Kawan S. Rakhra MD, Presenter: Nothing to Disclose

Adnan Mohammad Sheikh MD, Abstract Co-Author: Nothing to Disclose

David Allen MD, Abstract Co-Author: Nothing to Disclose

Paul E. Beaule MD, Abstract Co-Author: Consultant, Wright Medical Group, Inc, Memphis, TN Consultant, GETINGE AB, Ontario, Canada

The aim of this study was to compare the alpha angle measurement using the oblique axial plane with the maximal alpha value obtained using radial plane images in patients being investigated for femoroacetabular impingement(FAI).

A retrospective review was carried out in 41 subjects with clinically suspected FAI who had undergone magnetic resonance imaging(MRI).  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 were compared.  A clock face nomenclature was adopted for localization around the femoral head-neck junction with the anterior and superior locations designated 3 o’clock and 12 o’clock, respectively.  The alpha angle was measured on the oblique axial plane image, and compared to the alpha angles measured on radial images at the 12, 1, 2, and 3 o'clock vectors. 

The study included 41 hips of 18 male and 23 females(age range 17-60 years, mean 39 years).  The mean alpha angle value using the oblique axial plane was was 53.4°. The mean maximal alpha angle measurement using the radial images was 70.5°.  The mean difference between the two measurement planes was 17.1° (P<0.001).  Using the radial images, the mean alpha angle values were highest at the radial 1 and 2 o’clock vectors which had mean values of 64.7° and 65.9°, respectively. The differences between the oblique axial and the radial 1 and 2 o’clock values were statistically significant (both P<0.001).  The vast majority(37/41, 90%) of the maximal alpha angle values occurred in either the 1 or 2 o’clock positions.  In a significant proportion(22/41, 54%) of hips the alpha angle was <55° on the conventional oblique axial plane image but ≥55° on the radial images.

Radial imaging yields significantly higher alpha angle values than conventional oblique axial imaging. It interrogates the femoral head-neck junction circumferentially, thus being able to evaluate the entire anterosuperior quadrant.  In contrast, the oblique axial plane can only optimally asses the anterior aspect of the head-neck junction.  A large proportion of patients with clinically suspected FAI may have a significant osteochondral bump underestimated or missed if only oblique axial plane images are used.

Radial plane imaging should be standard in the MRI investigation of FAI.

Rakhra, K, Sheikh, A, Allen, D, Beaule, P, Comparison of MRI Oblique Axial and Radial Plane Alpha Angle Measurements in Femoroacetabular Impingement.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6011161.html