Distribution of Bacterial Contamination in Partial Penetration Surrogate Ballistic Wounds

With the rise in use of improvised explosive devices (IEDs) during armed conflicts, there has been an increase in the number of military personnel subjected to secondary blast injuries to the extremities. Shrapnel and debris ejected during the explosion of an IED become high-speed projectiles capable of producing ballistic trauma. When a projectile penetrates soft tissues, bacterial contamination from the skin and surrounding areas can be brought into the wound by the projectile itself or by the suction effect caused by the formation of the temporary cavity. If not properly treated, the presence of this initial bacterial contamination in the wound could lead to infection. Studies aimed at understanding the distribution of bacterial contamination along the permanent cavity could provide useful information to improve treatment protocols for these types of injuries. In this paper, a simple lower extremity surrogate model is used to investigate bacterial distribution in partial penetration ballistic wounds.

The targets used in the study were square blocks made of 11% by weight ballistic gelatin at 4 ^oC in which the sides had a length of 127-mm (5-in). That dimension was selected based on the size of the calf region of an adult human male. Spherical 4.5-mm (0.177-in) projectiles were fired into the targets using a standard multi-pump air rifle. The speed of the projectiles was adjusted to obtain three different partial penetration depths: approximately one quarter, one half and three quarters of the target length. Bacterial contamination was represented using a DH5-alpha Escherichia coli-laden filter paper placed on the front face of the target.

After each shot was fired, the speed and penetration depth of the projectile were measured. A gelatin strip containing the permanent cavity was extracted from the target and segmented into 12.7 mm (0.5-in) segments. Using a biopsy punch, the permanent cavity was removed from each segment and placed in a test tube with buffer solution. Then, the test tube was placed in a water bath to melt the gelatin and standard microbiology protocols were followed to determine the number of colony forming units (CFUs) present in each segment. The bacteria distribution for each test was represented by percent of total CFU in the wound versus segment number. In addition, using information from all the tests conducted, bacterial contamination as a function of the projectile penetration depth was explored. For the cases considered, the results obtained showed that most of the bacterial contamination occurred in the segments closer to the projectile entry point.