium, approximately 60° to each other.

General Application Steps for a Fractured Radius/Ulna

1) With the leg suspended, and axial length re-established, the carpus and elbow should be flexed and extended to determine if any rotational or angular joint malalignment exists. Any malalignment should be corrected as described for application of a linear fixator. The ring or partial ring is positioned over the juxta-articular bone segment and secured to the bone with a stopper wire. The wire should be positioned parallel to the joint, perpendicular to the longitudinal axis of the bone, and in the medial-lateral plane of the juxta-articular bone segment.
2) A half-pin is inserted into the primary bone segment through a release incision and a pre-drilled hole placed over the cranial-medial aspect of the antebrachium. This pin should be placed 1 to 2 cm from the corresponding joint surface. The half-pin should be positioned parallel to the joint surface and perpendicular to the longitudinal axis of the bone.
3) The half-pin is connected to the cranial-medial hybrid connecting rod using an SK clamp. The elbow and carpus should again be flexed and extended to evaluate and correct any rotational or angular malalignment, if present.
4) An additional wire is passed through the juxta-articular bone segment approximately 60° to the first wire and on the opposite flat surface of the ring. This wire is secured to the ring using wire fixation bolts and nuts. Both wires connected to the ring should be tensioned. A “drop wire” should be placed if the length of the short bone segment will allow.
5) Two or three additional SK clamps are placed on the hybrid connecting rod and half-pins inserted into the cranial-medial aspect of the radius to complete the frame.
6) Additional frame stiffness should be created by the addition of a diagnol strut.
7) A hybrid type I-b frame can be created by positioning a second hybrid connecting rod, approximately 60° from the first rod, over the cranial-lateral aspect of the antibrachium. The rod is connected to the ring. Additional SK clamps are added to the rod and half-pins are inserted into the cranial-lateral aspect of the radius through release incisions and pre-drilled pilot holes. These half-pins should be placed between the half- pins placed from the cranial-medial hybrid rod (Figure 53-76).
 

Application of a Multiplaner 1-B HCF to a Distal Humeral or Femur Fracture

Due to the extensive soft tissues surrounding the stifle and elbow joint, a full ring cannot be positioned over the distal humerus or femur without impingement of joint motion. Partial or stretch rings are used instead. Furthermore, placement of divergent wires from a ring in these locations causes substantial penetration of the flexor and extensor muscle groups. In the author’s experience, this often results in significant permanent loss of joint motion. However, if fixation elements are only placed in the medial-lateral plane of the juxta-articular bone segment, joint motion is preserved in both the elbow and stifle joints. In addition, fixation pins should only be placed into the proximal-lateral and cranial-lateral regions of the humerus and femur to minimize penetration of major muscles near the hip and shoulder joint. Safe soft tissue corridors have been described for the humerus and femur in the linear external fixation section.

General Application Steps for a Distal Humeral or Femur Fracture

1) A pilot hole is pre-drilled through the condyle in the true medial-lateral plane of the distal juxta-articular bone segment. A positive profile full-pin with cortical (humerus) or cancellous (femur) thread profile is inserted through the condyle using low speed.
2) The full-pin is attached to the medial and lateral aspect of a stretch ring using two pin fixation bolts and 6.0 mm nuts. The open surface of the ring can be directed cranial or caudally to allow normal motion of the joint. The author preferes to direct the open surface of the ring caudaully in order to provide an anterior platform available for additional connecting elements (Figure 53-77). Alternatively, the open end of the ring can be orientated cranial.
3) An intramedullary pin is inserted in a normograde manner from the proximal aspect of the bone the intramedullary pin can be retrograded from the fracture site, through a mini-approach. The pin is inserted into the center of the femoral condyle or into the medial half of the humeral condyle. If necessary, a mini-approach can be used to facilitate placement of the intramedullary pin into the condyle. The pin is advanced until the axial length of the bone is re-established. The pin also prevents translational malalignment of the condyle and the primary bone segment. Once the fracture is aligned, the pin is then passed distally into the condyle.
4) The stretch ring is manipulated to correct rotational and angular malalignment of the condylar bone segment. A hybrid connecting rod is secured to the lateral aspect of the stretch ring using two 6.0 mm nuts. To allow proper angulation of the hybrid rod, it may be necessary to attach the hybrid rod to the ring using either paired spherical nuts and washers, or by creating an articulation using two posts.
5) The proximal end of the hybrid rod is attached to the proximal aspect of the primary bone segment using two - three positive profile half-pins and SK clamps. The hybrid rod can be tied into the intramedullary pin using an articulation. Alternatively the IM pin can be contoured and directly attached to the hybrid rod with an SK clamp (Figure 53-78).
6) To provide additional support of the condyle, a positive profile half-pin is inserted into the condylar or supracondylar region, distal to the fracture. The pin can be inserted into either the medial or lateral aspect of the condyle. The author has found that a medially placed half-pin is clinically well tolerated, penetrates less soft tissue and results in less restriction of joint motion than a laterally placed pin. The half-pin is placed from either a post or a short hybrid connecting rod (Figure 53-79).
7) One or two diagonal struts can be created using articulations secured to the anterior region of the stretch ring to improve frame stiffness. Additional half-pins can be placed into the proximal cranial-lateral aspect of the humerus or femur from SK clamps positioned along the diagonal strut (Figure 53-80).
8) The frame can easily be destabilized to enhance fracture healing postoperatively by: a) removal of the intramedullary pin; b) removal of diagonal struts; or c) by converting the HCF into a lateral 1-a linear external fixator by removing the ring, diagonals, medial half-pin, and cutting the medial aspect of the full pin.
 

Postoperative Care of the HCF

Postoperative care of a HCF is similar to the care previously described for circular and linear fixators. If destabilization of the frame is desired, it is generally performed 6 to 8 weeks after surgery. Staged disassembly is usually not necessary if fixation wires have been used in the short bone segment. If a wire or pin causes significant drainage or becomes loose it should be removed, or replaced if necessary. Orthogonal radiographs should be performed every 6 to 8 weeks until fracture healing is complete and the frame removed. The supporting, connecting and assembly elements of the HCF can be cleaned and reused numerous times.

Suggested Readings

Cross AR, Lewis DD, Rigaud S, Rapoff AJ: Effect of various distal ring-block configurations on the biomechanical properties of circular external skeletal fixators for use in dogs and cats. J Am Vet Res 65; 4: 393, 2004.

Lewis DD, Bronson DG, Cross AR, et al.: Axial characteristics of circular external skeletal fixator single ring constructs. Vet Surg 30: 386, 2001.

Marcellin-Little DJ, Roe SC, Rovesti GL, et al.: Are circular external fixators weakened by the use of hemispherical washers? Vet Surg 31: 367, 2002.

Toombs JP, Bronson DG, Ross D, Welch RD: The SKTM external fixation system: description of components, instrumentation, and application techniques. Vet Comp Ortho Traumatol 2:76, 2003.

Worsar MA, Marcellin-Little DJ, Roe SC: Influence of bolt-tightening torque, wire size, and component reuse on wire fixation in circular external fixation. Vet Surg 31: 571, 2002.