Coronal Plane Alignment of the Knee Type I knees are not a homogeneous phenotype: differences in coronal alignment sensitivity following total knee arthroplasty

Aims The Coronal Plane Alignment of the Knee (CPAK) classification categorizes knees into nine phenotypes based on the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO). CPAK Type I is highly prevalent in East Asian populations, but encompasses a broad spectrum of varus deformity. We hypothesized that CPAK Type I knees are not a homogeneous phenotype, and that the severity of constitutional varus may modify the relationship between coronal alignment changes and clinical outcomes after total knee arthroplasty (TKA). Methods This retrospective study included 258 knees undergoing primary TKA using navigation with a mechanical alignment strategy. CPAK Type I knees were sub-classified into Type 0 (aHKA < −6°) and Type Ia (−6°≤ aHKA < −2°), termed Asia-CPAK (A-CPAK). Absolute changes in alignment (ΔaHKA and ΔJLO) were then calculated. Clinical outcomes were assessed using the Knee Injury and Osteoarthritis Outcome Score (KOOS) at one year. Associations between alignment changes and KOOS sub-scales were analyzed. Results Across all knees, ΔJLO and ΔaHKA were not associated with KOOS outcomes. No associations were identified within Type I knees using the conventional CPAK classification. In contrast, within the A-CPAK classification, Type Ia knees demonstrated significant negative correlations between ΔJLO and improvements in activities of daily living, sports, and quality of life (r = −0.21 to −0.37, p < 0.05), indicating worse outcomes with greater deviation from native JLO. No significant associations were observed in Type 0 knees. Conclusion Sub-classification of CPAK Type I into Type Ia and Type 0 reveals phenotype-dependent differences between coronal alignment changes and outcomes. In mild varus (Type Ia), deviation from native JLO adversely affects outcomes. In severe varus (Type 0), sensitivity to joint line changes is reduced, suggesting the extent of native alignment correction may be individualized according to deformity severity. Cite this article: Bone Jt Open 2026;7(11):1471–1480.