Effects of composition 1 and trinitrotoluene explosive pressure on auditory tissue: an ovine cadaveric assessment

Background: Eardrum rupture is one of the most common injuries associated with explosive pressures. Estimates for pressures at rupture vary widely and are challenging to determine because of (I) a range of variables that can degrade the structural integrity of the auditory tissue, and (II) the difficulties of minimizing their impact. Improving predictions on the pressures at which eardrum tissue ruptures would provide useful measures in avoiding or reducing the chances of these injuries. Although a variety of animal models could provide useful data, ovine specimens are uniquely positioned to capture representative behavior for humans, given anatomical similarities, while ameliorating some of the pragmatic issues related to specimen preparation and analysis. The objective of this study is to refine estimates of explosive pressure that initiate rupture of the tympanic membrane in sheep. Methods: Fresh cadaveric sheep ears were directly exposed to outdoor blast pressures generated by either composition 1 (C-1) or trinitrotoluene (TNT) explosives. A variety of imaging methods were used to determine the range of pressures at which eardrum rupture occurs. The pressures were measured at a variety of scaled distances for three separate tests of C-1 (n=5), TNT (n=6), and C-1 (n=4). Direct imaging with a boroscope was used for the first two tests, and with more detailed photographic imaging for the third. Results: There were three ranges of pressure that bounded eardrum rupture. For pressures under 30 kPa, no ruptures were observed. Pressures between 30 and 40 kPa resulted in a transition range where both ruptures and intact eardrums were observed. For pressures over 40 kPa, all eardrums ruptured. This included three ruptures that were on the opposite side of the blast direction, where the exposed ear canal entrance measured 75 kPa. Conclusions: Blast pressures that result in cadaveric ovine tissue rupture were narrowed from prior estimates to under 30 kPa (no rupture) to over 40 kPa (rupture) based on direct measurements (n=15). Ears on the opposite side of the blast direction were not spared from rupture.