Beta-angle measurements valid, cost effective for assessing bony anatomy in patients with FAI

Brunner A. J Bone Joint Surg (Br). 2010;92(9):1203-1208. doi:10.1302/0301-620X.92B9.24410.

Measurement of the ß-angle on plain radiographs may be an alternative to open MRI for assessing bony anatomy in patients with femoroacetabular impingement.

Alexander Brunner, MD, and colleagues at Cantonal Hospitals Lucerne, Switzerland, included 50 consecutive patients with symptomatic femoroacetabular impingement (FAI) and 50 healthy asymptomatic controls in this trial. Standard anteroposterior pelvic radiographs were obtained in all patients so that the researchers could measure the center-edge angle. The researchers measured the alpha-angle with MR arthrograms with slices parallel to the axis of the femoral neck passing through the center of the head. Their goal was to determine the relationship between range of movement and the ß-angle.

The results showed that patients with FAI had a mean ß-angle of 15.6· (95% CI, 13.3·-17.7·) compared with 38.7· for the asymptomatic group (95% CI, 36.5·-41.0·). There was high correlation between internal rotation and the ß-angle in the FAI group; it was moderate in the asymptomatic group, the authors found. The inter- and intra-observer reliability was excellent for both groups.

“Our findings suggest that the measurement of the ß-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,” the authors wrote.

Femoroacetabular impingement (FAI) is recognized as a source of hip pain, and due to its pathomechanism could lead to early osteoarthritis in young adults. Correct and early identification of patients with FAI can allow for early intervention and therefore prevention of early osteoarthritis in these young patients.

In this well-designed study, the authors have demonstrated that the ß-angle measured on radiographs with the hip in 90° of flexion and 20° of abduction is a valid, reliable and cost-effective tool in FAI diagnosis. The big advantage is that measurements are done with the hip in a position where impingement takes place in patients with FAI, therefore mimicking the clinical situation. Although 3-D imaging is very useful to evaluate the full extent of the lesion and to plan surgery, the ß-angle measured on plain radiographs is an excellent screening tool for the early detection of patients with FAI.

– Geert Meermans, MD
University College London Hospital
London