Lateral disk herniation into the lumbar intervertebral foramen: differential diagnosis.

Lateral disk herniation into the intervertebral foramen if not recognized may result in a negative surgical exploration. The condition is readily recognized by computed tomography (CT), which demonstrates a soft-tissue mass in the intervertebral foramen, focal protrusion of the posterolateral margin of the disk into the foramen, absence of dural sac deformity, and occasionally the presence of calcium or gas in the intervertebral foramen. The differential diagnosis includes metastatic neoplasm, primary neoplasm, and conjoined root sheath anomaly. Features that enable CT differentiation of these conditions from lateral disk herniation are described. Myelography is useful in the identification of a conjoined root sheath.