Accurate simulation of the herniated cervical intervertebral disc using controllable expansion: a finite element study

Cervical Disc Herniation Model 1

Goal of the Study?

The objective of this study 1 was to establish a three-dimensional finite element (3D-FE) model of the cervical disc and spinal cord to simulate an intervertebral disc compression injury of the spinal cord by controlling the expansion of specific parts of the disc model

herniated disc model

Why are they doing this study?

Cervical spondylotic myelopathy (CSM) is one of the most common spinal cord disorders in people older than 55. It is a degenerative disease that impairs the function of the spinal cord due to progressive and chronic compression. In advanced stages it can cause neurological issues ranging from neck stiffness, arm pain, numbness in the hands to more severe symptoms such as quadriplegia (tetraplegia).

To date, many of the in vivo and in vitro experimental studies focusing on impingement or compression are unable to simulate the sophisticated nature of spinal cord injuries. The authors speculate that computational models are better able to evaluate mechanical forces and spinal cord deformations. In particular, they argue that FE models may provide a way to more accurately simulate disc compression and therefore predict future deterioration and onset of CSM.

What was done?

The researchers developed a 3D FE (finite element) model of the cervical disc and spinal cord. This model was comprised of four distinct materials to represent the white matter, gray matter, pia matter and annulus ground. Cervical disc protrusions were simulated by applying thermal expansion to multiple FE unites to trigger bulging of the cervical disc either directly or indirectly.  

Three models of symmetric cervical disc herniation (median, paramedian and lateral) were created by evenly raising the temperature of corresponding FE units to 30 °C. The asymmetric hernia model was developed by rising the temperature of the paramedian type model gradually from 22 to 30 °C. They then used a linear regression analysis to determine the relationship between the various models and temperature changes.

What did they find?

Overall, the researchers found a correlation between rising temperatures and the gradual increase of severity of disc herniation. In all four models, herniated masses were observed at the region where thermal expansion occurred. In the asymmetric hernia model, the protrusion was more severe on the side with a high temperature increase than on the side with a lower temperature.  

They found that spinal cord compressions resulting from intervertebral disc protrusion were observed in all models, except the lateral type. The greater the cervical cord was compressed by the protruding disc, the larger the area with higher stress. Moreover, as the level of compression increased, the deformation of the spinal cord intensified and the stress was dispersed to the anterior horn and intermediate gray of the grey matter, which could explain the progression of neurological symptoms.  

Why do these findings matter?

A better understanding of the biologic mechanisms of spinal cord damage caused by chronic mechanical stress is required to improve diagnosis, treatment and clinical outcomes. In particular, FE models may provide a way to more accurately simulate disc compression  and herniated discs and therefore predict future deterioration and onset of CSM.


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