Combined assessment of coronal plane alignment classification and functional knee phenotype reveals sex differences in Chinese knee osteoarthritis patients: a retrospective cross-sectional study

Currently, the Coronal Plane Alignment of the Knee (CPAK) classification and Functional Knee Phenotype (FKP) classification are widely adopted criteria for evaluating lower limb coronal alignment of the knee. However, studies combining these two classifications to explore sex-based differences in knee morphology among Chinese populations remain scarce in recent years. This retrospective cross-sectional study enrolled a total of 1764 preoperative patients (374 men and 1390 women) with end-stage knee osteoarthritis undergoing total knee arthroplasty (TKA). Preoperative weight-bearing full-length radiographs were analyzed using deep learning-based software. Hip-knee-ankle angle, mechanical lateral distal femoral angle, mechanical medial proximal tibial angle, joint line convergence angle, arithmetic Hip-knee-ankle angle, joint line obliquity, femoral anatomical-mechanical angle, and tibial mechanical angle were measured. The distributions of CPAK classification and FKP combinations were compared between sexes. CPAK Type I was the most common phenotype (30.6%), followed by Types II (18.9%) and IV (18.1%); the overall CPAK distribution did not differ by sex ( P = 0.445). Women had slightly greater joint line convergence angle and femoral anatomical-mechanical angle. The distribution of the 20 most frequent FKP combinations differed by sex ( P = 0.0069; Cramér’s V = 0.148). After BH-FDR correction, only VAR HKA 6°+NEU FMA 0°+NEU TMA 0° remained different between sexes (7.22% in men vs. 3.31% in women; q = 0.036). None of the 18 within-CPAK femoral or tibial phenotype comparisons remained significant after BH-FDR correction (all q ≥ 0.175). CPAK and FKP provided complementary, non-interchangeable descriptions of preoperative coronal knee morphology. In this large Chinese TKA-selected cohort, sex differences were selective rather than uniform across classification levels. This study extends previous work by quantifying sex-specific FKP combinations and femoral and tibial phenotype distributions within CPAK subtypes; outcome-based studies are required before these findings can guide surgical targets. Level III.