The coronal plane alignment of the knee classification of a Japanese population indicates more varus deformity tendency compared with European countries

Purpose

The purpose of this study was to compare the distribution of the coronal plane alignment of the knee (CPAK) phenotype between the healthy population and the arthritic population in Japan.

Methods

The retrospective cross-sectional study included 1049 knees. There were 256 healthy individuals with a total of 512 knees and 310 individuals with a total of 537 arthritic knees who underwent around-knee osteotomy between June 2010 and January 2024. The CPAK phenotype was compared between the healthy population and the arthritic population. Furthermore, the CPAK phenotype rates were compared between Japan and the other countries.

Results

The most common type of CPAK was type II (51.2%), followed by type III (26.4%) and type I (16.6%) in the healthy population, and type I (63.4%), followed by type II (22.9%) and type IV (6.3%) in the arthritic population. There were significant differences in CPAK types I, II, III, IV, VI between the healthy and arthritic populations. The rates of CPAK phenotype in Japan were significantly different from those in the other countries. The rates of apex distal joint line obliquity in the healthy and arthritic population (94.2%, 88.9%) were significantly higher than in the other countries.

Conclusions

There were significant differences in distribution of the CPAK phenotype between the healthy and arthritic populations. Furthermore, the Japanese distribution of the CPAK phenotype was different from other countries. Japanese CPAK types were highly involved in distal joint line apex obliquity, which indicates varus deformity in the future.