Modeling Blast-Related Traumatic Brain Injury

1. Modeling Blast-Related Traumatic Brain Injury Michelle Nyein Department of Aeronautics and Astronautics Massachusetts Institute of Technology Cambridge, MA 02139 January 26, 2009 Research Adviser: Prof. Raul Radovitzky

2. Outline • Background • Blast-Related TBI • Why are we interested? • What is known? • Modeling Approach • Computational Framework • Biofidelic Models of Human Head & Helmet/Pads • Constitutive Models & Material Properties • Blast & Impact Conditions • Numerical Simulations • Head Simulations • Blast (30 atm) & Impact (5 m/s) • Helmet Simulations • Blast (30 atm) & Impact (5 m/s)

3. Motivation • High incidence of blast-related Traumatic Brain Injury (TBI) in military contexts • 59% of “at risk” veterans admitted to Walter Reed in 2003-2004 were diagnosed with at least mild TBI (Okie, 2005) • One study found ~11% of soldiers suffered blast-related TBI – 165,000+ soldiers could be affected (Hoge, 2008) • Need to improve fundamental understanding of blast effects, injury mechanisms caused by blast • Controversy over whether post-concussive symptoms are due to TBI or PTSD (Hoge, 2008) • Does the primary effect of a blast wave – the overpressure – constitute an independent cause of mild TBI? • Can we better protect soldiers from blast-related TBI? 3

4. What Is Known? • Impact-Related TBI • Types of Injury • Cerebral contusions – may be caused by high intracranial pressures (Zhang, 2004) • Diffuse axonal injury (DAI) – may be caused by large shear stresses, particularly in brainstem & corpus callosum (Nishimoto, 1998) • Injury Criteria • Zhang, 2004: Reconstructed NFL head-to-head collisions • Intracranial pressures of 48-130 kPa in injury cases • Shear stresses of 3.1-12 kPa in injury cases • Willinger, 2003: Reconstructed real world accidents • Von Mises stresses of 18 & 38 kPa for 50% chance of moderate & severe neurological lesions

5. What Is Needed? • Develop and validate models of brain response to blast informed with realistic brain tissue mechanical properties • Elucidate tissue and cell-level brain injury mechanisms due to primary blast effects • Derive primary blast TBI injury criteria • Develop basic strategies for design of protection systems against blast-related TBI

6. Approach • Modeling: • Development of anatomical models of the human head • Large-scale coupled simulation of blast-tissue interaction • Experimental characterization • Mechanical properties of brain tissue at high strain rates • Blast validation experiments on physical & animal models • Cell response to pressure waves • Clinical studies • Clinical evaluation of Iraq & Afghanistan veterans

7. Modeling approach

8. Computational framework: VTF • Lagrangian solid formulation ideal for: • Tracking of material interfaces • Materials with history: plasticity, fracture • Unstructured mesh, finite element approach • Mesh healing and optimization (HealMesh): eliminate mesh distortion • Eulerian fluid formulation: • Well-established shock capturing and advection schemes • Structured grid, finite volume approach • Automatic Mesh Refinement: capture pressure/density gradients/boundary geometry • FSI coupled approach: Virtual Test Facility (RR et al, 1998, 2001 Deiterding et al, Eng. with Computers 2006): • Based on ghost-fluid method/level sets (Fedkiw) • Coupling enforced at every fluid time step

9. DVBIC/MIT Full Human Head Computer Model + ACH • 11 differentiated tissues and anatomical structures: skin/fat, skull, muscle, sinus, eyes, CSF, GM, WM, venous, ventricles, glia • ACH: composite shell + standard padding geometry • Coarsest model: 900K elements

10. Constitutive Models • Existing models include: • Nonlinear/quasilinear viscoelastic (Drapaca, 2006; Shen, 2006; Brands, 2004; Miller & Chinzei, 2002) • Volumetric Response: • Mie-Gruneisen/Hugoniot equation of state for skull • Describes shock response of many solids • Tait equation of state for remaining fluid-embedded structures – commonly used for fluids • Deviatoric Response: Neohookean

11. Material Properties

12. Thorax tolerance to air blast From: Textbook of military medicine, Part 1, Chapter 7 The physics and mechanisms of primary blast injury

13. Numerical simulation

14. Blast Simulation (30 atm) • Equivalent to free air explosion of .57 kg TNT at 0.6 m standoff distance • Roughly corresponds to threshold for 99% lethality for unarmored blast lung injury • Run on SHC on 20 processors: 14 solid, 6 fluid

15. Blast Simulation (30 atm)

16. Impact Simulation (5 m/s) • Head strikes stationary, immovable object at 5 m/s • Run on SHC on 20 processors

17. Intracranial compressive pressure by structure Blast Simulation, 30 atm Impact Simulation, 5 m/s .81 ms, -21 MPa Gray Matter & Skull .29 ms, -52 MPa, Gray Matter & Skull

18. Intracranial tensile pressure by structure Impact Simulation, 5 m/s Blast Simulation, 30 atm .26 ms, 9 MPa CSF & Skull .81 ms, 5.7 MPa Sinus & Skull

19. Intracranial Von Mises Stress by structure Blast Simulation, 30 atm Impact Simulation, 5 m/s Gray Matter & Skull, .30ms, 101 MPa .81ms, 45 MPa Skull & Muscle

20. For helmet and padding, used: Neohookean model Material properties of Kevlar Helmet Simulations

21. Blast Simulation with ACH (30 atm): Pressure Field

22. Blast Simulation with ACH (30 atm)

23. Impact Simulation with ACH (5 m/s):Pressure Field

24. Blast Simulation (30 atm): Intracranial compressive pressure by structure Head Simulation Helmet Simulation Gray Matter & Skull, .37 ms, -44 MPa .29 ms, -52 MPa, Gray Matter & Skull

25. Blast Simulation (30 atm): Intracranial tensile pressure by structure Helmet Simulation Head Simulation Helmet, .34 ms, 17 MPa .26 ms, 9 MPa CSF & Skull Skull & Sinus, .38 ms, 7 MPa

26. Blast Simulation (30 atm): Intracranial Von Mises stress by structure Helmet Simulation Head Simulation Gray Matter & Skull, .30ms, 101 MPa Gray Matter & Skull, .37 ms, 74 MPa

27. Summary • Blast wave transmits complex patterns of stress to the head • Very short duration (<1ms) • In this time period there are no significant strains • But large localized strain rates and high volumetric & deviatoric stresses • Tissue interfaces are particularly vulnerable • Skull experiences largest stresses, thus providing some protection to brain tissues • Peak pressures & stresses slightly lower in helmet simulations

28. Conclusions • Simulations for blast conditions leading to lethal thoracic injury gave pressure and von Mises stress values that far exceed suggested brain injury criteria for impact conditions • Suggests primary blast effects constitute a plausible cause for TBI. • Suggests helmets may mitigate blast effects in the head

29. Future work • Validation against cadaver and animal test data • Construction of computer model for pig • Development and refinement of constitutive models • Based on experimental data from collaborators • Extensions of the Full Head Model • Mapping of DTI tractography to anisotropic constitutive model of white matter • Extension to neck and torso

30. Thank you

31. Meet the IED

32. Intracranial compressive pressure by structure Blast Simulation, 30 atm Impact Simulation, 5 m/s Gray Matter & Skull, .92 ms, -22 MPa Gray Matter & Skull, .37 ms, -44 MPa

33. Intracranial tensile pressure by structure Blast Simulation, 30 atm Impact Simulation, 5 m/s Helmet, .34 ms, 17 MPa Helmet, .8 ms, 11 MPa Skull & Sinus, .38 ms, 7 MPa Skull & Sinus, .8 ms, 4.7 MPa

34. Intracranial Von Mises stress by structure Blast Simulation, 30 atm Impact Simulation, 5 m/s Gray Matter & Skull, .37 ms, 74 MPa Muscle & Skull, .92 ms, 51 MPa

35. Constitutive Modeling of Brain Tissue Response (S. Socrate, MIT) Diffusion of interstitial fluid driven by pore pressure gradients (t < 0.1s) 3D Hyperelastic nonlinear network Short term “glassy” Viscoelastic resistance Long term relaxation (t~100 s linear) medium term relaxation (0.1s < t <<100 s nonlinear) Model Fit to LOW RATE and HIGH RATE experiments