Acute Pathophysiology of Blast Injury—From Biomechanics to Experiments and Computations: Implications on Head and Polytrauma

Traumatic brain injury (TBI) has been a major public health concern in the United States for decades; with an average of 1.4 million sustaining TBI, leading to 50,000 deaths, 235,000 hospitalizations, and 1.1 million emergency department visits. Among the several causes of TBI including fall accidents, motor vehicle crashes, sports collisions, and firearm assaults, explosions-induced TBI, commonly referred to as Blast-Induced Neurotrauma (BINT) has been frequently observed in recent warfare. In this chapter, we will discuss the origin, mechanics, mechanisms of BINT, and focus on the current state-of-the-art in terms of protection, diagnostics, prognostics and therapeutics involved in BINT. In addition, we will outline novel research approaches currently underway in terms of: experiments in laboratory-on animals, surrogate heads, cadaveric heads; as well as computer models of head-brain system. Finally, an analysis on the protective capabilities that have been introduced in BINTs against blast loading will be evaluated. TBI has been a major public health concern in the United States for decades; from 1995 to 2001 there was an average of 1.4 million Americans a year who sustained TBI, leading to 50,000 deaths, 235,000 hospitalizations, and 1.1 million emergency department visits (Langlois et al., 2004). Of these, an estimated 80,000–90,000 experienced the onset of long-term disability (Langlois et al., 2004; Thurman et al., 1999). These TBIs were largely caused by falls, motor vehicle crashes, sports collisions, and firearm assaults (Langlois et al., 2004). However, in this chapter, we are concerned with a new type of brain injury arising from explosions commonly referred to as BINT. We will focus on the origin, mechanics, and mechanisms of BINT, and the current state-of-the-art in terms of protection, diagnostics, prognostics, and therapeutics of the injury. We will outline the research approaches currently under way in terms of experiments in laboratory on animals, surrogate heads, cadaveric heads, computer models of the head–brain system, and finally an analysis on the protective capabilities of the current helmets against blast loading.BINT has been recognized as a major problem among U.S. service members since Operation Enduring Freedom (OEF) began in Afghanistan in October 2001 and Operation Iraqi Freedom (OIF) began in Iraq in May 2003. Blasts have been considered a threat to soldiers and civilians since the early 1970s, when researchers mainly considered lung damage and designed better protective body armors. Ironically, these body armors have saved many during explosions, making them more vulnerable to brain injuries. A recent RAND report estimates that 320,000 service members or 20% of the deployed force (total deployed 1.6 million) potentially suffer from TBI (Tanielian, 2008). However, of this population, approximately 60% have never been assessed by a health care provider specifically for TBI. The Department of Defense (DOD) based on data from 2004 to 2006 at selected military installations has estimated that 10%–20% of (the total deployed) OEF/OIF service members potentially sustain mild TBI (mTBI) (Reneer et al., 2011). Other studies also report the occurrence of TBI in OEF/OIF veterans. For example, a recent study has found that 22.8% of soldiers in an Army Brigade Combat Team returning from Iraq had clinically confirmed TBI (Terrio et al., 2009). A survey of OEF/OIF veterans, who had left combat theaters by September 2004, found that about 12% of the 2,235 respondents reported a history consistent with mTBI (Zhu et al., 2010). Among those who have been medically evacuated from theater, the proportions who have suffered TBI are predictably higher. For example, between January 2003 and February 2007, 29% of the patients evacuated from the combat theater to Walter Reed Army Medical Center in Washington, DC, had evidence of TBI (Defense and Veterans Brain Injury Center, 2007). Of 50 OEF/OIF veterans treated at the Tampa Veterans Affairs Polytrauma Rehabilitation Center, 80% had incurred combat-related TBI, with 70% of the injuries caused by improvised explosive devices (IEDs) (Clark et al., 2007). For active-duty military personnel in war zones, blasts are the primary cause of TBI (Tanielian, 2008). Recent statistics from the conflict in Iraq show that several thousands of active duty U.S. soldiers have sustained TBI, 69% of these as a result of blasts (Warden, 2006). Analysis of data collected (collection period: March 2004–September 2004) from 115 patients from the Navy–Marine Corps that were identified with TBI have found that IEDs were the most common mechanism of injury, responsible for 52% of TBI cases overall. The analysis also showed that intracranial injuries, particularly concussions, were the most common diagnosis, especially among patients with nonbattle injuries (94%). Although multiple TBI-related diagnoses were common, 51% of the patient group had only an intracranial injury with no accompanying head fracture or open wound of the head. It was also found that of 115 patients analyzed, about 63% of patients were wearing a helmet at the time of injury (Clark et al., 2007). In addition to the data reported previously, the DOD, in cooperation with the Armed Forces Health Surveillance Center and Defense and Veterans Brain Injury Center, has consolidated the data of clinically confirmed TBI cases among service members and categorized them based on the severity of injury as shown in Figure 18.1 (DoD, 2012). mTBI contributes to more than 80% of the total reported brain injuries (Figure 18.1) because exposure to repeated low-level blasts is a common feature of the war zone personnel/civilian populations. Indeed, blast-induced mTBI has been identified as the signature injury of OEF and OIF (Jones et al., 2007; Tanielian, 2008; Terrio et al., 2009; Warden, 2006). It should also be noted that the number of TBI cases reported by the DOD (DoD, 2012) is 10 times smaller than that estimated by the RAND (Tanielian, 2008) study. This discrepancy is attributed to methodology used for estimating the numbers. RAND estimates are often based on the subjective response to nonspecific screening questions asked to several hundred individuals; that resulting percentage was then generalized to the entire deployed population. On the contrary, DOD estimates are completed based on clinically confirmed TBI cases.