Medial joint line elevation of the tibia measured during surgery has a significant correlation with the limb alignment changes following medial unicompartmental knee arthroplasty

PURPOSE: This study aimed to determine the correlation between the change in joint line height calculated from the resected bone and insert thickness during surgery and the change in limb alignment following unicompartmental knee arthroplasty (UKA). It was hypothesized that joint line elevation is correlated with the change in limb alignment. METHODS: A total of 74 consecutive patients diagnosed with either isolated medial compartmental osteoarthritis (38 patients) or spontaneous osteonecrosis of the knee (36 patients) were included. The thickness of the proximal tibial and distal femoral bony cuts was measured during surgery; using these values, the medial joint line elevation of the tibia (MJLET) and medial joint line reduction of the femur (MJLRF) were defined. The correlation between the amount of change in the hip-knee-ankle (HKA) angle before and after surgery (δHKA) and the thickness of the tibial insert (TI), MJLET, or MJLRF were evaluated. RESULTS: The mean values of δHKA, TI, MJLET, and MJLRF were 5.0° ± 2.6°, 8.5 ± 0.8, 4.4 ± 1.3, and 0.0 ± 1.1 mm, respectively. The δHKA had a significant, but weak-positive correlation with the TI (P = 0.02), and moderate-positive correlation with MJLET (P < 0.001). However, no correlation was observed between δHKA and MJLRF. CONCLUSIONS: The MJLET measured during surgery had a significant moderate-positive correlation with the change in limb alignment following medial UKA. For clinical relevance, the surgeon can predict postoperative limb alignment after UKA by considering, intraoperatively, both the insert thickness and the depth of resection at the proximal tibia thus minimizing technical error to obtain an optimal alignment after UKA. LEVEL OF EVIDENCE: Diagnostic study, level II.