The Effects of Total Knee Replacement on the Extensor Retinaculum of the Knee

Abstract

INTRODUCTION: Patellofemoral dysfunction following total knee replacement (TKR) is significant. Due to the intimate relationship of the femoral component and patella, aspects of knee arthroplasty such as femoral component malrotation and patellofemoral overstuff have been sited as predisposing factors to such complications. The principal structures of the extensor retinaculum act as a checkrein for the patella as it tracks over the femur. Little biomechanical information is available on the behaviour of these structures, or the effects of TKR on them. AIM: The aim of this thesis was to measure retinacular behaviour in the normal knee and following TKR. We hypothesise that TKR will cause significant elongation of the retinacula and this would be further deranged with the addition of femoral component malrotation and patellofemoral overstuff. METHODS: Retinacular length changes were measured by threading fine sutures along the retinacula and attaching these to displacement transducers. The intact knee was flexed-extended on a custom built rig, while the quadriceps were tensed. Measurements were repeated post-TKR (Genesis II CR, Smith & Nephew Co.), following internal/external rotation of the femoral component 5° and finally altering the resurfaced patellar thickness by 2mm increments. RESULTS: The medial patellofemoral ligament (MPFL) was close to isometric, whereas the lateral retinaculum slackened significantly with knee extension. TKR did not cause statistically significant elongation of the retinacula. Internal rotation of the femoral component resulted in the MPFL slackening whereas external rotation resulted in the MPFL tightening as the knee extended. The lateral retinaculum showed no significant differences. Overstuffing the patellofemoral joint caused significant stretching of the MPFL at all angles of knee flexion, but very little change in the lateral retinaculum. CONCLUSION: This work has shown a correctly positioned TKR does not cause significant retinacular length changes sufficient to affect knee function. It has shown that small changes in femoral component rotation and patellofemoral overstuff of 4mm cause significant changes, particularly in the medial structures and not the lateral structures, contrary to current understanding. This work has described for the first time how the lateral retinaculum’s mobile attachments allow its principal fibres to move anteriorly and posteriorly with the patella, taking up any slack/tension produced by abnormal patellar shift/tilt. This work provides important insight into the contribution of the retinacula to patellofemoral biomechanics after knee replacements and may help in developing a more soft tissue friendly knee prosthesis.