The alpha angle in cam-type femoroacetabular impingement: new reference intervals based on 2038 healthy young adults.

We report on gender-specific reference intervals of the alpha angle and its association with other qualitative cam-type findings in femoroacetabular impingement at the hip, according to a population-based cohort of 2038 19-year-olds, 1186 of which were women (58%). The alpha angle was measured on standardised frog-leg lateral and anteroposterior (AP) views using digital measurement software, and qualitative cam-type findings were assessed subjectively on both views by independent observers. In all, 2005 participants (837 men, 1168 women, mean age 18.6 years (17.2 to 20.1) were included in the analysis. For the frog-leg view, the mean alpha angle (right hip) was 47 degrees (26 to 79) in men and 42 degrees (29 to 76) in women, with 97.5 percentiles of 68 degrees and 56 degrees, respectively. For the AP view, the mean values were 62 degrees (40 to 105) and 52 degrees (36 to 103) for men and women, respectively, with 97.5 percentiles of 93 degrees and 94 degrees. Associations between higher alpha angles and all qualitative cam-type findings were seen for both genders on both views. The reference intervals presented for the alpha angle in this cross-sectional study are wide, especially for the AP view, with higher mean values for men than women on both views.