Background: The shape of the dysplastic acetabulum has been generally determined by 2-dimensional computed tomography (CT) measurements (transaxial CT indices), such as the anterior and posterior acetabular indices and acetabular anteversion. However, their reliability and validity in evaluating the acetabular coverage have not been confirmed.
Methods: This study examined the reliability and validity of the transaxial CT indices using 2- and 3-dimensional CT analyses of the acetabulum before and after a Dega osteotomy. Twenty-two acetabuli from 16 patients with cerebral palsy with hip instability were examined. Interobserver and intraobserver reliability tests were performed, and the preoperative and postoperative measurements were compared for the concurrent and construct validity test.
Results: The transaxial CT indices revealed substantial intraobserver and interobserver reliability. The concurrent validity of the anterior acetabular and posterior acetabular indices before osteotomy was moderate (r = 0.45, 0.49), whereas the construct validity after osteotomy was low when compared with 3-dimensional methods.
Conclusion: The transaxial CT indices are reliable and valid for assessing the acetabular coverage in the preoperative status. However, care should be taken when using these indices to assess the outcome of a pelvic-reshaping osteotomy because of their insufficient construct validity.
Level of evidence: Level III of diagnostic study.