Association Between Postoperative Mechanical Alignment and Clinical Outcomes After Medial Opening-Wedge High Tibial Osteotomy: A Retrospective Cohort Study

Background: The optimal coronal mechanical alignment after medial opening-wedge high tibial osteotomy (MOWHTO) remains controversial. This study aimed to evaluate the association between postoperative hip–knee–ankle angle (HKA) and early- to mid-term clinical outcomes and to explore whether this relationship showed evidence of nonlinearity within the observed valgus range. Methods: This single-center retrospective cohort study included 188 patients who underwent MOWHTO with a 10- or 12.5-mm osteotomy opening. The primary exposure was HKA measured at 6 weeks postoperatively, and the final follow-up International Knee Documentation Committee (IKDC) score was designated as the primary clinical outcome. Lysholm and visual analog scale (VAS) scores were evaluated as secondary outcomes. Associations between postoperative HKA and clinical outcomes were assessed using multivariable linear regression with HC3 heteroskedasticity-robust standard errors, while potential nonlinear relationships were explored by incorporating a quadratic HKA term. Results: The mean age was 52.64 ± 4.74 years, the mean follow-up was 25.48 ± 6.80 months, and the mean HKA at 6-weeks postoperatively was 2.88 ± 1.21° of valgus. IKDC and Lysholm scores significantly improved, whereas VAS scores decreased (all p < 0.001). In multivariable analyses, each 1° increase in postoperative valgus HKA was associated with a 1.49-point-higher final IKDC score (95% CI, 0.04–2.93; p = 0.044) and a 2.61-point-higher final Lysholm score (95% CI, 1.10–4.13; p < 0.001). No statistically significant association was observed with final VAS (β = 0.004; 95% CI, −0.050 to 0.058; p = 0.883). The quadratic HKA term was not statistically significant for any clinical outcome, indicating no statistically detectable nonlinearity within the observed 0.4–4.5° valgus range. The IKDC association was attenuated in an influence sensitivity analysis, whereas the Lysholm association remained significant. Conclusions: Within the restricted postoperative HKA range observed in this cohort (0.4–4.5° of valgus), greater postoperative valgus HKA was associated with higher functional outcome scores, with a more pronounced and consistent association for Lysholm and a more modest association for the primary outcome, IKDC. No association was observed with pain, and no statistically detectable nonlinearity was identified within the observed HKA range. These observational findings do not establish a causal valgus dose–response relationship, a universal optimal postoperative HKA, or the superiority of individualized correction. Prospective studies across broader alignment ranges are needed to define clinically appropriate alignment targets for specific patient subgroups.