Pelvic rib

Clinical History

A 45-year-old patient reported as a follow up for a motor vehicle accident with history of vague right flank pain. A pelvic radiographwas performed which revealed a large bony protuberance arising from the right sacral region.

Imaging Findings

A 45-year-old patient reported as a follow up for a motor vehicle accident with history of vague right flank pain. His clinical examination was unremarkable except for deep tenderness in the right flank. A radiograph of pelvis was advised (as shown in the fig) which showed a large bony protuberance arising from the right sacral region. This appearance is consistent with a pelvic rib, which is a rare normal variant and usually asymptomatic. Surgical intervention was offered to the patient. However, due to the limited nature of the patient’s complaints, he did not opt for intervention. For this reason, a histopathologic exam could not be performed.

Discussion

In the spinal column, many anomalies related to formation or segmentation defects have been reported (1, 2). Anomalies that have a cortex and develop in the soft tissues around the vertebral column are very rare, and radiologically they look like ribs or digits (3). When seen within the pelvis or abdomen these anomalies are known as pelvic ribs or pelvic digits (4, 5, 6) Pelvic rib is a rare congenital anomaly in which bone develops in soft tissue adjacent to normal skeletal bone (5). On plain radiograph it typically appears as a rib- or a phalanx-like bone with a clear cortex and medulla related to the pelvis, often with a characteristic pseudoarticulation at the base (3). Clinically, cervical supernumerary ribs may cause neurovascular symptoms or thoracic outlet compression syndrome whereas pelvic ribs are usually asymptomatic, and, in most cases, found incidentally during the examinations for other problems (7, 8). In those rare cases in which sacral ribs cause symptoms, there is typically discomfort and diminished mobility during movement of the ipsilateral hip, and, possibly can cause compromise of the birth canal in female (9). Standard radiographs and computed tomography provide the best diagnostic information. Its differential diagnosis includes myositis ossificans, avulsion injuries, heterotopic bone formation, Fong’s disease, and osteochondroma. Pelvic digit can usually be radiologically differentiated from posttraumatic myositis ossificans and from heterotopic bone formation by its well corticated appearance and the absence of history of trauma (4, 10). CT of pelvic digit confirms the presence of cortical bone (11) and is useful in equivocal cases (12.). Myositis ossificans is characterized by a radiolucent core with a calcified periphery, which is clearly separated from adjacent bones (11). Avulsion injuries of the pelvis commonly occur during athletic activity, with peak incidence in adolescents and teenagers (13). Pain, diminished motion, and soft tissue haematoma correlate with a bony fragment (13). In some cases, new bone formation after surgery or ossification of the sacrotuberous ligament can resemble a pelvic digit (5). Fong’s disease is characterized by bilateral iliac horns arising posteriorly and centrally from the ilia (11). Osteochondroma is continuous with underlying bone (5) and a cartilaginous cap may be present (12). Pelvic digit is usually an asymptomatic, benign condition and is discovered incidentally (3). In the absence of symptoms, surgical excision is not required (10). Pelvic ribs are suggested as a rare cause of fetopelvic disproportion, and a case of scoliosis and hypoplasia of the ipsilateral gluteal musculature arising from pelvic rib has also been reported (7, 14). In conclusion, it is important to recognize and distinguish the pelvic rib from posttraumatic ossification and avulsion injuries and, thus, avoid unnecessary additional investigations. The radiographic entity of pelvic rib/ digit should be known by every radiologist as an incidental finding for which no further action is required.

Differential Diagnosis List

Final Diagnosis

The Pelvis Rib

URL:	https://www.eurorad.org/case/6344
DOI:	10.1594/EURORAD/CASE.6344
ISSN:	1563-4086