A rare anomaly of the axis: report of a case with shaded three-dimensional computed tomographic display.

A rare anomaly of the C-2 vertebra (axis) was found in a 54-year-old man with myelopathy. In addition to spina bifida of C-2 and abnormal articulations between C-2 and C-3, the lamina of the axis was separated from the rest of the posterior arch on each side, and the free dysplastic laminae and spinous process invaginated deeply into the spinal canal. The condition differed from the infrequent but previously described cervical spondylolysis in that the pars interarticularis and the pedicle were not dehiscent. Shaded three-dimensional computed tomographic display proved most useful in demonstrating this complicated osseous anomaly.