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Interrater and intrarater reliability of the alpha angle of the hip: is the alpha angle reliable?

Barlow, Briana; Reem, Jenniferb; Stanley, Markc; Craven, Charlesa; Bruneau, Pierrea

doi: 10.1097/BCO.0000000000000123
Diagnostic Imaging

Background: The purpose of the study was to determine the inter- and intra-rater reliability of the alpha angle measured on hip axial and axial oblique MRI, which has not been well defined in the existing orthopaedic literature.

Methods: Fifty MRIs were reviewed by one musculoskeletal radiologist and two orthopaedic hip surgeons. The examiners evaluated the alpha angle measurements in two rounds. In each round, the three examiners independently reviewed all MRIs in random order to establish intrarater and interrater reliability. Correlation coefficients were calculated between pairs of alpha angle readings.

Results: The pooled intrarater reliability when testing all 50 MRIs was 0.78 indicating strong agreement. The pooled intrarater reliability for only axial oblique images was 0.85, indicating almost perfect agreement. The interclass correlation coefficients for all 50 MRIs was 0.63–0.75, indicating strong agreement. The pooled interrater reliability using only the axial oblique images was 0.84, demonstrating near perfect agreement.

Conclusions: The alpha angle is a reliable measure when calculated on axial oblique MRI. Our study indicates that almost perfect agreement exists when measuring the alpha angle on axial oblique MRI.

aOrthopaedic Surgery Department, Naval Medical Center, San Diego CA

bOrthopaedic Surgery Department, Naval Hospital, Oak Harbor WA

cRadiology Department, Naval Medical Center, San Diego CA

Financial disclosure: The authors report no financial conflicts of interest.

Correspondence to Brian Barlow, MD, Orthopaedic Surgery Department, Naval Medical Center San Diego, 34800 Bob Wilson Drive, San Diego CA 92134 Tel: 619-453-6519; fax: 619-532-8567; e-mail:

© 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins