This research provided insight into the biomechanics of traumatic brain stem injury by characterizing the response of the brainstem to mechanical deformation. The brainstem is a region of the CNS characterized by axonal fibers longitudinally organized in parallel tracts. This distinct structure helps dictate the material response of the brainstem and suggests that this region may respond to mechanical insult as a transversely isotropic structure. The objective of this work was to investigate this assumption through material testing in three mutually perpendicular directions.
A custom designed stress relaxation, oscillatory shear testing apparatus (STA) for testing soft biological tissues in simple shear was constructed and validated. The STA was validated by measurements of complex shear moduli of three mixtures of silicone gel with viscoelastic properties similar to soft biological tissue that were tested in both the STA and a commercially available solids rheometer.
The STA was used to perform stress relaxation tests on adult porcine brainstem to characterize the mechanical response of the brainstem. These experiments did not confirm our hypothesis of the brainstem's transversely isotropic material response. Specifically, the calculated relaxation moduli did not vary with testing orientation.
We hypothesized that due to the presence of significant relaxation in these tissues, the mechanical response of the brainstem is highly loading rate dependent. A series of dynamic oscillating shear tests in three mutually perpendicular directions were performed to further characterize the anisotropic nature of the brainstem at high loading rates. These experiments confirmed our hypothesis that the brainstem may be described as a transversely isotropic material; the complex moduli in the transverse direction were significantly higher than the other two moduli that were indistinguishable from one another.
A novel fiber reinforced composite model composed of viscoelastic fibers surrounded by a viscoelastic matrix was developed to analyze the oscillatory shear data. The model predicted that the fibers are stiffer and more viscous than the surrounding matrix, a relationship reinforced by the results of the oscillatory shear tests. The predicted fiber modulus was confirmed by results of oscillatory shear tests on the optic nerve of the guinea pig.
|1993||Saatman KE. An Isolated Single Myelinated Nerve Fiber Model for the Biomechanics of Axonal Injury [PhD thesis]. Philadelphia, PA: University of Pennsylvania; 1993.|
|1970||Yamada H. Strength of Biological Materials. Evans FG, ed. Baltimore, MD: Williams & Wilkins Company; 1970.|
|1968||Oppenheimer DR. Microscopic lesions in the brain following head injury. J Neurol Neutrosurg Psychiatry. August 1968;31(4):299-306.|
|1974||Ommaya AK, Gennarelli TA. Cerebral concussion and traumatic unconsciousness: correlation of experimental and clinical observations of blunt head injuries. Brain. December 1974;97(4):633-654.|
|1970||Estes MS, McElhaney JH. Response of brain tissue of compressive loading. ASME Biomechanical & Human Factors Conference; May 31–June 3, 1970; Washington, DC.1-4. ASME Publication 70-BHF-13.|
|1993||Galbraith JA, Thibault LE, Matteson DR. Mechanical and electrical responses of the squid giant axon to simple elongation. J Biomech Eng. February 1993;115(1):13-22.|
|1994||Bandak FA, Eppinger RH. A three-dimensional finite element analysis of the human brain under combined rotational and translational accelerations. In: Proceedings of the 38th Stapp Car Crash Conference. October 31–November 2, 1994; Fort Lauderdale, FL. Warrendale, PA: Society of Automotive Engineers:145-163. SAE 942215.|
|1964||Hashin Z, Rosen BW. The elastic moduli of fiber-reinforced materials. J Appl Mech. June 1964;31(2):223-232.|
|1946||Pudenz RH, Shelden CH. The lucite calvarium: a method for direct observation of the brain, II: cranial trauma and brain movement. J Neurosurg. 1946;3(6):487-505.|
|1969||Fallenstein GT, Hulce VD, Melvin JW. Dynamic mechanical properties of human brain tissue. J Biomech. 1969;2(3):217-226.|
|1995||DiMasi FP, Eppinger RH, Bandak FA. Computational analysis of head impact response under car crash loadings. In: Proceedings of the 39th Stapp Car Crash Conference. November 8-10, 1995; San Diego, CA. Warrendale, PA: Society of Automotive Engineers:425-438. SAE 952718.|
|1991||Myers BS, McElhaney JH, Doherty BJ. The viscoelastic responses of the human cervical spine in torsion: experimental limitations of quasi-linear theory, and a method for reducing these effects. J Biomech. 1991;24(9):811-817.|
|1991||Dimasi F, Marcus J, Eppinger R. 3-D anatomic brain model for relating cortical strains to automobile crash loading. In: Proceedings of the 13th International Technical Conference on Experimental Safety Vehicles (ESV). November 4-7, 1991; Paris, France.916-924.|
|1967||Fung YCB. Elasticity of soft tissues in simple elongation. Am J Physiol. December 1967;213(6):1532-1544.|
|1993||Fung YC. Biomechanics: Mechanical Properties of Living Tissues. 2nd ed. New York, NY: Springer-Verlag; 1993.|
|1985||Jane JA, Steward O, Gennarelli T. Axonal degeneration induced by experimental noninvasive minor head injury. J Neurosurg. January 1985;62(1):96-100.|
|1982||Gennarelli TA, Thibault LE, Adams JH, Graham DI, Thompson CJ, Marcincin RP. Diffuse axonal injury and traumatic coma in the primate. Ann Neurol. 1982;12(6):564-574.|
|1972||Shuck LZ, Advani SH. Rheological response of human brain tissue in shear. J Basic Eng. December 1972;94(4):905-911.|
|1995||Zhou C, Khalil TB, King AI. A new model comparing impact responses of the homogeneous and inhomogeneous human brain. In: Proceedings of the 39th Stapp Car Crash Conference. November 8-10, 1995; San Diego, CA. Warrendale, PA: Society of Automotive Engineers:121-137. SAE 952714.|
|1994||Zhou C, Khalil TB, King AI. Shear stress distribution in the porcine brain due to rotational impact. In: Proceedings of the 38th Stapp Car Crash Conference. October 31–November 2, 1994; Fort Lauderdale, FL. Warrendale, PA: Society of Automotive Engineers:133-143. SAE 942214.|
|1988||Galbraith JA. The Effects of Mechanical Loading on the Electrophysiology of the Squid Giant Axon [PhD thesis]. Philadelphia, PA: University of Pennsylvania; 1988.|
|1977||Adams JH, Mitchell DE, Graham DI, Doyle D. Diffuse brain damage of immediate impact type: its relationship to "primary brain-stem damage" in head injury. Brain. July 1977;100(3):489-502.|
|1978||Pamidi MR, Advani SH. Nonlinear constitutive relations for human brain tissue. J Biomech Eng. February 1978;100(1):44-48.|
|1983||Povlishock JT, Becker DP, Cheng CLY, Vaughan GW. Axonal change in minor head injury. J Neuropathol & Exp Neurol. 1983;42(3):225-242.|
|1956||Strich SJ. Diffuse degeneration of the cerebral white matter in severe dementia following head injury. J Neurol Neutrosurg Psychiatry. August 1956;19(3):163-185.|
|1970||Metz H, McElhaney J, Ommaya AK. A comparison of the elasticity of live, dead, and fixed brain tissue. J Biomech. July 1970;3(4):453-458.|
|1967||Peerless SJ, Rewcastle NB. Shear injuries of the brain. Can Med Assoc J. March 11, 1967;96(10):577-582.|
|1991||Ruan JS, Khalil T, King AI. Human head dynamic response to side impact by finite element modeling. J Biomech Eng. August 1991;113(3):276-283.|
|1970||Galford JE, McElhaney JH. A viscoelastic study of scalp, brain, and dura. J Biomech. 1970;3(2):211-221.|
|1976||McElhaney JH, Roberts VL, Hilyard JF. Handbook of Human Tolerance. Tokyo, Japan: Japan Automobile Research Institute (JARI) Inc; 1976.|
|1990||Margulies SS, Thibault LE, Gennarelli TA. Physical model simulations of brain injury in the primate. J Biomech. 1990;23(8):823-836.|
|1998||Arbogast KB, Margulies SS. Material characterization of the brainstem from oscillatory shear tests. J Biomech. 1998;31(9):801-807.|
|2006||Ning X, Zhu Q, Lanir Y, Margulies SS. A transversely isotropic viscoelastic constitutive equation for brainstem undergoing finite deformation. J Biomech Eng. December 2006;128(6):925-933.|
|1998||Shreiber DI. Experimental and Computational Modeling of Traumatic Brain Injury: In Vivo Thresholds for Mechanical Disruption of the Blood-Brain Barrier [PhD thesis]. Philadelphia, PA: University of Pennsylvania; 1998.|
|2000||Darvish KK. Characterization of Nonlinear Viscoelastic Properties of Brain Tissue Using Forced Vibrations [PhD thesis]. Charlottesville, VA: University of Virginia; January 2000.|