Reduced bearing excursion after mobile bearing unicompartmental knee replacement is associated with poor functional outcome.

BackgroundA small proportion of patients with Mobile unicompartmental knee replacement (UKR) report poor functional outcomes in spite of optimal component alignment on post-operative radiographs. The purpose of this study was to assess if there was a correlation between functional outcome and knee kinematics.MethodsFrom a cohort of consecutive cases of 150 Oxford medial UKR, patients with fair/poor functional outcome at one year post surgery (OKS < 34, n=15) were identified and matched for age, sex, pre-operative clinical scores, and follow-up period with a cohort of patients with good/excellent outcome (OKS 34, n=15). In vivo kinematic assessment was done using step-up and deep knee bend exercises under fluoroscopic imaging. The fluoroscopic videos were analyzed using MATLAB software to measure the variation in time taken to complete the exercises, Patellar Tendon Angle (PTA) and Bearing Position (BP) with Knee Flexion Angle (KFA).ResultsMean OKS in the fair/poor group was 29.9 and the mean OKS in the good/excellent group was 41.1. The tibial slope, time taken to complete the exercises and the PTA trend over the flexion range was similar in both the groups; however, BP as well as extent of bearing excursion differed significantly. The total bearing excursion in the OKS<34 group was significantly smaller than the OKS34 group (35%). Furthermore, on average the bearing was positioned 1.7 mm more posterior on the tibia in the OKS<34 group.ConclusionThis study provides evidence that abnormal knee kinematics, in particular bearing excursion and positioning, are associated with worse functional outcome after mobile UKR.