The injured canine cervical spine after six months of healing. An in vitro three-dimensional study

The cervical spine is a common site of spinal injuries. The stability of an injured cervical spine is not only dependent on injury severity, but also on the degree of healing time. Using a canine model, three injuries of varying degrees of severity were surgically produced at the C4-C5 level and allowed to heal for 6 months. No internal or external support was provided. The harvested cervical spines (C2-C7) were subjected to three-dimensional biomechanical testing by applying individually six pure moments. The resulting three-dimensional displacements were recorded using stereophotogrammetry, and the intervertebral motions were calculated. The results are compared with the in vivo behavior of the same specimens and with an in vitro control group. At 1 N-m, the average flexion-extension range of motion (ROM) for the intact C4-C5 level was 24.5 degrees (standard deviation [SD], 6.6 degrees). A facetectomy at this level significantly increased the in vitro ROM to 51.1 degrees (SD, 4.4 degrees). The in vitro ROM decreased to 19.8 degrees (SD, 7.3 degrees) in the facetectomized group of this study, which were allowed to heal for 6 months before death. Similar results were obtained in axial rotation and lateral bending. The findings show that after 6 months of healing, the injured canine spine, although acutely hypermobile, exhibited biomechanical characteristics that were not different from those of the normal intact specimens.