IMPLANT DESIGN ON PATELLAR TRACKING: A COMPARISON OF 2 DIFFERENT ANTERO-POSTERIOR STABILISED KNEE PROSTHESIS

Abstract

Studies have shown that the normal patella tracks laterally with flexion of the knee joint, consistent with the findings of Eckhoff et al. that the femoral sulcus is lateral to the mid-plane between the 2 femoral condyles. Patellar pain and instability is a known complication of Total Knee Arthroplasty (TKA). To date, several studies have identified the effect of femoral and tibial components on complication after TKA. However, there is very little work on how the design of the implant affects patellar tracking. Our study compares lateralization of the patella in two different AP stabilized knee implants.

A modified caliper was used to measure the width and position of the patella relative to the femur at different degrees of knee flexion. The relationship of the patella midpoint to that of the femur was subsequently assessed. Group 1 consisted of 25 native knees. Group 2 consisted of 25 patients with antero-posterior stabilized knee implant with a spherical medial condyle and a deep lateralized patellar groove (MRK, Finsbury Orthopaedics, UK). And Group 3 consisted of 25 patients with traditional cam-and-post posterior cruciate-substituting implant with a symmetrical patellar groove (PFC-Sigma, DePuy, UK). The mean follow-up for the 50 TKAs was 28 months.

Lateral tracking corresponded well in all groups, but the mean lateral displacement of the patella in group 2 correlated more closely to that of group 1. At 90 degrees of flexion, the patella was displaced a mean of 7mm laterally in both groups 1 and 2, but a mean of 4mm in group 3. Two-tailed Mann-Whitney U test (95% confidence interval) showed that the difference in lateral patellar displacement between groups 1 and 3, and that between groups 2 and 3 were statistically significant (p< 0.05). However, the patellar displacement between groups 1 and 2 was not statistically different.

Our results indicate that lateral patellar displacement in group 2 is similar to that of native knees (group 1). The effect of the underlying lateralized deep patellar groove of the femoral component in group 2 is more able to mimic that of the native femoral sulcus. This intrinsic implant design accommodates the natural tracking of the patella.