Imaging description

At birth, the primary ossification centers of the ilium, pubis, and ischium converge on the triradiate cartilage at the hip (Figure 97.1). On a poorly positioned radiograph (Figure 97.2), the ischium may appear to be displaced medially relative to the ilium, which should not be mistaken for a fracture through the triradiate cartilage. The triradiate cartilage gradually thins (Figure 97.3), and the roof of the acetabulum may appear irregular in children 7–12 years of age. Particularly when viewed on axial CT images, this should not be mistaken for comminuted acetabular fracture (Figure 97.4). The bony acetabulum fuses around 11 to 14 years of age, achieving its adult appearance slightly earlier in girls than boys [1].

Around puberty, three secondary ossification centers develop around the acetabulum, including the os acetabuli (epiphysis of os pubis along the anterior wall of the acetabulum), epiphysis of the ilium (forms the superior wall of the acetabulum), and a small epiphysis of the ischium (Figures 97.5 and 97.6) [2]. These contribute to the depth of the acetabulum but may be confused with avulsion injuries (Figure 97.7). The os acetabuli may persist into adulthood as a separate, well-corticated ossicle (Figure 97.8).

The body and alae of the sacrum develop from several separate primary ossification centers (Figure 97.1), which typically fuse between one and seven years of age [3]. Cartilage bordering the articular surfaces of the sacroiliac joints in young children makes them appear wider on radiographs than would be normal for an adult (Figure 97.1). Small triangular secondary ossification centers appear around puberty along the anterior sacroiliac joint spaces at the levels of S1 and S3 (Figure 97.6) [4]. These begin to fuse to the lateral os sacrum around 18 years of age.