Spondylolysis of the axis. Report of four cases.

Spondylolysis or spondylolisthesis of the cervical spine, especially of the upper cervical spine, is very rare. The authors report four cases of spondylolysis of the axis and outline its roentgenographic features for differential diagnosis. The clinical course of the cases and the unvarying roentgenographic findings throughout the course strongly suggest that the lesion is congenital in origin.