Medial center of rotation and 90-degree lateral laxity improve patient-reported outcomes in posterior cruciate retaining total knee arthroplasty.

Objectives: Physiologic knee kinematics are crucial for successful total knee arthroplasty (TKA) but are often not replicated. Using a medial stabilizing technique (MST) minimizes bone resection but results in lateral laxity. This study aimed to investigate the effects of lateral laxity on knee kinematics and symptoms after TKA.

Methods: Mobile-bearing cruciate-retaining MST-TKA was performed on 40 knees using a navigation system. In the kinematic analysis, the anteroposterior (AP) translations of the medial femoral condyle (MFC) and lateral femoral condyle (LFC), femoral rotation angles, and medial and lateral component gaps were recorded every 0.1 s. These data were extracted from the software from 0° to 120° flexion in 10° increments. Kinematics were classified as the medial center of rotation (MCR) or non-MCR between 0° to 90° of flexion. Lateral laxity was calculated by subtracting the medial component gap from the lateral component gap. The final follow-up Knee Injury and Osteoarthritis Outcome Scores (KOOS) were evaluated. The relationships between the pre and postoperative kinematics and between postoperative lateral laxity and kinematics were assessed using Spearman's correlation coefficients. Finally, the correlation between postoperative lateral laxity and KOOS symptoms was evaluated using linear regression analysis.

Results: Preoperative kinematics, including AP translation of the MFC and LFC and femoral rotation, correlated with postoperative kinematics (all P < 0.001). Additionally, postoperative lateral laxity correlated with postoperative AP translation of the MFC, LFC, and femoral rotation (all P < 0.001). Furthermore, the receiver operating characteristic analysis indicated a cutoff value of 0.9 mm on postoperative lateral laxity at 90° flexion for postoperative MCR (P < 0.001). Postoperative lateral laxity at 90° flexion was significantly correlated with KOOS symptoms (β = 0.465, P = 0.025).

Conclusion: Preoperative kinematics and postoperative lateral laxity correlated with postoperative kinematics after MST-TKA. Postoperative lateral laxity greater than 0.9mm at 90° flexion was associated with physiological kinematic motion, leading to fewer knee symptoms in the PROMs. The key to successful TKA was considered to be keeping the asymmetric gap balance with physiological lateral laxity, rather than the conventional symmetrical gap balance.

Level of evidence level iii: Retrospective study.