Modeling spinal cord contusion, dislocation, and distraction: characterization of vertebral clamps, injury severities, and node of Ranvier deformations

Nonlinear viscoelastic characterization of the porcine spinal cord

Although quasi-static and quasi-linear viscoelastic properties of the spinal cord have been reported previously, there are no published studies that have investigated the fully (strain-dependent) nonlinear viscoelastic properties of the spinal cord. In this study, stress relaxation experiments and dynamic cycling were performed on six fresh porcine lumbar cord specimens to examine their viscoelastic mechanical properties. The stress relaxation data were fitted to a modified superposition formulation and a novel finite ramp time correction technique was applied. The parameters obtained from this fitting methodology were used to predict the average dynamic cyclic viscoelastic behavior of the porcine cord. The data indicate that the porcine spinal cord exhibited fully nonlinear viscoelastic behavior. The average weighted root mean squared error for a Heaviside ramp fit was 2.8 kPa, which was significantly greater (p<0.001) than that of the nonlinear (comprehensive viscoelastic characterization method) fit (0.365 kPa). Further, the nonlinear mechanical parameters obtained were able to accurately predict the dynamic behavior, thus exemplifying the reliability of the obtained nonlinear parameters. These parameters will be important for future studies investigating various damage mechanisms of the spinal cord and studies developing high-resolution finite elements models of the spine.

Characterization of a cervical spinal cord hemicontusion injury in mice using the infinite horizon impactor

The majority of clinical spinal cord injuries (SCIs) are contusive and occur at the cervical level of the spinal cord. Most scientists and clinicians agree that the preclinical evaluation of novel candidate treatments should include testing in a cervical SCI contusion model. Because mice are increasingly used because of the availability of genetically engineered lines, we characterized a novel cervical hemicontusion injury in mice using the Infinite Horizon Spinal Cord Impactor (Precisions Systems & Instrumentation, Lexington, KY). In the current study, C57BL/6 mice received a hemicontusion injury of 75 kilodynes with or without dwell time in an attempt to elicit a sustained moderate-to-severe motor deficit. Hemicontusion injuries without dwell time resulted in sustained deficits of the affected forepaw, as revealed by a 3-fold decrease in usage during rearing, a ∼50% reduction in grooming scores, and retrieval of significantly fewer pellets on the Montoya staircase test. Only minor transient deficits were observed in grasping force. CatWalk analysis revealed reduced paw-print size and swing speed of the affected forelimb. Added dwell time of 15 or 30 sec significantly worsened behavioral outcome, and mice demonstrated minimal ability of grasping, paw usage, and overground locomotion. Besides worsening of behavioral deficits, added dwell time also reduced residual white and gray matter at the epicenter and rostral-caudal to the injury, including on the contralateral side of the spinal cord. Taken together, we developed and characterized a new hemicontusion SCI model in mice that produces sufficient and sustained impairments in gross and skilled forelimb function and produced primarily unilateral functional deficits.

Effects of vertebral column distraction on transcranial electrical stimulation-motor evoked potential and histology of the spinal cord in a porcine model

BACKGROUND

Spinal cord injury can occur following surgical procedures for correction of scoliosis and kyphosis, as these procedures produce lengthening of the vertebral column. The objective of this study was to cause spinal cord injury by vertebral column distraction and evaluate the histological changes in the spinal cord in relationship to the pattern of recovery from the spinal cord injury.

METHODS

Global osteotomy of all three spinal columns was performed on the ninth thoracic vertebra of sixteen pigs. The osteotomized vertebra was distracted until transcranial electrical stimulation-motor evoked potential (TES-MEP) signals disappeared or decreased by >80% compared with the baseline amplitude; this was defined as spinal cord injury. The distraction distance at which spinal cord injury occurred was measured, the distraction was released, and the TES-MEP recovery pattern was observed. A wake-up test was performed, two days of observations were made, and histological changes were evaluated in relationship to the recovery pattern.

RESULTS

Spinal cord injury developed at a distraction distance of 20.2 ± 4.7 mm, equivalent to 3.6% of the thoracolumbar spinal length, and the distraction distance was correlated with the thoracolumbar spinal length (r = 0.632, p = 0.009). No animals exhibited complete recovery according to TES-MEP testing, eleven exhibited incomplete recovery, and five exhibited no recovery. During the two days of observation, all eleven animals with incomplete recovery showed positive responses to sensory and motor tests, whereas none of the five animals with no recovery had positive responses. On histological evaluation, three animals that exhibited no recovery all showed complete severance of nerve fibers (axotomy), whereas six animals that exhibited incomplete recovery all showed partial white-matter injury.

CONCLUSIONS

Parallel distraction of approximately 3.6% of the thoracolumbar length after global osteotomy resulted in spinal cord injury and histological evidence of spinal cord damage. The pattern of recovery from the spinal cord injury after release of the distraction was consistent with the degree of axonal damage. Axotomy was observed in animals that exhibited no recovery on TES-MEP, and only hemorrhagic changes in the white matter were observed in animals that exhibited incomplete recovery.

Traumatic brain injury in young rats leads to progressive behavioral deficits coincident with altered tissue properties in adulthood

Traumatic brain injury (TBI) affects many infants and children, and results in enduring motor and cognitive impairments with accompanying changes in white matter tracts, yet few experimental studies in rodent juvenile models of TBI (jTBI) have examined the timeline and nature of these deficits, histologically and functionally. We used a single controlled cortical impact (CCI) injury to the parietal cortex of rats at post-natal day (P) 17 to evaluate behavioral alterations, injury volume, and morphological and molecular changes in gray and white matter, with accompanying measures of electrophysiological function. At 60 days post-injury (dpi), we found that jTBI animals displayed behavioral deficits in foot-fault and rotarod tests, along with a left turn bias throughout their early developmental stages and into adulthood. In addition, anxiety-like behaviors on the zero maze emerged in jTBI animals at 60 dpi. The final lesion constituted only ∼3% of brain volume, and morphological tissue changes were evaluated using MRI, as well as immunohistochemistry for neuronal nuclei (NeuN), myelin basic protein (MBP), neurofilament-200 (NF200), and oligodendrocytes (CNPase). White matter morphological changes were associated with a global increase in MBP immunostaining and reduced compound action potential amplitudes at 60 dpi. These results suggest that brain injury early in life can induce long-term white matter dysfunction, occurring in parallel with the delayed development and persistence of behavioral deficits, thus modeling clinical and longitudinal TBI observations.

A contusive model of unilateral cervical spinal cord injury using the infinite horizon impactor

While the majority of human spinal cord injuries occur in the cervical spinal cord, the vast majority of laboratory research employs animal models of spinal cord injury (SCI) in which the thoracic spinal cord is injured. Additionally, because most human cord injuries occur as the result of blunt, non-penetrating trauma (e.g. motor vehicle accident, sporting injury) where the spinal cord is violently struck by displaced bone or soft tissues, the majority of SCI researchers are of the opinion that the most clinically relevant injury models are those in which the spinal cord is rapidly contused.(1) Therefore, an important step in the preclinical evaluation of novel treatments on their way to human translation is an assessment of their efficacy in a model of contusion SCI within the cervical spinal cord. Here, we describe the technical aspects and resultant anatomical and behavioral outcomes of an unilateral contusive model of cervical SCI that employs the Infinite Horizon spinal cord injury impactor. Sprague Dawley rats underwent a left-sided unilateral laminectomy at C5. To optimize the reproducibility of the biomechanical, functional, and histological outcomes of the injury model, we contused the spinal cords using an impact force of 150 kdyn, an impact trajectory of 22.5° (animals rotated at 22.5°), and an impact location off of midline of 1.4 mm. Functional recovery was assessed using the cylinder rearing test, horizontal ladder test, grooming test and modified Montoya's staircase test for up to 6 weeks, after which the spinal cords were evaluated histologically for white and grey matter sparing. The injury model presented here imparts consistent and reproducible biomechanical forces to the spinal cord, an important feature of any experimental SCI model. This results in discrete histological damage to the lateral half of the spinal cord which is largely contained to the ipsilateral side of injury. The injury is well tolerated by the animals, but does result in functional deficits of the forelimb that are significant and sustained in the weeks following injury. The cervical unilateral injury model presented here may be a resource to researchers who wish to evaluate potentially promising therapies prior to human translation.

Design and biomechanical evaluation of a rodent spinal fixation device

Study Design

An in vitro and in vivo study in rats.

Objectives

To design a novel rat spinal fixation device and investigate its biomechanical effectiveness in stabilizing the spine up to eight weeks post injury.

Methods

A fixation device made of polyetheretherketone was designed to stabilize the spine via bilateral clamping pieces. The device effectiveness was assessed in a Sprague-Dawley rat model after it was applied to a spine with a fracture-dislocation injury produced at C5–C6. Animals were euthanized either immediately (n=6) or eight weeks (n=9) post-injury and the C3-T1 segment of the cervical spine was removed for biomechanical evaluation. Segments of intact spinal columns (C3-T1) (n=6) served as uninjured controls. In these tests, anterior-posterior shear forces were applied to the C3 vertebra to produce flexion and extension bending moments at the injury site (peak 12.8Nmm). The resultant two-dimensional motions at the injury site (i.e. C5–C6) were measured using digital imaging and reported as ranges of motion (ROM) or neutral zones (NZ).

Results

Flexion/extension ROMs (average ± S.D.) were 18.1 ± 3.3°, 19.9 ± 7.5°, and 1.5 ± 0.7°, respectively for the intact, injured/fixed, and injured/8-week groups, with the differences being highly significant for the injured/8-week group (p=0.0002). Flexion/extension NZs were 3.4 ± 2.8°, 5.0 ± 2.4°, and 0.7 ± 0.5°, respectively for the intact, injured/fixed, and injured/8-week groups, with the differences being significant for the injured/8-week group (p =0.04).

Conclusion

The device acutely stabilizes the spine and promotes fusion at the site of injury.

Translational spinal cord injury research: preclinical guidelines and challenges

Advances in the neurobiology of spinal cord injury (SCI) have prompted increasing attention to opportunities for moving experimental strategies towards clinical applications. Preclinical studies are the centerpiece of the translational process. A major challenge is to establish strategies for achieving optimal translational progression while minimizing potential repetition of previous disappointments associated with clinical trials. This chapter reviews and expands upon views pertaining to preclinical design reported in recently published opinion surveys. Subsequent discussion addresses other preclinical considerations more specifically related to current and potentially imminent cellular and pharmacological approaches to acute/subacute and chronic SCI. Lastly, a retrospective and prospective analysis examines how guidelines currently under discussion relate to select examples of past, current, and future clinical translations. Although achieving definition of the "perfect" preclinical scenario is difficult to envision, this review identifies therapeutic robustness and independent replication of promising experimental findings as absolutely critical prerequisites for clinical translation. Unfortunately, neither has been fully embraced thus far. Accordingly, this review challenges the notion "everything works in animals and nothing in humans", since more rigor must first be incorporated into the bench-to-bedside translational process by all concerned, whether in academia, clinical medicine, or corporate circles.

Care of rats with complete high-thoracic spinal cord injury

The complications of spinal cord injury (SCI) increase in number and severity with the level of injury. A recent survey of SCI researchers reveals that animal models of high SCI are essential. Despite this consensus, most laboratories continue to work with mid- or low-thoracic SCI. The available data on cervical SCI in animals characterize incomplete injuries; for example, nearly all studies published in 2009 examine discrete, tract-specific lesions that are not clinically-relevant. A primary barrier to developing animal models of severe, higher SCI is the challenge of animal care, a critical determinant of experimental outcome. Currently, many of these practices vary substantially between laboratories, and are passed down anecdotally within institutions. The care of animals with SCI is complex, and becomes much more challenging as the lesion level ascends. In our experience, the care of animals with high-thoracic (T3) SCI is much more demanding than the care of animals with low-thoracic SCI, even though both injuries result in paraplegia. We have developed an animal care regimen for rats with complete high-thoracic SCI. Our practices have been refined over the past 7 years, in collaboration with animal care centre staff and veterinarians. During this time, we have cared for more than 300 rats with T3 complete transection SCI, with experimental end-points of up to 3 months. Here we provide details of our animal care procedures, including acclimatization, housing, diet, antibiotic prophylaxis, surgical procedures, post-operative monitoring, and prevention of complications. In our laboratory, this comprehensive approach consistently produces good outcomes following T3 complete transection SCI: using body weight as an objective indicator of animal health, we have found that our rats typically return to pre-operative weights within 10 days of T3 complete SCI. It is our hope that the information provided here will improve care of experimental animals, and facilitate adoption of models that directly address the complications associated with higher level injuries.

Effects of advanced age on the morphometry and degenerative state of the cervical spine in a rat model

Aging causes changes in the geometry of the human cervical spine that may influence the tissue response to applied loads. Rat models are often used to study spinal cord injuries (SCI) and have the potential to enhance our understanding of the effect of age on SCI. The goal of this study was to characterize the morphometry and degenerative state of the cervical spine in Fisher 344 rats, and to determine the influence of age on these variables. Fifteen rats were split into three age groups: young adult (3 months of age), aged (12-18 months) and geriatric (30 months). Following tissue harvest we used a μCT scanner to image the cervical and upper thoracic spine from each specimen. Analysis software was used to measure variables including canal pinch diameter (the most rostral point on the dorsal aspect of a vertebral body to the most caudal aspect of the lamina on the immediately rostral vertebra), vertebral canal depth, width, and area, vertebral body height, depth, width, and area, and intervertebral disc thickness. Orthopaedic surgeons used midsagittal images to rate the degenerative state of the intervertebral discs. For all measures except disc thickness there was a significant increase (mean (SD) = 15.0 (9.7)%) for the aged compared to young specimens (P < 0.05). There were significant differences between the aged and geriatric specimens for only vertebral body depth (P = 0.016) and area (P = 0.020). Intervertebral disc degeneration was significantly greater on the ventral aspect of the spinal column (P < 0.001), with a trend toward increased degeneration in the geriatric specimens (P = 0.069). The results suggest that age-related morphometric differences may need to be accounted for in experimental aging models of SCI in rats.

Pharmacological interventions for spinal cord injury: where do we stand? How might we step forward?

Despite numerous studies reporting some measures of efficacy in the animal literature, there are currently no effective therapies for the treatment of traumatic spinal cord injuries (SCI) in humans. The purpose of this review is to delineate key pathophysiological processes that contribute to neurological deficits after SCI, as well as to describe examples of pharmacological approaches that are currently being tested in clinical trials, or nearing clinical translation, for the therapeutic management of SCI. In particular, we will describe the mechanistic rationale to promote neuroprotection and/or functional recovery based on theoretical, yet targeted pathological events. Finally, we will consider the clinical relevancy for emerging evidence that pharmacologically targeting mitochondrial dysfunction following injury may hold the greatest potential for increasing tissue sparing and, consequently, the extent of functional recovery following traumatic SCI.

Biomarkers for severity of spinal cord injury in the cerebrospinal fluid of rats

One of the major challenges in management of spinal cord injury (SCI) is that the assessment of injury severity is often imprecise. Identification of reliable, easily quantifiable biomarkers that delineate the severity of the initial injury and that have prognostic value for the degree of functional recovery would significantly aid the clinician in the choice of potential treatments. To find such biomarkers we performed quantitative liquid chromatography-mass spectrometry (LC-MS/MS) analyses of cerebrospinal fluid (CSF) collected from rats 24 h after either a moderate or severe SCI. We identified a panel of 42 putative biomarkers of SCI, 10 of which represent potential biomarkers of SCI severity. Three of the candidate biomarkers, Ywhaz, Itih4, and Gpx3 were also validated by Western blot in a biological replicate of the injury. The putative biomarkers identified in this study may potentially be a valuable tool in the assessment of the extent of spinal cord damage.

Characterization of a novel bidirectional distraction spinal cord injury animal model

Scoliosis corrective surgery requires the application of significant multidirectional stress forces, including distraction, for correction of the curved spine deformity and the application of fixation rods. If excessive, spine distraction may result in the development of new neurological deficits, some as severe as permanent paralysis. Current animal models of spinal cord injury, however, are limited to contusion, transection, or unidirectional distraction injuries, which fail to replicate the multidirectional forces that occur during spine corrective surgery. To address such limitation, we designed a novel device that relies on intervertebral grip fixation and linear actuators to induce controllable bidirectional distraction injuries to the spine. The device was tested in three (i.e., 3, 5, and 7 mm) distention paradigms of the rat T9-T11 vertebra, and the resulting injuries were evaluated through electrophysiological, behavioral, and histological analysis. As expected, 3mm bilateral spine distractions showed no neurological deficit. In contrast, those with 5 and 7 mm showed partial and complete paralysis, respectively. The relationship between the severity of the spine distraction and injury to the spinal cord tissue was determined using glial fibrillary acidic protein immunocytochemistry for visualization of reactive astrocytes and labeling of ED1-positive activated macrophages/microglia. Our results demonstrate that this device can produce bidirectional spine distraction injuries with high precision and control and, thus, may be valuable in contributing to the testing of neuroprotective strategies aimed at preventing unintended new neurological damage during corrective spine surgery.