Reliability of acetabular measures in developmental dysplasia of the hip

The purpose of this study was to determine the reliability in the measurement of the acetabular index and the acetabular angle in children with developmental dysplasia of the hip. Seventeen children with unilateral developmental dislocation of the hip treated by closed reduction were reviewed. The acetabular index and the acetabular angle of 34 hips were measured twice by two observers. The method of Bland and Altman as outlined by Loder was used to calculate reliability. Mean age at reduction was 9 months. Radiographs were reviewed at a mean of 58 months following reduction. The intraobserver reliability of the acetabular index in involved hips was +/-4.1 degrees. The intraobserver reliability of the acetabular angle for involved hips was +/-3.6 degrees. The interobserver reliability of the acetabular index in involved hips was +/-13.7 degrees. The interobserver reliability of the acetabular angle for involved hips was +/-7.8 degrees. To ensure true change, a single observer should document at least an 8 degrees change in the acetabular index or a 7 degrees change in acetabular angle between two radiographs.