Coronal plane alignment of the knee phenotypes distribution varies significantly as a function of geographic, osteoarthritic and sex-related factors: A systematic review and meta-analysis

Purpose: The coronal plane alignment of the knee (CPAK) classification is a nine-phenotype matrix based on limb alignment and joint line obliquity. This study aimed to provide a global overview of CPAK distribution, hypothesising significant geographic, osteoarthritic and sex-related variations. Methods: A systematic literature search (Embase, Medline/PubMed and Cochrane Library) following PRISMA guidelines was conducted, utilising the search terms “Coronal Plane Alignment of the Knee” OR “CPAK”. Studies considering image modalities other than long-leg radiographs were excluded. A random-effects meta-analysis of proportions was performed, and statistical significance was defined as p < 0.05. Results: A total of 38 studies comprising 46,966 knees were analysed. The most common phenotypes worldwide were CPAK I (33.1%), II (25.9%) and III (14.4%) in the osteoarthritic population and CPAK II (34.9%), I (21.5%) and III (19.3%) in the healthy population. Among osteoarthritic populations, CPAK type I was predominant in Europe (29.2%), Asia (41.9%) and America (33.6%), type II in Australia (32.6%) and type III in Africa (28.6%). In healthy populations, type II was predominant in Europe (42.8%) and Asia (35.3%), whereas type I was most common in South America (44.8%). Significant regional differences were observed among both osteoarthritic and healthy knees, and between osteoarthritic and healthy knees in individual countries. In Europe, significant sex differences were observed in the distribution of types I (39.1% M; 23.5% F) and III (11.4% M; 24.6% F) in the osteoarthritic population, and in the distribution of types I (26.7% M; 9.4% F), II (43.9% M; 34.4% F) and III (11.3% M; 20.6% F) in the healthy population. In Asia, significant sex differences were found for type III in osteoarthritic knees (6.3% M; 11.4% F). Conclusion: CPAK distribution varies significantly as a function of geographic, osteoarthritic, and sex-related factors. A personalised approach to TKA may be desirable to better accommodate these differences. Level of Evidence: Level IV.