Following exposure of the knee, it is preferred to maintain the residual patellar tendon and surrounding fibrous tissue, because this provides a vascular tissue layer for later closure over the allo-graft. If the femoral and tibial components require revision, it is best to perform this step prior to placement of the allograft. Any hardware about the tibial tubercle should be removed. Whether to resurface the patella allograft remains debatable. It is our current preference not to resurface the patella.

The tibia is prepared by creating a trough about 60 to 80 mm long, which is fashioned along the tibial tubercle and tibial crest. The trough is created by removing the anterior cortex and compressing the underlying cancellous bone. Distally the osteotomy should be oblique in order to reduce the stress riser. Additionally, if possible, a rim of cortical bone should be maintained beneath the tibial component. The allograft then can be "keyed" into place (Fig. 13.4).

Figure 13.4. The extensor allograft in place.

At this point, the patella height needs to be determined in order to set the position of the tibial bone graft. With the knee in full extension, the patella should sit over the anterior flange of the femoral component and the inferior border of the patella approximately 1 cm above the joint line. Once the patella position is selected, the tibial bone graft is secured either with two bicortical screws or two cerclage wires. If the tibial component is being revised, the cerclage wires should be passed through drill holes in the tibial diaphysis and placed posterior to the tibial stem. With a stem extension in place, it may be difficult to set the screws or pass the wires.

The allograft quadriceps tendon is then passed through a transverse slit in the host quadriceps tendon. With the knee in full extension, the quadriceps tendon allograft is secured to the quadriceps expansion with multiple nonabsorbable sutures. The original patella is thinned to a wafer, or cortical shell. A patellectomy is not performed because the residual patella bone facilitates healing and serves as a useful landmark. It usually sits over the patella allograft and makes an interesting postoperative radiograph because two patellae can be seen (Fig. 13.5).

At this point, the range of motion and tension are checked. The range of motion is usually 45 to 60 degrees of flexion, and if properly oriented, the patella tracks centrally without a tilt. While the medial quadriceps retinaculum is sutured to the medial margin of the allograft, the lateral retinacular release is left open. The knee is then closed in a routine fashion and immobilized in extension with a cast or brace for 6 weeks. During this time the patient is allowed to ambulate full weight-bearing and encouraged to practice quadriceps setting exercises. After 6 weeks, the knee is braced and gradual range-of-motion exercises are initiated. The brace is discontinued when there is radiographic evidence that the tibial bone graft is healed, and the quadriceps muscle power is difficult to support the leg.

Figure 13.5. Postoperative radiograph showing the extensor allograft in place. Note the double patella.

Our clinical experience with this reconstruction technique includes six cases of extensor mechanism allografts for chronic rupture of the patellar tendon. Although four patients have full active extension, there are two patients who have an extensor lag less than 10 degrees. The average knee flexion is 90 degrees. All six patients are ambulating independently.

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