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Comparison of MRI Alpha Angle Measurement Planes in Femoroacetabular Impingement

  • Symposium: Femoroacetabular Impingement: Current Status of Diagnosis and Treatment
  • Published:
Clinical Orthopaedics and Related Research

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° and 70.5°, respectively. In 54% of subjects, the alpha angle was less than 55° on the conventional oblique axial plane image but 55° 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.

Level of Evidence: Level III, diagnostic study. See the Guidelines for Authors for a complete description of levels of evidence.

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Acknowledgments

We thank Anna Fazekas and Simon Dagenais for assistance in study design and statistical analysis and Natasha La Russa for postprocessing reformations of the MRI studies.

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Authors and Affiliations

  1. Department of Diagnostic Imaging, The Ottawa Hospital—General Campus, University of Ottawa, 501 Smyth Road, Ottawa, ON, Canada, K1H 8L6

    Kawan S. Rakhra MD, FRCPC & Adnan M. Sheikh MD

  2. Division of Orthopaedic Surgery, The Ottawa Hospital, University of Ottawa, Ottawa, ON, Canada

    David Allen MD, FRCSC & Paul E. Beaulé MD, FRCSC

Authors
  1. Kawan S. Rakhra MD, FRCPC
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  2. Adnan M. Sheikh MD
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  3. David Allen MD, FRCSC
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  4. Paul E. Beaulé MD, FRCSC
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Corresponding author

Correspondence to Kawan S. Rakhra MD, FRCPC.

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Each author certifies that he or she has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

Each author certifies that his or her institution has approved or waived approval for the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

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Rakhra, K.S., Sheikh, A.M., Allen, D. et al. Comparison of MRI Alpha Angle Measurement Planes in Femoroacetabular Impingement. Clin Orthop Relat Res 467, 660–665 (2009). https://doi.org/10.1007/s11999-008-0627-3

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  • DOI: https://doi.org/10.1007/s11999-008-0627-3

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