P4. Cervical kyphosis increases spinal cord stress and strain in the stenotic cervical spine during neck motion

Abstract

Background Context

Spinal cord stress and strain contributes to the pathophysiology of degenerative cervical myelopathy (DCM) and progressive cervical kyphosis can lead to worsening myelopathy. In DCM, the combination of spinal cord biomechanics, sagittal alignment and cord compression is known to increase spinal cord damage. However, the relationship between these biomechanical factors is not well understood. Quantifying spinal cord biomechanics and its relationship to sagittal alignment in DCM can guide surgical strategies that address adverse spinal cord stress and strain in addition to cord compression.

Purpose

To quantify the effect of cervical sagittal alignment on spinal cord stress and strain in the stenotic cervical spine.

Study Design/Setting

Finite element analysis.

Patient Sample

N/A.

Outcome Measures

Spinal cord stress and strain.

Methods

A previously validated three-dimensional finite element model of the human cervical spine with spinal cord was used. Three models of cervical alignment were created: lordosis (C2-C7 Cobb angle: 20 degrees), straight (0 degrees) and kyphosis (-9 degrees). Spinal cord prestress and prestrain due to spinal alignment was quantified. Progressive spinal stenosis was simulated at the C5-C6 segment with ventral disk protrusion that reduced the anteroposterior spinal canal diameter to 10mm, 8mm and 6mm. Flexion and extension of the cervical spine was simulated with a pure moment load of 2 Nm. The model was constrained at the inferior surface of the T1 vertebra in all degrees-of-freedom, and the sagittal moment loads were applied at the superior vertebra. An additional follower force of 75N to simulate the head mass and muscle force was applied. Von Mises stress and maximum principal strain of the whole cervical spinal cord was calculated during flexion and extension and added to the prestress and prestrain. The relationship between spinal cord biomechanics, alignment and cord compression was analyzed using linear regression analysis.

Results

Spinal cord prestress and prestrain was greatest for the kyphotic spine (7.53 kPa, 5.4%) and least for the lordotic spine (0.68 kPa, 0.3%). Progressive kyphosis and stenosis were associated with increase in spinal cord stress (R²=0.99) and strain (R²=0.99). For every 1 degree increase in kyphosis, average cervical spinal cord stress increased by 0.196 kPa and for every 1% increase in spinal cord compression, the von Mises stress increased by 1.86 kPa. Compared to straight and lordotic alignment, cervical kyphosis was associated with greater spinal cord stress and strain during neck flexion-extension and the magnitude of the difference was greater with increasing stenosis.

Conclusions

Cervical kyphosis increases spinal cord stress and strain and the effect is magnified with cord compression and neck motion. The results of this study provide the quantitative biomechanical basis for greater spinal cord damage in DCM patients with cervical kyphosis. Incorporating the effect of sagittal alignment on spinal cord biomechanics is necessary to accurately quantify spinal stress and strain during neck motion.

FDA Device/Drug Status

This abstract does not discuss or include any applicable devices or drugs.