Speaking the same language? A direct cartography between functional knee phenotypes and CPAK.

Abstract

This study aimed to analytically map functional knee phenotypes (FKP) to the Coronal Plane Alignment of the Knee (CPAK) classification, evaluate CPAK's ability to represent native coronal alignment variability in non-osteoarthritic (NO) and osteoarthritic (OA) populations and propose a simplified translational framework between both systems. It was hypothesized that CPAK represents a discretized form of native coronal alignment and that its relationship with functional phenotypes is inherently probabilistic. Arithmetic hip-knee-ankle angle and joint line obliquity were analytically derived from femoral and tibial mechanical angles, enabling direct conversion of functional phenotypes into CPAK types. A theoretical cartography was constructed using mean values and full angular intervals (±1.5°). Four assignment strategies were applied: deterministic assignment, interval propagation, stochastic Monte Carlo simulation and least probable CPAK scenario. Conversions were performed in 308 NO and 2692 OA knees from the functional phenotype cohort and compared with 500 NO and 507 OA knees from the CPAK cohort. A synthetic bidimensional framework (Hirschmann-CPAK Translational grid [HCT-9]) was analytically derived. Considering angular intervals, each functional phenotype corresponded to multiple CPAK types (mean 1.7 per phenotype), showing that deterministic conversion underestimated variability. In NO knees, distributions obtained using the mean, interval, stochastic and rare methods differed significantly from the original cohort (all <i>p</i> < 0.001). In OA knees, the distribution obtained using stochastic conversion showed no significant difference (<i>p</i> = 0.188), suggesting probabilistic convergence. Extreme types were overrepresented in the rare scenario. The HCT-9 framework identified zones of low and high ambiguity, particularly in central phenotypic regions. The relationship between FKP and CPAK is inherently probabilistic. The HCT-9 framework enables structured translation between both systems and supports a shared alignment language for total knee arthroplasty. Level III, comparative retrospective study.