It is well understood that loss of motion following spinal fusion increases strain in the adjacent motion segments. However, it is unclear if to date, studies on cervical spine biomechanics can be affected by the role of coupled motions in the lumbar spine. Accordingly, we investigated the biomechanics of the cervical spine following cervical fusion and lumbar fusion during simulated whiplash.

A validated whole-human finite element model was used to investigate whiplash injury. The cervical spine before and after spinal fusion was subjected to simulated whiplash exposure in accordance with Euro NCAP testing guidelines, and the strains in the anterior longitudinal ligaments of the adjacent motion segments were computed.

In the models of cervical arthrodesis, peak ALL strains were higher in the motion segments adjacent to the level of fusion, and strains directly increased with longer fusions. The mean strain increase in the motion segment immediately adjacent to the site of fusion from C2-C3 through C5-C6 was 26.1% and 50.8% following single- and two-level cervical fusion (p=0.03). On average, peak strains experienced in a lumbar-fused spine were 1.0% less than those seen in a healthy spine (p=0.61). The C3-C4 motion segment had disproportionately high increases in strain following cervical fusion. The C6-C7 motion segment experienced high absolute strain under all tested conditions but the increase in strain following fusion was very small. This study provides support for both the hypothesis that adjacent segment disease is associated with post-arthrodesis biomechanical influences and the hypothesis that adjacent segment disease is a result of natural history, and inherent structures at risk.