In vivo kinematics of a low contact stress rotating platform total knee arthroplasty system under weight bearing and non-weight bearing condition.

BACKGROUND: The patterns and magnitudes of axial femorotibial rotation are variable due to the prosthesis design, ligamentous balancing, and surgical procedures. We aimed to investigate the effects of the weight bearing (WB) condition on the kinematics of mobile-bearing total knee arthroplasty (TKA). METHODS: We examined 12 patients (19 knees) implanted with a low contact stress (LCS) mobile-bearing TKA system using a two-dimensional to three-dimensional registration technique. The in vivo kinematics of dynamic deep knee flexion under WB and non-WB (NWB) conditions were compared. We evaluated the knee range of motion, femoral axial rotation relative to the tibial component, anteroposterior translation, and kinematic pathway of the femorotibial contact point for both the medial and lateral sides. RESULTS: Under the WB condition, the mean range of motion was 117.8° ± 16.7°. Under the NWB condition, the mean range of motion was 111.0° ± 4.4°. The mean range of axial rotation from full extension to maximum flexion was 3.0° ± 1.5° under the WB condition and 2.2° ± 1.0° under the NWB condition. With regard to the anteroposterior translation, the LCS mobile-bearing TKA system showed the same kinematic patterns under both conditions, except for axial rotation at 0°, 10°, and 110°. From hyperextension to maximum flexion, the kinematic pattern reflected a central pivot under both conditions. CONCLUSIONS: In conclusion, this study demonstrated that, in an LCS mobile-bearing TKA system, knee kinematics showed the same patterns under NWB and WB conditions, except for axial rotation at the early phase. Further understanding of knee kinematics could provide us with useful information for future design concepts of TKA implants.