Comparison of MRI, CT, Dunn 45° and Dunn 90° alpha angle measurements in femoroacetabular impingement



To determine if a significant difference existed among alpha angle measurements between 4 imaging techniques, axial oblique CT and MRI, Dunn 45° and Dunn 90° plain radiographs, in patients with symptomatic cam femoroacetabular impingement (FAI) and labral tear.


A single-surgeon prospective radiographic analysis of consecutive non-arthritic and non-dysplastic patients with symptomatic FAI and labral tears who underwent surgery was performed. Alpha angle was measured using standard techniques as described by Nötzli. Cam morphology was defined via alpha angle measurement of >50.5 degrees. Group comparisons were made using ANOVA and chi-squared test. Sample size calculation was performed prior to study enrollment.


31 subjects (16 female; 33.5 ± 10.5 years mean age) were included. There was a significant difference in alpha angle measurements between all 4 imaging techniques (F [3,120] = 8.144; p<0.001), with the Dunn 45° view (66.3 ± 11.4°) significantly greater than all 3 other techniques (Dunn 90° [57.5 ± 10.7°; p = 0.015], MRI [53.3 ± 11.5°; p<0.001], and CT (54.9 ± 11.6°; p = 0.001). There was no significant difference in alpha angle between Dunn 90°, MRI, and CT. There was a significant difference in the observed number of hips with cam morphology between imaging techniques (χ2 9.4; p = 0.025).


The Dunn 45° radiograph yielded a significantly higher alpha angle than Dunn 90°, axial oblique MRI, and CT imaging modalities. Use of the Dunn 90° or axial oblique MRI or CT as the threshold for cam osteoplasty may result in untreated symptomatic cam FAI. The authors recommend the Dunn 45° radiograph as the most sensitive evaluation of cam morphology.

Post author correction


Article Subject: Arthroscopy



Kevin M. Smith, Brayden J. Gerrie, Patrick C. McCulloch, David M. Lintner, Joshua D. Harris

Article History


Financial support: None.
Conflict of interest: None.

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  • Houston Methodist Orthopaedics and Sports Medicine, Houston Methodist Hospital, Houston, Texas - USA

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