Wound ballistics and mechanisms of Injury

As a missile traverses the body it causes injury by transferring some or all of its available energy, and this is manifested by lacerating and crushing tissues in its path and, in some cases, injury remote from the missile path (see below). The amount of energy transferred may be expressed by the formula:

                                                                       

                               K.E - = 1/2M (v12-v22 )

where KE is the available energy, M is the mass, and V1 and V2 are the velocities at entry and at exit, respectively. In general, bullets fired from handguns and most modern fragment munitions are propelled at low velocity, have low available energy (100—5 00 J) and result in low-energy transfer wounds. Missiles with high available energy (2000—3000 J) include high-velocity assault rifle bullets (> 900 m/second) and some large fragments, and have potential to cause high-energy transfer wounds (Figs 19.1 and 19.2). Some modern high-performance handguns are now capable of firing high-velocity bullets with high available energy.

By convention, missile wounds are now described in terms of energy transfer, not velocity as was the custom, recognising that velocity is merely one factor determining energy available and its transfer to tissues. Low-energy transfer wounds are characterised by injury confined to the wound track. High-energy transfer wounds also cause local laceration and crush injury but have, in addition, the potential to cause injury remote from the wound track associated with a phenomenon known as temporary cavitation (Fig. 19.3).

The extent of cavitation depends upon the density and elasticity of the target organ or structure, and in certain circumstances is associated with injury many centimetres away from the missile wound track.

Cavitation within solid organs such as the liver, spleen and kidney results in shattering with high morbidity and mortality. The extent of injury to bowel is variable. In general, the small bowel fares better than the colon, particularly if the latter is loaded with faeces. A similar event in an elastic tissue   such as the lung may result in quite modest injury. In the limb the position is more complex and controversial. While voluntary muscle may merely stretch if injured in isolation, bone fares badly. As a rule, bone involvement results in severe injury due to high-energy transfer with disruption of the missile and involved bone, with generation of secondary missiles. Extensive devitalisation of muscle is a typical finding. Devitalised muscle in the depths of a missile wound provides the perfect culture medium for the growth of pathogenic bacteria, a fact recognised by military surgeons for centuries. Nerves and blood vessels respond unpredictably with injury, ranging from minimal bruising to complete disruption.

Within the closed skull there is, in addition, a rapid, high-pressure shock wave causing widespread disruption and injury at a distance. Thus, vital centres at the base of the brain may be injured by a wound of the cranium.