Distribution of coronal plane alignment knee phenotypes in Japanese patients with knee osteoarthritis: a comparison of two-dimensional and three-dimensional measurements.

Background: This study aimed to determine whether the distribution of coronal plane alignment of the knee (CPAK) classification differs in Japanese patients with knee osteoarthritis (OA) when assessed using two-dimensional (2D) standing long-leg radiographs versus three-dimensional (3D) supine computed tomography-based measurements. Methods: This retrospective study included 100 knees of 86 patients who underwent primary total knee arthroplasty for OA (mean age, 72.9 years; 27 men, 73 women; body mass index, 27.3 kg/m Results: All parameters differed significantly between 2D and 3D measurements. Mean lateral distal femoral angle was 88.2° on 2D and 87.9° on 3D (p < 0.01), medial proximal tibial angle was 85.3° and 83.8° (p < 0.01), aHKA was -2.9° and -4.1° (p < 0.01), and JLO was 173.5° and 171.7° (p < 0.01), respectively. The distribution of CPAK phenotypes did not differ significantly between 2D- and 3D-based classifications, with good agreement (Cohen's kappa = 0.615). All parameters demonstrated strong correlations between 2D and 3D measurements (r > 0.80). Conclusion: In Japanese patients with knee OA, the distribution of CPAK phenotypes showed good agreement between 2D- and 3D-based assessments. Although 2D measurements may underestimate varus alignment and JLO, they appear sufficient for CPAK-based phenotypic assessment in clinical practice.