Mechanism of the spinal cord injury and the cervical spondylotic myelopathy: new approach based on the mechanical features of the spinal cord white and gray matter

Deformative stress associated with an abnormal clivo-axial angle: A finite element analysis

Background:

Chiari malformation, functional cranial settling and subtle forms of basilar invagination result in biomechanical neuraxial stress, manifested by bulbar symptoms, myelopathy and headache or neck pain. Finite element analysis is a means of predicting stress due to load, deformity and strain. The authors postulate linkage between finite element analysis (FEA)-predicted biomechanical neuraxial stress and metrics of neurological function.

Methods:

A prospective, Internal Review Board (IRB)-approved study examined a cohort of 5 children with Chiari I malformation or basilar invagination. Standardized outcome metrics were used. Patients underwent suboccipital decompression where indicated, open reduction of the abnormal clivo-axial angle or basilar invagination to correct ventral brainstem deformity, and stabilization/ fusion. FEA predictions of neuraxial preoperative and postoperative stress were correlated with clinical metrics.

Results:

Mean follow-up was 32 months (range, 7-64). There were no operative complications. Paired t tests/ Wilcoxon signed-rank tests comparing preoperative and postoperative status were statistically significant for pain, bulbar symptoms, quality of life, function but not sensorimotor status. Clinical improvement paralleled reduction in predicted biomechanical neuraxial stress within the corticospinal tract, dorsal columns and nucleus solitarius.

Conclusion:

The results are concurrent with others, that normalization of the clivo-axial angle, fusion-stabilization is associated with clinical improvement. FEA computations are consistent with the notion that reduction of deformative stress results in clinical improvement. This pilot study supports further investigation in the relationship between biomechanical stress and central nervous system (CNS) function.

Flexion model simulating spinal cord injury without radiographic abnormality in patients with ossification of the longitudinal ligament: the influence of flexion speed on the cervical spine

BACKGROUND/OBJECTIVE

It is suspected that the speed of the motion of the spinal cord under static compression may be the cause of spinal cord injury (SCI). However, little is known about the relationship between the speed of the motion of the spinal cord and its stress distributions. The objective was to carry out a biomechanical study of SCI in patients with ossification of the longitudinal ligament without radiologic evidence of injury.

METHODS

A 3-dimensional finite element spinal cord model was established. After the application of static compression, the model underwent anterior flexion to simulate SCI in ossification of the longitudinal ligament patients without radiologic abnormality. Flexion of the spine was assumed to occur at 1 motor segment. Flexion angle was 5 degrees, and flexion speeds were 0.5 degrees/s, 5 degrees/s, and 50 degrees/s. Stress distributions inside of the spinal cord were evaluated.

RESULTS

Stresses on the spinal cord increased slightly after the application of 5 degrees of flexion at a speed of 0.5 degrees/s. Stresses became much higher at a speed of 5 degrees/s and increased further at 50 degrees s.

CONCLUSIONS

The stress distribution of the spinal cord under static compression increased with faster flexion speed of the spinal cord. High-speed motion of the spinal cord under static compression may be one of the causes of SCI in the absence of radiologic abnormality.

Effects of white, grey, and pia mater properties on tissue level stresses and strains in the compressed spinal cord

Recent demographics demonstrate an increase in the number of elderly spinal cord injury patients, motivating the desire for a better understanding of age effects on injury susceptibility. Knowing that age and disease affect neurological tissue, there is a need to better understand the sensitivity of spinal cord injury mechanics to variations in tissue behavior. To address this issue, a plane-strain, geometrically nonlinear, finite element model of a section of a generic human thoracic spinal cord was constructed to model the response to dorsal compression. The material models and stiffness responses for the grey and white matter and pia mater were varied across a range of reported values to observe the sensitivity of model outcomes to the assigned properties. Outcome measures were evaluated for percent change in magnitude and alterations in spatial distribution. In general, principal stresses (114-244% change) and pressure (75-119% change) were the outcomes most sensitive to material variation. Strain outcome measures were less sensitive (7-27% change) than stresses (74-244% change) to variations in material tangent modulus. The pia mater characteristics had limited (<4% change) effects on outcomes. Using linear elastic models to represent non-linear behavior had variable effects on outcome measures, and resulted in highly concentrated areas of elevated stresses and strains. Pressure measurements in both the grey and white matter were particularly sensitive to white matter properties, suggesting that degenerative changes in white matter may influence perfusion in a compressed spinal cord. Our results suggest that the mechanics of spinal cord compression are likely to be affected by changes in tissue resulting from aging and disease, indicating a need to study the biomechanical aspects of spinal cord injury in these specific populations.

Finite element analysis of spinal cord injury in the rat

A three-dimensional (3D) finite element model (FEM) that simulates the Impactor weight-drop experimental model of traumatic spinal cord injury (SCI) was developed. The model consists of the rat spinal cord, with distinct element sets for the gray and white matter, the cerebrospinal fluid (CSF), the dura mater, a rigid rat spinal column, and a rigid impactor. Loading conditions were taken from the average impact velocities determined from previous parallel weight-drop experiments employing a 2.5-mm-diameter, 10-g rod dropped from either 12.5 or 25 mm. The mechanical properties were calibrated by comparing the predicted displacement of the spinal cord at the impact site to that measured experimentally. Parametric studies were performed to determine the sensitivity of the model to the relevant material properties, loading conditions, and essential boundary conditions, and it was determined that the shear modulus had the greatest influence on spinal cord displacement. Additional simulations were performed where gray and white matter were prescribed different material properties. These simulations generated similar drop trajectories to the homogeneous model, but the stress and strain distributions better matched patterns of acute albumin extravasation across the blood-spinal cord barrier following weight-drop SCI, as judged by a logit analysis. A final simulation was performed where the impact site was shifted laterally by 0.35 mm. The off-center impact had little effect on the rod trajectory, but caused marked shifts in the location of stress and strain contours. Different combinations of parameter values could reproduce the impactor trajectory, which suggests that another experimental measure of the tissue response is required for validation. The FEM can be a valuable tool for understanding the injury biomechanics associated with experimental SCI to identify areas for improvement in animal models and future research to identify thresholds for injury.

Biomechanics of spinal cord injury: a multimodal investigation using ex vivo guinea pig spinal cord white matter

The primary injury phase of traumatic spinal cord injury (SCI) was investigated using a novel compression injury model. Ventral white matter from adult guinea pigs was crushed to 25%, 50%, 70%, and 90% ex vivo. During compression, the physical deformation, applied force and the compound action potentials (CAP) were simultaneously recorded. In addition, axonal membrane continuity was analyzed with a horseradish peroxidase (HRP) exclusion assay. Experimental results showed both a CAP decrease and increased HRP uptake as a function of increased compression. The percent CAP reduction was also consistent to the percent HRP uptake, which implies that either metric could be used to assess acute axon damage. Analysis of the HRP stained axon distribution demonstrated a gradient of damage, with the highest levels of staining near the gray matter. The patterns of axon damage revealed by histology also coincided with higher levels of von Mises stress, which were predicted with a recently developed finite element model of ventral white matter. Numerical values obtained from the finite element model suggest stress magnitudes near 2 kPa are required to initiate anatomical tissue injury. We believe that data from this study could further elucidate the deformation-function relationship in acute spinal cord injury.

Cervical spondylotic myelopathy: a brief review of its pathophysiology, clinical course, and diagnosis

Degenerative disease of the cervical spine commonly occurs in the natural process of aging. This can lead to compression of the spinal cord and symptomatic myelopathy. We review the pathophysiological factors that lead to myelopathy and the controversial natural history of untreated myelopathy. Signs and symptoms at presentation, examination findings, differential diagnosis, and diagnostic studies are also discussed.

Investigation of anteroposterior head-neck responses during severe frontal impacts using a brain-spinal cord complex FE model

Injuries of the human brain and spinal cord associated with the central nervous system (CNS) are seen in automotive accidents. CNS injuries are generally categorized into severe injuries (AIS 3+). However, it is not clear how the restraint conditions affect the CNS injuries. This paper presents a newly developed three-dimensional (3D) finite element head-neck model in order to investigate the biomechanical responses of the brain-spinal cord complex. The head model consists of the scalp, skull, and a detailed description of the brain including the cerebrum, cerebellum, brainstem with distinct white and gray matter, cerebral spinal fluid (CSF), sagittal sinus, dura, pia, arachnoid, meninx, falx cerebri, and tentorium. Additionally, the neck model consists of the cervical vertebral bodies, intervertebral discs, muscles, ligaments, spinal cord with white and gray matter, cervical pia, and CSF. The two models were linked together to construct a finite element (FE) model of the brain-spinal cord complex. The material stiffness and failure properties of porcine cervical pia mater were measured from uniaxial tensile tests with various strain rates at Yamaguchi University. The head-neck model was validated against three sets of brain test data obtained by Nahum et al. (1977), Trosseille et al. (1992), and Hardy et al. (2001) and two sets of neck test data obtained from Thunnissen et al. (1995) and Pintar et al. (1995). Additionally, a series of parametric studies were conducted to investigate the effects of restraint conditions on CNS injuries. The injury criteria for brain injuries were based on Cumulative Strain Damage Measure, while those for spinal cord injuries were based on the ultimate strains of the spinal cord and pia mater. It was found that the brain-spinal cord model was useful to investigate the relationship between the restraint conditions and CNS injuries.

Behavioral and histological characterization of unilateral cervical spinal cord contusion injury in rats

Most experimental studies of spinal cord injury (SCI) in rats damage the thoracic cord, with the consequent functional loss being due to interruption of long tracts connecting the caudal spinal cord to the rostral nervous system. Less work has been done evaluating injury to the cervical cord, even though it is the most common level of human SCI. In addition to the long tracts, the cervical spinal cord contains the sensory and motor neurons responsible for upper extremity function. The purpose of this study was to further develop a rat model of cervical spinal cord contusion injury using a modified NYU/MASCIS weight drop device. Mild (6.25 mm) and moderate (12.5 mm) C5 unilateral injuries were produced. Behavioral recovery was examined using a grooming test, a paw preference test, a walkway test (The Catwalk), and a horizontal ladder test. Histological outcome measures included sparing at the lesion epicenter, sparing throughout the extent of the lesion, quantification of myelin loss rostral and caudal to the lesion, and motor neuron counts. Compared to controls, animals receiving SCI exhibited injury severity-specific deficits in forelimb, locomotor, and hindlimb function persisting for 6-weeks post-SCI. Histological analysis revealed ipsilateral containment of the injury, and differentiation between groups on all measures except motor neuron counts. This model has many advantages: (1) minimal animal care requirements post-SCI, (2) within subject controls, (3) functional loss involves primarily the ipsilateral forelimb, and (4) it is a behavioral and histological model for both gray and white matter damage caused by contusive SCI.