Objective: The description of femoral head sphericity and related risk for femoroacetabularimpingement is currently limited to an angular estimate-the alpha angle-whose relevance and accuracy have been challenged. We developed a three-dimensional approach for both automated digital measurement of the alpha angle and the detection of camdeformities. Accuracy and diagnostic relevance of the alpha angle estimated by means of the oblique axial and multiple radial plane protocol were compared with the computed results.
Materials and methods: Using subject-specific statistical information of the femur head and mid-neck region, a method was developed to accurately compute the maximum alpha angle and to define aspherical eccentric areas at the femoral head-neck junction. The method was evaluated on 102 dry cadaver femur specimens.
Results: Average detection limit for bony prominences at the head-neck transition was 0.98 mm. Pixel size of the investigated CT data was 0.79 mm. Mean maximum computed alpha angle of the femurs with cam-type morphology as identified by the morphological method was 67.72° (range 53.04-88.02°). Mean maximum computed alpha angle of the femurs without cam deformity was 47.65° (range 38.67-59.81°). Alpha angle estimates obtained by means of the multiple radial plane protocol correlated better (R = 0.88) and showed higher diagnostic agreement (phi = 0.77) with the 3D computational analysis compared to the oblique axial protocol (R = 0.60; phi = 0.67).
Conclusions: The alpha angle seems to be a relevant screening tool when obtained by 3D computed analysis or when estimated according to the multiple radial plane protocol. Estimates obtained by means of the oblique axial protocol have insufficient diagnostic and measurement accuracy.