Impact of Mechanical Axis Position and Coronal Plane Alignment Phenotypes on Clinical Outcomes in Medial Opening Wedge High Tibial Osteotomy

Introduction: Medial opening wedge high tibial osteotomy (MOWHTO) is a widely used surgical technique for treating varus-aligned medial compartment knee osteoarthritis. While traditional strategies emphasize lateralizing the mechanical axis toward the Fujisawa point, recent biomechanical insights suggest that excessive valgus correction (>2°) may increase stress on the lateral compartment without conferring additional clinical benefit. Moreover, restoring a horizontal joint line is essential to optimize load distribution and improve knee function. The Coronal Plane Alignment of the Knee (CPAK) classification provides a phenotype-based framework that incorporates both mechanical axis deviation and joint line obliquity (JLO), though its application in MOWHTO remains underexplored. Material and Methods: A retrospective study was conducted on 147 knees in 123 patients who underwent MOWHTO with a minimum follow-up of 24 months. Radiographic parameters evaluated included mechanical femorotibial angle (mFTA), medial proximal tibial angle (MPTA), mechanical lateral distal femoral angle (mLDFA), joint line convergence angle (JLCA), arithmetic hip-knee-ankle angle (aHKA), and JLO. Patients were categorized into five groups based on postoperative weight-bearing line (WBL) position across the tibial plateau. CPAK phenotypes were recorded pre- and postoperatively. Clinical outcomes were assessed using the Hospital for Special Surgery (HSS) knee score. Results: CPAK type I (varus alignment with distal apex) was the predominant preoperative phenotype (82.3%). Postoperatively, a notable proportion of knees transitioned to CPAK types V (24.5%) and VI (17.7%), both associated with significantly higher HSS scores (p < 0.001). The mean aHKA improved from −7.35° to +1.59° (p < 0.001), and JLO corrected from 172.4° to 180.8° (p < 0.001), reflecting joint line horizontalization. JLCA remained stable (p > 0.05). Clinical outcomes were most favourable in patients with postoperative WBL positions between 50% and 60%, comparable to outcomes achieved at the Fujisawa point. Conclusion: Successful outcomes in MOWHTO depend not only on lateralization of the mechanical axis but also on restoring joint line orientation. This study is among the first to apply the CPAK classification to MOWHTO and demonstrates its potential for guiding phenotype-based, individualized correction strategies. Targeting a WBL position between 50% and 60%, along with horizontal joint line restoration, appears to optimize clinical results. Level of evidence: Level III (retrospective comparative study)