Influence of limb position on femoral neck anteversion angle measurement during computed tomography imaging.

Background: The femoral neck anteversion angle has been used as a surgical indicator for hip and patellofemoral joint disorders. However, the influence of limb position on femoral neck anteversion angle measurements during imaging remains unclear. Therefore, this study aimed to investigate the influence of limb position on femoral neck anteversion angle measurements.

Methods: Computed tomography images of 20 femurs from 10 patients were obtained. The angle between the line passing through the center of the femoral head and the center of the femoral neck and the tangential line of the femoral posterior condyles on axial slices was measured as the femoral neck anteversion angle. Raw femoral neck anteversion angle data was defined as the original femoral neck anteversion angle. The cutting direction of the axial plane was changed from -20° to 20° in 5° increments to simulate limb position changes for each of the following measurements: hip flexion/extension, abduction/adduction angles, and their combined directions. The femoral neck anteversion angle was measured under each condition, and the change in the angle was calculated. The correlation between hip angle and femoral neck anteversion angle change was analysed by Spearman's rank correlation coefficient.

Results: The mean original femoral neck anteversion angle was 17.6°. There was a strong negative correlation between hip flexion/extension change and femoral neck anteversion angle change ( = -0.96, p < 0.001). There was a weak correlation between hip adduction/abduction change and femoral neck anteversion angle change ( = 0.35, p < 0.001). The average maximum potential difference in femoral neck anteversion angle measurement combining flexion/extension and abduction/adduction was 21.0° ± 4.9°.

Conclusions: The femoral neck anteversion angle changed in association with changes in limb position, particularly with hip flexion and extension. Careful attention to limb position and conditions of the slice is needed to consistently evaluate the femoral neck anteversion angle.