Os acetabuli in femoro-acetabular impingement: stress fracture or unfused secondary ossification centre of the acetabular rim?

Ossicles located at the acetabular rim are generally referred to as unfused secondary ossification centres and are named "os acetabuli". They are also observed in severely dysplastic hips, where they are considered as fatigue fractures of the acetabular rim due to overload. In a retrospective study, we evaluated the radiographs of 495 patients, who were treated surgically for femoro-acetabular impingement. In 18 hips (15 patients) a large osseous fragment at the anterolateral acetabular rim was found. All patients presented radiographically with a femoral head showing an aspherical extension producing a "cam" impingement. Sixteen hips had a retroverted acetabulum, indicating anterior overcover. Preoperative MRIs available in 12 patients showed the presence of a fragment composed of labrum, articular cartilage and bone. The gap between the stable acetabulum and the rim fragment had a vertical orientation. All patients had been exposed to a physically demanding profession or contact sport and in 15 hips no memorable traumatic episode was present. The mechanism leading to this acetabular rim fragment is thought to be fatiguing due to femoro-acetabular impingement. The aspheric portion of the head is jammed into the acetabulum and with time causes a stress fracture of the retroverted portion of the acetabulum. True "Os acetabuli" are morphologically similar, but the orientation of the cartilaginous growth plate is more parallel to the joint surface, in contrast to the hips in the current study, where the separation line was perpendicular to the joint surface. The clinical importance is that the presence of an acetabular rim fragment in the non-dysplastic hip must raise suspicion of femoro-acetabular impingement.;