Graduate Research Assistant University of Virginia Center for Applied Biomechanics
Abstract Text: Each year in the United States, 1.5 million Americans survive a traumatic brain injury (TBI) [1]. In 2020, over 64,000 people had a TBI-related death in the United States [2]. Understanding how head impact exposure causes TBI is a key piece of knowledge that will improve TBI prevention. From a biomechanics perspective, we seek to understand how head impacts cause the brain to deform and strain. Sonomicrometry is an experimental technique developed to investigate dynamic brain deformation in postmortem human surrogates (PMHS). Sonomicrometry involves measuring dynamic distances between piezoelectric crystals through a series of ultrasonic pulses transmitted through a material, and in this case brain tissue [3]. Previous work embedded transmitter crystals on the inner table of the skull and receiving crystals buried deep within the brain tissue, enabling a finite set of data points consisting of high temporal resolution brain deformation during the application of controlled rotational head kinematics [4]. A major challenge to studying the mechanics of the brain in a postmortem model is that the integrity of brain tissue decreases rapidly following death. The time course of this degradation and how it affects the mechanical response of the brain are not well understood. It has been shown in ex vivo experiments that intact and hydrated brain tissue does not change in mechanical properties for at least 5 days postmortem [5], but how long postmortem brain tissue remains viable for high-rate in situ testing is unknown. The objective of this study is to utilize the sonomicrometry method to measure brain deformation during a severe head impact and to quantify the effect of post-mortem time on the measured deformation.
Testing was performed with a human cadaveric head-neck specimen over a seven-day period postmortem. This specimen had no prior brain abnormalities, and the cause of death was esophageal cancer. A total of 32 neutral-density sonomicrometry crystals were implanted throughout the specimen’s head, with 24 crystals as receivers within the brain and 8 as transmitters attached to the skull. A closed loop-controlled test device was used to apply dynamic rotations to the head in the axial direction. Four sets of dynamic rotations of varying rotational velocity (20 and 40 rad/s) and duration (30 and 60 ms) were applied for each testing day. Testing was performed on three, five- and seven-days postmortem and compared against original testing that was completed within 42 hours postmortem.
Prior to testing, a head CT was performed to acquire the initial crystal configuration and their respective distances. Each test produced 192 potential distance time-history traces. The processed data from each testing day were compared to the results from the original testing day using the maximum displacement of every distance trace. The difference in maximum displacement for each trace was found by subtracting the maximum displacements of the original data from the maximum displacements from days three, five and seven to directly compare the changes due to post-mortem time. For each case, the data showed a slight increase in the maximum displacements when compared to the maximum displacements of the original data. However, it was found that in general the results from days three, five and seven correlated well with the results from the original testing day. For testing at 20 rad/s for 30 ms, the R^2 values for days three, five, and seven were 0.975, 0.931, 0.996, respectively. For testing at 20 rad/s for 60 ms, the R^2 values for days three, five and seven were 0.951, 0.994, 0.995, respectively. For testing at 40 rad/s for 30 ms, the R^2 values for days three, five and seven were 0.990, 0.996, 0.995, respectively.
The results of this pilot study suggest that, with appropriate material handling and perfusion, the in situ human brain tissue remains mechanically viable between two to seven days postmortem. Given the lack of biomechanical data on human brain deformation at injurious impact conditions, these results are encouraging as they may facilitate different types of test conditions within the same specimen.
