Andrew C. Merkle

The pathobiology of blast injuries and blast-induced neurotrauma as identified using a new experimental model of injury in mice

Current experimental models of blast injuries used to study blast-induced neurotrauma (BINT) vary widely, which makes the comparison of the experimental results extremely challenging. Most of the blast injury models replicate the ideal Friedländer type of blast wave, without the capability to generate blast signatures with multiple shock fronts and refraction waves as seen in real-life conditions; this significantly reduces their clinical and military relevance. Here, we describe the pathophysiological consequences of graded blast injuries and BINT generated by a newly developed, highly controlled, and reproducible model using a modular, multi-chamber shock tube capable of tailoring pressure wave signatures and reproducing complex shock wave signatures seen in theater. While functional deficits due to blast exposure represent the principal health problem for todays warfighters, the majority of available blast models induces tissue destruction rather than mimic functional deficits. Thus, the main goal of our model is to reliably reproduce long-term neurological impairments caused by blast. Physiological parameters, functional (motor, cognitive, and behavioral) outcomes, and underlying molecular mechanisms involved in inflammation measured in the brain over the 30 day post-blast period showed this model is capable of reproducing major neurological changes of clinical BINT.

Assessing Behind Armor Blunt Trauma (BABT) Under NIJ Standard-0101.04 Conditions Using Human Torso Models

BACKGROUND Although soft armor vests serve to prevent penetrating wounds and dissipate impact energy, the potential of nonpenetrating injury to the thorax, termed behind armor blunt trauma, does exist. Currently, the ballistic resistance of personal body armor is determined by impacting a soft armor vest over a clay backing and measuring the resulting clay deformation as specified in National Institute of Justice (NIJ) Standard-0101.04. This research effort evaluated the efficacy of a physical Human Surrogate Torso Model (HSTM) as a device for determining thoracic response when exposed to impact conditions specified in the NIJ Standard. METHODS The HSTM was subjected to a series of ballistic impacts over the sternum and stomach. The pressure waves propagating through the torso were measured with sensors installed in the organs. A previously developed Human Torso Finite Element Model (HTFEM) was used to analyze the amount of tissue displacement during impact and compared with the amount of clay deformation predicted by a validated finite element model. All experiments and simulations were conducted at NIJ Standard test conditions. RESULTS When normalized by the response at the lowest threat level (Level I), the clay deformations for the higher levels are relatively constant and range from 2.3 to 2.7 times that of the base threat level. However, the pressures in the HSTM increase with each test level and range from three to seven times greater than Level I depending on the organ. CONCLUSIONS The results demonstrate the abilities of the HSTM to discriminate between threat levels, impact conditions, and impact locations. The HTFEM and HSTM are capable of realizing pressure and displacement differences because of the level of protection, surrounding tissue, and proximity to the impact point. The results of this research provide insight into the transfer of energy and pressure wave propagation during ballistic impacts using a physical surrogate and computational model of the human torso.

Human head–neck computational model for assessing blast injury

A human head finite element model (HHFEM) was developed to study the effects of a blast to the head. To study both the kinetic and kinematic effects of a blast wave, the HHFEM was attached to a finite element model of a Hybrid III ATD neck. A physical human head surrogate model (HSHM) was developed from solid model files of the HHFEM, which was then attached to a physical Hybrid III ATD neck and exposed to shock tube overpressures. This allowed direct comparison between the HSHM and HHFEM. To develop the temporal and spatial pressures on the HHFEM that would simulate loading to the HSHM, a computational fluid dynamics (CFD) model of the HHFEM in front of a shock tube was generated. CFD simulations were made using loads equivalent to those seen in experimental studies of the HSHM for shock tube driver pressures of 517, 690 and 862 kPa. Using the selected brain material properties, the peak intracranial pressures, temporal and spatial histories of relative brain-skull displacements and the peak relative brain-skull displacements in the brain of the HHFEM compared favorably with results from the HSHM. The HSHM sensors measured the rotations of local areas of the brain as well as displacements, and the rotations of the sensors in the sagittal plane of the HSHM were, in general, correctly predicted from the HHFEM. Peak intracranial pressures were between 70 and 120 kPa, while the peak relative brain-skull displacements were between 0.5 and 3.0mm.

Human Surrogate Head Response to Dynamic Overpressure Loading in Protected and Unprotected Conditions

The ballistic performance of helmets has contributed to increased soldier survivability through the prevention of penetrating injuries. However, the efficacy of helmets in mitigating primary blast–induced traumatic brain injury (bTBI) is unclear. The objective of this effort was to utilize the Human Surrogate Head Model (HSHM) to investigate brain response to shock tube overpressure loading conditions, both with and without personal protective equipment (PPE). The HSHM is a physical surrogate which includes a brain, skull, facial structure and skin, all fabricated using biosimulant materials. The system was mounted to a Hybrid III Anthropomorphic Test Device neck to allow head motion during overpressure exposure. Pressure sensors were embedded along the sagittal plane in the anterior and posterior regions of the biosimulant brain. A series of shock tube tests using driver pressures at four levels (ranging from 420 to 1150 kPa) were conducted to simulate blast loading conditions. Internal pressure response was highly correlated to driver pressure, thus demonstrating the surrogate models sensitivity to load conditions. Characteristic features observed in the archetypical pressure waveform were used to evaluate differences between test parameters, including the effects of a helmet system on response. Results suggest that the helmet does not alter the initial peak pressure response. However, certain subsequent pressure peaks were found to undergo statistically significant reductions when a helmet was placed on the HSHM. The results of this test series demonstrate the use of a surrogate head system in characterizing the brain response to overpressure loading. Future studies will further evaluate the efficacy of PPE and contribute to the understanding of blast-induced injury mechanisms.

Development and validation of a statistical shape modeling-based finite element model of the cervical spine under low-level multiple direction loading conditions

Cervical spinal injuries are a significant concern in all trauma injuries. Recent military conflicts have demonstrated the substantial risk of spinal injury for the modern warfighter. Finite element models used to investigate injury mechanisms often fail to examine the effects of variation in geometry or material properties on mechanical behavior. The goals of this study were to model geometric variation for a set of cervical spines, to extend this model to a parametric finite element model, and, as a first step, to validate the parametric model against experimental data for low-loading conditions. Individual finite element models were created using cervical spine (C3–T1) computed tomography data for five male cadavers. Statistical shape modeling (SSM) was used to generate a parametric finite element model incorporating variability of spine geometry, and soft-tissue material property variation was also included. The probabilistic loading response of the parametric model was determined under flexion-extension, axial rotation, and lateral bending and validated by comparison to experimental data. Based on qualitative and quantitative comparison of the experimental loading response and model simulations, we suggest that the model performs adequately under relatively low-level loading conditions in multiple loading directions. In conclusion, SSM methods coupled with finite element analyses within a probabilistic framework, along with the ability to statistically validate the overall model performance, provide innovative and important steps toward describing the differences in vertebral morphology, spinal curvature, and variation in material properties. We suggest that these methods, with additional investigation and validation under injurious loading conditions, will lead to understanding and mitigating the risks of injury in the spine and other musculoskeletal structures.

An Enhanced Articulated Human Body Model Under C4 Blast Loadings

Previously we had developed an articulated human body model to simulate the kinematic response to the external loadings, using CFDRC’s CoBi implicit multi-body solver. The anatomy-based human body model can accurately account for the surface loadings and surface interactions with the environment. A study is conducted to calibrate the joint properties (for instance, the joint rotational damping) of the articulated human body by comparing its response with those obtained from the PMHS test under moderate loading conditions. Additional adjustments in the input parameters also include the contact spring constants for joint stops at different joint locations. By comparing the computational results with the real scenarios, we fine tune these input parameters and further improve the accuracy of the articulated human body model. In order to simulate the effect of a C4 explosion on a human body in the open field, we employ a CFD model with a good resolution and the appropriate boundary treatment to obtain the blast loading condition on the human body surface more accurately. The numerical results of the blast simulation are shown to be comparable to the test data. With the interface to apply the blast pressure loading from the CFD simulation on the articulated human body surface, the articulated human body dynamics due to the C4 explosions are modeled and the simulation results are shown to be physiological reasonable.

Correlating Tissue Response with Anatomical Location of mTBI Using a Human Head Finite Element Model under Simulated Blast Conditions

Mild traumatic brain injury (mTBI) has recently been shown to include deficits in cognitive function that have been correlated to changes in tissue within regions of the white matter in the brain. These localized regions show decreased anisotropy in water diffusivity, which are thought to be related to local mechanical damage. However, a specific link to mechanical factors and tissue changes in these regions has not been made. This study is an initial attempt at such a correlation. A human head finite element model, verified against experimental data under simulated blast loading conditions, was used to estimate strains within regions in the brain that are correlated to functional deficits. Strain values from the most anterior and posterior extent of the corpus callosum (the rostrum and the splenium), the right and left anterior and posterior limb of the internal capsule (ALIC and PLIC), and the left cingulum bundle were calculated under frontal blast loading at overpressure intensities below those typically known to cause injury. Strain peaks of approximately 1 percent were noted in regions associated with cognitive brain injury, indicating that loading conditions which involve higher pressures could raise strains to significant levels.

Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury.

Traumatic brain injury (TBI) caused by explosive munitions, known as blast TBI, is the signature injury in recent military conflicts in Iraq and Afghanistan. Diagnostic evaluation of TBI, including blast TBI, is based on clinical history, symptoms, and neuropsychological testing, all of which can result in misdiagnosis or underdiagnosis of this condition, particularly in the case of TBI of mild-to-moderate severity. Prognosis is currently determined by TBI severity, recurrence, and type of pathology, and also may be influenced by promptness of clinical intervention when more effective treatments become available. An important task is prevention of repetitive TBI, particularly when the patient is still symptomatic. For these reasons, the establishment of quantitative biological markers can serve to improve diagnosis and preventative or therapeutic management. In this study, we used a shock-tube model of blast TBI to determine whether manganese-enhanced magnetic resonance imaging (MEMRI) can serve as a tool to accurately and quantitatively diagnose mild-to-moderate blast TBI. Mice were subjected to a 30 psig blast and administered a single dose of MnCl2 intraperitoneally. Longitudinal T1-magnetic resonance imaging (MRI) performed at 6, 24, 48, and 72 h and at 14 and 28 days revealed a marked signal enhancement in the brain of mice exposed to blast, compared with sham controls, at nearly all time-points. Interestingly, when mice were protected with a polycarbonate body shield during blast exposure, the marked increase in contrast was prevented. We conclude that manganese uptake can serve as a quantitative biomarker for TBI and that MEMRI is a minimally-invasive quantitative approach that can aid in the accurate diagnosis and management of blast TBI. In addition, the prevention of the increased uptake of manganese by body protection strongly suggests that the exposure of an individual to blast risk could benefit from the design of improved body armor.

Modeling Articulated Human Body Dynamics Under a Representative Blast Loading

Blast waves resulting from both industrial explosions and terrorist attacks cause devastating effects to exposed humans and structures. Blast related injuries are frequently reported in the international news and are of great interest to agencies involved in military and civilian protection. Mathematical models of explosion blast interaction with structures and humans can provide valuable input in the design of protective structures and practices, in injury diagnostics and forensics. Accurate simulation of blast wave interaction with a human body and the human body biodynamic response to the blast loading is very challenging and to the best of our knowledge has not been reported yet. A high-fidelity computational fluid dynamic (CFD) model is required to capture the reflections, diffractions, areas of stagnation, and other effects when the shock and blast waves respond to an object placed in the field. In this effort we simulated a representative free field blast event with a standing human exposed to the threat using the Second Order Hydrodynamic Automatic Mesh Refinement Code (SHAMRC). During the CFD analysis the pressure time history around the human body is calculated, along with the fragment loads. Subsequently these blast loads are applied to a fully articulated human body using the multi-physics code CoBi. In CoBi we developed a novel computational model for the articulated human body dynamics by utilizing the anatomical geometry of human body. The articulated human body dynamics are computed by an implicit multi-body solver which ensures the unconditional stability and guarantees the quadratic rate of convergence. The developed solver enforces the kinematic constraints well while imposing no limitation on the time step size. The main advantage of the model is the anatomical surface representation of a human body which can accurately account for both the surface loading and the surface interaction. The inertial properties are calculated using a finite element method. We also developed an efficient interface to apply the blast wave loading on the human body surface. The numerical results show that the developed model is capable of reasonably predicting the human body dynamics and can be used to study the primary injury mechanism. We also demonstrate that the human body response is affected by many factors such as human inertia properties, contact damping and the coefficient of friction between the human body and the environment. By comparing the computational results with the real scenario, we can calibrate these input parameters to improve the accuracy of articulated human body model.© 2011 ASME

Development of a Human Head Physical Surrogate Model for Investigating Blast Injury

The objective of this effort was to develop a Human Surrogate Head Model (HSHM) and measure its response to pressure loading conditions representative of a blast environment. The HSHM consists of skin, face, skull, and brain fabricated using biosimulant materials and mounted to the neck of a Hybrid III Anthropomorphic Test Device to allow head motion during loading. The HSHM instrumentation includes pressure and displacement sensors embedded in the anterior and posterior areas of the brain along the saggital plane. The displacement sensors are a custom solution developed for this particular application. A series of shock tube tests at three varying load levels were conducted with the HSHM to simulate blast loading conditions. As pressure loading levels increased, the intracranial pressures and brain displacements increased as well. However, the spatial response of the displacement sensors varied with location in the brain. The results of this test series provide the first instance of intracranial pressure and directly measured brain displacements recorded from an anatomically correct head surrogate exposed to conditions representative of blast loading.Copyright