THE INFLUENCE OF FEMORAL CONDYLAR LIFT OFF ON THE WEAR OF FIXED BEARING ARTIFICIAL KNEE JOINTS

Abstract

Introduction: In vivo fluoroscopic studies have shown considerable differences in kinematics between different designs of knee prostheses and compared to the natural knee. Most noticeably, lift off of the femoral condyles from the tibial insert has been observed in many patients (Dennis et al, 2003). The aim of this study was to simulate lateral femoral condylar lift off in vitro and to compare the wear of fixed bearing knee prostheses with and without lift off.

Materials and Methods: 12 PFC Sigma cruciate retaining fixed bearing knees (DePuy, Leeds, UK) were tested. The 10 mm thick inserts were manufactured from GUR1020 UHMWPE and gamma irradiated in a vacuum. The inserts snap fitted into titanium alloy tibial trays, and articulated against Co-Cr-Mo alloy femoral components. The testing was carried out on six station simulators (Prosim, Manchester, UK). Femoral axis loading (maximum 2.6 kN) and the flex-ion-extension profile (0–58°) were adopted from ISO 14243 (1999). The internal/external rotation was ± 5° and anterior/ posterior displacement 0–5 mm. Six of the knees were tested under these standard conditions for 4 million cycles. A further six knees were tested under these conditions with the addition of lateral femoral condylar lift off, for 5 million cycles. The lift off was achieved by introducing an adduction moment to the tibial carriage, producing a separation of approximately 1 mm during the swing phase of the simulator cycle. The simulator was run at 1 Hz and the lubricant used was 25% newborn calf serum. Wear was determined gravimetrically, using unloaded soak controls to adjust for moisture uptake. Statistical analysis was performed using Students t-test (p < 0.05).

Results: Under the standard kinematic conditions the mean wear rate with 95% confidence limits was 8.8 ± 4.8 mm3/million cycles. When femoral condylar lift off was simulated the mean wear rate increased to 16.2 ± 2.9 mm3/million cycles, which was statistically significantly higher (p < 0.01). The wear patterns on the femoral articulating surface of all the inserts showed more burnishing wear on the medial condyle than the lateral. However, in the simulation of lift off the medial condyle was even more aggressively worn with evidence of adhesion and surface defects.

Discussion: The presence of lateral femoral condylar lift off resulted in a higher wear rate on the medial compartment of the PFC Sigma fixed bearing knee. This could be due to elevated contact stresses as the lateral lift off produced uneven loading of the bearing. Further, additional medial/lateral sliding of the medial condyle whilst it remained in contact may have accelerated the wear by cross shearing of the polyethylene in the medial/lateral direction. This direction is weakened when the polyethylene is preferentially molecularly orientated by sliding in the flexion-extension axis. The implications of condylar lift off include premature wear of the polyethylene and possible component loosening.