Skull Flexure from Blast Waves: A Mechanism for Brain Injury with Implications for Helmet Design

📝 Abstract

Traumatic brain injury [TBI] has become a signature injury of current military conflicts, with debilitating, costly, and long-lasting effects. Although mechanisms by which head impacts cause TBI have been well-researched, the mechanisms by which blasts cause TBI are not understood. From numerical hydrodynamic simulations, we have discovered that non-lethal blasts can induce sufficient skull flexure to generate potentially damaging loads in the brain, even without a head impact. The possibility that this mechanism may contribute to TBI has implications for injury diagnosis and armor design.

💡 Analysis

Traumatic brain injury [TBI] has become a signature injury of current military conflicts, with debilitating, costly, and long-lasting effects. Although mechanisms by which head impacts cause TBI have been well-researched, the mechanisms by which blasts cause TBI are not understood. From numerical hydrodynamic simulations, we have discovered that non-lethal blasts can induce sufficient skull flexure to generate potentially damaging loads in the brain, even without a head impact. The possibility that this mechanism may contribute to TBI has implications for injury diagnosis and armor design.

📄 Content

1 Skull Flexure from Blast Waves: A Mechanism for Brain Injury with Implications for Helmet Design* William C. Moss1, Michael J. King1, and Eric G. Blackman2 1Lawrence Livermore National Laboratory, Livermore, CA 94551 2Department of Physics and Astronomy, University of Rochester, Rochester NY 14627

Abstract Traumatic brain injury [TBI] has become a signature injury of current military conflicts, with debilitating, costly, and long-lasting effects. Although mechanisms by which head impacts cause TBI have been well-researched, the mechanisms by which blasts cause TBI are not understood. From numerical hydrodynamic simulations, we have discovered that non-lethal blasts can induce sufficient skull flexure to generate potentially damaging loads in the brain, even without a head impact. The possibility that this mechanism may contribute to TBI has implications for injury diagnosis and armor design.

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LLNL-JRNL-412717 2 Traumatic brain injury [TBI] results from mechanical loads in the brain, often without skull fracture, and causes complex, long lasting symptoms (1,2). TBI in civilians is usually caused by head impacts resulting from motor vehicle (3,4) and sports accidents (5,6). TBI has also emerged to be endemic among military combat personnel exposed to blasts. As modern body armor has substantially reduced soldier fatalities from explosive attacks, the lower mortality rates have revealed the high prevalence of TBI (1,7,8). There is an urgent need to understand the mechanisms by which blasts cause TBI, to better diagnose injury and design protective equipment, such as helmets.
Impact-induced TBI [ITBI] has been extensively studied, primarily through animal testing and analyses of human trauma data (9), and has been linked to accelerations of the head. By contrast, the damage producing mechanisms for blast- induced TBI [BTBI] are not well understood (10,11). Mechanical loads from the blast pressure, accelerations, or impacts, as well as electromagnetic or thermal exposure have all been proposed (12). Because blasts can cause head impacts by propelling a soldier into another object (or vice versa), protection research has traditionally focused on reducing the acceleration of the head during an impact. However, shock tube experiments in which restrained animals were subjected to blast-like conditions confirmed that blast pressures, without subsequent impacts, can cause TBI (13). Several mechanisms by which the blast alone can damage the brain have been proposed, including bulk acceleration of the head (12), transmission of loads through orifices in the skull, and compression of the thorax, which generates a vascular surge to the brain (13). Surprisingly, blast-induced deformation of the skull has been neglected, perhaps due to the perception that the hard skull protects the brain from non-lethal blast waves (14). Here we show via three-dimensional hydrodynamical simulations that direct action of the blast wave on the head causes skull flexure, producing mechanical loads in brain tissue comparable to those in an injury-inducing impact, even at non-lethal blast pressures as low as 1 bar above ambient.
We studied head impacts and blast waves on the head using ALE3D (15), an arbitrary Lagrangian-Eulerian [ALE] finite element hydrocode. Figure 1 shows our blast simulation geometry. The charge size and standoff distance from the simulated head were chosen to generate a non-lethal blast wave (16). The skull is modeled as a hollow 3 elastic ellipsoid that contains a viscoelastic brain surrounded by a layer of cerebrospinal fluid [CSF]. The tensile stress that the CSF layer can carry is capped at one bar below atmospheric pressure to capture cavitation-like effects (17,18), although it is not clear if the CSF itself cavitates due to the presence of impurities and dissolved gas (19), or if the interfaces between the CSF and the subarachnoid walls cannot support tensile stresses. Because the CSF layer is thin, capping its tensile strength models either scenario. A simplified face (with no lower jaw),

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