Peter D. Fabricant, MD1; Brandon P. Hirsch, MD2; Ian Holmes, BS1

Bryan T. Kelly, MD1; Dean G. Lorich, MD1; David L. Helfet, MD1

Eric A. Bogner, MD1; Daniel W. Green, MD, MS1

Abstract
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.

Clinical Relevance:  
Prior to closure of the triradiate cartilage (typically at the age of twelve years in girls and fourteen years in boys), magnetic resonance imaging, rather than radiographs and computed tomography, should be used to evaluate a child with hip pain or to measure and characterize acetabular morphology when cross-sectional imaging is indicated. This study characterizes a secondary ossification center along the edge of the posterior wall of the acetabulum that becomes ossified just prior to fusion. This secondary rim ossification center should not be confused with pathologic conditions such as labral avulsion or posterior acetabular wall injury.