Introduction and Aims: There is a relationship between the anatomy of the hip joint and the development of arthritis. A common cause of hip pain in the young adult that can lead to arthritis is acetabular dysplasia. More recently, femoroacetabular impingement has been described as another cause of hip pain. The purpose of our study was to evaluate the applicability of pelvic computed tomography (CT) with three-dimensional surface rendering to evaluate femoro-acetabular impingement.

Method Thirty-six hips (30 patients; 17 males; 13 females) with persistent hip pain, mean age 41 (37–52), underwent three-dimensional CT of the pelvis, as well as MRI arthrography with gadolinium enhancement. On 3D CT, the concavity of the femoral head-neck junction (offset), alpha angle as described by Notzli was calculated to depict the anterior femoral neck contour. The concavity of the posterior aspect of the head neck junction was measured as the beta angle. The same measurements were made in 20 hips, consisting of randomly selected patients with no prior history of hip pathology or pain (mean age 37; 13 males; eight females).

Results The mean alpha angle for the symptomatic group was 66.4 (39–94) and 43.8 (39.3–48.3) for the control group (p=0.001). All symptomatic hips had abnormal findings on MRA: labral tears in all; cartilage delamination/ulceration in 14 hips; herniation pits in six hips. The majority of labral tears and delamination were located in the antero-superior quadrant. In the surgical treated group, all MRA findings were confirmed. The mean beta angle was significantly smaller (increase concavity) in the symptomatic versus the controls: 40.2 versus 43.8 (p=0.011). Interestingly in the symptomatic group the beta angle was significantly lower than the alpha angle (p< 0.02), but not in the controls.

Conclusion: 3D CT with surface rendering and multiplanar reformation is useful to determine degree of bone buttressing of the anterior femoral head-neck junction quantitatively assessed by alpha angle measurement, which is elevated in patients with femoro-acetabular impingement. With a greater posterior concavity i.e. small beta angle in the symptomatic group versus the control, subclinical slipped femoral epiphysis remains a plausible cause of this deformity.