Background <p>Medial opening wedge high tibial osteotomy (MOWHTO) shifts the mechanical axis laterally to relieve medial knee stress. The CPAK classification, which categorizes knee alignment into nine phenotypes, has been scarcely applied in studies involving MOWHTO. The aim of this study is to explore the effect of CPAK phenotypes on cartilage regeneration and functional outcome after MOWHTO.</p> Methods <p>This was retrospective study involving 186 patients who underwent MOWHTO, with 103 of these patients receiving second-look arthroscopy. We analyzed pre- and postoperative CPAK phenotypes, along with factors such as age, sex, BMI, laterality, and undercorrection, to identify contributors to clinical outcomes. Outcomes were assessed through arthroscopic evaluation of cartilage regeneration and the Knee Injury and Osteoarthritis Outcome Score (KOOS) for the entire cohort.</p> Results <p>This study included 81 men and 105 women, with a mean age of 60.3 years. The mean KOOS was 69.0 ± 12.1 at an average follow-up of 40.6 months. Among 103 patients who underwent second-look arthroscopy, 55.3% showed cartilage improvement. Univariate analysis revealed that postoperative CPAK phenotype and undercorrection correlated with KOOS, while age and postoperative CPAK phenotype correlated with cartilage improvement. Multivariate analysis identified undercorrection as the sole significant contributor to both postoperative KOOS and cartilage improvement.</p> Conclusions <p>Undercorrection is the main factor affecting clinical outcomes and cartilage regeneration, rather than the specific CPAK phenotype. When proper correction is achieved, neither preoperative nor postoperative CPAK phenotypes significantly influence results, indicating that MOWHTO can achieve favorable outcomes even in the presence of femur-driven deformities or intra-articular wear.</p>