As originally described (sometimes called the 90° Dunn view), it is an anteroposterior view of the hip with the patient supine and with the hips and knees flexed at 90°, the legs abducted 15°-20° from the midline, and the femur in neutral rotation. The beam is centered at the midway point between the anterior superior iliac spine and the pubic symphysis, and the tube-to-film distance is ~40 in (102 cm). Imagine a patient lying flat on an examination table with the legs in stirrups.
The modified (45°) Dunn view is the same, except that the hip is flexed to 45°. Imagine a patient lying flat on a table with the knee bent and the foot flat on the table. The paper by Clohisy et al has nice pictures of these views.
The Dunn views (45° or 90°) are best at demonstrating femoral head asphericity. A cross-table view in internal rotation can also be used, but anteroposterior or externally rotated cross-table views are likely to miss asphericity.
Using MRI as the standard, and a cut-off alpha angle of 50.5° for diagnosis of cam-type FAI, the 90° Dunn view was found to be 91% sensitive and 88% specific, with a positive predictive value of 93%, negative predictive value of 84%, and accuracy of 90%. There was also strong correlation between alpha angles on the Dunn view and on MRI (Pearson correlation of 0.7).
- Barton C, Salineros MJ, Rakhra KS, Beaulé PE. Validity of the alpha angle measurement on plain radiographs in the evaluation of cam-type femoroacetabular impingement. Clin Orthop Relat Res. 2011 Feb;469(2):464-9.
- Clohisy JC, Carlisle JC, Beaulé PE, Kim YJ, Trousdale RT, Sierra RJ, Leunig M, Schoenecker PL, Millis MB. A systematic approach to the plain radiographic evaluation of the young adult hip. J Bone Joint Surg Am. 2008 Nov;90 Suppl 4:47-66.
- Dunn DM. Anteversion of the neck of the femur; a method of measurement. J Bone Joint Surg Br. 1952 May;34-B(2):181-6.
- Meyer DC, Beck M, Ellis T, Ganz R, Leunig M. Comparison of six radiographic projections to assess femoral head/neck asphericity. Clin Orthop Relat Res. 2006 Apr;445:181-5.