Incidental discovery of a sacral rib: a case report.

PURPOSE: The observation of an accessory bone joined to the sacrum (suggestive of a sacral rib) prompted us to describe the structure's morphology and its anatomic relationships and to consider its morphogenesis and clinical implications.

METHOD: A 38-year-old woman underwent computed tomography, to characterize the extension of a thoracic mass. We compared our observations with the literature data.

RESULTS: We observed a voluminous accessory bone located behind and to the right of the sacrum. The bone was articulated with the third sacral vertebra and featured a head and three processes. These characteristics were suggestive of a sacral rib. We also observed involution of the gluteus maximus.

CONCLUSION: This accessory bone probably resulted from overdevelopment of a costal process and an absence of fusion with the primitive vertebral body. Sacral ribs are rare and usually asymptomatic but appear to be more prevalent in young women. The adjacent muscles are often abnormal. Awareness of the potential presence of this bone is essential for surgeons who operate on the lumbosacral junction.