Joint line orientation angle and related parameters in healthy knees: Association with CPAK types and bilateral symmetry.

Abstract

This study aimed to evaluate the joint line orientation angle (K-JLOA) and related radiographic parameters in a healthy population. Specifically, we sought to identify factors influencing K-JLOA, compare K-JLOA across Coronal Plane Alignment of the Knee (CPAK) types, and analyze bilateral concordance of these parameters and CPAK phenotypes. Standing long-leg radiographs from 100 healthy adults (200 knees) aged 20-50 years were examined. K-JLOA and related parameters were measured, including the mechanical hip-knee-ankle angle (mHKA), medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA), arithmetic hip-knee-ankle angle (aHKA), joint line obliquity (JLO), ankle joint line orientation angle (A-JLOA), joint line convergence angles of the knee and ankle (K-JLCA, A-JLCA), and intercondylar and intermalleolar distances (K-ICD, A-IMD). Multiple regression analysis was used to identify determinants of K-JLOA. Differences in K-JLOA across CPAK types were assessed by one-way ANOVA. Bilateral symmetry was evaluated using Pearson correlation coefficients, and concordance of CPAK phenotypes between right and left knees was expressed as percentage agreement. The mean K-JLOA was -3.2° ± 2.7°, while the mean JLO was 173.3° ± 3.5°. K-JLOA was positively associated with MPTA (β = 0.317, p < 0.001) and LDFA (β = 0.441, p < 0.001), but negatively associated with K-JLCA (β = -0.306, p < 0.001), A-JLCA (β = -0.111, p = 0.033), and A-IMD (β = -0.316, p < 0.001). Although statistically significant differences in K-JLOA were observed across CPAK types, their magnitude was small. Most radiographic parameters demonstrated strong bilateral correlations (e.g., mHKA r = 0.762, p < 0.001; A-JLOA r = 0.782, p < 0.001), whereas the concordance of CPAK phenotypes between contralateral knees was only 31%. In healthy adults, the mean K-JLOA was slightly medially inclined and was significantly influenced by both tibial (MPTA) and femoral (LDFA) geometry as well as joint space parameters (K-JLCA, A-JLCA) and foot position (A-IMD). Differences in K-JLOA across CPAK types were small. Most radiographic parameters demonstrated strong bilateral concordance, whereas JLO and aHKA showed weaker agreement. This, combined with classification boundary effects, resulted in poor concordance of CPAK phenotypes. These findings suggest that while K-JLOA and related parameters can be reliably measured, clinicians should interpret phenotype classifications cautiously and integrate quantitative radiographic measures when considering personalized alignment strategies. Level III.