Managing gunshot wounds and serious bite wounds (Proceedings)
Gunshot wounds can be a common injury in veterinary medicine varying from shotgun and rifle injuries in rural areas to handguns and air powered weapons in cosmopolitan areas. The severity of injury resulting from gunshots is variable as well based on the type and composition of the bullet, the projectile (flight) characteristics, kinetic energy absorbed on impact and tissues involved. A basic understanding in ballistics and tissue reaction can be helpful in determining the extent of injury and the appropriate treatment necessary for a given projective wound.
The characteristics of the bullet and weapon influence the type and severity of injury inflicted. These can be classified into Internal Ballistics, External Ballistics, and Terminal Ballistics. Internal Ballistics refers to the characteristics within the weapon such as bullet design (bullet, shotgun pellets, air-powered pellets), weapon design, and barrel effects. External Ballistics refers to the in flight projectile characteristics such as projectile design, wind velocity, drag, and gravity. Terminal Ballistics are the characteristics that affect impact including the projectile velocity, kinetic energy of the projectile and absorption by the particular tissue, and projectile movement/ position on impact.Bullets typically have a lead core, however they vary in size, shape, and weight. The shape and casing of the bullets (fully jacketed, partially jacketed, hollow point) affects the degree the bullet is deformed/ fragmented on impact, which slows the velocity of the projectile causing soft tissue destruction along its path as well as increasing the kinetic energy absorbed by the soft tissues and thereby increasing the amount of soft tissue destruction. For example, a partially jacketed hunting bullet designed to deform and mushroom on impact can create 40X more soft tissue destruction than a military bullet of similar size and mass.
The bullet and barrel design also affects the flight characteristics of the projectile. Typically, most modern bullets are fired though barrels which are rifled with helical grooves which elicits a spin on the bullet improving its motion through flight and accuracy. Bullets can become unstable during flight resulting in a yaw or tumble of the bullet as it travels through the air. This altered motion slows the velocity of the projectile but overall increases the soft tissue damage by increasing the area of the bullet on contact and may facilitate fragmentation of the bullet further enhancing the amount of tissue destruction.
Shotgun pellets are available in steel and lead and come in a variety of pellet sizes and number. BB and pellet guns fire low mass and low velocity projectiles. Shotguns pellets are designed to disperse increasing the target area. The pattern of dispersion can be changed by modifying the barrel length and diameter with a cone shaped dispersion of pellets extending from the barrel. Even though they are fired at a low velocity, the typical hourglass or cone shape of these pellets can incur severe soft tissue injury at a close range. The closer the range the more destructive this pattern becomes as a concentrated density of pellets with high kinetic energy strikes a small target area.
The Kinetic Energy of a projectile is a measure of its destructive potential as is measured by its mass and velocity.
Kinetic energy (KE) = Mass X Velocity 2 / 2
Looking at this equation, doubling the mass of the projectile doubles its kinetic energy whereas doubling the velocity will quadruple its kinetic energy. As a result, a smaller projectile with a higher velocity may have a greater kinetic energy and destructive potential compared to a slower projectile with greater mass at impact. This is important, as the velocity of a projectile is the most important factor or determinant for wound severity.
Wound Severity is determined also by the target tissue type and its ability to absorb or resist kinetic energy, which is a function of the tissues specific gravity, elasticity, and relative cohesive properties. Tissues with a lower specific gravity and greater elasticity, such as skin and lung, are better able to absorb a portion of the kinetic energy of a projectile, compared to bone. Although liver and muscle have similar specific gravities, liver has more relative tissue destruction because the tissue is less cohesive and less resilient. Dense tissues with limited compliance, such as bone, absorb a larger amount of kinetic energy and can shatter.
When a bullet interacts with tissues several mechanisms of tissue injury can occur including Lacerations and Crushing injuries, Shockwaves, Cavitation, and Secondary Missiles. Crushing injuries and lacerations are the primary method of injury with low velocity projectiles. Shockwaves are waves of high energy and short duration which propagate in all directions and tend to move ahead of the projectile causing not only adjacent but tissue damage distant to the path of the projectile. Cavitation refers to the transient rapid expansion of tissues adjacent to the bullet. This distension can be up to 30X the diameter of the bullet. The marked tissue distension creates a negative pressure resulting in a vacuum into which debris can be sucked deep into the wound it its entry and exit points. The result of this rapid expansion of tissues is a permanent cavity that can harbor necrotic and contaminated tissues. Tissues that have a lower specific gravity and greater elasticity are less affected by cavitation. Secondary Missiles are either primary projectile fragments or bone fragments that arise from the shattering of bone and these missiles are driven into adjacent tissues resulting in further tissue injury.