Femoral head‐neck offset measurements in 339 subjects: distribution and implications for femoroacetabular impingement

PURPOSE: The α-angle is a frequently used measurement to quantify head-neck offset pathology in cam-type femoroacetabular impingement. However, diverging norm values and reference intervals have been described, especially in men. METHODS: The α-angle, the head-neck offset ratio and the triangular index were measured on anteroposterior and lateral radiographs of 339 (170 men and 169 women) subjects without evident underlying hip pathology. RESULTS: For male subjects, the mean α-angle was 49.4° (SD 10.5°) on anteroposterior and 49.1° (SD 10.6°) on lateral radiographs, whereas for female subjects, it was 45.0° (SD 8.0°) and 46.1° (SD 9.9°), respectively. Thus, calculated upper limits of reference intervals in the population screened were 70° for both radiographic projections in men and 61° (anteroposterior) and 66° (lateral) in women. CONCLUSION: These data substantiate recent findings that the pathological threshold of the α-angle in male subjects may be lower than that previously assumed. LEVEL OF EVIDENCE: Diagnostic, Level III.