The effect of primary blast shock wave on skeletal muscle
Keywords: skeletal muscle
shock waves
in vitro
RNA sequencing
combat casualties
blast injury
blast

Explosives continue to be one of the main mechanisms of injury of military personnel and civilians in conflict zones. The range of explosive devices (e.g., mines, artillery shells, improvised explosive devices (IEDs) and grenades) and the variety of environments in which they detonate (e.g., buried in the ground, on the surface, in the air or in confined spaces) means that the mechanical insult to the body and the subsequent injuries are highly variable. One unexplained phenomenon that can follow a blast injury is progressive tissue loss (PTL) in which apparently healthy soft tissue decays in the days and weeks afterwards. The overarching goal of this thesis is to study the effects of primary blast shock wave on the cellular microenvironment of skeletal muscle, in order to understand the role that a blast wave has on the onset of PTL. RNA analysis was carried out on legacy tissue samples from two in vivo experiments involving the exposure of skeletal muscle to a shock wave. The analysis showed that there were several common pathways between the models, and no single mechanism of cell death predominated. Instead, apoptosis, necrosis and necroptosis were upregulated to similar degrees. Therefore indicating that there is a cellular response in skeletal muscle when exposed to a blast wave.
Following a review of the literature a 3D model of skeletal muscle was developed by seeding a hydrogel with muscle cells, which demonstrated alignment and myotube fusion after 13 days. Sequencing of the hydrogel RNA revealed expression of key myotube differentiation markers, including Myosin heavy chain isoforms, Paired box proteins 7 and Myogenin. The 3D model was attached to deformable posts and the deflection of the post used to determine the tension generated by the muscle model. Hydrogen peroxide was used as a positive control for injury and the response was characterised using metabolic and RNA sequencing data.
The response of the 3D hydrogels to a blast wave was investigated by placing them in an explosively-driven frustum shock tube. The tube was calibrated for a range of explosive charges and measurements showed that 30 g of PE7 explosive produced a blast wave at the target location with a peak overpressure of 697 kPa and a positive phase duration of 2.39ms-1. The blast wave was equivalent to 3.23 kg-TNT at 2 m (assuming a hemispherical blast) and using a blast lethality model was predicted to be more than 50% lethal. A charge of 5 g produced a blast wave that was predicted to be non-lethal. Skeletal muscle hydrogels showed different responses to explosive loading: the 30 g charge resulted in a significant drop in cellular metabolic activity at 2 hours which failed to return to baseline by day 7. The 5 g charge resulted in an increase in metabolic activity over baseline by day 7 indicating that the cells has recovered. There was no detectable change in the tension generated by the hydrogels for either blast condition. In conclusion, this thesis has demonstrated that there is cellular injury from primary blast shock waves, reflected in alterations in metabolic activity and creatinine kinase release, and that this cellular response can be seen in analysis of in vivo tissue samples and captured with a 3D in vitro tissue scaffold model using an experimental blast tube. This suggest that primary blast wave could be responsible for initiating the signalling pathway that results in PTL.