In vivo kinematics of mobile-bearing posterior-stabilized total knee arthroplasty developed from three dimensional computed tomography system measurements of Japanese knee joints.

Abstract

The mobile-bearing posterior-stabilized (PS) total knee arthroplasty (TKA) tailored to the bone morphology of Japanese patients has the kinematic features such as medial pivot motion (MP), external femoral rotation relative to the tibia during knee flexion, and bicondylar rollback, the backward movement of the femur relative to the tibia, both resembling those of a normal knee. However, these concepts have not been confirmed in vivo. This study aimed to analyze the in vivo kinematics of this TKA and confirm the reproducibility of the concepts. A two dimensional to three dimensional (2D/3D) registration technique was used to analyzed the in vivo kinematics of mobile-bearing PS TKA designed based on morphological characteristics of Japanese patients. We also investigated the relationship between the setting position of implants and in vivo kinematics. The setting position of the implants was evaluated using the hip-knee-ankle angle and the alpha, beta, gamma, and delta angles. Analysis was performed on 17 knees in 11 patients with a mean age of 73.0 ± 6.7 years. Loaded flexion caused 8.39 ± 3.4° of external rotation of the femoral component relative to the tibial component. Regarding kinematics, an MP pattern was observed during the initial flexion, and bicondylar rollback was observed beyond approximately 80° of flexion. Patients were subdivided into two groups according to the presence of the MP pattern (MP and non-MP groups). No evident relationship between the setting position of the implants and in vivo kinematics was observed, except for γ angle. This study verified the in vivo kinematics of the PS-mobile-type implant and demonstrated that it reproduced its intended design in most cases. Although a significant difference was found in the γ angle, no consistent relationship was observed between the presence of the MP pattern and other implant positioning parameters. Therefore, the factors contributing to the absence of medial pivot motion in some cases remain unclear. Despite this limitation, our findings suggest that the design principles of the implant were found to be reproducible to a certain extent, suggesting potential applicability in replicating natural knee motion in clinical settings.