title: Comparison of MRI alpha angle measurement planes in femoroacetabular impingement.
abstract: Insufficient femoral head-neck offset is common in femoroacetabular impingement (FAI) and reflected by the alpha angle, a validated measurement for quantifying this anatomic deformity in patients with FAI. We compared the alpha angle determined on magnetic resonance imaging (MRI) oblique axial plane images with the maximal alpha angle value obtained using radial images. The MRIs of 41 subjects with clinically suspected FAI were reviewed and alpha angle measurements were performed on both oblique axial plane images parallel to the long axis of the femoral neck and radial images obtained using the center of the femoral neck as the axis of rotation. The mean oblique axial plane and mean maximal radial alpha angle values were 53.4 degrees and 70.5 degrees, respectively. In 54% of subjects, the alpha angle was less than 55 degrees on the conventional oblique axial plane image but 55 degrees or greater on the radial plane images. Radial images yielded higher alpha angle values than oblique axial images. Patients with clinically suspected FAI may have a substantial contour abnormality that can be underestimated or missed if only oblique axial plane images are reviewed. Radial plane imaging should be considered in the MRI investigation of FAI.
authors.affiliation: Department of Diagnostic Imaging, The Ottawa Hospital-General Campus, University of Ottawa, 501 Smyth Road, Ottawa, ON, Canada K1H 8L6. krakhra@ottawahospital.on.ca
authors.forename: Kawan S
authors.initials: KS
authors.lastname: Rakhra
authors.forename: Adnan M
authors.initials: AM
authors.lastname: Sheikh
authors.forename: David
authors.initials: D
authors.lastname: Allen
authors.forename: Paul E
authors.initials: PE
authors.lastname: Beaulé
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D000293:Adolescent; D000328:Adult; D001175:Arthrography; D005260:Female; D005270:Femur Head / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D005272:Femur Neck / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D006801:Humans; D007592:Joint Diseases / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D016059:Range of Motion, Articular; D012189:Retrospective Studies; D055815:Young Adult