Classification of coronal plane alignment of arthritic and healthy knees in Japan

There are geographic differences in the prevalence of coronal plane alignment of the knee (CPAK) phenotypes. This study aimed to evaluate the CPAK classification of healthy and arthritic knees in Japan. A cohort of arthritic knees (150 knees in 125 patients who underwent primary total knee arthroplasty) and a cohort of healthy knees (150 knees in 150 patients who underwent total hip arthroplasty or bipolar hip arthroplasty with healthy knees) underwent radiological assessment with long-leg radiographs and were categorized into nine phenotypes. The lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), mechanical and arithmetic hip–knee–ankle angles (mHKA and aHKA, respectively), and joint line obliquity were measured. The mean LDFA was 88.1° and 86.9° in the arthritic and healthy knee cohorts, respectively, and the mean MPTA was 85.2° and 86.6°, respectively. The mean mHKA was −8.0° and −1.2° and the mean aHKA was −2.9° and −0.3° in the arthritic and healthy knee cohorts, respectively. There was no significant difference in the distribution of varus/valgus knee on mHKA between the arthritic and healthy knee cohorts. In the arthritic knee cohort, most were CPAK type I (97 knees, 64.7%). Healthy knees were most commonly classified as type II (71 knees, 47.3%). The distribution of varus/valgus knee was similar in Japanese arthritic and healthy knees. However, the distribution of the CPAK classification in the arthritic and healthy knees of Japanese individuals was different. • Study of coronal plane alignment of the knee (CPAK) phenotypes. • Evaluation of CPAK classification of healthy and arthritic knees in Japan. • Most arthritic knees were classified as CPAK type I. • Most healthy knees were classified as CPAK type II. • The CPAK classification in osteoarthritis might be hard to use directly in Japan.