Predicting Unconsciousness from a Pediatric Brain Injury Threshold

An exploration of higher-level language comprehension deficits and factors influencing them following blast TBI in US veterans

PRIMARY OBJECTIVE

The objective of this study was to investigate the factors that might have a negative influence on auditory processing and higher-level language processing in the US veterans of the recent foreign wars (Iraq and Afghanistan).

RESEARCH DESIGN

Exploratory, cross-sectional, correlational, prospective, cohort-design.

METHODS AND PROCEDURES

The experimental group consisted of 12 US veterans of war (10 males and 2 females) with blast exposure. The control group consisted of six US veterans (5 males and 1 female) without the history of blast exposure. Both groups were matched in mean age. Both groups were tested on Boston Assessment of Traumatic Brain Injury, Consonant Trigrams Test, Symbol Digit Modality Test, Trail Making Test, SCAN-3, CELF-5-Metalinguistics, CASL, and an unpublished test on the processing of sentence prosody.

MAIN OUTCOMES AND RESULTS

Significant group differences in attention, and time-compressed sentence processing were found. For those veterans (in the experimental group) who were not wearing their helmets at the time of blast, additional significant differences were noted with inferencing and auditory figure-ground tasks.

CONCLUSIONS

Findings support the importance of including speech/language pathologists in all stages of recovery for veterans post-blast exposure.

The Transverse Isotropy of Spinal Cord White Matter Under Dynamic Load

The rostral-caudally aligned fiber-reinforced structure of spinal cord white matter (WM) gives rise to transverse isotropy in the material. Stress and strain patterns generated in the spinal cord parenchyma following spinal cord injury (SCI) are multidirectional and dependent on the mechanism of the injury. Our objective was to develop a WM constitutive model that captures the material transverse isotropy under dynamic loading. The WM mechanical behavior was extracted from the published tensile and compressive experiments. Combinations of isotropic and fiber-reinforcing models were examined in a conditional quasi-linear viscoelastic (QLV) formulation to capture the WM mechanical behavior. The effect of WM transverse isotropy on SCI model outcomes was evaluated by simulating a nonhuman primate (NHP) contusion injury experiment. A second-order reduced polynomial hyperelastic energy potential conditionally combined with a quadratic reinforcing function in a four-term Prony series QLV model best captured the WM mechanical behavior (0.89 < R2 < 0.99). WM isotropic and transversely isotropic material models combined with discrete modeling of the pia mater resulted in peak impact forces that matched the experimental outcomes. The transversely isotropic WM with discrete pia mater resulted in maximum principal strain (MPS) distributions which effectively captured the combination of ipsilateral peripheral WM sparing, ipsilateral injury and contralateral sparing, and the rostral/caudal spread of damage observed in in vivo injuries. The results suggest that the WM transverse isotropy could have an important role in correlating tissue damage with mechanical measures and explaining the directional sensitivity of the spinal cord to injury.

Biofidelic white matter heterogeneity decreases computational model predictions of white matter strains during rapid head rotations

The finite element (FE) brain model is used increasingly as a design tool for developing technology to mitigate traumatic brain injury. We developed an ultra high-definition FE brain model (>4 million elements) from CT and MRI scans of a 2-month-old pre-adolescent piglet brain, and simulated rapid head rotations. Strain distributions in the thalamus, coronal radiata, corpus callosum, cerebral cortex gray matter, brainstem and cerebellum were evaluated to determine the influence of employing homogeneous brain moduli, or distinct experimentally derived gray and white matter property representations, where some white matter regions are stiffer and others less stiff than gray matter. We find that constitutive heterogeneity significantly lowers white matter deformations in all regions compared with homogeneous properties, and should be incorporated in FE model injury prediction.

A Porcine Model of Traumatic Brain Injury via Head Rotational Acceleration

Unique from other brain disorders, traumatic brain injury (TBI) generally results from a discrete biomechanical event that induces rapid head movement. The large size and high organization of the human brain makes it particularly vulnerable to traumatic injury from rotational accelerations that can cause dynamic deformation of the brain tissue. Therefore, replicating the injury biomechanics of human TBI in animal models presents a substantial challenge, particularly with regard to addressing brain size and injury parameters. Here we present the historical development and use of a porcine model of head rotational acceleration. By scaling up the rotational forces to account for difference in brain mass between swine and humans, this model has been shown to produce the same tissue deformations and identical neuropathologies found in human TBI. The parameters of scaled rapid angular accelerations applied for the model reproduce inertial forces generated when the human head suddenly accelerates or decelerates in falls, collisions, or blunt impacts. The model uses custom-built linkage assemblies and a powerful linear actuator designed to produce purely impulsive non-impact head rotation in different angular planes at controlled rotational acceleration levels. Through a range of head rotational kinematics, this model can produce functional and neuropathological changes across the spectrum from concussion to severe TBI. Notably, however, the model is very difficult to employ, requiring a highly skilled team for medical management, biomechanics, neurological recovery, and specialized outcome measures including neuromonitoring, neurophysiology, neuroimaging, and neuropathology. Nonetheless, while challenging, this clinically relevant model has proven valuable for identifying mechanisms of acute and progressive neuropathologies as well as for the evaluation of noninvasive diagnostic techniques and potential neuroprotective treatments following TBI.

Impact depth and the interaction with impact speed affect the severity of contusion spinal cord injury in rats

Spinal cord injury (SCI) biomechanics suggest that the mechanical factors of impact depth and speed affect the severity of contusion injury, but their interaction is not well understood. The primary aim of this work was to examine both the individual and combined effects of impact depth and speed in contusion SCI on the cervical spinal cord. Spinal cord contusions between C5 and C6 were produced in anesthetized rats at impact speeds of 8, 80, or 800 mm/s with displacements of 0.9 or 1.5 mm (n=8/group). After 7 days postinjury, rats were assessed for open-field behavior, euthanized, and spinal cords were harvested. Spinal cord tissue sections were stained for demyelination (myelin-based protein) and tissue sparing (Luxol fast blue). In parallel, a finite element model of rat spinal cord was used to examine the resulting maximum principal strain in the spinal cord during impact. Increasing impact depth from 0.9 to 1.5 mm reduced open-field scores (p<0.01) above 80 mm/s, reduced gray (GM) and white matter (WM) sparing (p<0.01), and increased the amount of demyelination (p<0.01). Increasing impact speed showed similar results at the 1.5-mm impact depth, but not the 0.9-mm impact depth. Linear correlation analysis with finite element analysis strain showed correlations (p<0.001) with nerve fiber damage in the ventral (R(2)=0.86) and lateral (R(2)=0.74) regions of the spinal cord and with WM (R(2)=0.90) and GM (R(2)=0.76) sparing. The results demonstrate that impact depth is more important in determining the severity of SCI and that threshold interactions exist between impact depth and speed.

The mechanics of traumatic brain injury: a review of what we know and what we need to know for reducing its societal burden

Traumatic brain injury (TBI) is a significant public health problem, on pace to become the third leading cause of death worldwide by 2020. Moreover, emerging evidence linking repeated mild traumatic brain injury to long-term neurodegenerative disorders points out that TBI can be both an acute disorder and a chronic disease. We are at an important transition point in our understanding of TBI, as past work has generated significant advances in better protecting us against some forms of moderate and severe TBI. However, we still lack a clear understanding of how to study milder forms of injury, such as concussion, or new forms of TBI that can occur from primary blast loading. In this review, we highlight the major advances made in understanding the biomechanical basis of TBI. We point out opportunities to generate significant new advances in our understanding of TBI biomechanics, especially as it appears across the molecular, cellular, and whole organ scale.

Maximum principal strain correlates with spinal cord tissue damage in contusion and dislocation injuries in the rat cervical spine

The heterogeneity of the primary mechanical mechanism of spinal cord injury (SCI) is not currently used to tailor treatment strategies because the effects of these distinct patterns of acute mechanical damage on long-term neuropathology have not been fully investigated. A computational model of SCI enables the dynamic analysis of mechanical forces and deformations within the spinal cord tissue that would otherwise not be visible from histological tissue sections. We created a dynamic, three-dimensional finite element (FE) model of the rat cervical spine and simulated contusion and dislocation SCI mechanisms. We investigated the relationship between maximum principal strain and tissue damage, and compared primary injury patterns between mechanisms. The model incorporated the spinal cord white and gray matter, the dura mater, cerebrospinal fluid, spinal ligaments, intervertebral discs, a rigid indenter and vertebrae, and failure criteria for ligaments and vertebral endplates. High-speed (∼ 1 m/sec) contusion and dislocation injuries were simulated between vertebral levels C3 and C6 to match previous animal experiments, and average peak maximum principal strains were calculated for several regions at the injury epicenter and at 1-mm intervals from +5 mm rostral to -5 mm caudal to the lesion. Average peak principal strains were compared to tissue damage measured previously in the same regions via axonal permeability to 10-kD fluorescein-dextran. Linear regression of tissue damage against peak maximum principal strain for pooled data within all white matter regions yielded similar and significant (p<0.0001) correlations for both contusion (R(2)=0.86) and dislocation (R(2)=0.52). The model enhances our understanding of the differences in injury patterns between SCI mechanisms, and provides further evidence for the link between principal strain and tissue damage.

MATERIAL PROPERTIES OF ADULT RAT SKULL

Development of advanced computational rat head models requires accurate material properties of the rat brain, meninges, skull, and other soft tissues. This study investigated adult rat skull material properties, which are very limited in the current literature. A total of 20 skull specimens were harvested from 10 adult rats. High resolution (16 μm) microcomputed tomography scans were performed for each specimen to observe dimensional changes within each specimen and internal porosities through the cross sections. The specimens were tested in three-point bending at loading velocities of 0.02 and 200 mm/s. The elastic modulus, energy absorbed to failure, energy density, and bending stress were calculated using classical beam theory. Results demonstrated that bending velocity (strain rate) had significant effect on elastic modulus and bending stress, but not on energy and energy density. The Young's moduli of rat skull measured in this study were comparable to those measured from the adult human skull.

Injury biomechanics and child abuse

Child abuse is a leading cause of morbidity and mortality in young children and infants in the United States. Medical care providers, social services, and legal systems make critical decisions regarding injury and history plausibility daily. Injury plausibility judgments rely on evidence-based medicine, individualized experiences, and empirical data. A poor outcome may result if abuse is missed or an innocent family is accused, therefore evidence and science-based injury assessments are required. Although research in biomechanics has improved clinical understanding of injuries in children, much work is still required to develop a more scientific, rigorous approach to assessing injury causation. This article reviews key issues in child abuse and how injury biomechanics research may help improve accuracy in differentiating abuse from accidental events. Case-based biomechanical investigations, human surrogate, and computer modeling biomechanics research applied to child abuse injury are discussed. The goal of this paper is to provide an overview of key research studies rather than on review or commentary articles. Limitations and future research needs are also reviewed.

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.

Neuropsychology and clinical neuroscience of persistent post-concussive syndrome

On the mild end of the acquired brain injury spectrum, the terms concussion and mild traumatic brain injury (mTBI) have been used interchangeably, where persistent post-concussive syndrome (PPCS) has been a label given when symptoms persist for more than three months post-concussion. Whereas a brief history of concussion research is overviewed, the focus of this review is on the current status of PPCS as a clinical entity from the perspective of recent advances in the biomechanical modeling of concussion in human and animal studies, particularly directed at a better understanding of the neuropathology associated with concussion. These studies implicate common regions of injury, including the upper brainstem, base of the frontal lobe, hypothalamic-pituitary axis, medial temporal lobe, fornix, and corpus callosum. Limitations of current neuropsychological techniques for the clinical assessment of memory and executive function are explored and recommendations for improved research designs offered, that may enhance the study of long-term neuropsychological sequelae of concussion.

Linking impact to cellular and molecular sequelae of CNS injury: modeling in vivo complexity with in vitro simplicity

Traumatic brain injury (TBI) represents one of most common disorders to the central nervous system (CNS). Despite significant efforts, though, an effective clinical treatment for TBI is not yet available. The complexity of human TBI is modeled with a broad group of experimental models, with each model matching some aspect of the human condition. In the past 15 years, these in vivo models were complemented with a group of in vitro models, with these in vitro models allowing investigators to more precisely identify the mechanism(s) of TBI, the different intracellular events that occur in acute period following injury, and the possible treatment of this injury in vitro. In this paper, we review the available in vitro models to study TBI, discuss their biomechanical basis for human TBI, and review the findings from these in vitro models. Finally, we synthesize the current knowledge and point out possible future directions for this group of models, especially in the effort toward developing new therapies for the traumatically brain injured patient.