Distribution of Coronal Plane Knee Alignment Phenotypes in an African American Population: A Deep Learning-Based Analysis

Background: The coronal plane alignment of the knee (CPAK) classification integrates the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO) to describe knee morphology. Although CPAK phenotype distributions have been reported across regions, data from North American populations, particularly African American patients, remain limited. Purpose: We sought to characterize the CPAK phenotype distribution and alignment in an African American population and to assess sex differences. Methods: We conducted a retrospective cross-sectional study analyzing African American participants enrolled in the Osteoarthritis Initiative (OAI), a large U.S.-based dataset complied and funded by the National Institute of Health. Patients who underwent standardized weight-bearing lower-limb radiography were eligible for analysis. All radiographic measurements were performed on standardized, full-length, weight-bearing lower-limb radiographs obtained at the 12-month OAI follow-up visit. Alignment parameters were obtained using a deep learning model for landmark detection and angle measurement. Given the descriptive objective of the study, both knees were included to characterize the full spectrum of CPAK phenotypes within the cohort. Patients with knee flexion contractures or hyperextension exceeding 10° at the baseline visit were excluded. Results: A total of 516 knees from 258 African American patients were included. CPAK II was the most prevalent phenotype (37.8%), followed by CPAK III (19.2%) and CPAK I (14.3%). Neutral phenotypes accounted for 51.2% of knees, while valgus phenotypes (26.7%) were more common than varus phenotypes (22.1%). Males demonstrated greater varus alignment than females (mean aHKA -1.2° vs 0.8°) and a higher prevalence of varus CPAK phenotypes, whereas females more frequently exhibited valgus phenotypes, particularly CPAK III (23.5% vs 9.1%). JLO was predominantly apex distal in both sexes. Conclusions: This retrospective analysis of data from a large database found that the knees of African American patients predominantly exhibited CPAK II and III phenotypes, with more valgus alignment than the original CPAK description. Significant sex-based differences were observed. These findings provide population-specific CPAK reference data and highlight the importance of diversity when applying the CPAK classification. Level of Evidence: Level IV: retrospective cross-sectional study.