Developmental dysplasia of the hip (DDH) affects approximately 1 % of live births. Dislocated hips require reduction and stabilisation in a spica cast, and reduction efficacy is assessed radiologically. Numerous measurements are used to ascertain the adequacy of reduction but can be inconsistent in evaluating femoral head position. This study describes the morphology of the developing acetabulum in DDH and validates a novel method to assess adequate reduction of the dysplastic hip following closed or open reduction.

A retrospective review was performed of 66 consecutive patients undergoing reduction of hip dislocation over a 2-year period. Three independent reviewers evaluated postoperative CT scans to assess anterior-posterior (AP) displacement and modified Shenton's line. Acetabular morphology was also assessed along with hip congruency using a described novel 'posterior neck line'.

Dislocated hips were successfully identified using the posterior neck line with a sensitivity of 0.71 and specificity of 0.88 giving a negative predictive value of 0.97. The interobserver reliability of this technique was higher in comparison against both (AP) displacement and modified Shenton's line.

We have shown a novel approach in assessing the acetabular morphology of DDH and a novel technique to accurately confirm the reduction of dislocated hips following open or closed reduction.