Atypical hangman's fractures.

Nineteen cases of traumatic spondylolisthesis of the axis, including 13 standard hangman's fractures and 6 anterolistheses of the C2 vertebral body associated with Effendi's atypical hangman's fractures, were studied. Unlike the standard Effendi Type I or Type II fractures, atypical hangman's fractures, occurring through the posterior aspect of the vertebral body with unilateral or bilateral continuity of the posterior cortex or pedicle, routinely narrow the spinal canal because of the fracture pattern and degree of subluxation. These atypical C2 injuries were more frequent and more often accompanied by paralysis (33%) than was previously anticipated. Because of their greater potential for neurologic compromise, it is essential that these fractures be recognized as distinct from standard Types I and II C2 fractures. Larger series should clarify both the true incidence of paralysis and long-term results from these unique C2 injuries.