Abstract
Radiodense structures resembling ossicles at the acetabular rim have received multiple names including “Os acetabuli, Os supertilii, Os marginale superius acetabuli, and Os coxae quartum”.
Various theories regarding their origin have been postulated. These structures commonly are observed in dysplastic hips and hips suffering from femoro-acetabular impingement and represent fractures of the acetabular rim. In our series we observed acetabular rim fragments in 4.9% of the dysplastic hips and in 6.4% of the hips with femoro-acetabular impingement.
Two different pathomechanics are responsible for the occurrence of these rim fragments. In dysplasia the short acetabular roof reduces the amount of available loading surface which leads to an overload on the lateral margin of the acetabulum, propagating the development of a fatigue fracture. However, as in all hips additional cysts were visible, it must be postulated, that cysts have to be present additionally and act as stress risers through which the rim bone eventually will fail. In hips with femoro-acetabular impingement the mode of failure is different. The relative anterior overcover in retroverted hips is subjected to stress during flexion of the hip, which is further increased by the frequent presence of an non-spheric extension of the femoral head as seen in cam impingement. The nonspheric femoral head-neck junction is jammed into the rim area. By repetitive traumatization the anterior rim eventually will fracture.
The clinical importance of acetabular rim fractures in the dysplastic hip is readily understood even by an unexperienced observer. However, it has to be considered as a sign that the hip has decompensated and it usually goes with significant articular cartilage damage. Because the radiographic appearance of the hip with femoro-acetabular impingement seems normal at first sight, the mechanism leading to anterior rim fracture may be overlooked. However, recognition and adequate treatment is important to prevent further degeneration of the hip.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
