CT Appearance of the retroisthmic cleft.

A 51-year-old woman with no complaint or history of back pain volunteered for CT examination of the lower lumbar spine in connection with a study of normal spinal measurements. Contiguous 5 mm sections at the two lowest lumbar levels were obtained parallel to the intervertebral disks with a GE CTIT 8800 scanner. Two defects were found in the neural arch of L5. On the right side a narrow defect was found in the lamina behind the isthmus (pars interarticularis) (fig . 1). The retroisthmic defect was characterized by regular osseous margins, small spurs of bone near the defect, and sclerosis and thickening of the contiguous neural arch. On the left side was a pars interarticularis defect with no sclerotic reaction. The intervertebral disks, facet joints, and vertebral alignment were normal.