Femoro-acetabular impingement: anatomic study of reliability and accuracy of alpha angle and offset ratio on fifty consecutive peri-operatively resected femoral heads.

Purpose: The purpose of this study was to determine whether X-ray attributes of the femoro-acetabular impingement (FAI) - alpha angle (AA) and offset ratio (OR) reflect real measurements on resected femoral heads.

Methods: FAI AA and OR were assessed on 50 consecutively-resected femoral heads. The parameters were measured on heads cut through the maximum range of the deformity and compared to the same parameters on standardized X-ray projections (anterior-posterior [AP] and axial views).

Results: Mean AA was 76.47° on dissected heads vs. 75.81° on axial X-ray (p = 0.688). Mean OR was 0.132 on dissected heads vs. 0.220 on axial X-ray (p < 0.001). Mean AA on ideal AP X-ray was 79.46° vs. 81.51° on AP standing plain X-ray view (p = 0.431). AA measurements on plain X-ray AP and axial view of halved femoral heads correlated highly.

Conclusions: AA on axial X-ray view reflected the real AA in our series, but the risk of cartilage damage cannot be predicted.