The plain β-angle measured on radiographs in the assessment of femoroacetabular impingement

The beta-angle is a radiological tool for measuring the distance between the pathological head-neck junction and the acetabular rim with the hip in 90 degrees of flexion in patients with femoroacetabular impingement. Initially it was measured using an open-chamber MRI. We have developed a technique to measure this angle on plain radiographs. Correlation analysis was undertaken to determine the relationship between the range of movement and the beta-angle in 50 patients with femoroacetabular impingement and 50 asymptomatic control subjects. Inter- and intra-observer reliability of the beta-angle was also evaluated. Patients with femoroacetabular impingement had a significantly smaller (p < 0.001) mean beta-angle (15.6 degrees, 95% confidence interval (CI) 13.3 to 17.7) compared with the asymptomatic group (38.7 degrees, 95% CI 36.5 to 41.0). Correlation between internal rotation and the beta-angle was high in the impingement group and moderate in the asymptomatic group. The beta-angle had excellent inter- and intra-observer reliability in both groups. Our findings suggest that the measurement of the beta-angle on plain radiography may represent a valid, reproducible and cost-effective alternative to open MRI in the assessment of the pathological bony anatomy in patients with cam, pincer and mixed femoroacetabular impingement.