Injury biomechanics, neuropathology, and simplified physics of explosive blast and impact mild traumatic brain injury

Explosive blast shock waves and blunt impact to the head are two types of loading shown to result in mild traumatic brain injury (mTBI). While mTBI from these two causes shares some common features behaviorally, there are distinct differences in the pathophysiology of the underlying injury mechanisms. Various elucidations have been offered in the literature to explain the organic damage associated with mTBI resulting from both types of loading. The current state of understanding in this field is somewhat limited by the degree of appreciation of the physics and biomechanics governing the effects of explosive blast shock waves and blunt impact on the head, which has resulted in the various approaches to the investigation of the operative brain injury "wounding mechanisms". In this chapter we provide a simplified description of terminology associated with forces on the head from explosive blast shock waves and blunt impact, to assist readers in the field in evaluating interpretations of brain injury "wounding" processes. Remarkably, mTBI from either loading is shown generally to result in only a small loss of neurons, with hippocampal neurons appearing to be particularly vulnerable to explosive blast shock waves. Explosive blast studies in large animal models show a unique pattern of periventricular injury, which is different from the classic diffuse axonal injury. Both astrocyte and microglial activation are also seen in explosive blast as well as impact trauma, but this may be a general secondary brain injury response, nonspecific to explosive blast or blunt trauma. Additionally, while moderate to severe impact closed head injuries sometimes result in petechial hemorrhages or hematomas, they do not appear to be associated with explosive blast mTBI even with repeated exposure to blasts.

Brain injury from explosive blast: description and clinical management

Accumulating clinical experience is indicating that explosive blast brain injury is becoming recognized as a disease distinct from the penetrating form of blast injury as well as the classic closed head injury (CHI). In recent US conflicts in Iraq and Afghanistan, over 60% of combat casualties were from explosive blast with the hallmark explosive weapon being the improvised explosive device (IED). Explosive blast TBI is a condition afflicting many combat injured warfighters potentially constituting another category of TBI. Clinically, it shares many features with conventional TBI but possesses some unique aspects. In its mild form, it also shares many clinical features with PTSD but here again has distinct aspects. Although military medical providers depend on civilian standard of care guidelines when managing explosive blast mTBI, they are continually adapting their medical practice in order to optimize the treatment of this disease, particularly in a theater of war. It is clear that further rigorous scientific study of explosive blast mTBI at both the basic science and clinical levels is needed. This research must include improved understanding of the causes and mechanisms of explosive blast TBI as well as comprehensive epidemiologic studies to determine the prevalence of this disease and its risk factors. A widely accepted unambiguous clinical description of explosive blast mTBI with diagnostic criteria would greatly improve diagnosis. It is hoped that through appropriate research meaningful prevention, mitigation, and treatment strategies for explosive blast mTBI can be speedily realized.

Advances in imaging explosive blast mild traumatic brain injury

In the past, direct physical evidence of mild traumatic brain injury (mTBI) from explosive blast has been difficult to obtain through conventional imaging modalities such as T1- and T2-weighted magnetic resonance imaging (MRI) and computed tomography (CT). Here, we review current progress in detecting evidence of brain injury from explosive blast using advanced imaging, including diffusion tensor imaging (DTI), functional MRI (fMRI), and the metabolic imaging methods such as positron emission tomography (PET) and magnetic resonance spectroscopic imaging (MRSI), where each targets different aspects of the pathology involved in mTBI. DTI provides a highly sensitive measure to detect primary changes in the microstructure of white matter tracts. fMRI enables the measurement of changes in brain activity in response to different stimuli or tasks. Remarkably, all three of these paradigms have found significant success in conventional mTBI where conventional clinical imaging frequently fails to provide definitive differences. Additionally, although used less frequently for conventional mTBI, PET has the potential to characterize a variety of neurotransmitter systems using target agents and will undoubtedly play a larger role, once the basic mechanisms of injury are better understood and techniques to identify the injury are more common. Finally, our MRSI imaging studies, although acquired at much lower spatial resolution, have demonstrated selectivity to different metabolic and physiologic processes, uncovering some of the most profound differences on an individual by individual basis, suggesting the potential for utility in the management of individual patients.

An imaging-based computational and experimental study of skull fracture: finite element model development

The development of a three-dimensional finite element model of the human skull is presented as a part of a novel imaging-based experimental and computational approach to study skull fracture. The rationale for this approach and its potential utility are presented. Mathematical model development, using computed tomography (CT) imaging data as a basis for collecting geometric characteristics of the skull, is given. The model includes anatomical detail of the cranial structure, accurate thickness distributions of the bone and soft tissue components, and density information that can be related to the elastic moduli of the various constituent materials. A procedure for generating the model from CT scan image data is presented. This procedure segments the head into cranial bone, facial, and scalp structures, and the brain.

On the mechanics of impact neurotrauma: a review and critical synthesis

A computational modeling technique for the simulation of head impact mechanics is proposed as a tool for the study of traumatic brain injury (TBI). The capabilities, limitations, and consequently the utilities of this technique are discussed in light of the current computing and imaging technologies and their prospective advancement. A significant portion of the mechanics, relevant to head impact simulation, has been incorporated into this computational technique resulting in automated devices that ease the technical burden of biomechanical analysis of TBI mechanisms. A brief exposition of the concepts utilized in these devices is given and their basis in experiment is discussed.

Mechanical characteristics of vascular tissue and their role in brain injury modeling: a review

A review of the mechanical characterization of vascular tissue is presented in the context of its relevance to the computational modeling of traumatic brain injury (TBI). Mathematical representations of experimentally observed vascular behavior are described. Several material models, for large vessels, and their applicability to blood vessels in the brain are discussed. Integration of vascular tissue behavior into an existing computational model of the brain, to account for the anisotropic effects they produce, is discussed.