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:
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
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.