Fixation with Screws and Bone Plates

Cortical and cancellous screws are commonly used for fracture repair in small animals. Cortical screws are fully threaded and are designed for use in cortical bone (Figure 51-1). Cancellous screws are fully or partially threaded and are used where cortical bone is thin and cancellous bone predominates (Figure 51-1). Cancellous screws have a steeper thread pitch, deeper threads, and a thinner core as compared with cortical screws.

Partially threaded cancellous screws are generally not used in cortical bone because removal of the screw is difficult as bone grows around the unthreaded shank. Both types of screws can be used for different purposes, including lag screws, positional screws, and plate fixation screws.

Lag screws are used for interfragmentary compression of fracture fragments (Figure 51-2). Compression occurs if the screw engages the far cortex and glides in the near cortex adjacent to the screw head. Cortical screws are selected for stabilization of cortical fragments in the diaphyseal region of the bone. The fragments are reduced and are secured temporarily with an appropriate bone clamp. Predrilling of the guide hole or thread hole before reduction and temporary stabilization is sometimes advantageous because it allows accurate placement of the hole in narrow segments of the bone fragment. If predrilling is done, a pointed drill guide is used to align the predrilled hole with the opposite hole to be drilled. The use of cortical screws requires drilling of a glide hole in the near cortex, equal in size to the thread diameter of the screw, to prevent the screw from making purchase. Screw holes should be drilled in the center of the fragment to prevent shifting during tightening. The hole should be drilled in a direction that bisects the angle formed by perpendicular lines to the fracture line and the longitudinal axis of the bone in fragments having less than 40° inclination. If inclination of the fracture is greater than 40°, the hole should be drilled perpendicular to the fracture line. The holes should also be placed an adequate distance away from the edge and tip of the fragment to prevent fracture of the fragment at the screw hole. A countersink tool is optimally used in the near cortex to distribute loads transferred by the screw head to the bone more evenly, thus making fracture less likely. A drill sleeve (outer diameter equal in size to the glide hole, inner diameter equal in size to the thread hole) is inserted in the glide hole until it meets the opposite cortex. A thread hole equal in size to the core of the screw is drilled in the far cortex. A depth gauge is used to measure the length of screw needed. The selected screw should be 1 to 2 mm longer than the measured hole depth to ensure adequate thread purchase in the far cortex. The hole is carefully threaded with the appropriate tap. The surgeon must insert the tap at the same angle as the drill bit and must avoid excessive wobble during tapping to prevent stripping or microfracture of the screw hole. The appropriate screw is then inserted and tightened. Overtightening can lead to stripping of the screw threads or fracture of the bone fragment; appropriate tightness can usually be attained by grasping the screwdriver with the thumb and the first two fingers, instead of the entire hand, when tightening.
 

Cancellous screws are often used to stabilize fragments in the metaphyseal or epiphyseal regions (Figure 51-3). When using cancellous screws in lag fashion, a glide hole is not needed if partially threaded screws are used. The smooth shaft should traverse the near fragment completely. Compression does not occur if screw threads engage the near fragment. The diameter of the hole in both cortices should be equal to the diameter of the core of the screw. Predrilling one fragment is often helpful for alignment of the hole in the center of the fragment. The fragments are reduced and temporarily are stabilized with a bone clamp. The hole is drilled, measured and tapped. Tapping of the hole is optional; it is often helpful to tap only the first few millimeters of the hole to assist with insertion of the screw. Pullout strength of these screws is enhanced if the entire length of the hole is not tapped. The screw is inserted and is tightened as described earlier for cortical screws.

Positional screws can be placed to hold fragments in alignment while a method of primary stabilization is applied. Small cortical fragments can be secured to the diaphysis with a screw that engages the cortices of both fragments. A glide hole is not used; therefore, compression of the fragment does not occur. This type of application is useful when compression of the fragment is likely to lead to its collapse into the medullary cavity or shifting of the fragment out of reduction.

Plate fixation screws are used to fasten a plate to bone. Glide holes are not used unless compression of fragments beneath the plate is desired. Both cortical and cancellous screws can be used, depending on the region of bone. The screws glide in the holes of the plate, thereby compressing the plate against the bone.
 

This topic is written based on the available literature through 2010 and does not cover the most current literature on this topic.

Biology of Fracture Healing

Interfragmentary compression and internal fixation, leading to direct bone healing, have been the gold standard for treating long bone fractures in small animals for many years. Recent developments however, have led to the principle of biological fracture healing. It is characterized by minimal biological damage together with flexible fixation. The minimal biological damage is achieved by indirect reduction techniques and pure alignment of the fragments without the need for precise reduction. Maximal blood supply is preserved to avoid iatrogenic bone necrosis. Flexible fixation is achieved by wide bridging of the fracture zone using locked nails, bridge plating, internal or external fixators. Such fixation leads to indirect bone healing with callus formation.

Plate Function

If the surgeon is able to generate axial compression by the use of a tension device or with eccentric loaded screws, the plate functions as a compression plate. In most instances, this mode is only possible in simple transverse fractures. Whenever the internal fixation of a diaphyseal fracture consists of a lag screw or screws in combination with a plate (to protect the lag screw fixation), the plate functions in a neutralization mode. Such a plate protects the interfragmentary compression achieved with the lag screw or screws from all torsional, bending, and shearing forces. In comminuted fractures of the metaphysis or diaphysis, the application of axial compressive forces may lead to collapse and or angular deviation of the fractured bone. Lag screws can not overcome these forces. In order to prevent loss of bone length or proper alignment in comminuted fractures, it is necessary to supplement the fixation with a buttress plate. The function of the buttress (or bridging) plate is simply to prevent axial deformity as a result of shear or bending. This type of plate fixation is subjected to full loading. Therefore, every possible effort should be made to maintain all the soft tissue attachments and blood supply to the fragments, since healing will depend on the formation of a bridging callus rather than primary bone union. The proximal and distal ends of the plate ends must each be solidly fixed to the corresponding major bone segments by at least 3 screws. The addition of an intramedullary rod (plate-rod fixation) decreases the risk of plate fatigue by micromotion (Figure 51-4).
 

Dynamic Compression Plates

DCP

The dynamic compression plate (DCP; Synthes, Solothurn, Switzerland) was introduced in 1969. The veterinarian may chose from 4.5 mm (giant breed dogs), broad 3.5 mm (heavy and giant breed dogs), regular 3.5 mm (large dogs), 2.7 mm (medium and small dogs, cats) and 2.0 mm size (toy breed dogs, cats). The screw holes are best described as a portion of an inclined and angled cylinder. Tightening of a screw, which is inserted eccentrically at the inclined shoulder of the plate hole leads to movement of the bone fragment relative to the plate, and consequently, compression at the fracture site (Figure 51-5) The design of the screw holes allows for a displacement of up to 1.0 mm. Two eccentric screw insertions per fragment are possible. Depending on the application technique used, a DCP may function in compression mode, as a neutralization plate, or as a buttress plate.
 

LC-DCP

The limited contact dynamic compression plate (LC-DCP; Synthes, Solothurn, Switzerland) represents a further development of the DCP. Compared to the DCP, the area of the plate-bone contact (the plate “footprint”) of the LC-DCP is greatly reduced. The capillary network of the periosteum is thereby less compromised, leading to a relative improvement of cortical perfusion, which reduces the osteoporotic changes underneath the plate. The geometry of the plate, with its structured undersurface, results in an even distribution of stiffness, making contouring easier, and minimizing the tendency to kink at the holes when bent. The plate holes are evenly distributed over the entire length of the plate, which adds to the versatility of application (Figure 51-6). The plate is available both in stainless steel and in pure titanium. Titanium exhibits outstanding tissue tolerance.
 

Application Techniques

When using a 3.5 mm DCP or LC-DCP, the following steps are undertaken. The correct plate length and thickness is estimated from the radiograph. The plate is contoured with bending irons, bending pliers or a bending press. Special bending templates are available. Repeated bending is avoided, because this weakens the plate. The plate should be bent between the holes. The desired function of the screw must be determined (neutral or compression). The screw hole is drilled with the corresponding drill sleeve (standard or universal), which is slightly larger (2.5 mm) than the core of the screw (2.4 mm). The length is measured with the depth gauge. If the correct screw length is not available, the next longer screw is chosen. The hole is tapped (3.5 mm) and the screw is inserted with the screw driver (Figure 51-7).
 

As a rule of thumb, the following maximal forces on the screwdriver are recommended when inserting a plate screw: two fingers for a 2.0 mm screw, 3 fingers for a 2.7 mm screw and the whole hand for a 3.5 mm screw. For perfect force application, torque limiting screw drivers are available. Plate screws are applied first at the ends of the plate, then close to the fracture and finally, the remaining plate holes are filled. The screws are retightened until they are seated firmly.

Miniplates

The increasing demand for fracture treatment in cats and toy breed dogs and the ability of the veterinary surgeon, together with modern diagnostic aids, led to the development of small sized implants for stabilizing fractures in delicate areas such as the maxillofacial region.

The mini-fragment plates (Synthes, Solothurn, Switzerland) are designated for use with the 2.0 mm or 1.5 mm cortex screw. They are available as DCP, round hole plates, angled miniplate, T-miniplate or adaption plate. They are used in long bone fractures, mandibular fractures or pelvic fractures of toy breed dogs and cats (Figure 51-8).

The human Compact system (Synthes, Solothurn, Switzerland) was developed for hand and maxillofacial orthopedic surgery. The smaller sizes (1.0 mm, 1.3 mm and 1.5 mm) and varying plates are now available for veterinary use. The screws are self-tapping and are inserted with the stardrive screw driver. A similar system (Stryker, Kalamazoo MI, USA) is available with 1.3 mm, 1.7 mm and 2.3 mm plates and self-tapping screws, all made of titanium. It is especially helpful in feline orthopedics. The 2.3 mm system perfectly fits the demands, when long bones of cats or toy breed dogs are stabilized (Figure 51-9).
 

Special Plates

Reconstruction plates are characterized by deep notches between the holes that allow accurate contouring. The plate is considered not to be as strong as the compression plates, and may be further weakened by heavy contouring. The holes are oval, to allow for dynamic compression. These plates are especially useful in fractures of bones with complex 3-D geometry, as encountered in the pelvis, especially the acetabulum (See Figure 51-8). Veterinary T- and L-plates are available in different sizes from 2.0 mm to 3.5 mm (See Figure 51-8). Double hook plates are used in proximal femur fractures as well as for intertrochanteric osteotomies. Right and left triple pelvic osteotomy plates with different torque are available in 2.7 mm and 3.5 mm sizes. Tubular plates are useful in areas with minimal soft tissue coverage, such as the olecranon, distal ulna or the malleoli. In scapula fractures, the tubular plate can be applied with its convex surface laid against the scapula spine (See Figure 51-8).

Internal fixators

Biomechanics of internal fixators

The introduction of locking bone plate/screw systems has generated certain advantages in fracture fixation over other plating methods. Locking plate/screw systems are appropriately classified as internal fixators. The stability is given by the locking mechanism between the screw and the plate. The plate does not need to have intimate contact with the underlying bone, making exact plate contouring less crucial. Diminished contact between the plate and the bone may also preserve the periosteal blood supply, thereby reducing the extent of bone resorption under the plate. Internal fixators are used in neutralization or buttress mode. Bone healing under internal fixators is by callus formation (indirect healing).

Experimental studies have shown, that internal fixators offer greater stability than standard reconstruction plates without locking screws. The screws must only be inserted in the cis-cortex. This increases the versatility of internal fixators, which become extremely helpful in acetabular fractures, carpal or tarsal fractures, or in situations where double plating is indicated.

LCP and UniLock

For veterinary use, the locking compression plate (LCP) und the UniLock are available (Synthes, Solothurn, Switzerland). They both have a locking system with threads. The LCP has a so called combination plate hole, which can accommodate either a conventional screw or the new locking head screw. All standard AO plates from 2.7 to 4.5 are available with the combination hole (Figure 51-10). The UniLock comes as 2.0 mm or 2.4 mm system, together with locking screws, non-locking screws and emergency screws. All screws are self-tapping. The locking screws are inserted perpendicular to the plate. A special drill guide, which is screwed into the hole and centers the drill precisely, facilitates the locking mechanism between screw and plate (Figure 51-11).
 

Technical failures and their Prevention

Some common factors leading to technical failures and strategies to avoid them are listed below. Technical failures are usually due to incomplete assessment of the fracture patient, which in turn leads to suboptimal fixation.

Consider the following factors before performing osteosynthesis:

• Animals, which sustain injuries on more than one limb, need more stable fixations than those, which are able to protect a single limb injury by non-weight bearing. 
• In case of an infected and unstable fracture, rigid fixation is mandatory.
• Whenever possible, the least invasive treatment is chosen.

Consider the following factors during osteosynthesis:

• Inadvertent stripping of the bone or detachment of muscles from fragments should be avoided. It is important to preserve as much blood supply as possible to enable optimal fracture healing.
• While using power equipment, cooling with isotonic solutions is mandatory to prevent heat necrosis on the bone and subsequent loss of fixation at the implant-bone interface.

Consider the following factors after osteosynthesis:

• Postoperative resorption at the fragment ends, which were anatomically reduced, are mostly due to the fact, that plate osteosynthesis was not rigid enough for direct or indirect bone healing. Due to the strain theory, the fracture gap must be widened and callus formation will start.
• Implant related stress protection of a healing bone can lead to bone resorption and osteoporosity. Therefore, implants should be removed, as soon as clinical fracture healing has been completed.

Suggested Readings

Gauthier E, Perren SM, Ganz R: Principles of internal fixation, Curr Orthop 6: 220, 1992.

Keller M, Voss K: UniLock: Applications in small animals. Dialogue 2: 20, 2002.

Koch DA: Screws and plates. In Johnson AL, Houlton JEF, Vannini R, eds: AO principles of fracture management in the dog and cat, Duebendorf: AO foundation, 2005, p 26.

Perren SM, Russenberger M, Steinemann S, et al.: A dynamic compression plate. Acta Orthop Scand Suppl 125: 31, 1969.

Perren SM, Klaue K, Pohler OEM, et al: The limited contact dynamic compression plate (LC-DCP) Arch Orthop Trauma Surg 109: 304, 1990.

Perren SM: Evolution of internal fixation of long bone fractures. J Bone Joint Surg (Br) 84B: 1093, 2002.
 

Introduction

The SOP (String of Pearls) was designed to serve as a locking plate system for the veterinary and human orthopedic community. As with all locking plate systems, the SOP can be thought of mechanically as internal – external fixators. The SOP consists of a series of cylindrical sections (“internodes”) and spherical components (“pearls”). There are three system sizes which accommodate 3.5 mm, 2.7 mm and 2.0 mm screws. The cylindrical component, or internode, has an area moment of inertia greater than the corresponding standard DCPs. Mechanical testing using ASTM standards has demonstrated that the 3.5 SOP is 50% stiffer, and has a bending strength (load at which the plate plastically bends) of 16 to 30% greater than the 3.5 mm LCP, DCP, or LC-DCP.

The SOP can be contoured in six degrees of freedom; medial to lateral bending, cranial to caudal bending, and torsion using specially designed bending irons (Figure 51-12). Properly performed, contouring results in bending or torsion at the internode, preserving the locking function of the pearl. Mechanical testing has demonstrated that although bending a SOP will reduce its stiffness and strength by approximately one third, a SOP bent through 40 degrees remains almost (96%) as stiff as an untouched 3.5 DCP. Similarly, a SOP twisted through 20 degrees remains significantly stiffer than the new and untouched 3.5 DCP.

The spherical component of the SOP accepts a standard cortical bone screw. There is a section of standard threads within the spherical component, and a section into which the head of a standard screw recedes. As the screw head recedes into the spherical component, it comes into contact with a ridge causing the screw to press fit into the pearl. This press fitting prevents loosening of the screw during the cyclic loading of weight bearing, and results in a very rigid screw/plate construct. This concept removes critical limitations of locking plate designs employing a hole with either single, double, or conical threads. The larger diameter part of the pearl receives a drill/tap guide and allows drilling, measuring with a depth gauge, and tapping of the screw hole, with familiar ORIF instrumentation (Figure 51-13). The circular cross-section of the implant and the increased diameter of the pearls in comparison with the internodes gives the implant a relatively consistent stiffness profile – the screw holes are not notable “weak points”. The larger size of the pearl protects it against deformation during contouring or load bearing. The use of inserts (“golf tees”) placed into the pearls protects the pearl absolutely and preserves locking function completely during contouring.
 

There is theoretical potential for the screw to cold weld, making it difficult to remove. However, this has not been seen in practice but should it happen, a section of the plate can be simply cut through an internode using a bolt cutter and the offending section removed.

Not all screws are alike. The SOP is designed to be used with high quality screws manufactured to standard tolerances for screw head and thread sizes. Self tapping screws must have triple flutes so that consecutive screws will tap without lifting the plate away from the bone. Inferior screws with unconventional design or loose manufacturing tolerances are becoming more common in veterinary orthopedic surgery as most orthopedic companies outsource screw production. Such screws may not have sufficient quality control to work in the SOP system. For this reason orthopedic screws from the supplier of the SOP should be used, or if using them from another supplier they should be tested in the SOP to assure compatibility.

Biomechanics

The biomechanics of interlocking plate systems differs fundamentally from conventional bone plates – extrapolation of experience gained using non-locking, DCP systems, is not always appropriate. Screws in conventional bone plates press the plate onto bone as the screw is tightened. The threads of the screw pull and slightly deform the bone that the threads engage. As bone is viscoelastic and remodels, the pull lessens over the first several minutes after installation due to bone relaxation, then over the next period of days and weeks due to remodeling. Oval holes allow dynamic compression and load sharing since the screw can move slightly along the long axis of the plate. The screw can pivot in the hole of the plate.

In contrast, locking systems, including the SOP, will function invariably as “buttress” systems – even when they are applied to an anatomically reconstructed fracture. The screws of interlocking plates act as transverse supporting members, subjected to cantilever bending. The primary loads on bone during weight bearing are axial, along the long axis of the bone. Axial loads of a bone encounter a screw and the load is transferred at the bone/screw interface to the screw, then to the plate, then back to the screw on the other side of the fracture, then to bone. Here, there is no pulling of the plate down to the bone so the resistance to pullout of a screw is less relevant. Importantly, the screw is integrally and always part of the transmission of forces across areas of fracture. Locking plate systems rarely utilize dynamic compression, and are acting as buttress devices. The result of die back of bone in the initial healing phase, and the reliance upon lag screws, wires or other mechanically inferior components within the reconstruction means that even where load sharing is achieved at surgery, locking systems invariably function in buttress mode.

With the difference in transmission of forces across the area of fracture, pullout strength of bone screws becomes far less important, making locking screw systems preferred choices in cancellous or osteoporotic bone. Conversely, the fatigue life of the screw/plate interface increases in importance. Clinically, this is of relatively less importance in engaging two cortexes with a bone screw, and much greater importance in increasing the number of bone screws, unicortical or bicortical, to enhance fatigue life. However, while adding a unicortical screw may be of limited benefit with conventional plates, unicortical screws within a locking system function effectively and are appropriate.

This highlights an important mechanical feature of all interlocking plate systems including the SOP. Specifically, there is a distinct stress riser at the screw/plate interface where forces are transferred from a less stiff element (the screw) to a much stiffer element (the plate or SOP). If excessive force is cyclically applied across the fracture, the shaft of the screw will cold work and become brittle. The yield point from elastic to plastic deformation will become less, and cracks will develop and propagate across the screw. This is fatigue failure and ultimately the screw will break. Theoretical considerations suggest that 4 screws in each major fragment is appropriate to protect the screws against fatigue failure. The cross sectional area of the SOP is pi r² or 20 mm². That of the shaft of a screw is about 5 mm². Therefore, by installing four screws on either side of the fracture the shear area of the screws will approximately equal that of the SOP. Again, the screws may be unicortical. This may be achieved by application of an additional SOP for example if the distal segment is short. A second SOP can be on the contralateral or orthogonal side of the bone, or two SOPs can be nested side by side. The use of an intramedullary pin (SOP-rod technique) enhances the stiffness of a construct to an extent which is not appreciated by many surgeons. This increased stiffness substantially protects implants and protects against fatigue failure. The use of SOP in pairs (for example, in the spine) or in conjunction with a rod (for example, in long bone fractures) should be considered the norm.

Bone slicing is a potential problem associated with the use of locking systems in poor quality bone. With conventional plating systems applied to weak cancellous or osteoporotic bone, screw pullout is the critical factor. However, with locking screw systems screws cannot pullout, especially if there is some divergence or convergence with screws. Instead, failure will occur through slow creep of the screw through the weak bone, known as “bone slicing.” Therefore, as locking plate systems are preferred in weak or osteoporotic bone, they may still exhibit this mode of failure if the bone / implant system used is not sufficiently robust. Bone slicing has not been identified in SOP cases so the importance of this phenomenon in veterinary patients is not yet known.

Application Techniques: Appendicular Skeleton

The primary utility of the SOP in the femur, humerus, tibia, radius, and ulna is in comminuted fractures. Although the SOP can be used in conventional “open approach” fracture surgery, it is especially valuable with biologic fixation methods and minimally invasive techniques. For example, techniques involving SOP and screws installed with stab incisions or mini approaches, or more open approaches where the area of comminution is preserved. The comminuted, diaphyseal femoral fracture will be used as an example of standard SOP methods (Figure 51-14).
 

Comminuted diaphyseal femoral fractures are best repaired using the SOP in combination with under sized intramedullary pins, also known as a Rod and Beam fixation. A standard surgical approach appropriate to the specifics of the fracture is made. An intramedullary pin of 1/3rd to 1/2 the diameter of the medullary cavity is placed normograde from the intra-trochanteric fossa, threading the area of comminution, into the distal femoral segment. The limb is aligned with reference to adjacent anatomical landmarks. In the femur the coxofemoral joint should be in slight anteversion while the stifle is flexed. An elevator is passed along the lateral aspect of the femur, under the biceps and vastus. Inserts should be placed into the SOP holes before contouring to prevent distortion of the holes. An SOP of appropriate length is contoured: it is helpful to have radiographic images of the opposite, un-fractured femur to guide the contour. The contour does not have to be perfect, as the SOP does not need to lie directly on bone. The distal aspect of the SOP can be contoured to follow the femoral condyles caudally and the proximal SOP can be twisted directing the screws antegrade to the femoral neck. The SOP is placed in the soft tissue tunnel, and contour is reviewed. Four screws should be engaged on each side of the fracture. Unicortical screws are appropriate and “empty” screw holes – even over the fracture – are acceptable. The IM pin will prevent bending of the SOP, so there may be a long area without screws in the center of the femur.

The drill guide is placed into a screw hole on one end of the bone and the remaining screw holes observed to make sure the SOP is positioned properly. Remember that the screw will always be directed perpendicular to the spherical component of the SOP. Though you can twist the SOP to change screw direction, this is done prior to installation of screws. The drill and tap guide will direct the drill and tap in the proper direction. The insert is removed from the SOP at the first screw location, either proximal or distal. The drill hole is made using the drill guide, then the depth is measured. A screw is placed. Standard or self tapping screws can be used according to surgeon preference. It is possible for the tap/self-tapping screw to not engage the bone hole immediately. This results in the SOP being pulled too far away from the bone. This can be prevented by applying gentle axial pressure during early placement of the tap/self-tapping screw. Note also that when using a bone tap, care must be taken subsequently when placing the screw to ensure that the screw threads engage in the bone as desired, and not 360° later. The screw should be tightened so that the screw head seats firmly into the spherical component of the SOP. If a unicortical screw is placed, the depth gage measures the minimal length the screw needs to be by the standard method of hooking the near cortex. Then the depth gage is advanced to the trans cortex or, in some cases, the intramedullary pin. A screw 0 to 2 mm longer than the measured minimum distance is chosen. Measuring the distance to the transcortex or intramedullary pin will assure that an oversized screw will not interfere with any structure. The same procedure is repeated for all screws.

Applying a SOP is similar to standard ORIF principles and procedures with these notable exceptions. First, the SOP does not need to lie directly on the outer cortical surface. It should be placed close to the bone to keep its profile as low as possible, but might contact the bone in a few locations or not at all. This preserves the periosteal blood supply of the bone and healing callus. The screw will tighten into the plate, this does not assure that the screw is in solid bone. However, locking screws are better for soft or osteoporotic bone as screw thread holding power is not the method of transmission of forces. Some divergence of screws is desirable. The SOP can be contoured in six degrees of freedom. It is possible, and sometimes desirable, to contour the SOP in non-standard shapes, to follow the fracture configuration or tension surface of a bone. The SOP can be contoured into a spiral for example.

Technical Guidelines

Note that these are guidelines and not rules. They are provided to experienced, knowledgeable and sensible surgeons with the assumption that such experience, knowledge and common sense will be brought to bear on each individual case.

Femur

SOP-rod:
IM pin 20%-40% diameter of medullary canal,
    Normograde or retrograde
    Open or closed placement
4 screws in distal and 4 screws in proximal fragments
Single 2.7 SOP (plus rod) in patients up to 10 kg (lateral aspect)
Single 3.5 SOP (plus rod) in patients up to 35 kg (lateral aspect)
Double 3.5 SOP (plus rod) in patients over 35 kg (lateral aspect)

Humerus – diaphysis

SOP-rod:
IM pin 20%-40% diameter of medullary canal,
    Normograde or retrograde
    Open or closed placement
    Bed into medial epicondyle
    Consider reverse placement through medial epicondyle in very distal fractures
4 screws in distal and 4 screws in proximal fragments
Single 2.7 SOP (plus rod) in patients up to 10 kg (medial aspect, lateral aspect or “spiral”)
Single 3.5 SOP (plus rod) in patients up to 35 kg (medial aspect, lateral aspect or “spiral”)
Double 3,5 SOP (plus rod) in patients over 35 kg. (medial aspect, lateral aspect or “spiral”)

Humerus – elbow “Y” or “T”

Combined medial and lateral approaches or transulnar approach (Figure 51-15)
Anatomic reconstruction with lag screws, K wires etc
Two SOPs, one medial and one lateral
Total of 4 SOP screws in reconstructed condylar fragment (not necessary to have all 4 screws in the same SOP)
Total of 4 screws in proximal major fragment (not necessary to have all 4 screws in the same SOP)
Two x 2.7 SOPs in patients up to 20 kg
Two x 3.5 SOPs in patients over 35 kg
 

Tibia – diaphysis

IM pin 20% to 40% diameter of medullary canal,
    Normograde
4 screws in distal and 4 screws in proximal fragments
Single 2.7 SOP (plus rod) in patients up to 10 kg (medial aspect)
Single 3.5 SOP (plus rod) in patients up to 35 kg (medial aspect)
Double 3.5 SOP (plus rod) in patients over 35 kg.(medial aspect)

Ulna – Radius

Small IM pin in ulna
   Normograde or retrograde
SOP on radius
4 screws in proximal and 4 screws in distal fragment
SOP on medial or dorsal aspect distally
SOP on cranial aspect proximally
Avoid overlong screws transfixing radius and ulna
2.7 SOP in patients up to 10 kg
3.5 SOP in patients over 10 kg

Spine – Fractures or Distraction-fusion

The SOP serves well as a locking spinal fixation system, much like a pedicle screw system or locking cervical fusion devise (Figure 51-16). It does not lag onto bone which accommodates irregularities of the vertebral column. The SOP is applied to the dorsal lateral aspect of the spine, directing the screws at 30 to 40 degrees from the mid saggital plane into the vertebral bodies. Two SOP plates are applied to the left and right sides of the spine. With vertebral luxations, two three hole SOPs are applied with four screws engaging the vertebral bodies on either side of the luxation. With vertebral fractures and instabilities, longer plates are applied and may engage two vertebrae on either side of the instability. As the SOP is not lagged onto bone, the irregularities do not pose a problem as seen in applying standard orthopedic plates. The cylindrical shape lies on the pedicle and avoids compression of nerve roots exiting the intervertebral foramen.
 

As the angle of screw placement is greater in the thoracolumbar area compared to the lower lumbar area, the SOP can be twisted to vary the screw angles.

The SOP can be used for cervical fracture repair, or cervical fusion in cases of instability. Two SOPs are applied to 4 adjacent vertebrae. In this way a minimum of 4 screws are on either side of the fracture or instability. The screws are directed slightly laterally. The screws need not penetrate the vertebral canal. It is important to direct the screws without damaging the spinal cord, nerve roots, venous sinus, or vertebral artery.
   Always use SOPs in pairs
      Cervical – ventral aspect of vertebrae
      Thoracic, T-L, Lumbar - SOPs bilaterally on lateral aspects with screws directed ventro – medially
      Lumbo-sacral – bilateral SOPs with screws directed ventro-medially into lumbar vertebral bodies. Caudally the SOP can be twisted and contoured to engage the iliac shaft.
Minimum of 3 scews in each vertebral body (not necessary to have all screws in the same SOP)
Use longest possible screws to engage maximum amount of vertebral bone
Penetration of far cortex is not essential but should be performed when possible
Stand SOP off spine to avoid damage to emerging nerve roots
2.7 SOP in patients up to 10 kg
2.7 and 3.5 SOPs can be used in combination

Pelvis

SOP can be used successfully in most pelvic fractures. The reconstructed pelvis is inherently fairly stable by virtue of its shape and extensive musculature. Potentially disruptive forces tend to be very much smaller than those encountered in long bone fractures. Consequently, pelvic implants can be relatively smaller than those needed for long bones and, similarly, pelvic fracture fragments can often be effectively stabilized with relatively few screws.

Ilium

Gluteal roll-up approach – can be extended caudally by trochanteric osteotomy
SOP applied to lateral aspect of pelvis
Minimum 2 screws cranial and 2 screws caudal
Twist SOP cranially to optimise stability in thin bone
2.7 SOP in patients up to 20 kg
3.5 SOP in patients over 15 kg
Two SOP plates nested whenever possible (Figure 51-17)

Acetabulum

Open reduction and temporary fixation with K wires, bone forceps etc.
SOP applied to dorsal aspect of acetabulum (Figure 51-18). Minimum 2 screws cranial to fracture and 2 screws caudal to fracture
Single locked screw in stable butterfly fragment is acceptable
2.7 SOP in patients up to 35 kg
3.5 SOP in patients over 35 kg

Miscellaneous Applications

SOP has been used successfully in a variety of other situations including shoulder arthrodesis, pan-tarsal arthrodesis, augmentation of TPLO and TPO procedures and in the revision/salvage of failed fracture and arthrodesis surgeries. The information provided in these guidelines and the recommendations given for “standard” cases will provide the surgeon with a starting point for implant selection and surgical planning in non-routine applications.
 

Suggested Readings

DeTora MD, Kraus KH. Mechanical testing of locking and non-locking 3.5mm bone plates. Vet Comp Orthop Trauma 21: xx-xx, 2008.

Egol KA, Kubiak EN, Fulkerson E, Kummer F, Koval KJ. Biomechanics of locked plates and screws. J Orthop Trauma 18(8): 488-93, 2003.

Schutz M, Sudkamp NP. Revolution in plate osteosynthesis: new internal fixator systems. J Orthop Sci 8: 252–258, 2003.

Gardner MJ, Brophy RH, Campbell D et al. The mechanical behavior of locking compression plates compared with dynamic compression plates in a cadaver radius model. J Orthop Trauma 9: 597-603, 2005.

Sommer C, Gautier E, Muller M et al. First clinical results of the Locking Compression Plate (LCP) Injury; 34 (Suppl 2): B43-B54, 2003.