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Technical Paper

The Roles of Mechanical Compression and Chemical Irritation in Regulating Spinal Neuronal Signaling in Painful Cervical Nerve Root Injury

Both traumatic and slow-onset disc herniation can directly compress and/or chemically irritate cervical nerve roots, and both types of root injury elicit pain in animal models of radiculopathy. This study investigated the relative contributions of mechanical compression and chemical irritation of the nerve root to spinal regulation of neuronal activity using several outcomes. Modifications of two proteins known to regulate neurotransmission in the spinal cord, the neuropeptide calcitonin gene-related peptide (CGRP) and glutamate transporter 1 (GLT-1), were assessed in a rat model after painful cervical nerve root injuries using a mechanical compression, chemical irritation or their combination of injury. Only injuries with compression induced sustained behavioral hypersensitivity (p≤0.05) for two weeks and significant decreases (p<0.037) in CGRP and GLT-1 immunoreactivity to nearly half that of sham levels in the superficial dorsal horn.
Technical Paper

Development of a Duration Threshold for Modulating Evoked Neuronal Responses After Nerve Root Compression Injury

Cervical nerve roots are susceptible to compression injuries of various durations. The duration of an applied compression has been shown to contribute to both the onset of persistent pain and also the degree of spinal cellular and molecular responses related to nociception. This study investigated the relationship between peripherally evoked activity in spinal cord neurons during a root compression and the resulting development of axonal damage. Electrically evoked spikes were measured in the spinal cord as a function of time during and after (post-compression) a 15 minute compression of the C7 nerve root. Compression to the root significantly (p=0.035) reduced the number of spikes that were evoked over time relative to sham. The critical time for compression to maximally reduce evoked spikes was 6.6±3.0 minutes. A second study measured the post-compression evoked neuronal activity following compression applied for a shorter, sub-threshold time (three minutes).
Technical Paper

Digital Human Modeling Goals and Strategic Plans

Digital human modeling (DHM) progress worldwide will be much faster and cohesive if the diverse community now developing simulations has a global blueprint for DHM, and is able to work together efficiently. DHM developers and users can save time by building on each other's work. This paper highlights a panel discussion on DHM goals and strategic plans for the next decade to begin formulating the international blueprint. Four subjects are chosen as the starting points: (1) moving DHM into the public safety and internet arenas, (2) role of DHM in computer assisted surgery and automotive safety, (3) DHM in defense applications, and (4) DHM to improve workplace ergonomics.
Technical Paper

Pediatric Rotational Inertial Brain Injury: the Relative Influence of Brain Size and Mechanical Properties

Head injury is the most common cause of death and acquired disability in childhood. We seek to determine the influence of brain mechanical properties on inertial pediatric brain injury. Large deformation material properties of porcine pediatric and adult brain tissue were measured and represented by a first-order Ogden hyperelastic viscoelastic constitutive model. A 3-D finite element mesh was created of a mid-coronal slice of the brain and skull of a human adult and child (2 weeks old). Three finite element models were constructed: (1) a pediatric mesh with pediatric brain properties, (2) a pediatric mesh with adult tissue properties, and (3) an adult mesh with adult tissue properties. The skull was modeled as a rigid solid and an angular acceleration was applied in the coronal plane with center at C4/C5. The brain is assumed to be homogeneous and isotropic.
Technical Paper

Comparing Experimental Data to Traumatic Brain Injury Finite Element Models

Validating a traumatic brain injury finite element model is often limited by a lack of extensive animal injury data that may be used to examine the conditions under which the model is accurate. Given that most published reports specify only general descriptions of injury, this study examined potential evaluation strategies and assessed the ability of a finite element model to simulate the general descriptions of injury in an animal model. The results of this study showed that 1) the results from a simplified finite element model could estimate trends that were similar to the injury patterns observed in a set of animal experiments, 2) a parameter (Z parameter), which quantified the comparison process between computational and animal data, estimated trends that would help in the model evaluation process, and 3) a more complete evaluation process would occur if multiple testing methods were included in the evaluation procedure.
Technical Paper

Finite Element Modeling Approaches for Predicting Injury in an Experimental Model of Severe Diffuse Axonal Injury

Traumatic brain injury finite element analyses have evolved from crude geometric representations of the skull and brain system into sophisticated models which take into account distinct anatomical features. However, two distinct finite element modeling approaches have evolved to account for the relative motion that occurs between the skull and cerebral cortex during traumatic brain injury. The first and most common approach assumes that the relative motion can be estimated by representing the cerebrospinal fluid inside the subarachnoid space as a low shear modulus, virtually incompressible solid. The second approach assumes that the relative motion can be approximated by defining a frictional interface between the cerebral cortex and dura mater. This study presents data from an experimental model of traumatic brain injury coupled with finite element analyses to evaluate the modeling approach's ability to predict specific forms of traumatic brain injury.
Technical Paper

Regional Differences in Mechanical Properties of the Porcine Central Nervous System

Computational modeling is a potentially powerful tool to provide information about the mechanisms of traumatic brain injury. In order to ensure that the estimates calculated by these computer models provide the most useful information, it is essential that these models contain accurate central nervous system (CNS) tissue properties. Previous material property measurements lack strict control over crucial experimental parameters that may influence material properties and tail to examine any regional variation in the measured response. To address these issues, we measured the material response of two regions of the CNS, the brainstem and the cerebrum. Specifically, adult porcine tissue was subjected to high loading rate mechanical deformation using a custom designed oscillatory shear device. Complex shear moduli were calculated over a range of frequencies (20-200 Hz) at two engineering strain amplitudes (2.5%, and 5.0%).
Technical Paper

Biomechanics of Diffuse Brain Injuries

This report discusses the development of brain injury tolerance criteria based on the study of three model systems: the primate, inanimate physical surrogates, and isolated tissue elements. Although we are equally concerned with the neural and neurovascular tissue components of the brain, the report will focus on the former and, in particular, the axonal elements. Under conditions of distributed, impulsive, angularacceleration loading, the primate model exhibits a pathophysiological response ranging from mild cerebral concussion to massive, diffuse white matter damage with prolonged coma. When physical models are subjected to identical loading conditions it becomes possible to map the displacements and calculate the associated strains and stresses within the field simulating the brain. Correlating these experimental models leads to predictive levels of tissue element deformation that may be considered as a threshold for specific mechanisms of injury.