Abstract
Objective: The aim of this study was to test the hypothesis that malrotation of the femoral component following total knee replacement (TKR) may lead to patellofemoral complications as a consequence of excessive stretching of the retinaculae.
Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed allowing the tibia to move freely through an arc of flexion. The quadriceps and iliotibial tract were loaded to 205N in their physiologic lines of action using a cable, pulley and weight system. Tibiofemoral flexion and extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus small changes in ligament length were recorded by the transducers. Ligament length changes were recorded every 10° from 90° to 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted following insertion of a cruciate retaining TKR (Genesis II). The femoral component was rotated using a custom built intramedullary device. Ligament length changes were measured at neutral rotation, 5° internal and 5° external rotation. Statistical analysis was performed using a two way ANOVA test.
Results: Internal rotation resulted in the MPFL slackening a mean of 1.7mm from 70-0° extension (p< 0.001). External rotation resulted in the MPFL tightening a mean of 1.5mm over the same range (p< 0.01). The lateral retinaculum showed less significant differences.
Conclusion: External rotation resulted in smaller length changes than internal rotation. Patellar tilting as a result of internal rotation may be caused by MPFL slackening and not lateral retinacular tension, contrary to popular understanding.
Correspondence should be addressed to Mr T Wilton, c/o BOA, BASK at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE, England.
