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

Objective: 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). Materials and methods: 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. Results: 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. Conclusions: 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. © 2019, ISS.