Traumatic instability in the previously fused cervical spine.

The effect of fused segments in the cervical spine has been documented to cause chronic changes in adjacent levels. This article reports an association between the presence of fused cervical segments and the predisposition to acute, traumatic instability at adjacent levels. Patients with cervical fractures were reviewed during a 12-year period. Fifteen patients were identified who sustained cervical fractures in the presence of previously fused segments. The presence of fusion was reviewed for its effect of neurologic injury, delay in diagnosis, and patterns of fractures. The diagnostic studies used to document instability were reviewed. We found that preexisting cervical fusions often result in a delay of diagnosis because of altered anatomy and atypical fracture patterns. The fractures occurred within one or two levels from the fused segment. There are different fracture patterns associated with fusions in the upper cervical spine and those fusions in the lower cervical spine. The presence of fusions significantly affected treatment choices in this group of patients.