(B-73) Relationship between cervical spinal cord and column biomechanics in disc herniations: finite element modeling results

Introduction:: Many studies have been conducted using finite element modeling to determine the biomechanical responses of the human cervical spine under normal, diseased, surgically altered, and injured conditions. A considerable majority of the models have simulated the cervical spine as an osteoligamentous structure with varying degrees of complexity: from isolated disc segments to functional units - one and two level - to segmented columns to subaxial spinal columns to head-neck and intact whole-body human models. Muscle modeling has been incorporated in some studies, and this includes simulations of the passive musculature to mimic human cadaver experimental results, a data set used for validation of these models. While these models have improved our understanding of human cervical spine biomechanics, spinal column and spinal cord modeling as a unit has not been a subject of routine investigation. Because the osteoligamentous spinal column houses the spinal cord and its associated soft tissue complex, there is an interaction between cord and the column – defined by the complex array of disc and facet joints connecting the spinal vertebrae via ligaments in the anterior and posterior regions. It is important to develop and analyze finite element models that incorporate the spinal cord to investigate the interrelationship of cord-column mechanics during injury loading such as whiplash. The objective of this study was to develop a cord-column model and determine the combined biomechanical responses under whiplash loading.


Materials and Methods:: We used our previously developed and validated osteoligamentous cervical spinal column model and incorporated the spinal cord based on the clinical anatomy and components. Briefly, spinal cord components consisted of the white mater, the grey mater, the pia mater, the dura mater, the denticulate ligaments, and the cerebrospinal fluid encasing these components of the cord inside the spinal canal. The spinal column components included the cortical and cancellous bones of the vertebral body, endplates, anulus fibers and ground substance and nucleus pulposus of the disc, bilateral facet joints, all major spinal ligaments that included the two longitudinal, bilateral capsular, flava, and spinous ligaments, and muscles and head. The material properties used in previously published literature studies were adopted for all the column and cord components. The intact spine model underwent mesh convergence and element quality checks that included the aspect ratio, warpage, Jacobian, and skew angle checks. The model was validated in flexion, extension, and lateral bending, simulating normal physiological conditions. The cord-column model was modified to simulate disc herniation by constricting the spinal canal space at the C5-C6 segment to 14 mm (mild impingement of the disc, case A) and 6 mm (severe impingement of the disc, case B), and these two cases represented clinical degenerative cervical myelopathy states for surgical potential. The effect of whiplash loading at a velocity of 1.8 m/s on these spines were assessed by determining range of motion, disc pressure and cord stress at caudal and cephalad (C4-C5 and C6-C7) and index (C5-C6) segments.


Results, Conclusions, and Discussions:: Results: Data were normalized to the intact spine in the following analysis: For spine in case A, at the cephalad, index and caudal segments, the range of motion changes were +15%, +1%, and +9%, respectively, intradiscal pressures changes were +74%, +45%, and 227%, respectively, and spinal cord stress changes were -16%, -30% and 3%, respectively (negative values represent decrease and positive represent increase). For spine in case B, at the cephalad, index and caudal segments, the range of motion changes were +17%, +3%, and +7%, respectively, intradiscal pressures changes were +83%, +77%, and 189%, respectively, and spinal cord stress changes were -26%, 70% and -2%, respectively. These ratios were different when normalized with respect to case A, and those data will be presented if accepted.

Discussion: These results show that the responses of the spinal cord as determined by the stresses in the cord and responses of the osteoligamentous cervical column as determined by the external segmental range of motion, and disc pressure, the intrinsic biomechanical parameter that describes the intervertebral disc-body loading in the anterior of the spine, bear a nonuniform or inconsistent relationships. For example, while the external clinical measure of range of motion varied up to 17%, internal disc loading variations were considerably greater, and cord stress, another intrinsic parameter, also responded with greater variability. In addition, cord stress changes were not unidirectional, with both caudal and cephalad segments responding differently from the index level where the disc impingement/herniation was present.

Conclusions: The present finite element model-predicted findings, on the effects of disc herniation of spinal column and cord biomechanics, can be used as an adjunct by clinicians in their decision-making process. As MRI scans are a routine for diagnosis and treatment of disc herniation, patient-specific models can be developed using the present morphable column-cord finite element model to generate patient-specific risk for spinal cord injury during whiplash.


Acknowledgements (Optional): : This research study was supported in part by the Office of the Assistant Secretary of Defense for Health Affairs, through the Broad Agency Announcement under Award W81XWH-16-1- 0010 and the Department of Veterans Affairs Medical Research.


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