Validity of the alpha angle measurements on plain radiographs in the evaluation of cam-type femoroacetabular impingement in patients with slipped capital femoral epiphysis.

The purpose of the study was to investigate the correlation of two different alpha angle (a-angle) measurements ("anatomical method and "three-point method") with the anterior offset ratio (AOR), femoral head ratio (FHR), and lateral femoral head ratio (LFHR) in patients with slipped capital femoral epiphysis (SCFE). We included 39 hips of 26 patients. The a-angles were measured on the frog-leg lateral view (Lat) and anteroposterior (Ap) view, FHR was measured on the Ap view, and LFHR and AOR were measured on the Lat view. A t test was performed to analyze the means of the alpha angles measured using the three-point method and the anatomical method, and also, a correlation was conducted to assess the association of the a-angles among the FHR, LFHR, and AOR. The mean a-angles in the Ap plane in the three-point method and anatomical method were 76° ± 15° and 64° ± 10° respectively (p < 0.001). The mean a-angles in the Lat plane in the three-point method and anatomical method were 67° ± 13° and 56° ± 11° respectively (p < 0.001). The AOR showed a significant correlation only with the anatomical method a-angle values in the Lat plane (p = 0.026). The a-angles in the three-point method in the Lat plane did not show any significant correlation with the AOR, FHR, and LFHR. Both the FHR and LFHR values correlated significantly with the Ap plane a-angles in the three-point method and anatomical method. However, none of these correlations was strong. The a-angle measurement methods described in patients without femoral head-neck axis disruption may not be valid in patients with a disorder such as SCFE.