Standing Long-Leg Radiographs Underestimate Varus Alignment and Joint Line Obliquity Compared With Computed Tomography in Japanese Patients.

Objective This study aimed to determine whether standing long-leg radiographs (LLR) underestimate the medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) compared with CT in a Japanese population, and to evaluate the resulting coronal plane alignment of the knee (CPAK) classification based on the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO). Methods This retrospective study included 90 patients (97 knees) who underwent robot-assisted total knee arthroplasty (TKA) between April 2024 and October 2025. Patients with previous surgery on the affected limb or post-fracture malunion were excluded. The MPTA and LDFA were measured using LLR and CT. The aHKA and JLO were calculated, and the CPAK classification was determined. Results There were significant differences between LLR and CT measurements for the MPTA, LDFA, aHKA, and JLO (p < 0.05). Compared with CT, LLR underestimated varus lower limb alignment and JLO. Although there was no significant difference between the two imaging modalities in the distribution of CPAK classifications, CT showed a higher proportion of types I-III and a lower proportion of types IV-V. Conclusions In a Japanese population, LLR underestimated the degree of varus lower limb alignment and JLO compared with CT. Although the distribution of CPAK classifications did not significantly differ between the two imaging modalities, CT showed greater varus alignment and lower JLO values than LLR, suggesting that it may be useful for evaluating knees near the boundaries between CPAK classification types.