Interlocking Nailing of Canine and Feline Fractures

This chapter was submitted in 2006 and was based upon the available literature through that year. Other interlocking nail devices have emerged since that time, but are not covered in this chapter.

Introduction

The principles of management of diaphyseal fractures of the femur, tibia and humerus by internal fixation have evolved considerably from the original AO concepts of complete anatomical reduction and rigid stabilization of all the fractured fragments. Nowadays, the concept of biological management of diaphyseal fractures places greater emphasis on less invasive surgical approaches, and preservation of the bone blood supply and the fracture hematoma, especially in cases of comminuted non-reducible fractures. Overall alignment and stabilization of the proximal and distal fragments are obtained without interference with the intermediate comminuted fracture fragments. Interlocking nail fixation is the method of choice for the stabilization of diaphyseal fractures of the femur and tibia in adult humans.¹ In recent years, it has become more widely accepted as a method of treating diaphyseal fractures in small animals as well.^2-8

Principles of Interlocking Nailing

Interlocking nailing evolved as a modification of intramedullary fixation using Steinmann pins for the stabilization of diaphyseal fractures in small animals. Intramedullary pinning of fractures in animals was first introduced about sixty years ago. While this method often resulted in successful fracture healing, complications due to fracture instability, fracture collapse, pin migration, and sciatic nerve entrapment were not uncommon. Intramedullary pins provide good stability against bending loads during the fracture healing period, provided that the chosen pin is of adequate diameter and stiffness. This is due to the fact that intramedullary pins are located in the neutral axis of loading within the medullary canal, and as such they are more resistant to bending loads than bone plates and other extra-cortical fixation devices. However, intramedullary pins are quite ineffective at counteracting axial compressive and torsional loads, especially in the case of comminuted fractures. Therefore the basic principle of interlocking nail fixation is that insertion of locking bolts securing the proximal and distal fragments to the nail, counteracts the axial and torsional loads, making the fracture fixation construct much more stable overall.

Types and Sizes of Nails

Several different systems of interlocking nails designed for canine and feline fractures have been developed by surgeons from various countries, world wide.^2-9 In principle, all these interlocking nails function in a similar manner, but they differ somewhat in regard to the instrumentation used for their application. The most widely used interlocking nails in North America (Innovative Animal Products, Rochester, MN) are round in cross section and made from 316L stainless steel that has been cold worked to increase stiffness and fatigue life in vivo. The nails are available in various diameters (4.0, 4.7, 6.0, 8.0 and 10mm) and lengths. An implant of appropriate dimensions (diameter and length) must be selected to match the patient’s fractured bone because the nail is not usually cut to length during surgery. One end of the nail has a sharpened trocar point to facilitate insertion into the medullary canal. The other end of the nail is machined with two flanges and an internal thread to allow for precise attachment of the drill-aiming guide during surgery. Typically each nail has two non-threaded transverse holes near to each end, for the insertion of locking bolts. The spacing between these pairs of holes is either 11 mm or 22 mm. The closer hole spacing allows for the insertion of two locking bolts when there is limited metaphyseal bone available. In addition, nails are also available with just one hole proximally or one hole distally for the stabilization of fractures near to the metaphyses, in which case there is less available bone for interlocking (Figure 50-1).

Locking bolts are inserted transversely through the bone and holes in the nail with the aid of a special drilling aiming guide. This instrumentation is described in further detail in the following section about application. The locking bolts have a smooth shaft with four self-tapping threads under the bolt head, to engage the near bone cortex. The shaft of the bolt is almost the same diameter as the nail holes, with just a small under-sized tolerance to prevent jamming during insertion. Prior to the availability of locking bolts, conventional cortical bone screws were used for locking. However, the use of screws for this purpose is no longer recommended, because of their inferior mechanical performance; failure of screws by bending or breakage was occasionally a problem clinically.^10,11 During the course of fracture healing, the locking bolts are mainly loaded by bending or quasi-bending forces. Under these conditions, the stiffness and fatigue life of the locking bolt are determined by its area moment of inertia which is calculated using the formulae of π x radius⁴/4. For example, the calculated area moment of inertias for the 2.7 mm diameter locking bolts and cortical screws are 2.61 mm⁴ and 0.64 mm⁴ respectively.¹² In the case of the cortical screw, this value is much lower because the core diameter of the screw is only 1.9 mm. Under conditions of cyclic bending the fatigue life of the 2.7 bolt is over 140 times greater than that of the 2.7 cortical screw.¹³ The diameter of the medullary canal at the level of the locking bolt is also an important factor when considering the mechanical performance of the locking bolt.¹⁴ The bending moment on the bolt is proportional to the unsupported length of the bolt within the medullary canal. Therefore in large breed dogs, the bending load on the locking bolt may be considerable in the metaphyseal region where the bone has a relatively greater diameter. In bones that are ovoid in cross section, it may be possible to reduce the bending load on the locking bolt by orienting it in the direction of the shorter cross-sectional axis of the bone.
 

Another advantage of using locking bolts instead of cortical screws is that bolts more effectively control torsional instability of the construct. With loading, the threads of cortical screws in the region of contact within the nail hole become deformed and flattened.¹⁵ This effectively reduces the outside diameter of the screw, and allows for greater torsional slack in the construct in comparison to locking bolts.^16,17

Techniques of Application of Interlocking Nails

Preoperative radiographs of the fractured bone are needed for surgical planning and selection of an appropriately sized nail. The radiographic views need to be true medio-lateral and cranio-caudal projections, with minimal magnification or distortion of bone length. In case of comminuted fractures, radiographs of the contralateral intact bone may be more useful for preoperative planning. The length and diameter of the nail to be inserted can be estimated by overlaying the radiograph with a transparent plastic sheet with the outline of the nail templates printed on it. When using digital radiography, it is necessary to use an internal radiographic marker of known length for calibration of the radiographic magnification, and to import digital templates of the nails for planning.¹⁸

In case of diaphyseal fractures that are near to the metaphysis, there must be sufficient bone available for seating of the nail and the locking bolt(s), without invading the adjacent joint. Some juxta-articular fractures will not be suitable candidates for interlocking nailing because there is insufficient bone stock for implant fixation. In these cases, alternative means of fixation such as bone plating or hybrid external fixation may provide better stability.

An open surgical approach using appropriate aseptic surgical technique is needed for insertion of the interlocking nail and screws. The extent of the surgical exposure required is influenced by factors such as ease of fracture reduction, the volume of musculature in the region, and how readily the bone can be palpated. Fluoroscopically guided closed nailing of tibial fractures is possible, but closed nailing of femoral fractures in dogs is more challenging. Even if a closed nailing is performed, it will be necessary to make some limited incisions over the proximal and distal ends of the bone for insertion of the nail and locking bolts. Bone holding forceps can be applied to bone through these incisions as well, to allow alignment of the fractured bone. Axial traction is applied to the bone using these bone holding forceps to obtain fracture reduction. For fractures in the metaphyseal region, indirect traction by ligamentotaxis is applied. As far as possible, direct exposure of the fracture hematoma and elevation of the soft tissue attachments of the fractured bone fragments should be minimized.

A small diameter Steinmann pin held in a Jacob’s chuck is introduced into the medullary canal to establish axial alignment of the fractured bone. Normograde insertion of the pin is recommended for femoral and tibial fractures. The pin is introduced into the femur through the trochanteric fossa. In the tibia, it is inserted into the proximal end of the bone through a cranio-medial surgical approach, at a point half way between the tibial tuberosity and the medial collateral ligament. In humeral fractures, either retrograde pin insertion from the fracture site or normograde insertion from the greater tubercle is equally appropriate and safe. The opening in the medullary canal can be progressively enlarged by the sequential insertion of Steinmann pins of progressively larger diameter. Alternatively, the medullary cavity can be opened with a reamer. The reamer should only remove cancellous bone from the metaphyseal region. Aggressive reaming of the endosteal cortical bone in the diaphysis should not be performed because cortical bone is much thinner in dogs and cats than in humans. In humans, extensive reaming of the medullary canal is often performed to improve the mechanical performance of an interlocking nail because a large diameter, stiffer nail can be used that is more resistant to fatigue and breakage. However, on the other hand, reaming can cause fracturing of the diaphyseal cortex and damage to the medullary bone blood flow with conse- quential delayed or nonunion of the fracture.

In preparation for insertion of the nail, an extension piece is attached to the end of the nail (Figure 50-2). The flanges on the end of the nail must interdigitate with those on the extension piece, and the connection is secured by tightening of the threaded, internal spindle with a hexagonal screw driver. The insertion handle is then attached to the extension piece, and used in a manner similar to a Jacobs chuck to drive the nail into the medullary cavity (Figure 50-3). The nail has to be inserted by normograde technique because only one end of the nail has a trocar point. Care should be taken to ensure that the nail is adequately seated into the distal metaphysis of the bone, without accidentally going too far and penetrating the articular cartilage surface of the joint. Depth of penetration is judged by overlying a second nail of the same length, or with intra-operative fluoroscopy. After the nail is inserted, do not attempt to correct any offsets in the fracture reduction by the application of bone holders or cerclage wiring, until after the locking bolts have been inserted. Clamps or cerclage wires placed across the fracture can result in slight bending of the nail which may in turn result in inaccurate drilling and the locking bolts missing the holes in the nail.
 

After the nail is seated, the insertion handle is removed and the drill aiming guide is attached to the nail extension piece (Figure 50-4). Accurate drilling of the holes is the most technically challenging part of the procedure, and can be the greatest source of intra-operative frustration. Inaccurate drilling can result in the locking bolt being inserted adjacent to the nail, rather than through it. A tissue protection sleeve is inserted into the drill aiming guide in a position that corresponds to one of the distal nail holes. Then the appropriate drill guide is inserted, and a hole is drilled though the bone and the hole in the nail. Sharp drill bits with a “stick-tight” point are used to minimize the risk of the drill migrating to one side of the bone. Particular care is taken when the drill is entering the periosteal surface at an acute angle, as it has a tendency to migrate “down-hill”. The diameter of the drill hole is the same as the shaft diameter of the locking bolt. The bone diameter is measured with a depth gauge, and an appropriate length locking bolt is inserted. After the two distal locking bolts have been inserted, the fracture alignment is corrected with respect to bone length and torsion with reference to anatomical landmarks. The proximal locking bolts are then inserted, and the drill aiming guide and extension piece are removed (Figure 50-5). If possible, two locking bolts should be inserted into the proximal and the distal fragments. Careful planning is needed to ensure that there are no empty holes in the nail near the fracture zone because of the risk of nail breakage. Additionally, the minimum distance from the fracture zone to the locking bolts should be 2 cm or more.

Adjunctive fixation is not required unless there are cortical fissures in close proximity to the locking bolts in which case cerclage wire can be applied. Generally comminuted fracture fragments are not disturbed. Autologous cancellous bone graft harvested from the proximal metaphysis of the humerus or tibia should be inserted at the fracture site in adult dogs if an open fracture reduction has been performed. In case of massive bone defects, large quantities of bone graft can be harvested from the wing of the ilium by using an acetabular reamer (BioMedtrix, Boonton, NJ).
 

Specific Fractures

Femur

Insertion of the nail by normograde technique in the trochanteric fossa allows it to be lateralized and thus avoid damage to the femoral head and coxo-femoral joint. The nail can be inserted by blind insertion through the gluteal muscles, or under direct visualization. The trochanteric fossa is exposed by transecting the tendon of the superficial gluteal muscle and retracting it proximally, and cranial retraction of the middle and deep gluteal muscles. Care is taken to avoid iatrogenic damage to the sciatic nerve that lies just caudal to the hip joint. Normally the femoral diaphysis of dogs has a cranio-caudal bend, or procurvatum. To overcome this curvature, two piece diaphyseal fractures may need to be axially aligned in slight recurvatum to allow the nail to be adequately seated in the distal metaphysis and condyles of the femur. In comminuted fractures in which anatomic reduction of the fragments is not the goal, this curvature is not an important factor in determining nail placement. In cats, the femoral diaphysis is generally quite straight, and can readily accommodate a small diameter nail without loss of normal bone alignment.

For more distal diaphyseal fractures, the nail can be introduced into the femur from the intercondylar notch and driven proximally. This allows the nail and locking bolts to engage more of the bone in the femoral condyles, and thus improve the stability of the fixation. Depending on the diameter of nail, and the amount of curvature in the femur, the nail may also be introduced through the articular cartilage surface at the very distal extent of the trochlear groove. However, it is important that the end of the nail is buried below the joint surface so it does not interfere with the patella. As an additional refinement to this technique, the buried end of the nail can be covered with a osteochondral plug that has been cut out of the trochlear groove with a bone trephine, prior to insertion of the nail.

Humerus

Fractures of the humeral diaphysis can be repaired via a limited lateral surgical approach to the diaphysis. It is not necessary to mobilize the brachialis muscle and radial nerve to the same extent needed for lateral bone plate fixation. For normograde insertion, the nail is started cranially on the ridge of the greater tuberosity, with the shoulder placed in slight flexion. It is not started on the most proximal point of the greater tuberosity because the inherent curvature of the humerus may prevent it from being adequately seated into the distal fragment. Alternatively the medullary canal can be reamed retrograde from the fracture site. Most humeral shaft fractures involve the distal one third of the diaphysis, and having adequate bone stock in the distal fragment and medial condyle for nail insertion will be a major consideration. In the majority of these types of fractures only single screw fixation distally is possible and thus a nail with one screw hole distally will be selected to avoid leaving an empty screw hole at the fracture site. In very large dogs the distal end of the nail can be directed medially and seated into the medial part of the humeral condyle. As with all intramedullary devices, implants should not impinge on the olecranon fossa. Distal interlocking screws are inserted with care, as they may be very close to the radial nerve.

Tibia

Closed nailing of tibial fractures may be possible, especially with the aid of fluoroscopic guidance, because the bone fragments are readily palpable. The entry point for the nail on the tibial plateau is located half way between the tibial tuberosity and the medial collateral ligament, and several mm inside the medial cortex. This point is centrally located with respect to the axis of the medullary cavity, and just cranial to the articular surface and insertion of the cranial cruciate ligament. To begin, a small diameter Steinmann pin is inserted normograde from this point and directed distally, ensuring it remains inside the medullary cavity. This hole is then enlarged with the reamer. If difficulty is encountered, retrograde reaming from the fracture site is then performed, to try to meet up with the proximal reaming tract. The tibial diaphysis is sigmoid in shape and narrowest distally, so nail diameter will tend to be smaller than that used in the femur.

A longer extension piece is used in tibial fracture so that the connection with the drill-aiming device does not impact upon the femoral condyle and patella. Due to the longer work distances, it should be recognized that there is an increased risk of deviation of the drill-aiming guide that may result in the drill missing the distal holes in the nail.

References

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3. Duhautois B. L’enclouage verrouille veterinaire: etude clinique retrospective sur 45 cas. Prat Med Chir Amin Comp 30:613-630, 1995.
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11. Suber JT, Basinger RR, Keller WG. Two unreported modes of interlocking nail failure: breakout and screw bending. Vet Comp Orthop Traumatol 15:228-232, 2002.
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13. Litsky AS, Johnson KA, Aper RL, Roe SC: A novel screw design for improving the fatigue life of interlocking nails. Proceedings Society for Biomaterials Annual Meeting, Sydney 2004.
14. Aper RL, Litsky AS, Roe SC, Johnson KA. Effect of bone diameter and eccentric loading on fatigue life of cortical screws used with interlocking nails. Am J Vet Res 64:569-573, 2003.
15. von Pfeil DJF, Dejardin LM, DeCamp CE, Meyer EG, Lansdowne JL, Weerts RJH, Haut RC. In vitro biomechanical comparison of a plate-rod combination-construct and an interlocking nail-construct for experimentally induced gap fractures in canine tibiae. Am J Vet Res 66:1535- 1543, 2005.
16. Landsdowne JL, Sinnott MT, Ting D, Haut RC, Dejardin LM. Design and in vitro evaluation of the structural properties of a novel and current interlocking nail systems. Proceedings American College of Veterinary Surgeons annual meeting, October 5-7, 2006.
17. Dejardin LM, Lansdowne JL, Sinnott MT, Sidebotham CG, Haut RC. In vitro mechanical evaluation of torsional loading in simulated canine tibiae for a novel hourglass-shaped interlocking nail with a self-tapping tapered locking design. Am J Vet Res 67:678-685, 2006.
18. Mattoon JS. Digital radiography. Vet Comp Orthop Traumatol 19:123-132, 2006.