Normative distribution of the arithmetic hip-knee-ankle angle in individuals with normal mechanical alignment

The arithmetic hip-knee-ankle angle (aHKAA) has been proposed as a bony geometry-based parameter reflecting constitutional coronal alignment. However, normative values of aHKAA in individuals with normal mechanical alignment remain insufficiently defined. This study aimed to determine the distribution of aHKAA in individuals with normal hip-knee-ankle angle (HKA) and joint-line convergence angle (JLCA), and to evaluate its relationship with mechanical alignment parameters.

In this retrospective radiographic reference study, 649 knees with normal HKA (1° valgus to 3.2° varus) and JLCA (0-2°) were included. mMPTA, LDFA, HKA, and JLCA were measured. aHKAA was calculated as mMPTA - mLDFA, and mechanical HKA (mHKA) was derived as 180° + aHKAA - JLCA. Normative reference intervals were defined using non-parametric methods. Correlations were assessed using Spearman analysis and agreement using Bland-Altman analysis.

Mean aHKAA was 1.14 ± 1.85°, with higher values in females than males (1.56 ± 1.81° vs 0.50 ± 1.74°, p < 0.001). Mechanical parameters showed narrow distributions (HKAA: 0.46 ± 1.18°; mHKA: 179.62 ± 1.53°). mHKA strongly correlated with HKAA (ρ = -0.756, p < 0.001) and with aHKAA (ρ = 0.719, p < 0.001). Bland-Altman analysis demonstrated a mean difference of -0.68° (95% limits -4.19° to 2.83°).

Despite normal mechanical alignment, aHKAA demonstrates inter-individual variability and sex-related differences. Mechanical neutrality does not necessarily reflect geometric neutrality.