Joint line convergence angle should be considered when constitutional alignment is predicted based on arithmetic hip-knee-ankle angle: a retrospective cross-sectional observational study.

Few studies have illustrated the role of the joint line convergence angle (JLCA) in the correlation between the arithmetic hip-knee-ankle angle (aHKA) and mechanical hip-knee-ankle angle (mHKA). We aimed to determine this role and to evaluate the effect of the JLCA in predicting constitutional alignment using the aHKA. A total of 107 normal participants (214 knees) and 246 patients (477 knees) with osteoarthritis (OA) were retrospectively examined using long-leg radiographs. The formula was calculated geometrically on plain radiographs of the knee based on the relationships among the mHKA, aHKA, and JLCA, and the distribution of the JLCA between the groups was determined. The effect of the JLCA on the prediction of constitutional alignment using the aHKA was evaluated. The geometric formula was determined to be 'mHKA=180°+aHKA-JLCA', and was verified. The median (quartile 1, quartile 3) JLCA was 2.0° (1.0°, 3.5°) and 2.5° (1.0°, 4.5°) in the normal and OA groups, respectively. The JLCA affected the prediction of constitutional alignment based on the aHKA. It should be considered when constitutional alignment is predicted based on the aHKA in patients with high JLCA values.