Femoroacetabular impingement and the cam-effect - A MRI-based quantitativeanatomical study of the femoral head-neck offset

We have observed damage to the labrum as a result of repetitive acetabular impingement in non-dysplastic hips, in which the femoral neck appears to ab ut against the acetabular labrum and a non-spherical femoral head to press against the labrum and adjacent cartilage. In both mechanisms anatomical va riations of the proximal femur may be a factor. We have measured the orient ation of the femoral neck and the offset of the head at various circumferen tial positions, using MRI data from volunteers with no osteoarthritic chang es on standard radiographs. Compared with the control subjects, paired for gender and age, patients showed a significant reduction in mean femoral ant eversion and mean head-neck offset on the anterior aspect of the neck. This was consistent with the site of symptomatic impingement in flexion and int ernal rotation, and with lesions of the adjacent rim. Furthermore, when str atified for gender and age, and compared with the control group, the mean f emoral head-neck offset was significantly reduced in the lateral-to-anterio r aspect of the neck for young men, and in the anterolateral-to-anterior as pect of the neck for older women. For patients suspected of having impingem ent of the rim, anatomical variations in the proximal femur should be consi dered as a possible cause.