Pathoanatomy of Acetabular Bicompartmentalization in Children with Avascular Necrosis of the Femoral Head
TJ Cho, IH Choi, CY Chung, WJ Yoo
Seoul National University Children's Hospital, Seoul, Korea

In severe Legg-Calve-Perthes disease with subluxated femoral head, the acetabulum sometimes takes bicompartmental appearance. Using a 3-D CT software program that affords the section of 2-D image in any wanted plane, we analyzed acetabular pathoanatomy, with special reference to the morphology of the inner surface of the acetabulum, in thirteen children with the bicompartmental acetabulum (12 LCPD and 1 AVN subsequent to septic hip arthritis). The anterior half of the acetabulum was concentric. However, the contour of the acetabular margin in the posterior half of the acetabulum consisted of two different arcs - an arc of the iliac acetabulum (superior) and the other arc of the acetabular fossa of the ischium (medial). The junction of these two arcs was located at the triradiate cartilage, which was increased in medio-lateral thickness at this point. The osteochondral articular margin of the ischium posterior to the non-articular acetabular fossa was thickened forming a ridge. The medio-lateral thickness of the non-articular acetabular fossa was thinner than that of the normal contralateral side. The combination of these focal morphologic changes of the acetabular fossa rendered the bicompartmental appearance on plain AP radiograph of the pelvis. It is concluded that bicompartmentalization of the acetabulum apparently reflects the altered biomechanics of the hip joint due to the subluxated femoral head, and the abnormal osteocartilaginous hypertrophy seems to be caused by synovial irritation and eccentric molding effect of the subluxated femoral head.