Summary

Preoperative kinematics and lateral laxity were associated with the kinematics after implantation in MST-TKA. Lateral laxity after implantation was associated with postoperative knee symptoms.

Abstract

Objectives: This study aimed to evaluate the effect of lateral laxity after total knee arthroplasty using the medial stabilizing technique (MST-TKA) on postoperative kinematics and patient-reported outcomes. It was hypothesized that lateral laxity was related to postoperative kinematics and knee symptoms.

Methods

Forty knees underwent cruciate-retaining mechanical alignment MST-TKA with a navigation system. The kinematics were evaluated pre-operatively and after implantation. The posterior translation of the medial femoral condyle (MFC), lateral femoral condyle (LFC), and femoral rotation angle were evaluated every 10° from 0° to 120°. The difference between the lateral and medial component gap was defined as the lateral laxity. The knee injury and osteoarthritis outcome score (KOOS) was assessed at the final follow-up in evaluating the patient-reported outcomes. The mean follow-up period was 24.2 months. Patients who demonstrated medial pivot motion (MP) after implantation were classified as the MP group, and the others as the non-MP group. All data were presented as medians [interquartile ranges]. The median values of kinematics parameters between the two groups were compared using the Mann-Whitney U test. The correlations between the kinematics at pre-operation and after implantation, and between lateral laxity and kinematics after implantation were evaluated using Spearman’s correlation test. Finally, the linear regression analysis determined the relationship between lateral laxity and the KOOS at the final follow-up.

Results

Twenty-two knees (55%) were classified into the MP group and 18 knees (45%) into the non-MP group. The maximum posterior translation of the LFC (6.8 [4.9, 11.0] mm), femoral rotation angle (4.6 [3.0, 7.8]°), and lateral laxity (1.1 [0.2, 1.8] mm) in the MP group were greater than those in the non-MP group (3.1 [1.6, 6.6] mm, -0.2 [-1.2, 0.7]°, 0.4 [-0.4, 0.9] mm, and p = 0.011, < 0.001, 0.048, respectively). In the correlation analysis, the posterior translations of the MFC (R = 0.29, p < 0.001), LFC (R = 0.52, p < 0.001), and femoral rotation angle (R = 0.35, p < 0.001) after implantation correlated with kinematics at pre-operation. Furthermore, lateral laxity also correlated with the posterior translation of the MFC (R = -0.21, p < 0.001), LFC (R = 0.21, p < 0.001), and femoral rotation angle (R = 0.41, p < 0.001) after implantation. In the ROC analysis, a cut-off value for medial pivot motion after implantation was calculated as lateral laxity of 0.6 mm at 90° of flexion (AUC = 0.676, Odds ratio = 4.550, p = 0.046). Furthermore, lateral laxity was associated with the KOOS symptoms (ß= 0.43, p=0.046).

Conclusions

Preoperative kinematics and lateral laxity were associated with the kinematics after implantation in MST-TKA. Lateral laxity after implantation was associated with postoperative knee symptoms. The medial gap is widened after the release of deep medial collateral ligament layers and osteophyte resection of the medial femorotibial joint. In MST-TKA, soft tissue balance should be considered to prevent loss of lateral laxity. However, the optimal maximum lateral laxity is not yet clear, nor is the influence of kinematic alignment TKA, which has been performed in recent years, so further research should be required.