An investigation into the redefinition of the alpha angle for 3D imaging.

Abstract

Cam-type femoroacetabular impingement (FAI) is traditionally diagnosed using the alpha angle, a two-dimensional metric that inadequately represents the three-dimensional nature of cam morphology. This study introduces a novel image-based framework that optimizes the alpha angle to better predict 3D cam deformity volume. Forty-five participants underwent MRI scans to obtain traditional alpha angle measurements and to develop a non-cam-deformed statistical shape model, which was fitted to all segmentations to calculate volumetric cam deviation. Automated alpha angles were calculated across radial planes, with the circle fitting radius varied systematically. Regression analyses compared alpha angle predictions of cam volume between traditional and optimized methods. Results demonstrated that adjusting the circle radius by +2 mm from the anatomical contour yielded the most accurate volume predictions. Sex-specific differences emerged, with the 2o'clock orientation optimal for males and 3o'clock for females. The optimized method outperformed traditional measurements, reducing cam volume prediction error by 30 % in females and 23 % in males. These findings highlight the limitations of current 2D imaging practices and advocate for sex-specific alpha angle parameters that retain clinical feasibility while improving diagnostic precision. By bridging the gap between 2D measurements and 3D deformity assessment, this approach enhances the clinical utility of the alpha angle in evaluating cam morphology.