Femoroacetabular cam-type impingement: global assessment of the femoral head-neck junction on a single reformatted MR image.

PURPOSE

To characterize the imaging appearance of femoral head-neck contour abnormalities on a short-axis magnetic resonance (MR) image compared with the usual anterior alpha angle measurements and multiple alpha angle measurements on radial reformatted MR images, with surgery as the reference standard.

MATERIALS AND METHODS

After institutional review board approval of this HIPAA-compliant study, 21 patients who underwent arthroscopy and 24 patients who did not, all of whom were evaluated with three-dimensional MR imaging, during 1 year were identified. Short-axis MR images of the femoral head-neck junction were reformatted with multiple radial images along the axis of the femoral neck. The following measurements were made at each hour of a clock face: (a) presence and size of bone contour abnormality visible beyond a best-fit circle and (b) femoral head-neck offset angles. Mann-Whitney, Fisher exact, and Wilcoxon matched-pair signed rank tests were performed. Intra- and interreader agreement were calculated as the Cohen κ.

RESULTS

Of the 21 subjects who underwent surgery, 16 were confirmed to have cam-type femoroacetabular impingement (FAI) at surgery. Comparing findings from short-axis images with those at surgery, average accuracy was 81%. Comparing findings from head-neck offset angles with those at surgery, average accuracy was 80%. On short-axis images, average bone elevation was 3.2 mm in patients with cam-type FAI and 1.4 mm in those without it. In eight of 24 subjects who did not undergo surgery, the alpha angle was normal but the short-axis MR image showed abnormal bone contour.

CONCLUSION

An abnormal bone contour identified on a short-axis MR image at the femoral head-neck junction correlates with surgical findings and may allow for a global characterization of the bone abnormality with regard to location, extent, and amount of elevation compared with the alpha angle and multiple head-neck offset angles.