A radiographic study of the ossification of the posterior wall of the acetabulum: implications for the diagnosis of pediatric and adolescent hip disorders.

BACKGROUND: Subtle variations in acetabular morphology have been implicated in several pathologic hip conditions. Although it is understood that the acetabulum forms at the junction of the ilium, ischium, and pubis at the triradiate cartilage, the ossification and development pattern of the posterior wall of the acetabulum is unknown. Standard radiographs and computed tomographic scans used in evaluation of the adolescent hip do not allow a complete assessment of the non-ossified portions of the developing acetabulum. The purpose of this study was to define the currently unknown ossification pattern and development of the posterior wall of the acetabulum and to determine when conventional imaging, with use of computed tomography and radiographs, is appropriate.
METHODS: One hundred and eighty magnetic resonance imaging examinations in patients who were four to fifteen years old were evaluated by a musculoskeletal radiologist for ossification patterns of the posterior wall of the acetabulum and triradiate cartilage. Correlations were made with available radiographs.
RESULTS: Posterior acetabular wall ossification lags behind anterior wall ossification throughout development. On average, the posterior wall of the acetabulum began to ossify at the chronological age of eight years, followed by a discrete rim of posterior calcification (posterior rim sign) at the patient age of twelve years, just prior to the fusion of the posterior acetabular wall elements to the pelvis. This preceded the closure of the triradiate cartilage in all subjects. On average, male patients had fusion of the posterior wall of the acetabulum one to 1.5 years after female patients.
CONCLUSIONS: The ossification of the posterior wall of the acetabulum is completed in a predictable manner prior to closure of the triradiate cartilage.