A 3-D Finite-Element Minipig Model to Assess Brain Biomechanical Responses to Blast Exposure

Abstract

Despite years of research, it is still unknown whether the interaction of explosion-induced blast waves with the head causes injury to the human brain. One way to fill this gap is to use animal models to establish “scaling laws” that project observed brain injuries in animals to humans. This requires laboratory experiments and high-fidelity mathematical models of the animal head to establish correlates between experimentally observed blast-induced brain injuries and model-predicted biomechanical responses. To this end, we performed laboratory experiments on Göttingen minipigs to develop and validate a three-dimensional (3-D) high-fidelity finite-element (FE) model of the minipig head. First, we performed laboratory experiments on Göttingen minipigs to obtain the geometry of the cerebral vasculature network and to characterize brain-tissue and vasculature material properties in response to high strain rates typical of blast exposures. Next, we used the detailed cerebral vasculature information and species-specific brain tissue and vasculature material properties to develop the 3-D high-fidelity FE model of the minipig head. Then, to validate the model predictions, we performed laboratory shock-tube experiments, where we exposed Göttingen minipigs to a blast overpressure of 210 kPa in a laboratory shock tube and compared brain pressures at two locations. We observed a good agreement between the model-predicted pressures and the experimental measurements, with differences in maximum pressure of less than 6%. Finally, to evaluate the influence of the cerebral vascular network on the biomechanical predictions, we performed simulations where we compared results of FE models with and without the vasculature. As expected, incorporation of the vasculature decreased brain strain but did not affect the predictions of brain pressure. However, we observed that inclusion of the cerebral vasculature in the model changed the strain distribution by as much as 100% in regions near the interface between the vasculature and the brain tissue, suggesting that the vasculature does not merely decrease the strain but causes drastic redistributions. This work will help establish correlates between observed brain injuries and predicted biomechanical responses in minipigs and facilitate the creation of scaling laws to infer potential injuries in the human brain due to exposure to blast waves.

A biomechanical-based approach to scale blast-induced molecular changes in the brain

“…In particular, using FE model-predicted ICPs and previously reported thresholds for blunt-induced mild TBI, they determined that the human requires a lower-magnitude BOP exposure to elicit a similar brain injury as that observed in the minipig. The fact that we observed the opposite when we compared humans and rats is not surprising because in addition to the numerous methodological differences (e.g., Saunders et al used a simplified boundary condition and the same material properties for both species), there are distinct anatomical differences between Yucatan minipigs and humans (e.g., skull thickness of ~ 6.9 mm for humans 60 vs. ~ 8.6 mm for Yucatan minipigs 29 ).…”

“…4 ). In addition to the distinct anatomical location of these regions, the inter-regional variability in biomechanical responses could be attributed to the unequal presence of vasculature and region-specific material properties assigned to the cerebrum, cerebellum, and brainstem in the FE model, which together influence and redistribute the brain-tissue responses to a blast insult 27 – 29 . Taken together, these regional disparities in molecular and biomechanical responses support the need for establishing region-specific correlates between experimentally measured GFAP changes and computationally predicted biomechanical responses, which we ultimately used to scale blast-induced changes between species.…”

“…Therefore, we expect that the scaling factors would change when different head orientations are considered. Fifth, similar to other studies 28 , 29 , 67 – 69 , we investigated the biomechanical responses in the brain only for the first 5 ms of the blast-wave propagation and did not represent the negative-pressure phase. While we acknowledge that the deviatoric responses (e.g., strain) in the brain may evolve further after this initial period, the objective of our study was to characterize the responses in the brain resulting from the initial interaction of the blast wave with the head, which potentially leads to the so-called primary blast injury.…”

“…While these efforts advanced our understanding of how to implement mechanics-based principles to relate blast-induced brain injuries across species, it is not clear whether and to what extent the predicted biomechanical responses correlate with the observed brain injuries. In addition, it is not known whether these results would hold had the models represented the detailed cerebrovascular network and used species-specific brain material properties, which are necessary to enhance model fidelity to blast loads 27 – 29 .…”