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Stress Fracture of the Acetabular Rim: Arthroscopic Reduction and Internal FixationA Case Report
Noah J. Epstein, MD1; Marc R. Safran, MD1
1 Department of Orthopaedic Surgery, Stanford University, 450 Broadway Street, M/C 6342, Redwood City, CA 94063
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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Department of Orthopaedic Surgery, Stanford University, Stanford, California

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2009 Jun 01;91(6):1480-1486. doi: 10.2106/JBJS.H.01499
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An anterolateral acetabular osseous fragment is often referred to as an "os acetabuli," and it may represent a secondary ossification center within the triradiate cartilage1. These fragments have been observed in dysplastic hips, in association with osteomyelitis and tuberculosis2, and following trauma3. The term acetabular rim syndrome has been used to describe fatigue fractures of the anterolateral acetabular rim in dysplastic hips2. These fragments are thought, in part, to relate to the abnormal shear stress that occurs in hips with type-II dysplasia2. Recently, in a retrospective review of 495 patients with femoroacetabular impingement, a large osseous fragment was found at the anterolateral aspect of the acetabular rim in eighteen hips in patients with cam-type impingement4. These patients were highly active and presented with an insidious onset of hip pain in the absence of specific trauma. Unlike the small labral ossicles that are typically attributed to fractured acetabular osteophytes, these fragments were shown to consist of labrum, articular cartilage, and bone on magnetic resonance imaging and histologic evaluation and were anatomically distinct from unfused secondary ossification centers4. The authors suggested that such fragments may represent stress fractures of the acetabular rim due to abnormal shear stress caused by cam impingement. In addition, these patients were found to have increased anterolateral acetabular coverage of the femoral head and relatively retroverted acetabuli. The authors proposed that an aspheric femoral head with acetabular retroversion, as seen with combined cam and pincer types of femoroacetabular impingement, should be considered a risk factor for the development of stress fractures of the acetabular rim in active patients. The patients in this series were managed with open surgical dislocation of the hip. Excision of the fragment was performed in twelve of the hips, and osteochondroplasty was performed in the remaining six hips. To our knowledge, there are no other published reports of such fragments in the setting of femoroacetabular impingement.
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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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