In vivo three-dimensional kinematics in unicompartmental knee arthroplasty: A sports activity cluster analysis.

PURPOSE: The specific kinematics relevant to sports activities after unicompartmental knee arthroplasty (UKA) remain unclear. We aimed to clarify the kinematic factors associated with sports activities after UKA. METHODS: Twenty-five knees that had undergone UKA were examined fluoroscopically during squatting (weight-bearing [WB]) and active-assisted knee flexion (non-weight-bearing [NWB]). Patients who were able to perform deep squatting were included. A two-dimensional/three-dimensional registration technique was used to evaluate tibiofemoral kinematics, including flexion angle, axial rotation angle, varus-valgus angle, and anteroposterior translation (APT) of the femur relative to the tibia. Patients were divided into two groups using hierarchical cluster analysis based on 1-year postoperative sports scores from the Knee Injury and Osteoarthritis Outcome Score and the 2011 Knee Society Score (low-to-moderate score group [n = 15] and high score group [n = 10]). RESULTS: The sports subscale Knee Injury and Osteoarthritis Outcome Scores, except for jumping, were significantly higher in the high-score group than in the low-score group. During WB, the flexion angle was significantly greater in the high-score group (136.2 ± 8.5°) than in the low-score group (126.3 ± 14.2°). In contrast, no significant difference in flexion angle was observed during NWB. Regarding lateral APT, the high-score group showed a more posterior position than the low-score group throughout the flexion range during WB. No significant between-group differences were observed in terms of axial rotation, varus-valgus angle, or medial APT. CONCLUSIONS: The ability to achieve greater flexion during weight-bearing activities is an important factor for sports performance after UKA. LEVEL OF EVIDENCE: Level III.