Analysis of Acetabular Ossification From the Triradiate Cartilage and Secondary Centers

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Acetabular development is a complex process that involves both endochondral growth from the triradiate cartilage (TRC) and intramembranous growth from the primary and secondary ossification centers of the innominate bones. Ponseti and others have described these centers including their contribution toward the development of normal acetabular shape. Prior studies have not utilized advanced imaging to study the appearance and closure of these secondary centers. The purpose of this study was to determine the chronological age at which the secondary ossification centers of the acetabulum appear and close and where there are any sex differences.


Patients who underwent abdominal and pelvic computed tomography (CT) scans between January 2009 and December 2014 at a pediatric hospital were retrospectively reviewed. Patients between age 6 and 16 years with adequate imaging of acetabulum were included. CT scans were assessed for the appearance and closure of the 3 acetabular secondary ossification centers [anterior (os pubis), superior (os ilium), and posterior (os ischium)] and closure of the TRC.


A total of 159 CT scans met inclusion criteria (66 males and 93 females). The median age of appearance of the secondary ossification centers was: posterior (10.1 females, 12.8 males), anterior (10.7 females, 13.4 males), and superior (11.1 females, 13.6 males). The median age of closure of the secondary ossification centers was: posterior (12.8 females, 13.6 males), anterior (12.8 females, 13.9 males), superior (14.5 females, 13.9 males), and TRC (14.5 females, 14.3 males). Most ossification centers in females appeared and closed approximately 2 to 3 years before males.


Secondary ossification centers in the acetabulum appear sequentially (first posterior, then anterior, then superior), with almost all centers closing just before TRC. Closure occurs earlier in females than males. Knowledge of these centers and their closure patterns allows better radiologic readings (especially CT studies) and understanding of acetabular growth, allowing more informed management of childhood hip conditions including dysplasia, trauma, and over-use sports injuries.

Level of Evidence:

Level III—Diagnostic.

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