Typical Development of the Secondary Ossification Centers of the Acetabulum and Their Effects on Acetabular Coverage of the Femoral Head

Background: We investigated the normal development of the secondary ossification centers of the acetabulum, focusing on their location and the amount of acetabular coverage increased by them. Methods: We enrolled 132 patients who were 7 to 16 years of age and had no pelvic deformity but did have ≥1 os ischium, os ilium, and/or os pubis on abdominal or pelvic computed tomographic (CT) scans. The locations of the ossification centers were evaluated by adopting an orientation using 0° for the superior acetabulum, 90° for the anterior acetabulum, 180° for the inferior acetabulum, and 270° for the posterior acetabulum, on a reconstructed 3-dimensional (3D) CT image. The acetabular coverage increase by the os ischium, os ilium, or os pubis was defined as the difference in the posterior acetabular sector angle (ΔPASA), posterosuperior acetabular sector angle (ΔPSASA), superior acetabular sector angle (ΔSASA), anterosuperior acetabular sector angle (ΔASASA), or anterior acetabular sector angle (ΔAASA) measured with and without each secondary ossification center. Patients were grouped into 3 age ranges: late childhood, preadolescence, and early adolescence. The location of each ossification center and the increase in acetabular coverage were compared between these groups. Results: In the late-childhood group, the median start-to-end positions in right hips were 269° to 316° for the os ischium, 345° to 356° for the os ilium, and 81° to 99° for the os pubis. These positions tended to be wider in the early-adolescence group at 252° to 328° for the os ischium (p < 0.001), 338° to 39° for the os ilium (p = 0.005), and 73° to 107° for the os pubis (p = 0.049) in right hips. In right hips in the late-childhood group, the median values were 8.1° for ΔPASA, 14.0° for ΔPSASA, 9.9° for ΔSASA, 11.1° for ΔASASA, and 3.9° for ΔAASA; and in the early-adolescence group, the median values in right hips were 10.7° for ΔPASA, 12.9° for ΔPSASA, 8.4° for ΔSASA, 7.4° for ΔASASA, and 5.6° for ΔAASA. Only the median ΔPASA was larger in the early-adolescence group than in the late-childhood group (p = 0.026). Similar results were observed in left hips. Conclusions: In early adolescence, the secondary ossification centers appeared at more extended areas along the acetabular rim, and the increase in acetabular coverage by the secondary ossification centers tended to be larger in the posterior area but not in the anterior or superior area. Level of Evidence: Diagnostic Level III . See Instructions for Authors for a complete description of levels of evidence.