Spinal cord deformation in flexion and extension-a finite element study

Several spinal disorders and traumatic loading situations are known to inflict damage to neurovascular components of the cervical spinal cord. An understanding of physiologic cervical spinal cord deformation is important for defining an injury threshold of the cervical spinal cord. Studies have shown that damage to the spinal cord can occur regardless of significant damage to surrounding structures. To understand the mechanics of spinal cord injury, one needs to quantify stresses and strains within the spinal cord and its components in response to external loads applied to the bony spine. Experimental studies can not address this issue. This study presents a finite element (FE) model to quantify the physiologic strains and stresses within the cervical spinal cord at the C5-C6 level. A ligamentous nonlinear three-dimensional FE model of the C5-C6 motion segment was developed from 1.5 mm thick serial computed tomography (CT) scan. The trend of the strain data indicates that the posterior surface of the cord was strained more than the anterior surface during flexion, which is in agreement with the data from Yuan et al. (1998). Also, the von Mises stress plots indicate an increase in stress on the dural sheath during flexion due to disc bulging. This model represents the first attempt the authors are aware of to quantify stress and strain in the spinal cord during physiologic spinal loading using a three-dimensional FE cervical spine model with spinal cord. Limitations of the spinal cord model include material property and boundary definitions, as well as the inclusion of fatty layers and other connective tissues around the cord in the canal.