The Pelvic Digit

Bony protuberance at the left anterior inferior iliac spine

A pelvic digit is a rare congenital anomaly in which bone develops in soft tissues adjacent to normal skeletal structures. On radiographs 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. It has been previously reported as an iliac rib or pelvic rib.

It is important to distinguish a pelvic digit from acquired anomalies such as myositis ossificans and avulsion injuries of the pelvis.

This anatomic variant can originate from a displaced costal process, a displaced sternal center, or the ossification center at the anterior superior iliac spine. These explanations, however, do not take into account the varied sites of attachment in the pelvis. As a pelvic digit can be found at the coccyx, the pelvic walls, and the inferior abdominal wall, it must arise from an embryonic mesoderm with rib-forming capacity that is disposed to these regions.

At the end of the third week of embryogenesis, embryonic mesoderm cells, with the potential to form ribs, migrate from the primitive streak and pass around the cloacal membrane, from the region of the future coccyx, through the region of the future pelvic walls, to the region of the lower abdominal wall.

References

• Casey MC, Phancao JP, Pressacco J. Answer to case of the month #106. Pelvic Digit. Can Assoc Radiol J. 2006 Feb;57(1):51-3.
• Rolando Reyna. MyPacs.net