Preoperative proximal tibial varus and varus limb alignment contribute to mid-term varus progression after fixed-bearing medial unicompartmental knee arthroplasty

Background Restoring native alignment or allowing slight residual varus is often considered optimal in medial unicompartmental knee arthroplasty (UKA). However, it remains unclear whether postoperative alignment remains stable over time and which preoperative factors contribute to varus progression. Methods We retrospectively reviewed 126 medial UKAs performed between 2017 and 2022 with a minimum follow-up of 3 years. Standing long-leg radiographs were obtained preoperatively, at 1–3 months postoperatively, and at final follow-up. Hip-knee-ankle (HKA), medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA), and tibial plateau tip-to-proximal tibial shaft (TPTPS) angle were measured. Change in alignment (ΔHKA) was defined as the difference between final and immediate postoperative HKA. Correlation and multivariate regression analyses were performed to identify predictors of ΔHKA, and clinical outcomes were analyzed relative to preoperative alignment. Results Mean HKA increased from 2.7° to 4.2° at final follow-up ( p < 0.001). The proportion of outliers (HKA >5°) increased from 17% to 39%. ΔHKA was significantly correlated with preoperative HKA (r = 0.471), MPTA (r = −0.388), and TPTPS (r = 0.355) (all p < 0.001). In multivariate regression analysis, preoperative HKA, LDFA, and MPTA independently predicted ΔHKA (R 2 = 0.295, p < 0.001). No association was found between ΔHKA and clinical outcomes. Conclusion Varus progression may occur after medial UKA, particularly in patients with preoperative varus alignment and proximal tibial varus. Surgeons should consider these factors when determining intraoperative alignment targets to avoid excessive residual varus.