SCIB Head/Brain Work Group University of Pennsylvania

1. SCIB Head/Brain Work GroupUniversity of Pennsylvania David Meaney and Susan Margulies

2. Southern Consortium for Brain Injury BiomechanicsOverall Objective The primary objective of the consortium is to generate the scientific foundation for a new method of predicting the probability of brain injury, given an applied input loading condition to the head

3. University of PennsylvaniaOverall Objective • To acquire experimental data for developing and validating the finite element model used in the NHTSA SIMon program • Brain Injury Thresholds • Boundary Conditions • Material Properties of Brain Tissue • Cell based models of Brain Tissue

4. Brain Injury Thresholds Progress To determine if a specific and serious injury, the breakdown of the blood brain barrier, can be predicted by the mechanical response of a simple finite element model of the rat brain • DCDmodel and data obtained - completed • idealized FEM development - completed • injury threshold derivation - completed • development of FEM - completed at Wayne State

5. Brain Injury ThresholdsOutcome • A simple animal model is used to study the threshold for brain injury (contusion) - examine the usefulness of different predictors for injury • The FEM is used to study other experimental TBI models (cortical impact) with other SCIB members • Continued cross-correlation of experimental models - one threshold does not seem to universally match

6. University of PennsylvaniaOverall Objective • To acquire experimental data for developing and validating the finite element model used in the NHTSA SIMon program • Brain Injury Thresholds • Boundary Conditions

7. Skull-Brain Boundary Conditions Specific Goals To measure the relative motion of the brain within the skull to provide more validation data for the SIMon model as it begins to transform from a research tool into a technology that is the basis for a new head impact protection standard.

8. Experimental Methods High Resolution T1-weighted Image SPAtial Modulation of Magnetization N=47 images from 5 subjects N=14 sets (from 11 subjects)

9. Postures and Motion Sequences Extension supine (ES) Neutral supine (NS) Flexion supine (FS) Neutral prone (NP) Flexion prone (FP)

10. Boundary Conditions Progress + Outcomes • Progress • SPAMM - completed. paper published • High Resolution T1 - completed. manuscript published • Outcomes: • Cerebellum rotates 1-4°; brainstem moves ≈ 1mm out of the foramen magnum during moderate neck flexion • Foramen magnum should NOT be modeled as a no-slip condition

11. University of PennsylvaniaOverall Objective • To acquire experimental data for developing and validating the finite element model used in the NHTSA SIMon program • Brain Injury Thresholds • Boundary Conditions • Material Properties of Brain Tissue

12. Brain Material Properties Specific Goals To provide biofidelic mechanical responses of the living brain that can be incorporated in finite element models of the head (like SIMon). • In vivo, in situ, in vitro indentation tests • Finite shear properties of Brainstem

13. “Ramp-and-Hold” d Pi P P P∞ R= 2mm t d In Vivo Testing Mechanical properties are determined by quickly indenting the exposed brain surface to a depth d with a rigid indentor while recording force P(t) and displacement d on computer. Assuming that the brain is a linear viscoelastic half-space, the shear modulus G can be calculated from the relationship: Lee  Radok (1960) The contact problem for viscoelastic bodies. J. Appl. Mech., 27: 438-444

14. brain excised and placed on pre-lubricated plate Comparing in vivo, in situ, in vitro IN VIVO testing Sodium pentobarbital overdose IN SITU testing IN VITRO testing

15. Major Findings • Preconditioning did not have a significant effect on the living brain, but significantly reduced the shear moduli in vitro. • Shear moduli in vitro are consistently lower than in situ (likely due to skull confinement). • The same boundary conditions exist in vivo and in situ, and there were no significant differences between these two comparable conditions. Overall, we conclude that the mechanical behavior of a living brain is similar to that of a dead brain

16. Brain Material Properties Specific Goals To provide biofidelic mechanical responses of the living brain that can be incorporated in finite element models of the head (like SIMon). • In vivo, in situ, in vitro indentation tests • Brainstem undergoing large strains

17. for parallel or perpendicular for cross-sectional 1 cm Brainstem properties(Finite shear) • 4-week porcine brainstems (N=15) • 3 specimens per subject parallel or perpendicular cross-sectional

18. Top Plate Force Transducer Humidity Chamber Linear Actuator Measured Force Tissue LVDT Tissue Measured Displacement To ramp and hold generator Bottom Plate MethodsBrainstem Testing in Simple Shear • Simple Stress Relaxation • Shear strains: 50, 40, 30, 20, 10, 5, 2.5, then 50% • Two preconditioning runs Arbogast et al., J Biomech. 1997

19. Results Anisotropic strain energy functions for instantaneous response W=Wmatrix +Wfiber G0initial shear modulus of matrix-controlled material g1, g2relative shear relaxation moduli t1, t2characteristic times qstiffness of axonal fibers

20. Brainstem Finite Shear Properties • Parameters fit simultaneously to 50% test data in all 3 directions predicted data from all other strains well (average R2 ≈85%) • Fiber stiffness at finite deformation is nearly 10x stiffer than brainstem “matrix” - previously we reported fiber 3x stiffer than matrix at 2.5% strain [Arbogast and Margulies, 1999] • Brainstem matrix component is the most compliant brain tissue region with a instantaneous shear modulus of 12.7 Pa. Previously we reported average cerebrum tissue modulus of 526.9 Pa [Prange and Margulies, 2002]

21. Brain Material Properties Progress To provide biofidelic mechanical responses of the living brain that can be incorporated in finite element models of the head (like SIMon). • In vivo, in situ, in vitro tests - completed, paper published • Brainstem - completed studies, manuscript submitted

22. Cell-based Models of Brain TissueSpecific Goals To provide approximations of the variation in cellular strains that occur within the brain during impact, estimates that can be incorporated in finite element models of the head (like SIMon).

23. Studying cellular kinematics in vitroOrganotypic Cultures • Use P4-P6 rat pups • Transverse sections 350 mm thick • Cultured on 0.005” think laminin treated silastic membranes • Fed 3X per week with Neurobasal-A supplemented with B27, glucose, and L-glutamine • Incubated for 10 days on a rocker (2 rocks per minute)

24. Planes imaged by confocal microscope Substrate Stretch Tracking bead/nuclei movement in gels and cultures Silicone gel or Organotypic tissue Labeled beads or nuclei Deformed Substrate Microscope Objective

25. k1 k1 k1 k1 k1 k1 k1 k1 k1 Approximating the constructs and tissue mm mc RVE

26. Assumptions • Cells follow hookean-type elastic behavior • Mechanical properties of the CNS matrix are similar to other soft tissues

27. Nodal equation and solution

28. The fraction of moving nuclei in organotypic tissue increases with applied strain

29. Predicted coefficient of variance indicates cells are much stiffer than extracellular matrix Non-linear matrix Kcell:Kmatrix = 16 sKcell~.2 Applied strain

30. Softening of brain tissue at finite strainsAre the cells softening? • Measured under simple shear conditions • Repeatable behavior • Prevents use of linear formulations • Approximate 35% decrease in tangential stiffness • Mechanism – cytoskeletal re-alignment/failure? Prange and Margulies, 2002

31. Brain Injury ThresholdsMajor Outcome • Thresholds for brain injury can be predicted with finite element approaches, but the cross-correlation of predictions among animal models continues • Brainstem motions indicate a mobile/free boundary condition at the foramen magnum • Brain properties measured from in vitro testing are reasonable approximations of the in vivo properties • Composite models suggest a very soft extracellular component integrated with stiffer cellular components

32. Brain Injury ThresholdsFuture Directions • Continue the development of animal models - in vivo imaging to track cell motion during impact • Material property testing - influence of age • Using material properties and finite element models - new physcial model validation studies to confirm model predictions • Developing better transformations between individual cell types and tissue - are individual cell populations at risk

33. Acknowledgements Southern Consortium for Injury Biomechanics Ashton Foundation