Wednesday, February 8, 2012

The Dunn View

The Dunn view, originally described in 1952 for measuring the anterversion of the femoral neck in children, is now commonly used for assessment of femoral head sphericity in young adults suspected of having cam-type femoroacetabular impingement (FAI).

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).

References

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