40. CERVICAL SPINAL MOTION THAT WOULD OTHERWISE BE SAFE, CAN CAUSE SPINAL CORD COMPRESSION IN A STENOTIC SPINE

Abstract

Purpose: The average age of people suffering spinal cord injuries in many countries is shifting toward an older population, with a disproportionate number occurring in the spondylotic cervical spine. These injuries are typically due to low energy impacts, such as a fall from standing height. Since a stenotic spinal canal (a common feature of a spondylotic cervical spine) can cause myelopathy when the spine is flexed or extended, traumatic flexion or extension likely causes the injury during the low energy impact. However, this injury mechanism has not been observed experimentally.

Method: To better understand this injury mechanism an in-vitro study, using six whole cervical porcine spines, was conducted. The following techniques were combined to directly observe spinal cord compression in a stenotic spine during physiologic and super-physiologic motion:

• A radio-opaque surrogate cord, with material properties matched to in-vivo specimens, replaced the real spinal cord.

• Sagittal plane X-rays imaged the surrogate cord in the spine during testing.

• Varying levels of canal stenosis were simulated by a M8 machine cap screw that entered the canal from the anterior by drilling through the C5 vertebral body.

• Pure moment loading and a compressive follower load were used to replicate physiologic and super-physiologic motion.

Results: Initial results show that a stenotic occlusion that removes all extra space in the canal in the neutral posture, without compressing the cord, can lead to spinal cord compression within physiologic ranges of flexion and extension. The spinal cord can also be compressed during slightly super-physiologic flexion and extension with only 25% canal occlusion. Physiologic loads and motions in the same spines did not cause cord compression when canal occlusion was 0%.

Conclusion: These results support the hypothesis that cervical spinal canal stenosis increases the risk of spinal cord injury because spinal cord compression was observed during motions and loads that would be safe for a non-stenotic spine. These results are limited primarily due to the use of a porcine spine. However, this new stenosis model and experimental technique will be applied to in-vitro human spine specimens in future work.