Single-Leg Weight-Bearing Reveals Hidden Varus Deformity in Knee Osteoarthritis

Background/Objectives: Accurate radiographic assessment of coronal alignment is essential for corrective osteotomies and knee arthroplasty. However, conventional bilateral weight-bearing radiographs may underestimate varus alignment under single-leg weight-bearing. This study aimed to describe the increase in knee varus alignment between bilateral and single-leg weight-bearing stances and to evaluate the factors associated with this increase. Methods: Standardized long-leg radiographs were obtained in bilateral and single-leg weight-bearing stances using a digital radiography system. ΔVarus was defined as the difference between single-leg bearing and bipedal varus angles (single-leg bearing varus—bipedal varus) and was reported in degrees (°). Patients were stratified by dominant lower extremity and sex for subgroup analyses. Results: The final sample study groups consisted of 101 patients (57 women, 44 men) with a mean age of 57.0 ± 12.7 years and a mean BMI of 25.1 ± 2.7 kg/m2. Analysis of the entire cohort revealed a statistically significant increase in varus alignment following single-leg weight-bearing (p < 0.001). Right dominant individuals had a significantly greater varus angle in the right lower extremity (10.9 ± 1.9°) than in the contralateral limb (9.9 ± 2.3°; p = 0.016). Similarly, in left-dominant individuals, the varus angle of the left knee (10.9 ± 2.3°) was significantly greater than that of the right knee (9.4 ± 1.8°; p = 0.009). The mean ΔVarus was 1.54° (SD, 0.75°; 95% CI, 1.39–1.69°; p < 0.001) for the right knee and 1.30° (SD, 0.70°; 95% CI, 1.16–1.44°; p < 0.001) for the left knee. Conclusions: This study has shown that standard bilateral weight-bearing long-leg radiographs may not fully reflect knee varus deformity, particularly the functional coronal alignment changes that occur during single-leg weight-bearing. An additional varus deformity of approximately 1.6° was observed in the dominant limb during single-leg weight-bearing. Single-leg bearing radiographs demonstrated greater varus alignment than bipedal radiographs. However, the clinical implications of this radiographic difference for surgical planning remain undetermined. In future prospective studies, the combined evaluation of gait analysis and single-leg loading radiographs will better elucidate the true contribution of the dominant limb to functional knee biomechanics.