Understanding the development of and theory behind the Coronal Plane Alignment of the Knee classification: Important concepts for contemporary total knee arthroplasty

The Coronal Plane Alignment of the Knee (CPAK) classification provides a standardized framework for characterizing constitutional knee alignment and joint line obliquity, independent of arthritic deformity. Conceptualized and validated in both healthy and osteoarthritic populations, the CPAK matrix integrates the arithmetic hip–knee–ankle angle and joint line obliquity to define nine coronal phenotypes. Since its introduction, CPAK has been instrumental in advancing personalized alignment strategies in total knee arthroplasty by informing surgical planning, imaging protocols, and global phenotype mapping. This review outlines the theoretical basis, geographic and sex-based differences, and clinical applications of CPAK, emphasizing its role in refining patient-specific alignment philosophies. • CPAK pragmatically defines constitutional knee alignment and joint line obliquity. • Patients fit into one of nine phenotypes using two straightforward calculations. • CPAK provides an understanding of geographical and sex variation in knee alignment. • CPAK offers clear insight into differing outcomes of knee reconstructive surgery. • Surgeons can tailor their alignment strategy to fit a patient's native phenotype.