In vivo kinematic comparison before and after mobile-bearing unicompartmental knee arthroplasty during high-flexion activities

Many patients who undergo unicompartmental knee arthroplasty (UKA) have an expectation that their knee flexion would increase following its replacement. Additionally, the survival rate of mobile-bearing UKA (MB-UKA) is high. However, the effect on the patient's kinematics remains unknown. This study aimed to clarify the kinematic effect of MB-UKA knees during high-flexion activities by comparing the in vivo kinematics before and after surgery. A squatting motion was performed under fluoroscopic surveillance in the sagittal plane before and after MB-UKA. To estimate the spatial position and orientation of the knee, a two-dimensional/three-dimensional registration technique was used. The femoral rotation and varus–valgus angle relative to the tibia and anteroposterior (AP) translation of the medial and lateral side of the femur on the plane perpendicular to the tibial mechanical axis in each flexion angle were evaluated. Regarding the varus–valgus angle, the preoperative knees indicated a significant varus alignment compared with the postoperative knees from 10° to 60° of flexion. There were no significant differences in the femoral rotation angle, AP translation, and kinematic pathway before and after MB-UKA in the mid-flexion of the range of motion. There were differences between the varus–valgus knee kinematics before and after MB-UKA, from 10 to 60° of flexion, but no difference from midrange of flexion to deep flexion. In addition, the rotational knee kinematics before and after MB-UKA was not significantly different. •High-flexion activities after UKA need to be evaluated in patients.•The kinematic effect of MB-UKA knees during high-flexion activities was evaluated.•Varus–valgus knee kinematics after MB-UKA differed from those before MB-UKA.