Translational constraint influences dynamic spinal canal occlusion of the thoracic spine: an in vitro experimental study

Mechanical constraints to spine motion can arise in a variety of real-world situations such as when shoulder belts prevent anterior translation of the thorax during automotive collisions. The effect of such constraint on spinal column-spinal cord interaction during injury remains unknown. The purpose of the present study was to compare maximal dynamic spinal canal occlusion, measured via a specialized transducer, in cadaveric upper thoracic spine specimens under a variety of anterior-posterior constraint conditions. Four injury models were produced using 24 cadaveric spine specimens (T1-T4). Incremental compressive trauma was applied under constrained (i.e. blocked anterior-posterior translation) flexion-compression, pure-compression and extension-compression, and under unconstrained (i.e. free anterior-posterior translation) flexion-compression. All displacements were applied at 500 mm/s. For all three constrained trauma groups, complete transducer occlusion occurred between 20 and 30 mm of compressive displacement. The extension-compression caused transducer occlusion significantly less than the other constrained models (p < 0.022) at 20 mm compression. For unconstrained flexion-compression, a compression of up to 50 mm resulted in a mean of 26% transducer occlusion. The constrained pure-compression tests led to burst fracture with significant body height loss at T2. The constrained flexion-compression and extension-compression tests caused fracture-dislocation injury at the T2-T3 level. Constrained trauma clearly led to more spinal canal occlusion than the unconstrained in these models, and more severe injury to the spinal column. The results add to our understanding of the effect of column injury pattern on spinal cord injury. This information has clear implications for the design of injury prevention devices.     

Measurement of canal occlusion during the thoracolumbar burst fracture process

Post-injury CT scans are often used following burst fracture trauma as an indication for decompressive surgery. Literature suggests, however, that there is little correlation between the observed fragment position and the level of neurological injury or recovery. Several studies have aimed to establish the processes that occur during the fracture using indirect methods such as pressure measurements and pre/post impact CT scans. The purpose of this study was to develop a direct method of measuring spinal canal occlusion during a simulated burst fracture by using a high-speed video technique. The fractures were produced by dropping a mass from a measured height onto three-vertebra bovine specimens in a custom-built rig. The specimens were constrained to deform only in the impact direction such that pure compression fractures were generated. The spinal cord was removed prior to testing and the video system set up to film the inside of the spinal canal during the impact. A second camera was used to film the outside of the specimen to observe possible buckling during impact. The video images were analysed to determine how the cross-sectional area of the spinal canal changed during the event. The images clearly showed a fragment of bone being projected from the vertebral body into the spinal canal and recoiling to the final resting position. To validate the results, CT scans were taken pre- and post-impact and the percentage canal occlusion was calculated. There was good agreement between the final canal occlusion measured from the video images and the CT scans.     

Effect of bone fragment impact velocity on biomechanical parameters related to spinal cord injury: A finite element study

Several experimental and computational studies have investigated the effect of bone fragment impact on the spinal cord during trauma. However, the effect of the impact velocity of a fragment generated by a burst fracture on the stress and strain inside the spinal cord has not been computationally investigated, even though spinal canal occlusion and peak pressure at various impact velocities were provided in experimental studies. These stresses and strains are known factors related to clinical symptoms or injuries. In this study, a fluid-structure interaction model of the spinal cord, dura mater, and cerebrospinal fluid was developed and validated. The von-Mises stress distribution in the cord, the longitudinal strain, the cord compression and cross-sectional area at the impact center, and the obliteration of the cerebrospinal fluid layer were analyzed for three pellet sizes at impact velocities ranging from 1.5 m/s to 7.5 m/s. The results indicate that stress in the cord was substantially elevated when the initial impact velocity of the pellet exceeded a threshold of 4.5 m/s. Cord compression, reduction in cross-sectional area, and obliteration of the cerebrospinal fluid increased gradually as the velocity of the pellet increased, regardless of the size of the pellet. The present study provides insight into the mechanisms underlying spinal cord injury.     

Radiography used to measure internal spinal cord deformation in an in vivo rat model

Little is known about the internal mechanics of the in vivo spinal cord during injury. The objective of this study was to develop a method of tracking internal and surface deformation of in vivo rat spinal cord during compression using radiography. Since neural tissue is radio-translucent, radio-opaque markers were injected into the spinal cord.Two tantalum beads (260 µm) were injected into the cord (dorsal and ventral) at C5 of nine anesthetized rats. Four beads were glued to the lateral surface of the cord, caudal and cranial to the injection site. A compression plate was displaced 0.5 mm, 2 mm, and 3 mm into the spinal cord and lateral X-ray images were taken before, during, and after each compression for measuring bead displacements. Potential bead migration was monitored for by comparing displacements of the internal and glued surface beads.Dorsal beads moved significantly more than ventral beads with a range in averages of 0.57–0.71 mm and 0.31–0.35 mm respectively. Bead displacements during 0.5 mm compressions were significantly lower than 2 mm and 3 mm compressions. There was no statistically significant migration of the internal beads.The results indicate the merit of this technique for measuring in vivo spinal cord deformation. The pattern of bead displacements illustrates the complex internal and surface deformations of the spinal cord during transverse compression. This information is needed for validating physical and finite element spinal cord surrogates and to define relationships between loading parameters, internal cord deformation, and biological and functional outcomes.     

Comparison of the anatomical morphology of cervical vertebrae between humans and macaques: related to a spinal cord injury model

Non-human primates are most suitable for generating cervical experimental models, and it is necessary to study the anatomy of the cervical spine in non-human primates when generating the models. The purpose of this study was to provide the anatomical parameters of the cervical spine and spinal cord in long-tailed macaques (Macaca fascicularis) as a basis for cervical spine-related experimental studies. Cervical spine specimens from 8 male adult subjects were scanned by micro-computed tomography, and an additional 10 live male subjects were scanned by magnetic resonance imaging. The measurements and parameters from them were compared to those of 12 male adult human subjects. Additionally, 10 live male subjects were scanned by magnetic resonance imaging, and the width and depth of the spinal cord and spinal canal and the thickness of the anterior and posterior cerebrospinal fluid were measured and compared to the relevant parameters of 10 male adult human subjects. The tendency of cervical parameters to change with segmental changes was similar between species. The vertebral body, spinal canal, and spinal cord were significantly flatter in the human subjects than in the long-tailed macaques. The cerebrospinal fluid space in the long-tailed macaques was smaller than that in the human subjects. The anatomical features of the cervical vertebrae of long-tailed macaques provide a reference for establishing a preclinical model of cervical spinal cord injury.     

The development of an improved physical surrogate model of the human spinal cord—Tension and transverse compression

To prevent spinal cord injury, optimize treatments for it, and better understand spinal cord pathologies such as spondylotic myelopathy, the interaction between the spinal column and the spinal cord during injury and pathology must be understood. The spinal cord is a complex and very soft tissue that changes properties rapidly after death and is difficult to model. Our objective was to develop a physical surrogate spinal cord with material properties closely corresponding to the in vivo human spinal cord that would be suitable for studying spinal cord injury under a variety of injurious conditions. Appropriate target material properties were identified from published studies and several candidate surrogate materials were screened, under uniaxial tension, in a materials testing machine. QM Skin 30, a silicone elastomer, was identified as the most appropriate material. Spinal cords manufactured from QM Skin 30 were tested under uniaxial tension and transverse compression. Rectangular specimens of QM Skin 30 were also tested under uniform compression. QM Skin 30 produced surrogate cords with a Young's modulus in tension and compression approximately matching values reported for in vivo animal spinal cords (0.25 and 0.20 MPa, respectively). The tensile and compressive Young's modulus and the behavior of the surrogate cord simulated the nonlinear behavior of the in vivo spinal cord.     

The effect of cerebrospinal fluid thickness on traumatic spinal cord deformation

A spinal cord injury may lead to loss of motor and sensory function and even death. The biomechanics of the injury process have been found to be important to the neurological damage pattern, and some studies have found a protective effect of the cerebrospinal fluid (CSF). However, the effect of the CSF thickness on the cord deformation and, hence, the resulting injury has not been previously investigated. In this study, the effects of natural variability (in bovine) as well as the difference between bovine and human spinal canal dimensions on spinal cord deformation were studied using a previously validated computational model. Owing to the pronounced effect that the CSF thickness was found to have on the biomechanics of the cord deformation, it can be concluded that results from animal models may be affected by the disparities in the CSF layer thickness as well as by any difference in the biological responses they may have compared with those of humans.     

Patient-specific finite element model of the spine and spinal cord to assess the neurological impact of scoliosis correction: preliminary application on two cases with and without intraoperative neurological complications

Scoliosis is a 3D deformation of the spine and rib cage. For severe cases, surgery with spine instrumentation is required to restore a balanced spine curvature. This surgical procedure may represent a neurological risk for the patient, especially during corrective maneuvers. This study aimed to computationally simulate the surgical instrumentation maneuvers on a patient-specific biomechanical model of the spine and spinal cord to assess and predict potential damage to the spinal cord and spinal nerves. A detailed finite element model (FEM) of the spine and spinal cord of a healthy subject was used as reference geometry. The FEM was personalized to the geometry of the patient using a 3D biplanar radiographic reconstruction technique and 3D dual kriging. Step by step surgical instrumentation maneuvers were simulated in order to assess the neurological risk associated to each maneuver. The surgical simulation methodology implemented was divided into two parts. First, a global multi-body simulation was used to extract the 3D displacement of six vertebral landmarks, which were then introduced as boundary conditions into the personalized FEM in order to reproduce the surgical procedure. The results of the FEM simulation for two cases were compared to published values on spinal cord neurological functional threshold. The efficiency of the reported method was checked considering one patient with neurological complications detected during surgery and one control patient. This comparison study showed that the patient-specific hybrid model reproduced successfully the biomechanics of neurological injury during scoliosis correction maneuvers.     

Atypical vertebral hemangioma: an aggressive form of a benign disease. Case Report and Literature Review

Vertebral hemangiomas are an incidental and relatively common radiological finding and a benign tumor of vascular origin. VH are the most common spine tumors with an estimated incidence of 1.9-27% in the general population. Rarely, vertebral hemangiomas can exhibit extraosseous expansion with resulting compression of the spinal cord. Such lesions are termed aggressive or atypical vertebral hemangiomas (AVH) and account for less than 1% of spinal hemangiomas. A 68-year-old female was referred with progressive walking difficulty and sensory disturbances in her lower extremities. MRI imaging of the thoracic spine revealed a T1- and T2-weighted hyperintense lesion involving the T10 vertebra. Additionally, there was extraosseous extension of the tumor into the spinal canal, located both anterior and posterior to the spinal cord, causing severe spinal cord compression. A combined endovascular and surgical approach (arterial coil embolization and en bloc resection) for treatment was decided. Although vertebral hemangiomas are an incidental and relatively common radiological finding, the findings of our case were consistent with an aggressive hemangioma with atypical MRI and clinical prognostic characteristics. In summary, the present case highlights the need for multidisciplinary approach and in-depth knowledge of this rare pathologic entity.     

Stretch along the craniocaudal axis improves shape recoverability of the spinal cord

The spinal cord is physiologically stretched along the craniocaudal axis, and is subjected to tensile stress. The purpose of this study was to examine the effect of the tensile stress on morphological plasticity of the spinal cord under compression and decompression condition. The C1-T2 spinal column was excised from 4 rabbits. The laminae and lateral masses were removed. After excision of surrounding structures, a small rod was placed on the spinal cord. The rod was connected with a pan of the scale balance. Varying the weight between 0 and 20g on the other scalepan, the indentation of the rod was measured. Then, the spinal cord was cut transversely to remove longitudinal tensile stress. The samples were measured again with the same protocol at point 10mm caudal to each pre-measured point on the spinal cord. The shape recovery rate was calculated. The length of the spinal cord decreased by 9.7% after the separation. The maximum indentation was 2.1mm (mean) at 20g, and did not differ between the separated and un-separated cords. The recovery rate was not significantly different between the separated and un-separated cords until 3g. At the load under 3g, the recovery rate after the separation was significantly lower than that before the separation. The tensile stress along the craniocaudal axis in the spinal cord did not affect the spinal cord deformation in response to the compression, but it did affect the shape recoverability after the decompression.     

New approach for graded compression spinal cord injuries in Rhesus macaque: method feasibility and preliminary observations

Background Current models of spinal cord injury (SCI) have been ineffective for translational research. Primate blunt SCI, which more closely resembles human injury, could be a promising model to fill this gap. Methods Graded compression SCI was produced by inflating at T9 an epidural balloon as a function of spinal canal dimensions in a non‐uniform group of monkeys. Results Sham injury and cord compression by canal invasion of 50–75% produced minimal morpho‐functional alterations, if at all. Canal invasion of 90–100% resulted in proportional functional deficits. Unexpectedly, these animals showed spontaneous gradual recovery over a 12‐week period achieving quadruped walking, although with persistent absence of foot grasping reflex. Histopathology revealed predominance of central cord damage that correlated with functional status. Conclusions Our preliminary results suggest that this model could potentially be a useful addition to translational work, but requires further validation by including animals with permanent injuries and expansion of replicates.     

A one-dimensional model of the spinal cerebrospinal-fluid compartment

Modeling of the cerebrospinal fluid (CSF) system in the spine is strongly motivated by the need to understand the origins of pathological conditions such as the emergence and growth of fluid-filled cysts in the spinal cord. In this study, a one-dimensional (1D) approximation for the flow in elastic conduits was used to formulate a model of the spinal CSF compartment. The modeling was based around a coaxial geometry in which the inner elastic cylinder represented the spinal cord, middle elastic tube represented the dura, and the outermost tube represented the vertebral column. The fluid-filled annuli between the cord and dura, and the dura and vertebral column, represented the subarachnoid and epidural spaces, respectively. The system of governing equations was constructed by applying a 1D form of mass and momentum conservation to all segments of the model. The developed 1D model was used to simulate CSF pulse excited by pressure disturbances in the subarachnoid and epidural spaces. The results were compared to those obtained from an equivalent two-dimensional finite element (FE) model which was implemented using a commercial software package. The analysis of linearized governing equations revealed the existence of three types of waves, of which the two slower waves can be clearly related to the wave modes identified in previous similar studies. The third, much faster, wave emanates directly from the vertebral column and has little effect on the deformation of the spinal cord. The results obtained from the 1D model and its FE counterpart were found to be in good general agreement even when sharp spatial gradients of the spinal cord stiffness were included; both models predicted large radial displacements of the cord at the location of an initial cyst. This study suggests that 1D modeling, which is computationally inexpensive and amenable to coupling with the models of the cranial CSF system, should be a useful approach for the analysis of some aspects of the CSF dynamics in the spine. The simulation of the CSF pulse excited by a pressure disturbance in the epidural space, points to the possibility that regions of the spinal cord with abnormally low stiffness may be prone to experiencing large strains due to coughing and sneezing.     

Hydralazine inhibits compression and acrolein-mediated injuries in ex vivo spinal cord

We have previously shown that acrolein, a lipid peroxidation byproduct, is significantly increased following spinal cord injury in vivo , and that exposure to neuronal cells results in oxidative stress, mitochondrial dysfunction, increased membrane permeability, impaired axonal conductivity, and eventually cell death. Acrolein thus may be a key player in the pathogenesis of spinal cord injury, where lipid peroxidation is known to be involved. The current study demonstrates that the acrolein scavenger hydralazine protects against not only acrolein-mediated injury, but also compression in guinea pig spinal cord ex vivo . Specifically, hydralazine (500 μmol/L to 1 mmol/L) can significantly alleviate acrolein (100–500 μmol/L)-induced superoxide production, glutathione depletion, mitochondrial dysfunction, loss of membrane integrity, and reduced compound action potential conduction. Additionally, 500 μmol/L hydralazine significantly attenuated compression-mediated membrane disruptions at 2 and 3 h following injury. This was consistent with our findings that acrolein-lys adducts were increased following compression injury ex vivo , an effect that was prevented by hydralazine treatment. These findings provide further evidence for the role of acrolein in spinal cord injury, and suggest that acrolein-scavenging drugs such as hydralazine may represent a novel therapy to effectively reduce oxidative stress in disorders such as spinal cord injury and neurodegenerative diseases, where oxidative stress is known to play a role.     

Finite Element Study of the Mechanical Response in Spinal Cord during the Thoracolumbar Burst Fracture

Background

The mechanical response of the spinal cord during burst fracture was seldom quantitatively addressed and only few studies look into the internal strain of the white and grey matters within the spinal cord during thoracolumbar burst fracture (TLBF). The aim of the study is to investigate the mechanical response of the spinal cord during TLBF and correlate the percent canal compromise (PCC) with the strain in the spinal cord.

Methodology/Principal Findings

A three-dimensional (3D) finite element (FE) model of human T12-L1 spinal cord with visco-elastic property was generated based on the transverse sections images of spinal cord, and the model was validated against published literatures under static uniaxial tension and compression. With the validated model, a TLBF simulation was performed to compute the mechanical strain in the spinal cord with the PCC. Linear regressions between PCC and strain in the spinal cord show that at the initial stage, with the PCC at 20%, and 45%, the corresponding mechanical strains in ventral grey, dorsal grey, ventral white, dorsal white matters were 0.06, 0.04, 0.12, 0.06, and increased to 0.14, 0.12, 0.23, and 0.13, respectively. At the recoiled stage, when the PCC was decreased from 45% to 20%, the corresponding strains were reduced to 0.03, 0.02, 0.04 and 0.03. The strain was correlated well with PCC.

Conclusions/Significance

The simulation shows that the strain in the spinal cord correlated well with the PCC, and the mechanical strains in the ventral regions are higher than those in the dorsal regions of spinal cord tissue during burst fracture, suggesting that the ventral regions of the spinal cord may susceptible to injury than the dorsal regions.     

A viscous tolerance criterion for soft tissue injury assessment

Experiments in our laboratory have documented that high-speed impact can cause severe injury to internal organs before either of the currently accepted chest injury criteria, which are based on spinal acceleration or chest compression, approach their tolerance limit. Those studies demonstrate an interdependence between the velocity of deformation and compression of the body on injury risk. A tolerable level of chest compression at a low velocity can prove to be fatal at higher velocities of deformation. The observation of a rate-sensitive tolerable compression led to the introduction of the Viscous criterion, VCmax, which accounts for the importance of both parameters. VCmax is the maximum of the product of velocity of deformation (V) and compression (C), and is derivable from the chest deflection response. This paper presents the empirical evidence and theoretical basis supporting the Viscous criterion, and shows it to be an indicator of the energy dissipated by soft tissue deformation. The Viscous criterion accurately predicts the risk of vital organ and soft tissue injury when other criteria fail.     

胸腰段骨折外科治疗的研究进展

胸腰椎骨折是骨科临床工作中常见的损伤。胸腰段骨折往往伴发不同程度的脊髓损伤,很大程度上影响患者的生活质量。准确把握其分型,并制定恰当、及时、有效的治疗方案,对伤员进行救治,恢复椎体的高度、序列与曲度,解除神经压迫,重建脊柱稳定性,减少近远期并发症,改善预后有着深远的意义。本文通过对胸腰段骨折的分类、手术入路、手术方式、不同类型骨折的治疗等四个方面的国内外近期研究成果作一综述。     

Optical Monitoring and Detection of Spinal Cord Ischemia

Spinal cord ischemia can lead to paralysis or paraparesis, but if detected early it may be amenable to treatment. Current methods use evoked potentials for detection of spinal cord ischemia, a decades old technology whose warning signs are indirect and significantly delayed from the onset of ischemia. Here we introduce and demonstrate a prototype fiber optic device that directly measures spinal cord blood flow and oxygenation. This technical advance in neurological monitoring promises a new standard of care for detection of spinal cord ischemia and the opportunity for early intervention. We demonstrate the probe in an adult Dorset sheep model. Both open and percutaneous approaches were evaluated during pharmacologic, physiological, and mechanical interventions designed to induce variations in spinal cord blood flow and oxygenation. The induced variations were rapidly and reproducibly detected, demonstrating direct measurement of spinal cord ischemia in real-time. In the future, this form of hemodynamic spinal cord diagnosis could significantly improve monitoring and management in a broad range of patients, including those undergoing thoracic and abdominal aortic revascularization, spine stabilization procedures for scoliosis and trauma, spinal cord tumor resection, and those requiring management of spinal cord injury in intensive care settings.     

The effect of lateral eccentricity on failure loads,kinematics, and canal occlusions of the cervical spine in axial loading

Current neck injury criteria do not include limits for lateral bending combined with axial compression and this has been observed as a clinically relevant mechanism, particularly for rollover motor vehicle crashes. The primary objectives of this study were to evaluate the effects of lateral eccentricity (the perpendicular distance from the axial force to the centre of the spine) on peak loads, kinematics, and spinal canal occlusions of subaxial cervical spine specimens tested in dynamic axial compression (0.5 m/s). Twelve 3-vertebra human cadaver cervical spine specimens were tested in two groups: low and high eccentricity with initial eccentricities of 1 and 150% of the lateral diameter of the vertebral body. Six-axis loads inferior to the specimen, kinematics of the superior-most vertebra, and spinal canal occlusions were measured. High speed video was collected and acoustic emission (AE) sensors were used to define the time of injury. The effects of eccentricity on peak loads, kinematics, and canal occlusions were evaluated using unpaired Student t-tests. The high eccentricity group had lower peak axial forces (1544±629 vs. 4296±1693 N), inferior displacements (0.2±1.0 vs. 6.6±2.0 mm), and canal occlusions (27±5 vs. 53±15%) and higher peak ipsilateral bending moments (53±17 vs. 3±18 Nm), ipsilateral bending rotations (22±3 vs. 1±2°), and ipsilateral displacements (4.5±1.4 vs. −1.0±1.3 mm, p<0.05 for all comparisons). These results provide new insights to develop prevention, recognition, and treatment strategies for compressive cervical spine injuries with lateral eccentricities.     

Regenerative treatment in spinal cord injury

Spinal cord injury is a devastating, traumatic event, and experienced mainly among young people. Until the modern era, spinal cord injury was so rapidly fatal that no seriously injured persons would survive long enough for regeneration to occur. Treatment of spinal cord injury can be summarized as follows: prevent further cord injury, maintain blood flow, relieve spinal cord compression, and provide secure vertebral stabilization so as to allow mobilization and rehabilitation, none of which achieves functional recovery. Previous studies have focused on analyzing the pathogenesis of secondary injury that extends from the injury epicenter to the periphery, as well as the tissue damage and neural cell death associated with secondary injury. Now, there are hundreds of current experimental and clinical regenerative treatment studies. One of the most popular treatment method is cell transplantation in injured spinal cord. For this purpose bone marrow stromal cells, mononuclear stem cells, mesenchymal stem cells, embryonic stem cells, neural stem cells, and olfactory ensheathing cells can be used. As a result, cell transplantation has become a promising therapeutic option for spinal cord injury patients. In this paper we discuss the effectiveness of stem cell therapy in spinal cord injury.