Response of Thoraco-Abdominal Tissue in High-Rate Compression.

Body armor is used to protect the human from penetrating injuries, however, in the process of defeating a projectile, the back face of the armor can deform into the wearer at extremely high rates. This deformation can cause a variety of soft and hard tissue injuries. Finite element modeling (FEM) represents one of the best tools to predict injuries from this high-rate compression mechanism. However, the validity of a model is reliant on accurate material properties for biological tissues. In this study, we measured the stress-strain response of thoraco-abdominal tissue during high-rate compression (1000 and 1900 s-1) using a split Hopkinson pressure bar (SHPB). High-rate material properties of porcine adipose, heart, spleen, and stomach tissue were characterized. At a strain rate of 1000 s-1, adipose (E = 4.7 MPa) had the most compliant stress-strain response, followed by spleen (E = 9.6 MPa), and then heart tissue (E = 13.6 MPa). At a strain rate of 1900 s-1, adipose (E = 7.3 MPa) had the most compliant stress-strain response, followed by spleen (E = 10.7 MPa), heart (E = 14.1 MPa), and stomach (E = 32.6 MPa) tissue. Only adipose tissue demonstrated a consistent rate dependence for these high strain rates, with a stiffer response at 1900 s-1 compared to 1000 s-1. However, comparison of all these tissues to previously published quasi-static and intermediate dynamic experiments revealed a strong rate dependence with increasing stress response from quasi-static to dynamic to high strain rates. Together, these findings can be used to develop a more accurate finite element model of high-rate compression injuries.

Time Delta Head Impact Frequency: An Analysis on Head Impact Exposure in the Lead Up to a Concussion: Findings from the NCAA-DOD Care Consortium.

Sport-related concussions can result from a single high magnitude impact that generates concussive symptoms, repeated subconcussive head impacts aggregating to generate concussive symptoms, or a combined effect from the two mechanisms. The array of symptoms produced by these mechanisms may be clinically interpreted as a sport-related concussion. It was hypothesized that head impact exposure resulting in concussion is influenced by severity, total number, and frequency of subconcussive head impacts. The influence of total number and magnitude of impacts was previously explored, but frequency was investigated to a lesser degree. In this analysis, head impact frequency was investigated over a new metric called 'time delta', the time difference from the first recorded head impact of the day until the concussive impact. Four exposure metrics were analyzed over the time delta to determine whether frequency of head impact exposure was greater for athletes on their concussion date relative to other dates of contact participation. Those metrics included head impact frequency, head impact accrual rate, risk weighted exposure (RWE), and RWE accrual rate. Athletes experienced an elevated median number of impacts, RWE, and RWE accrual rate over the time delta on their concussion date compared to non-injury sessions. This finding suggests elevated frequency of head impact exposure on the concussion date compared to other dates that may precipitate the onset of concussion.

A Preclinical Rodent Model for Repetitive Subconcussive Head Impact Exposure in Contact Sport Athletes.

Repetitive subconcussive head impact exposure has been associated with clinical and MRI changes in some non-concussed contact sport athletes over the course of a season. However, analysis of human tolerance for repeated head impacts is complicated by concussion and head impact exposure history, genetics, and other personal factors. Therefore, the objective of the current study was to develop a rodent model for repetitive subconcussive head impact exposure that can be used to understand injury mechanisms and tolerance in the human. This study incorporated the Medical College of Wisconsin Rotational Injury Model to expose rats to multiple low-level head accelerations per day over a 4-week period. The peak magnitude of head accelerations were scaled from our prior human studies of contact sport athletes and the number of exposures per day were based on the median (moderate exposure) and 95th percentile (high exposure) number of exposures per day across the human sample. Following the exposure protocol, rats were assessed for cognitive deficits, emotional changes, blood serum levels of axonal injury biomarkers, and histopathological evidence of injury. High exposure rats demonstrated cognitive deficits and evidence of anxiety-like behaviors relative to shams. Moderate exposure rats did not demonstrate either of those behaviors. Similarly, high exposure rats had histopathological evidence of gliosis [i.e., elevated Iba1 intensity and glial fibrillary acidic protein (GFAP) volume relative to shams] in the basolateral amygdala and other areas. Blood serum levels of neurofilament light (NFL) demonstrated a dose response relationship with increasing numbers of low-level head acceleration exposures with a higher week-to-week rate of NFL increase for the high exposure group compared to the moderate exposure group. These findings demonstrate a cumulative effect of repeated low-level head accelerations and provide a model that can be used in future studies to better understand mechanisms and tolerance for brain injury resulting from repeated low-level head accelerations, with scalable biomechanics between the rat and human.