Experimental study on pressure response to graded spinal canal compromise in an in vitro burst fracture mode

Abstract

Background: Spinal cord compression is a known cause of spinal cord injury. The purpose of this study is to measure pressure response during graded spinal cord compression. This information will be important in evaluating the amount of canal compromise that can be tolerated before risking neurological injury secondary to cord compression. To date, there is no published study that has evaluated pressure response to graded canal compromise in the thoracic and lumbar spine.

Materials and methods: A comparative biomechanical investigation using an in vitro burst fracture model of graded spinal canal compromise was performed. Four porcine spines, sectioned into four thoracics and four lumbar segments, were harvested from 30 kg pigs. Graded spinal canal compromise (0.75 mm/30 s) was achieved using a modified 12.7 mm dynamic hip screw. The real-time ventral epidural pressure was measured at each 0.75 mm of canal compromise.

Results: A significant increase in spinal cord pressure was recorded during graded spinal cord compression (P < 0.0001), and there were no statistical differences between the increase in pressure measured in the thoracic and lumbar spinal segments (P = 0.83). The pressure to degree of canal compromise curve exhibited an initial rapid rise in pressure followed by incrementally smaller increases in pressure as canal compromise increased.

Conclusions: Spinal cord pressure increased with any degree of canal compromise, the most important rise occurring with initial compression. Future studies will evaluate the usefulness of laminectomy in vivo to completely restore ventral epidural pressure in the thoracic and lumbar spine.

Keywords: Graded spinal cord pressure; lumbar spine; pig model; spinal canal compromise; thoracic spine.

Figure 1

Example of the experimental setup. Each spinal segment underwent graded spinal canal compromise with the use of a modified 12.7 mm dynamic hip screw. Part of the vertebral body has been removed for demonstrative purpose

Figure 2

Example of a lateral fluoroscopic/computed tomography image of a spinal segment when the dynamic hip screw reached approximately 70% of spinal cord compression. (1) An example measurement of the dynamic hip screw width, (2) width of the noncompressed spinal cord, and (3) width of the compressed spinal cord. Two lateral computed tomography images were taken during each experiment: the first at timestamp 0 (opening pressure), and a final one following the last compression. Pre- and post-images were used to correlate pressure response to graded degrees of canal compromise

Figure 3

The pressure curves. (a) Ventral epidural pressures recorded after each advancement of the screw; X-axis represents total distance traveled by the screw, Y-axis represents ventral epidural pressure. There were no significant differences between the pressures recorded in the thoracic (black squares) or lumbar (gray circles) spinal segments. (b) Distance traveled by the screw was converted to percent canal compromise (X-axis) and used for nonlinear fit, producing a single-phase curve that demonstrated an immediate rise in pressure with any amount of canal compromise. Gray shaded area represents the 95% confidence interval produced during the nonlinear regression (black, curved line). Our measures are plotted within this 95% confidence interval; thoracic (open circles), lumbar (black triangles)