Multi-radius posterior-stabilized mobile-bearing total knee arthroplasty partially produces in-vivo medial pivot during activity of daily living and high demanding motor task

PURPOSE: The purpose of the present study was to assess the kinematical behavior of a multi-radius posterior-stabilized (PS) mobile-bearing (MB) total knee arthroplasty (TKA) during an activity of daily living (Sit-To-Stand-STS) and a high demanding motor task (Deep-Knee-Lunge-DKL) using model-based dynamic RSA. We hypothesized the achievement of medial pivoting movement in both motor tasks due to the congruent geometry of the inlay with the femoral component, which should allow good stability of the medial compartment, and to the high magnitude of rotations guaranteed by the MB on the tibial side. METHODS: Twenty-two randomly selected patients were recruited and prospectively evaluated. The PS MB cemented TKA was implanted with the standard technique (medial parapatellar approach, adjusted mechanical alignment). At minimum 9-month follow-up, patients were examined with model based Dynamic RSA developed in our Institute (BI-STAND DRX 2) during the execution of two motor tasks: STS and DKL. The motion parameters were evaluated using the Grood and Suntay decomposition and the low-point kinematics methods. RESULTS: In the extension phase of DKL femur performed a greater antero posterior translation of 3.8 mm compared to STS between 0° and 20° of knee flexion (p < 0.05). Low-point analysis showed a medial pivoting movement in both motor tasks: in 62% of patients during STS and 48% during DKL. Varus-valgus rotations were lower than 1° during all the range-of-motion in both motor tasks without differences. CONCLUSIONS: Medial pivot was partially produced by this multi-radius PS MB TKA with some differences during activity of daily living (STS) and high demanding motor task (DKL).