There is a pressing need for a comprehensive explanation of the mechanism of brain injury after exposure to blast and several hypotheses have been suggested. The focus of this research was to investigate one of the hypotheses for primary brain injury due to blast: multimodal skull flexure. The significance of this research is twofold. First, resolution of the mode of energy transfer and of the induced stresses within the skull-brain system will allow for creations of mitigation/protective techniques/equipment, as well as design of experiments investigating live-cell response using more reliable physical models. Second, the data obtained experimentally will be available to validate computational models already developed, as well as future blast injury models.
Initially, to examine the mechanical response to shock wave exposure, studies were conducted with three polyurethane spheres, used as simplified models of a human skull/brain system. The spheres had identical geometry, but differed in key characteristics: shell thickness and composition of the filling. The spheres were placed in an inverted position inside a shock tube and were exposed to a first series of fifteen simulated blasts, changing pressure magnitudes and orientation of the sample. Subsequently, apertures were introduced in the spheres and a second series of fifteen simulated blasts was conducted, reproducing the same shock wave overpressure magnitudes and orientations of the sample. Internal pressures in three regions of the filling and strain values in four regions of the shell were collected.
All installation and testing details followed a scheme that was also applied to the cadaveric study conducted concurrently. This scheme was designed to allow comparison of the cadaveric data and the sphere data, as a means to help identify the primary components of the biomechanical response of the skull/brain system.
The specific aims of the sphere study were to map the internal pressures in different regions of the filling; to compare pressure distribution patterns with surface strain data recorded at the same time, for evaluation of gross deformations of the shell in relation to internal pressure profiles; to determine the relationship between magnitude levels of incident pressure and values of internal pressures in the simplified models; to investigate the effects of orientation, shell thickness, and apertures on internal pressures in the models.
Concurrently, four unembalmed cadavers heads were placed in an inverted position inside a shock tube and were exposed to fifteen simulated blasts, changing pressure magnitudes and orientation of the head. Intracranial pressures (ICP) in four regions of the brain and strain values in five regions of the skull were collected. The strain values were analyzed to evaluate gross deformations of the skull in relation to ICP profiles. The specific aims of the cadaveric study were to ascertain the relationship between magnitude levels of incident pressure and values of ICP in different regions of the brain, to investigate the effects of orientation on ICP in the same regions, and to compare pressure distribution patterns with surface strain data recorded at the same time.
Results from both studies suggested that internal pressure values were linked to the mechanical response of the coupled skull/brain (or shell/fluid) system, and that the distribution of the internal pressures supported the multimodal skull flexure theory. Pressure and strain results suggested that a shock wave interacting with the skull/brain (shell/fluid) system produces skull deformation, surface ripples, relative motion between the skull and brain, and a global skull compression as forecasted by the multimodal skull flexure theory.
In conclusion, the presence of the spacial and temporal pressure distributions caused by the multimodal skull flexure is undoubtedly creating stresses in the brain tissue, and it is reasonable to suspect that these stresses could be a primary source of injury.
Furthermore, results showed that significant values of internal pressure were recorded even in the absence of a fully functioning vasculature and/or an intact body, refuting previous studies that uphold the thoracic mechanism as a primary mechanism of brain injury during exposure to blast.
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