Bifid intrathoracic rib: a case report and classification of intrathoracic ribs.

Bifid intrathoracic rib is a very rare anomaly of the ribs that is characterized by an osseous prominence of a rib into the thoracic cavity. We report a 21-year-old woman with bifid intrathoracic rib arising from the anterior-lateral portion of a depressed 4th rib, based on findings from chest radiography and computed tomography (CT). This is only the second reported case of this type of intrathoracic rib worldwide. We discuss differential diagnoses for this case and suggest a classification of intrathoracic rib from the perspective of morphology and developmental biology.