The contribution of intraoperative medial compartment stability to post-operative knee flexion angle in unicompartmental knee arthroplasty.

PURPOSE: Given the knee kinematics and soft tissue balance in unicompartmental knee arthroplasty (UKA), it was hypothesised that intraoperative medial compartment stability will result in good functional outcome. The purpose of this study was to test the influence of soft tissue balance on post-operative knee flexion in UKA. METHODS: The influence of soft tissue balance on post-operative knee flexion in UKA was first examined retrospectively by using a newly developed tensor device in 30 consecutive patients diagnosed with either isolated medial compartmental osteoarthritis or idiopathic necrosis. The intraoperative component gap of the medial compartment was measured by using the tensor while applying a 20-lb joint distraction force at 0°, 10°, 30°, 45°, 60°, 90°, 120°, and 135° of knee flexion, with calculation of the joint looseness. Correlations between the soft tissue parameters and post-operative knee flexion angles were analysed 1 year after surgery. RESULTS: The post-operative knee flexion angle was negatively correlated with the component gap at 45°, 60°, and 90° of flexion (R = -0.41, P < 0.05; R = -0.44, P < 0.05; and R = -0.44, P < 0.05, respectively). Furthermore, the post-operative knee flexion angle was negatively correlated with joint looseness at 45°, 60°, and 90° of flexion (R = -0.40, P < 0.05; R = -0.41, P < 0.05; and R = -0.36, P < 0.05, respectively). CONCLUSIONS: The intraoperative medial compartment stability of knee flexion in midrange resulted in increasing post-operative knee flexion angle in UKA. Medial soft tissue release should be minimised, and assessment of soft tissue balance using a tensor can be performed to predict the post-operative knee flexion angle during surgery for UKA. LEVEL OF EVIDENCE: III.