MRI assessment of the importance of the acetabular labrum: a comparative radiographic and MRI study in children's hips.

The aim of this study was to investigate the correlation between radiograph [acetabular index (AI)] and MRI measurements of the angles of children's hips and the percentage of coverage that labrum provides to the acetabulum in healthy hips of children. The healthy hips of 38 children, of a mean age of 7.7 years, were studied using anteroposterior radiographs and coronal plane MRI. We used picture archiving and communication systems software to measure the AI in hip radiographs and the acetabular bone index (ABI) and acetabular labrum index (ALI) in hip MRIs. The Kolmogorov-Smirnov test and t-test were performed. Pearson's correlation and Bland and Altman plots were determined for analysis of measurement error and interobserver and intraobserver errors. Statistical significance was set at P value less than 0.05. Interobserver and intraobserver agreement was between 0.8 and 0.98. AI was 13.7°. ABI was 16.4° and ALI was 6.7°. There was a correlation between the angles of the hips determined by radiographs and MRI. The ALI accounted for 40.8% of the mean value of the ABI. The measurements of hip angles by radiographs were similar to the ones derived from MRI. In addition, the labrum adds stability to the healthy hip in a child, indicating a significant portion of the total coverage of the acetabulum to the femoral epiphysis. AI measurements from radiographs were lower than ABI measurements obtained from MRI. ALI was 41% of the ABI, which means that the labrums, in healthy hips of children significantly increase the coverage of the femoral epiphysis and also increase the stability of the acetabulum.

LEVEL OF EVIDENCE: III.