Fractures involving the distal extremity of the femur: Part 2 — The mature patient

Fractures of the supracondylar region of the femur in the adult animal are frequently comminuted and place great demands on the orthopedic surgeon. The anatomy of the area presents several challenges: there is little bone available distal to the fracture in which to seat orthopedic implants, what bone is present is soft cancellous bone, and the distal epiphysis of the femur is bowed caudally with respect to the diaphyseal bony column (1). This caudal bowing limits the distal purchase that can be obtained with implants, such as intramedullary pins and conventional bone plates. In addition, the distal extremity of the femur is a “high traffic area” in the greatest weight-bearing bone of the body. A distal fracture produces a short lever arm from the critical stifle joint, which predisposes any fracture repair to significant bending forces. These anatomic and biomechanical challenges are magnified in chondrodystrophoid or large patients.

Conventional bone plate repair, which is often the implant of choice in diaphyseal femoral fractures, may not permit enough screws to be placed in the distal fragment of a metaphyseal fracture. Normally at least 2 and preferably 3 plate screws must be present on either side of a fracture. Even if this is possible, the distal end of the plate may interfere with proper closure of the stifle joint capsule and may affect the function of the distal part of the quadriceps-patellar mechanism (2).

A viable alternative is the Arbeitsgemeinschaft für Osteosynthesefragen/Association for the Study of Internal Fixation (AO/ASIF) reconstruction plate (Synthes (Canada), Mississauga, Ontario) (Figure 1), which is more malleable than conventional plates and has notches between the screw holes, so that the plate can be bent in 3 planes (Figure 2). If the reconstruction plate is curved perpendicular to its long axis to contour it to the caudal bow of the distal extremity of the femur, it allows the surgeon to place at least 1 or 2 more screws in the distal fragment. The trade- off is that the ductile reconstruction plate is not as strong as a conventional plate and will bend or break if subjected to excessive loads. The bony column must be reconstructed sufficiently to allow load sharing between the implant and the bone. Small defects in the bony column underneath the plate may be tolerated, especially in smaller patients; however, fracture gaps in the bony cortex opposite the plate (trans cortex) force the reconstruction plate to act as a buttress, a task to which it is ill-suited (2,3).

Figure 1. Continued.

Figure 1. Lateral Tibial Head Buttress Plate (courtesy of Synthes (Canada) Ltd.).

Figure 2. A distal femoral fracture in a 9-month-old coonhound stabilized with a reconstruction plate. The fracture involved the joint surface and was approached using a tibial crest osteotomy. The tibial crest has been reattached by using 2 pins and a tension-band wire.

Other plating techniques that may lend themselves to repair of fractures of the distal extremity of the femur (distal femoral fractures) in the mature animal include a “hook” plate, the buttress plate for the lateral condyle of the human tibia (Lateral Tibial Head Buttress Plate, Synthes (Canada)), and the combination of a plate with an IM pin.

The 3.5-mm double hook plate (Synthes (Canada)) (Figure 3), designed for use in the intertrochanteric osteotomy procedure for dysplastic hips, can be turned upside down and adapted to distal femoral fractures (1). A procedure for making a double hook plate from a standard dynamic compression bone plate has also been described, whereby the terminal 2 holes of the plate are bent towards the ventral surface of the plate and perpendicular to its long axis. A hacksaw is then used to make longitudinal cuts along the inside margins of the terminal 2 holes, thus forming 2 projections like the tines (prongs) on a fork. The “hooks” (prongs) are gently hammered into parallel drill holes in the distal femoral fragment, perpendicular to the long axis of the femur. The intent is to place both hooks and at least 1 screw, proximal to the hooks, in the distal fragment (2).

Figure 3. Double Hook Plate (Courtesy of Synthes (Canada) Ltd.).

The lateral tibial head buttress plate was designed for use in humans in the repair of comminuted fractures of the proximal extremity of the tibia. The plate is flared and has a caudal bow at its proximal end. This proximal modification allows a triangular arrangement of 3 screw holes in a relatively small area. By using the plate designed for the left tibia and turning it upside down, the caudal bow and 3-screw hole configuration can be adapted nicely to the distal extremity of the right femur (2). The downside is that these plates accept 4.5-mm screws and, consequently, are suited only to patients weighing over 25 kg. Their price, at 2.5 to 3 times the price of a conventional plate, may also limit their use in some cases.

In severely comminuted supracondylar fractures, a bone plate and an IM pin combination may provide a viable option. Placing an IM pin, usually in a normograde fashion, allows the surgeon to regain femoral length, as the pin is seated in the distal fragment and so pushes it away from the proximal fragment. Normal axial alignment of the limb can also be reestablished at this stage. A bone plate is then contoured and applied to the bone. Proper contouring may be accomplished preoperatively with the aid of a ventrodorsal radiograph of the normal femur (4). The pin should occupy approximately 40% of the medullary diameter to maximize load protection for the plate and to minimize interference with plate screws. Even at this size, the pin will force the use of some monocortical screws. It is recommended that at least 1 bicortical and 3 monocortical screws be used on each side of the fracture (4). This technique is very hard on drill bits and taps as they encounter the pin. The surgeon would be well advised to figure replacement of these items into any estimate of material costs for the surgery. Usually, no attempt is made to reduce and immobilize comminuted bone fragments. The intent is to use a biologic fracture repair strategy where as much as possible of the soft tissue envelope and fracture environment is preserved. Vascularity of the healing fracture is also enhanced by efforts to promote controlled weight-bearing in the early postoperative period.

The use of external fixators (EF) with or without an IM pin has been advocated for these fractures and may be a viable option in small patients (1). As the patient grows larger, achieving adequate rigidity with an EF construct and avoiding draining pin tracts and loose pins as the EF pins impinge upon the soft tissues of the thigh become ever greater problems.

Circular external fixators and interlocking nails may be applicable in some cases of distal femoral fractures, but these techniques require specialized equipment and expertise. Recently, there has been evidence to suggest that interlocking nail repairs may provide superior rigidity to bone plates, which have long been the gold standard in femoral fracture repair (5).