Fixation with Pins and Wires

Definition and Indications

Cerclage refers to a wire used to encircle a bone. Cerclage wire typically is used to provide interfragmentary compression in a manner similar to that of interfragmentary screws, and is useful in situations where space is limited and screw application would be difficult, or when screws of the correct type or size are not available. Properly applied cerclage wires have been shown not to interfere with the blood supply to bone,^1,2 and may be used in the immature as well as the mature animal.^3,4 Types of cerclage wire application include full cerclage, where the wire completely encircles a complete cylinder of bone, and hemicerclage, where wire is passed through a hole or holes drilled through bone.

For the vast majority of fractures, cerclage wire is used as adjunctive, rather than primary, fixation. The surgeon should keep in mind that while properly applied cerclage wire in selected cases is very effective, errors in application or case selection can be disastrous. Careful attention to detail in fracture repair planning, fixation and post-operative assessment is critical. Cerclage wire is typically applied to long spiral or oblique fractures where the length of the fracture is roughly two and one-half to three times the diameter of the bone (Figure 49-1), and a single cerclage wire is avoided as it acts as a stress concentrator and becomes a fulcrum for motion of the fracture fragments. Cerclage wire may be used both as a temporary reduction device, for example, to hold the fracture in reduction while applying a plate, external skeletal fixator, or interlocking nail, or as a permanent device, often in combination with one of the above or with an intramedullary pin. Although some investigators have commented that cerclage wire may inhibit the surgeon’s ability to accurately contour a plate to bone, in practice properly applied cerclage wires are commonly left under plates and mechanical performance in one study showed that cerclage wires used under the plate performed as well as lag screws and were easier to apply.⁵ In certain circumstances, cerclage wire can also be used over a plate,^5,6 although mechanically they may not perform as well as they do under the plate.⁷

An exception to the single cerclage wire rule for long bones may be made if the intent is to prevent a non-displaced fissure fracture from propagating or fracturing further during manipulation of the fracture ends during open reduction (Figure 49-2).

Cerclage wire is made of relatively soft (usually annealed) 316L stainless steel that is available on spools, in coils or as preformed loops. Sizes of cerclage wire typically used in cats and dogs include 22, 20 and 18 gauge. On rare occasions, it may be appropriate to use 24 gauge wire in very tiny patients and 16 gauge wire in very large dogs. A special type of cerclage wire, cable cerclage, uses large diameter braided titanium alloy or stainless steel along with a special clamp system. Cable cerclage is designed for use over total hip replacements or for fixation of trochanteric osteotomies in humans. Successful use of cable cerclage after total hip revision in the dog has been reported.⁸

Wire diameter can exponentially increase load to failure (a 50% increase in diameter may increase load to failure by up to 169%),⁹ and the largest diameter wire that the surgeon can apply to the bone without technical difficulty is recommended.¹⁰ The area moments of inertia (I = πr4/4) of the various common wire sizes are listed in Table 49-1 and give the user an indication of how strength is greatly decreased as the size of the wire decreases, and also why the use of stainless steel suture material (eg, 30 gauge stainless steel wire) in any configuration is strictly contra-indicated for fracture repair. Method of wire application and type of knot have been extensively studied in both the human and veterinary literature in order to maximize both initial tightness of the wire and identify configurations that will stand up to cyclic load and maximize load to failure. When reviewing the literature, one should be aware that some studies are designed to evaluate cerclage wire used for spinal or tension band applications rather than for long bone applications, and be cautious when trying to apply results of those studies toward applications which they were not designed to evaluate.

Types of Knots and Types of Twisters

Cerclage wire in veterinary surgery is generally applied either as a twist wire or loop wire (single or double loop). Clinical advantages of twist wires include ease of application with a simple wire twister (Figure 49-3), the ability to tighten and fasten the wire at the same time, and the ability to retighten the wire if it loosens during fracture reduction,¹¹ as frequently occurs during the placement of multiple cerclage wires. Loop wires have the advantage of better initial tension or tightness when properly applied, and do not have a protruding twisted end that may irritate soft tissues. Loop wires that loosen during fracture reduction must be removed and replaced.

There are many different types of wire tighteners available which have been reported in the literature. They fall into categories of twist tighteners, loop tighteners and there are instruments available that can actually tie a square knot in stainless steel cerclage wire (Table 49-2).
 

Application of Full Cerclage Wire

For fixation of a long bone fracture, cerclage wire should only be used where the fracture can be anatomically reconstructed to complete the original, 360° cylinder of bone. Although wire can keep bone fragments compressed, it cannot keep them apart as a plate and screw construct can. If even a tiny piece of bone is missing, the fracture will collapse as the wire is tightened and a loose wire with loss of reduction will result. In general, cerclage is reserved for two-piece fractures, although occasionally a third piece may be successfully incorporated. It is important to minimize dissection and soft tissue trauma to the musculature attached to the bone while applying cerclage wire. Cerclage wire may be passed around the bone either directly or using an instrument such as a cerclage wire passer (Figure 49-4) or aneurysm needle (Figure 49-5). Soft tissue inadvertently trapped under the wire will undergo necrosis and this will subsequently lead to wire loosening, however, there is no need to attempt to place the wires subperiosteally.⁴ Wires are generally placed approximately 1 cm apart and at least 5 mm from the ends of the fracture. Some surgeons recommend that cerclage wires should be placed no nearer that one bone diameter away from the ends of the fracture.

After the cerclage wire has been carefully passed around the diaphysis of the bone and the fracture reduced, it must be tightened while maintaining reduction. For twist wire application, the wire may be twisted by hand for the first one or two twists loosely, leaving about 0.5 to 1 cm between the bone and the twist. A locking wire twister should be used for applying cerclage wire, as use of an ordinary pair of pliers allows loss of tension as the wire is being twisted. Using a locking wire twister, both wire ends should be grasped where they intersect (See Figure 49-3), and the wire pulled firmly up while twisting at the same time to avoid the complication of one wire wrapping around the other, which will drastically weaken the construct. It is of critical importance that the wire be tight. The surgeon should watch as the gap between the wire and the bone disappears, and should periodically check the wire for looseness by pushing firmly on it with a Freer periosteal elevator or other suitable instrument. If the wire is loose, tightening should continue. With practice, the operator will develop a “feel” for the mechanics of stainless steel cerclage wire, as a rule, it is common to break wires as they are being applied, particularly for the inexperienced surgeon. If the wire breaks between the 2nd and 3rd twist or higher and is tight, it may be left in place, otherwise, it is removed. The wire should be left without cutting or otherwise manipulating the ends until all cerclage wires have been placed, and then checked again for tightness. Care should be taken not to notch or otherwise damage the wire that is going to stay in the animal as even a small notch will greatly decrease the fatigue resistance of the wire.¹² Loose wires should either be retightened or removed and replaced. If the wires were placed for temporary fixation, for example to hold the fracture in reduction while applying a bone plate, they may be removed prior to final tightening of the plate screws.
 

Twist wires should be cut to preserve at least 2 to 3 twists. It has been shown that wiggling the wire during cutting can substantially decrease the tension in the twist wire.^13,14 Wire ends should not be bent over with twist wires utilized for full cerclage (as opposed to hemicerclage or wire used in pin and tension band fixations).

Loop wires may be applied using either commercially available or hand-made loops. The cerclage wire is placed as described above for twist wires, and the free end passed through the eye of the loop. The loop wire tightener is passed over the free end of the wire, which is passed through the crank of the tightener. The wire is tightened by turning the crank until it can no longer be moved. Tightness of the wire can be checked with a periosteal elevator or other suitable instrument. The wire is then bent over until the free end folds back on itself, maintaining tension on the wire during this step. The crank is then reversed until enough length of wire is exposed so that it can be cut, and the arm is pressed flat to the bone. A double loop wire is made by taking a suitable length of wire, folding it in half, passing it around the bone as described above, and passing the two free ends through the loop. A double loop tightener with two cranks is used to tighten the wire as described above for the single loop wire.¹⁵

For all types of wire, it is important that they are tightened perpendicular to the long axis of the bone, rather than perpendicular to the fracture line as they will slip down perpendicular to the bone when exposed to weight-bearing forces and become loose. In an area where the bone diameter is changing and wire slippage may occur, the use of a Kirschner wire to prevent slippage as a “skewer pin” may be indicated (Figure 49-6). The K-wire is placed perpendicular to the fracture line, and the cerclage wire is placed around it and tightened so that the ends of the K-wire prevent it from slipping. Skewer pin configurations are not as strong as lag screw fixations, but may be considered for the treatment of short oblique fractures if supported by another device.¹⁶

Cerclage wires should be placed at least one-half of the diameter of the bone apart. Multiple cerclage wires should always be used unless they are being used to prevent propagation of fissures. In the author’s opinion, the operator should also keep in mind that if more than four or five cerclage wires are being placed, that the possibility for excessive stripping of the soft tissues exists and another type of fixation should be considered (See figure 49-6).
 

Contraindications

Full cerclage wires are contraindicated in the treatment of transverse, short oblique (with the possible exception of a skewer pin configuration), segmental or multi-fragmented fractures. When evaluating preoperative radiographs of fractures, all of the fragments, even tiny ones, should be counted and if there are more than three, another method of fixation should be considered. Full cerclage are also contraindicated if, for any reason, the full 360 degrees of the shaft cannot be reconstructed, or the shape of the bone is such that wires cannot be applied so that they will sit perpendicular to the long axis of the bone without slipping. Loose or damaged cerclage wires should always be removed. Finally, the surgeon should balance the risk of damage to the blood supply and potential for a nidus of infection in high-velocity, open or infected fractures and as a general rule, cerclage wire fixation is contraindicated for these types of fractures.

Complications and their Prevention

Properly applied cerclage wires rarely cause problems, however, improperly applied wires are almost always problematic. Loose wires, the most common complication, usually occur either as a result of failure to completely reduce the fracture or because of improper tightening techniques. In the author’s experience, cerclage wire failure most frequently occurs when wires are utilized inappropriately on short-oblique or multi-fragmented femur fractures, usually combined with an intramedullary pin in large breed dogs. Loose wires very effectively prevent revascularization of the area around the fracture, and sequestration of dead bone fragments with collapse and rotational instability of the fracture are the end results (Figure 49-7). Prevention of complications depends upon careful case selection and proper application techniques, as described above. A failed pin and cerclage wire fixation can be devastating for the animal and in some cases may be irreparable, even if referred to a specialist with access to all types of orthopedic equipment.

Hemicerclage wire

Hemicerclage wire refers to wire that has been passed through at least one hole drilled through the bone. Although hemicerclage configurations have been reported for the treatment of rotational instability in long bone fractures, in practice they are very weak,¹⁷ reaching only about 3% of the load in Nm of an intact construct and absorbing only 2% of the energy that an intact construct can absorb prior to failure during mechanical testing.¹⁸ Hemicerclage wire applied to long bone fractures may also only be effective if rotational instability occurs in only one direction. Biomechanical testing of a variety of interfragmentary wire designs, either with hemicerclage wire, or combined cerclage wire and K-wire applications showed that a biplanar 90° configuration with wire and cross pin configuration had the highest torsional yield load and safe load,¹⁸ however, this configuration would be difficult to apply clinically and has yet to be tested in vivo.

Hemicerclage is primarily used where applied loads are low, for example in the treatment of mandibular, maxillary and some skull fractures. Considerations for applying these wires include avoiding tooth roots and angling drill holes such that it is easy to grasp the wires and pull them through the bone to allow tightening. Holes drilled for application of hemicerclage wires should be at least 2 implant diameters away from the fracture line to prevent them pulling through or fracturing the fragment as they are carefully tightened. Unlike full cerclage wire, hemicerclage wire is not prone to loosening after being tightened down to the bone and it is acceptable to bend the wire ends over. Overtightening of hemicerclage wire will cause bone failure and pull-out of the wire. Attention should be paid to pulling as much “slack” from the wire prior to tightening, and also to twisting the wire halfway between two points of fixation so the twist does not sit at the level of the drill hole and prevent further tightening.
 

References

1. Blass CE, van Ee RT, Wilson JW. Microvascular and histological effects on cortical bone of applied double-loop cerclage. J Am Anim Hosp Assoc 27:432,1991.
2. Rhinelander FW, Wilson JW. Blood supply to developing, mature and healing bone. In: Sumner-Smith G, ed. Bone in clinical orthopedics. Philadelphia: WB Saunders, 1979, p.162.
3. Ellison GW, Piermattei DL, Wells MK. The effects of cerclage wiring on the immature canine diaphysis: a biomechanical analysis. Vet Surg 11:44, 1982.
4. Wilson JW. Effect of cerclage wires on periosteal bone in growing dogs Vet Surg 16:299, 1987.
5. Nye R, Egger E, Huhta J, Histand M, Mallinckrodt C. Acute failure characteristics of six methods for internal fixation of canine femoral oblique fractures. Vet Comp Orthop Traum 9:106, 1996.
6. Kanakis TE, Cordey J. Is there a mechanical difference between lag screws and double cerclage. Injury 22:185, 1991.
7. Willer RL, Schwarz PD, Powers BE, Histand ME. Comparison of cerclage wire placement in relation to a neutralization plate: a mechanical and histological study. Vet Comp Orthop Traum 3:90, 1990.
8. Blaeser LL, Cross AR, Lanz OI. Revision of aseptic loosening of the femoral implant in a dog using cable cerclage. Vet Comp Orthop Traum 12:97, 1999.
9. Meyer DC, Ramseier LE, Lajtai G, Notzli H. A new method for cerclage wire fixation to maximal pre-tension with minimal elongation to failure. Clin Biomech 18:975, 2003.
10. Wilson JW. Knot strength of cerclage bands and wires. Acta Orthop Scand 59:545, 1988.
11. Bostrom MPG, Asnis SE, Ernberg JJ et al. Fatigue testing of cerclage stainless steel wire fixation. J Orthop Traum 8:422, 1994.
12. Oh I, Sander TW, Treharne RW. The fatigue resistance of orthopaedic wire. Clin Orthop Rel Res 192:228, 1985.
13. Roe SC. Evaluation of tension obtained by use of three knots for tying cerclage wires by surgeons of various abilities and experience. J Am Vet Med Assoc 220:334, 2002.
14. Rooks RL, Tarvin GB, Pijanowski GJ, Daly B. In vitro cerlage wiring analysis. Vet Surg 11:39, 1982.
15. Roe SC. Mechanical characteristics and comparisons of cerclage wires: introduction of the double-wrap and loop/twist tying methods. Vet Surg 26:310,1997.
16. Smith BA Kerwin SC, Hosgood G, et al. Mechanical comparison of two methods for interfragmentary fixation in a short oblique fracture model. Vet Comp Orthop Traum. 9:4, 1996.
17. Blass CE, Caldarise SG, Torzilli PA, Arnoczky SP. Mechanical properties of three orthopedic wire configurations. Am J Vet Res 46:1725, 1985.
18. Metelman LA, Schwarz PD, Hutchison JM, et al. A mechanical evaluation of the resistance of various interfragmentary wire configurations to torsion. Vet Surg 25:213, 1996.
19. Willer R. Cerclage wiring. In: Bojrab MJ (ed): Current techniques in small animal surgery 4th ed. Baltimore, Williams & Wilkins, 1998, p. 921.
20. Blass CE, Piermattei DL, Withrow SJ, Scott RJ. Static and dynamic cerclage wire analysis. Vet Surg 15:181,1986.
21. Cheng SL, Smith TJ, Davey JR. A comparison of the strength and stability of six techniques of cerclage wire fixation for fractures. J Orthop Traum 7:221,1993.
     

Definition and Indications

Devices used in the medullary cavity of long bones, such as intramedullary rods or nails, are designed to act as non-compressing splints. A gliding splint allows compression caused by physiologic loading conditions, while a non-gliding splint incorporates features that prevent fragment compaction, for example an interlocking nail. The terms “rod” and “nail”, while sometimes interchanged, are not equivalent. A rod is loosely applied, so that contact with endosteal bone is limited. Examples of use of a rod would be a rod suspending a roll of paper towels, allowing free movement between the paper towel tube and the rod, or typical veterinary use of an intramedullary Steinmann pin. In the veterinary literature, the term “pin” is often used interchangeably with “rod”. A nail is tightly applied to the endosteal bone to the point of firm wedging, just like a carpenter’s nail driven into a board, displacing wood and becoming firmly wedged.¹

Types of Implants Available

Intramedullary pins (IM pins) used in animals range from 1⁄4 inch diameter (6.3 mm) down to 5/64 inch diameter (2.0 mm). Intramedullary pins in this size range are called Steinmann pins. Smaller pins are usually referred to as Kirschner wires (K-wires), and although they may be used as intramedullary devices in very tiny animals, they are generally used as interfragmentary devices. K-wires are available in .035, 0.045, .054 and .062 inch diameters. Intramedullary pins and K-wires can be obtained as fully threaded, partially threaded or nonthreaded. Although some surgeons use partially (end) threaded pins for intramedullary pins with the intention of increasing rotational stability, in fact those pins do not provide additional rotational stability (Figure 49-8) and are at risk for breakage at the thread-shaft interface or in the weaker threaded portion (Figure 49-9). In addition, fully or partially threaded Steinmann pins and K-wires are more difficult to remove as the bone tends to grow into the threads. For these reasons, the use of threaded pins as intramedullary devices is not recommended.
 

Steinmann pins and Kirschner wires are available in a variety of lengths, usually from 6 to 12 inches long, and may have points on one or both ends. Although most are manufactured from surgical grade 316L stainless steel, pure titanium K-wires are also available. The pins are easily cut, and there is no advantage to the veterinary surgeon in purchasing single pointed pins.² Pins may be manufactured with a trocar, chisel, diamond, or bayonet point (Figure 49-10). Trocar points are by far the most commonly used and consist of a three-sided tip with a long bevel and good ability to penetrate cortical bone. Chisel points (also called diamond points) are broad, flat two-sided points with a short bevel and are designed to deflect the pin away from the cortex during drilling rather than engage the opposite cortex. Some pins are available with a trocar point on one end and a chisel point on the other: the starting hole through the cortex can be drilled using the trocar point, and then the pin can be reversed if the surgeon desires for it to deflect off the far cortex. Bayonet points are “single-lipped” or “free cutting” points found on pins designed for transfixation pins for circular external fixators. The ends are such that they can easily penetrate soft tissues and can be tapped through cortical bone with a hammer after being partially drilled through the bone in an attempt to preserve blood supply. These pins also come with “stoppers” in the middle and may be called “olive wires”, which may be used to pull a fragment into alignment or allow decreased translation of a bone segment within an external fixator frame (Figure 49-11).
 

Intramedullary pins excel in resisting bending forces in 360 degrees, can be placed with relatively little in the way of specialized equipment, and often can be placed with a limited approach. Intramedullary pin placement, unless a very large pin is placed or reaming of the medullary cavity is performed, has limited impact on the intramedullary blood supply. Intramedullary pins are relatively easy to remove, in contrast to fixation devices such as lag screws or plates. Intramedullary pins do not prevent rotation or counteract axial forces, and therefore are rarely used alone but combined with other types of fixation, for example cerclage wires, external fixators, plates, and lag screws. From a mechanical standpoint, use of the largest pin possible will result in the stiffest construct and most resistance to bending (Table 49-3). However, use of an excessively large pin has several disadvantages: difficult placement in a curved bone, for example the canine tibia and femur, damage to the intramedullary blood supply, and risk of creating additional fractures if the pin exceeds the diameter of the bone at its’ narrowest point, or isthmus. In general, a pin that is approximately 70% of the diameter of the long bone at the isthmus is chosen. If the surgeon anticipates combining the pin with another type of device, such as an external fixator, lag screw or plate, a slightly smaller intramedullary pin is chosen. Use of a pin that is too small may result in failure by pin bending or breakage (Figure 49-12). Use of multiple small pins to fill the medullary cavity, also called “stack pinning” to increase resistance to rotational stability, has been shown not to increase rotational stability significantly more than single intramedullary pinning.³
 

In addition to using ancillary devices to control rotational and axial forces on the bone with IM pins, modifications to the pins themselves, including placement of screws through holes across the pin (interlocking nail construct) and modification of the pin itself can be used. A recent example of this in the veterinary literature is the Trilam nail, a stainless steel intramedullary device designed with three “lamellae” extending down its length to counteract rotational forces. The nail is driven with a mallet into the medullary cavity without reaming, such that the three lamellae cut into the inner cortical bone, making it a true nail. Successful use of the Trilam nail in dogs and cats for the treatment of femoral, tibial and humeral fractures has been reported.⁴

K-wires, while they can be used as intramedullary devices, are usually used as interfragmentary devices, often to maintain temporary fracture reduction while the primary fixation (eg a plate) is applied. K-wires by themselves are relatively weak implants (See Table 49-1) and are not generally used alone. In certain fractures, for example physeal fractures in small dogs and cats, cross-pinning with K-wires can be sufficient when fracture healing is expected to be rapid. K-wires are also commonly utilized in combination with cerclage wire for tension band fixations and to support full cerclage wires in areas of changing bone diameter (“skewer wires”).

K-wires have also been modified to improve their anti-rotational characteristics, for example, the Orthofix self-compressing pin recently reported in the treatment of humeral condylar fractures in small breed dogs.⁵ These pins are small diameter (1.2 to 2.2 mm threaded segment, 1.5 to 3 mm shaft) pins are designed for use in cancellous bone. As the pin is drilled, the threaded portion cuts a thread into the cancellous bone. When the pin’s chamfer (location of the thread-shaft interface where the diameter of the pin increases) contacts the near cortex, further advancement of the implant partially strips the threads cut in the bone in the near fragment, while the threads in the far cortex maintain purchase, leading to interfragmentary compression.
 

Application Techniques for Intramedullary Pins and Interfragmentary Wires

Intramedullary pins may be inserted either from the fracture site (retrograde insertion) or from either the proximal or distal end of the bone itself (normograde insertion). The local anatomy of the bone often dictates how the pin is driven, for example, retrograde pin insertion in the tibia often results in damage to the articular cartilage or cruciate ligaments. An estimation of appropriate pin size (60 to 75%)⁶ may be made from the post-operative radiographs and may be confirmed by observation of the pin as it is gently introduced into the fracture site, even if normograde insertion is planned. If in doubt, a smaller pin should be used initially and replaced with a larger pin if necessary. The pin may be inserted either open or closed. Although closed pinning, based on palpation, can be performed by the experienced surgeon this can become more difficult in larger animals with soft tissue swelling, or in fractures greater than 72 hours old. The increased use of intraoperative imaging (fluoroscopy) can greatly facilitate IM pin placement in a minimally invasive fashion, with less damage to the soft tissues.

Intramedullary pins may be placed either by hand, using a Jacobs’ chuck (Figure 49-13), or with a drill. Hand insertion may allow the surgeon to feel whether or not the pin is advancing down the medullary cavity and whether it is up against or about to penetrate cortical bone. When placing pins by hand with a Jacobs’ chuck, the smallest amount of pin possible that will allow the pin to advance should be used, in order to prevent “wobble”, particularly when smaller pins are used. The chuck should be firmly tightened with the key in at least two separate places to avoid sudden loosening during pin advancement. Although Jacobs’ chucks are sold with protective devices that are designed to protect the surgeon’s hand from inadvertent pin penetration resulting from sudden pin loosening, in practice many surgeons place the palm of their gloved hand over the end of the pin to gain mechanical advantage. The tip of the pin should firmly engage bone, and the pin rotated back and forth with quarter turns (rather than driven consistently clockwise or counterclockwise) while avoiding any “wobble” that may enlarge the proximal hole made by the pin and predispose to pin loosening. In larger animals with hard cortical bone, hand placement can be very difficult. In addition, the smaller the pin the more difficult it is to drill by hand and power insertion is mandatory for inter-fragmentary K-wires.
 

When using a drill to insert an intramedullary pin or K-wire, a cannulated drill should be used to drive the pin to decrease the amount of wobble and the risk of pin bending or breakage during drilling. A high-torque, low speed drill should be used (as opposed to a high speed drill) to decrease heat generation and subsequent bone necrosis. Saline lavage is also important to decrease heating of the bone, particularly with K-wire insertion. When driving an intramedullary pin, whether by hand or with power, it is important to carefully line up the pin with the shaft of the long bone. During open reduction, the surgeon may find it easiest to drive the pin with one hand and stabilize the fracture segment by using a bone-holding forcep gripped with the non-dominant hand. Having an assistant hold a second pin parallel to the shaft of the long bone may be helpful, or in certain cases, placing an “aiming pin” retrograde just a few cm into the medullary cavity so the surgeon can attempt to drive the normograde pin along the axis provided by the aiming pin. When attempting to seat an intramedullary pin into metaphyseal bone, it is important not to accidentally penetrate the articular surface, for example in the distal femur. The surgeon will note an increase in resistance as the pin begins to seat into the metaphysis. In addition, the fracture fragments may begin to distract apart as the pin over-lengthens the bone as it is driven into the metaphysis. In a comminuted fracture, an IM pin may be used to help distract fracture fragments and assist with fracture reduction. As the sharp tip of the pin passes through the first of the two major fracture segments, it may be cut to help prevent penetration of the pin into the joint and will help distract the fracture. The distance that the pin has advanced may be judged by using a second pin of equal length and lining the two pins up after the Jacobs chuck has been removed. After the pin has been seated, the proximal and distal joints should be put through a complete range of motion, as it is easy to inadvertently place a pin into a joint. If available, intraoperative fluoroscopy is useful and much more efficient than closing the surgical approach, traveling to radiology, and returning to the operating room to redirect an implant. Immediately prior to wound closure, the surgeon should carefully palpate the soft tissues surrounding the bone for evidence of overly long or misplaced pins (Figure 49-14), which can be difficult to feel as they unexpectedly exit cortical bone during drilling.

After the pin is judged to be in the correct position, it is cut using specialized pin cutters. Pin cutters are generally designed only to cut pins of a certain range in diameter, and inadvertent use of small pin cutters to cut a large pin may result in permanent damage to an expensive piece of equipment. The surgeon should check the range of diameters listed on the side of the cutter (Figure 49-15). It is helpful to have an autoclavable pin guide (Figure 49-16) in the pack to determine the size of the pin in surgery. The surgeon should also avoid cutting the pin with bone cutters, which look very similar to pin cutters but will be irreparably damaged if used to cut stainless steel pins or wires. Autoclaved “hardware store” bolt cutters are acceptable but can be bulky and difficult to get into a surgical approach in some cases. Pins accumulate a lot of energy when cut and have the potential to cause serious injury to the surgeon, assistant or circulating technicians if the free end is not firmly grasped or covered with a Huck towel to prevent it from becoming a projectile. The fracture should be carefully observed during and after pin cutting to make sure that alignment is not disrupted.
 

Controversy exists over whether to cut pins short or leave them long to facilitate pin removal. Leaving pins long can lead to problems with iatrogenic nerve damage (proximal femur), damage to nearby articular cartilage or patellar tendon (tibia), or soft tissue irritation with subsequent seroma formation or erosion of the tip of the pin through the soft tissues. The alternative to leaving pins long is to cut them short and countersink them, using a countersink and mallet. Many surgeons accomplish this by retracting the pin approximately 1 cm, cutting the pin as short as possible, and then placing the countersink over the top of the pin and tapping it in approximately 1 cm or until it is level with the proximal aspect of the bone (eg, greater trochanter of the femur).

Pin Migration

Steinmann pins and Kirschner wires can loosen and “migrate” out of the bone over time, and in fact have been reported to migrate large distances and penetrate organs including the brain and heart (noted after the use of DeVita pins for reduction of hip luxations),⁷ or into a joint, as after pinning of a proximal femoral epiphyseal fracture. Where possible, it is helpful to bend the pin over using either a pin bender designed specifically for bending pins (Figure 49-17) or if such a device is not available, the Jacobs chuck or a metal Freer suction tip can be used to bend K-wires. The surgeon should be cautious when bending larger pins in soft bone or small fragments as the bone could fracture as the pin is being bent, or loss of reduction could occur if stabilization is marginal. Once the pin is bent, it is impossible for it to migrate towards its point, however, it can still migrate in the opposite direction.
 

Pin Removal

Pins should be removed if they are loose, irritate soft tissues, are in a joint or are infected. Although many surgeons routinely remove intramedullary pins, it has been our experience that stable pins do not need to be removed after the fracture has healed. In humans, there is controversy over whether or not pins should be removed, with one author noting that orthopedic surgeons with implants did not have their own hardware removed, citing a higher refracture rate after implant removal and no documented downside to leaving implants in.⁸

Contraindications

Intramedullary pins alone should never be used for comminuted fractures that will collapse around the pin or fractures that will be rotationally unstable. Pins and cerclage, while effective if properly applied, are useful only for certain types of simple, closed fractures in animals with good healing potential. Although use of an intramedullary pin in an open or infected fracture has been thought to have the potential to spread infection along the medullary cavity, in fact, intramedullary devices can safely be used in infected fractures as long as they are stable.⁹

Complications and their Prevention

Complications associated with intramedullary pinning most frequently include damage to adjacent structures. Bones most amenable to intramedullary pinning include the femur, tibia, humerus and ulna. Pinning of the mandible results in damage to tooth roots and an unstable repair. Pinning of the radius cannot be performed without damaging an articular surface, and bent, broken or migrating pins in the radius are difficult to retrieve. As discussed above, specifics for each bone as to whether the pin should be normograded or retrograded, and how the pin is directed in the medullary cavity, are detailed elsewhere but should be reviewed prior to IM pin fixation.

A common mistake when driving an IM pin is to be slightly off at an angle away from the center of the medullary canal, resulting in the pin penetrating cortical bone prior to crossing the fracture line. If this occurs, the pin must be completely withdrawn and a new entry site drilled, as the pin will tend to fall into the same track that was originally made. If the pin is inadvertently penetrated into a joint, merely withdrawing the pin back into the medullary canal is not sufficient, as once the animal begins to bear weight the pin will migrate into the joint. If possible, the pin may either be withdrawn and replaced with a slightly larger pin, or withdrawn and redriven at a slightly different angle to prevent it from entering the original hole into the adjacent joint.

Post-operative radiographs of the entire bone, in two orthogonal views must always be made post-operatively to confirm pin placement and fracture reduction. Pins that have penetrated a joint should always be removed, as severe damage can occur even within a few days to weeks. If pin migration is noted prior to fracture healing, the fracture is unstable and reinsertion of the implant will not solve the problem. It may lead to infection particularly if a tip of the implant has penetrated the skin. Repeat radiographs should be obtained, and an alternative plan made to address fracture instability with a different form of fixation.

Pulmonary fat embolism is a fairly common complication associated with the introduction of intramedullary devices in humans, and has been reported as a cause of morbidity in dogs during total hip replacement.¹⁰ Although pulmonary fat embolism is not commonly recognized in small animals associated with IM pins, one well-documented case has been reported to cause fatality in a cat,¹¹ and the surgeon and anesthetist should be aware of the risk and appropriate intra-operative monitoring performed, particularly in animals with pre-existing pulmonary trauma or disease.

References

1. Chandler RW. Principles of internal fixation. In: Rockwood CA, Green DP, Bucholz RW, Heckman JD (eds) Rockwood and Green’s Fractures in Adults. Philadelphia, Lippincott-Raven 1996: 165-179.
2. Howard PE. Principles of intramedullary pin and wire fixation. Seminars in Veterinary Medicine and Surgery (Small Animal) 6:52,1991.
3. Dallman MJ, Martin RA, Self BP, Grant WJ. Rotational strength of double-pinning techniques in repair of transverse fractures in femurs of dogs. Am J Vet Res 51:123, 1990
4. Hach V. Initial experience with a newly developed medullary stabilization nail (Trilam nail). Vet Comp Orthop Traum 13:109,2000.
5. Guille AE, Lewis DD, Anderson TP et al. Evaluation of surgical repair of humeral condylar fractures using self-compressing orthofix pins in 23 dogs. Vet Surg 33:314, 2004.
6. Piermattei DL, Flo GL. Brinker, Piermattei and Flo’s Handbook of Small Animal Orthopedics and Fracture Repair, 3rd ed. Philadelphia, WB Saunders, 1997: 95.
7. Nunamaker, DM. Fractures and dislocations of the hip joint. In: Textbook of Small Animal Orthopaedics. Philadelphia, J. B. Lippincott, 1985,403.
8. Beadling L. Nancy nailing: a pediatric innovation for contemporary society. Orthopedics Today 25:26, 2005.
9. Muir P, Johnson KA. Interlocking medullary nail stabilization of a femoral fracture in a dog with osteomyelitis. J Am Vet Med Assoc 209:397, 1996.
10. Terrell SP, Chandra AMS, Pablo LS, Lewis DD. Fatal intraoperative pulmonary fat embolism during cemented total hip arthroplasty in a dog. J Am Anim Hosp Assoc 40:345, 2004.
11. Schwarz T, Crawford PE, Owen MR et al. Fatal pulmonary fat embolism during humeral fracture repair in a cat. J Small Anim Pract 42:195, 2001.
12. Muir P, Johnson KA, Markel MD. Area moment of inertia for comparison of implant cross-sectional geometry and bending stiffness. Vet Comp Orthop Traum 8:146,1995.
     

Introduction

Tension banding is a technique by which tensile forces are converted into compressive forces. This principle can be applied to the repair of fractures in which a fragment is distracted from its original position by the pull of a muscle, tendon, or ligament. The area of fracture opposite the pull under tension is termed the tension side of the fracture. If the tension side of the fracture is fixed with a tension device, the device pulls in a vector which counters the distractive force. If the force of the distractive pull is not in a straight line with the tension device, the force of the distractive pull is redirected to a resulting vectoral force which is a compressive force across a fracture or osteotomy (Figure 49-18).
 

Indications

Indications for use of tension band wires include repair of fractures or osteotomies of the acromion of the scapula, supraglenoid tubercle, greater tubercle of the humerus, olecranon, greater trochanter of the femur, supracondylar epiphysis of the femur, medial malleolus of the tibia, tuber calcis, tibial tuberosity, and attachments of collateral ligaments. This is a commonly used technique because these are frequent sites of fracture and osteotomies for surgical approaches. A tension band wire can be successfully applied in many situations, if principles of application are followed and proper technique is used.

Technique

Before a tension band wire is applied, the direction of the distractive forces should be estimated. Because forces can change through the range of motion of a joint, the “average” distractive force should be estimated. The tension band should be applied to the side opposite the distractive forces, the tension side of the fracture or osteotomy.

After the fracture or osteotomy is reduced, two orthopedic pins (Kirschner wires) are inserted from the distracted fragment across the fracture line and into the attaching bone (Figure 49-19A). Two pins are used whenever possible to provide rotational stability. The pins should be applied parallel to the direction of desired compression and so that an orthopedic wire placed over them applies even, undeterred pressure to the tension side of the fracture. These pins should be seated in cortical bone in the opposite cortex to prevent migration.

With a drill or orthopedic pin, a hole is drilled through the cortex to accommodate the tension band wire. The distance of this hole from the fracture line should be such that the figure-of-eight wire does not cross directly over the osteotomy. A section of 0.8 mm, 1.0 mm or 1.2 mm orthopedic wire is looped one-third of the distance from one end. The short end is inserted through the hole in the cortical bone, and the long end with the loop is brought over the two orthopedic pins in a figure-of-eight pattern, and is twisted to the other loose end (Figure 49-19B). The preplaced loop and twisted ends are tightened alternatively or with the help of an assistant so the wire is evenly tightened (Figure 49-19C). The orthopedic wire should be cut, leaving three to four twists, and bent toward the bone. The Kirschner wires are bent over the tension band wire and are cut to secure it (Figure 49-19D and 49-19E). The ends of the wires are seated against bone. Aftercare of the tension band wire itself is minimal. No more that standard exercise restriction is required.
 

Complications

Complications are uncommon and are usually the result of improper technique. The six most common technical errors resulting in failure are depicted in Figure 49-20. The first error is having too small a fragment to accommodate an appropriately sized tension band device. Fractures and avulsions can be small, and applying a proper tension band may be difficult. More commonly, however, this error occurs when performing an osteotomy for a surgical exposure, such as an osteotomy of the greater trochanter of the femur or tibial tuberosity. Too small a fragment will break resulting in failure of the tension band. One usually avoidable technical error is the placement of only one pin. Because the vector of the distracting muscle, tendon, or ligament pull may change through a range of motion, there may be a torsional force across the fracture. Two pins prevent rotation. Small avulsion fragments may only accommodate a single pin. However, placing two smaller pins should be used before one larger pin whenever possible. Use of a loop instead of a figure-of-eight wire is an avoidable technical error. A loop tends to center the compression more toward the pin and allows the fracture line on the tension side to distract. Heavy-gauge wire should be used. Although 1.2 mm to 0.8 mm wire may seem difficult to manipulate, smaller wire is rarely appropriate even in small animals. The hole in the bone anchoring the tension band wire should engage enough material to counter the force of the tension device. These forces can be substantial. The pins should be anchored into the opposite cortex. Failure to do so can allow the pin to migrate.
 

Suggested Readings

Kraus KH. Tension band wiring. In: Bojrab MJ, ed. Current techniques in small animal surgery. 4th ed. Philadelphia: Williams & Wilkins, 1998:925.