Blast induced neurotrauma (BINT), and posttraumatic stress disorder (PTSD) are identified as the “signature injuries” of recent conflicts in Iraq and Afghanistan. The occurrence of mild to moderate traumatic brain injury (TBI) in blasts is controversial in the medical and scientific communities because the manifesting symptoms occur without visible injuries. Whether the primary blast waves alone can cause TBI is still an open question, and this work is aimed to address this issue. We hypothesize that if a significant level of intracranial pressure (ICP) pulse occurs within the brain parenchyma when the head is subjected to pure primary blast, then blast induced TBI is likely to occur.
In order to test this hypothesis, three post mortem human heads are subjected to simulated primary blast loading conditions of varying intensities (70 kPa, 140 kPa and 200 kPa) at the Trauma Mechanics Research Facility (TMRF), University of Nebraska-Lincoln. The specimens are placed inside the 711 mm × 711 mm square shock tube at a section where known profiles of incident primary blast (Friedlander waveform in this case) are obtained. These profiles correspond to specific field conditions (explosive strength and stand-off distance). The specimen is filled with a brain simulant prior to experiments. ICPs, surface pressures, and surface strains are measured at 11 different locations on each post mortem human head. A total of 27 experiments are included in the analysis.
Experimental results show that significant levels of ICP occur throughout the brain simulant. The maximum peak ICP is measured at the coup site (nearest to the blast) and gradually decreases towards the countercoup site. When the incident blast intensity is increased, there is a statistically significant increase in the peak ICP and total impulse (p<0.05). Even after five decades of research, the brain injury threshold values for blunt impact cases are based on limited experiments and extensive numerical simulations; these are still evolving for sports-related concussion injuries. Ward in 1980 suggested that no brain injury will occur when the ICP<173 kPa, moderate to severe injury will occur when 173 kPa<ICP<235 kPa and severe injury will occur when ICP>235 kPa for blunt impacts. Based on these criteria, no injury will occur at incident blast overpressure level of 70 kPa, moderate to severe injuries will occur at 140 kPa and severe head injury will occur at the incident blast overpressure intensity of 200 kPa. However, more work is needed to confirm this finding since peak ICP alone may not be sufficient to predict the injury outcome.