Prevalence of Cam-Type Femoroacetabular Impingement Morphology in Asymptomatic Volunteers

Background: Femoroacetabular impingement is a cause of hip pain in adults and is a possible precursor of osteoarthritis, with the cam type of impingement being the most common. The purpose of this study was to determine the prevalence of cam-type morphology of the hip in asymptomatic patients. Methods: Two hundred asymptomatic volunteers with no prior hip surgery or childhood hip problems underwent magnetic resonance imaging targeted to both hips. The subjects were examined at the time of magnetic resonance imaging for internal rotation of the hips at 90 degrees of hip flexion and for a positive impingement sign. The contour of the femoral head-neck junction was quantified with use of the alpha angle. A value of >50.5 degrees was considered positive for cam morphology. Measurements were performed independently by two musculoskeletal radiologists. Results: The mean age of the individuals was 29.4 years (range, 21.4 to 50.6 years); 79% were white, and 55.5% were women. The mean alpha angle anteriorly at the three o'clock position was 40.9 degrees +/- 7.0 degrees on the right and 40.6 degrees +/- 7.1 degrees on the left, whereas the mean alpha angle anterosuperiorly at the 1:30 position was 50.2 degrees +/- 8.0 degrees on the right and 50.1 degrees +/- 8.3 degrees on the left. Fourteen percent of the volunteers had at least one hip with cam morphology: 10.5% had an elevated alpha angle on either the right or the left side, and 3.5% had the deformity in both hips. Seventy-nine percent (twenty-two) of twenty-eight individuals who had an elevated alpha angle were men, and 21%(six) were women. Individuals with an elevated alpha angle on at least one side tended to be male (p < 0.001), with 24.7% (twenty-two) of eighty-nine men having cam morphology compared with only 5.4% (six) of 111 women. Conclusions: The prevalence of cam-type femoroacetabular impingement deformity is higher in men as well as in individuals with decreased internal rotation. Defining what represents a normal head-neck contour is important for establishing treatment strategies in patients presenting with prearthritic hip pain.