The complex problem of developing a predictive transfer function between the forces and/or motions experienced by vehicle occupants in the hostile mechanical environment of a collision and indicies relating to extent and severity of the various forms of central nervous system trauma can be made mom tractable by understanding neural injury as a function of mechanical deformation at the cellular level.
A comparison of results obtained from animal, physical, and isolated tissue models indicates that an injury threshold for neural tissue can be expressed in terms of a maximum strain under conditions of dynamic loading.
Nonhuman primates were subjected to controlled inertial loading of the head which produced a spectrum of pathophysiological consequences, ranging from mild cerebral concussion to severe diffuse axonal injury with prolonged coma. Physical models of the skull/brain structural conplex were loaded under identical kinematical conditions over the same range of accelerations and with precisely the same acceleration wave shapes. Nodal displacements of a printed grid contained within the surrogate brain material were measured komhighspeuifilmframsandthe associated strains were computed. In parallel studies isolated single axons were subjected to high strain rate uniaxial elongation with extension ratios comparable to the levels suggested by the physical model experiments. The physiological response of the axon was examined by measuring membrane potential, voltage-clamp current and cytosolic free calcium concentrations pre and post loading.
Correlations are made between the maximum extension ratio and the physiologic variabies which suggest an exponential dependence of injury severity upon the level of mechanical insult. At the isolated axon level the injury specnum varies from mild and spontaneously reversible depolarization, to irreversible changes in membrane potential and accumulation of intracellular free calcium The magnitude of the extension tatios which ’ describes this spectrum of pathophysiological response is then correlated with the computed strain field in the physical model studies. A transfer function is developed which permits one to relate the inertial loading conditions under which the deformation response of the tissue will yield a specific level of neural tissue injury severity.