Advanced Trauma Life Support ATLS Student Course Manual 2018

405
BIOMECHANICS OF INJURY
which is caused by a shock wave initiated by impact of
the bullet.
Cavitation is the result of energy exchange between
the moving missile and body tissues. The amount of
cavitation or energy exchange is proportional to the
surface area of the point of impact, the density of the
tissue, and the velocity of the projectile at the time of
impact. (See n FIGURE 2.) Depending on the velocity of
the missile, the diameter of this cavity can be up to 30
times that of the bullet. The maximum diameter of
this temporary cavity occurs at the area of the greatest
resistance to the bullet. This also is where the greatest
degree of deceleration and energy transfer occur. A
bullet fired from a handgun with a standard round can
produce a temporary cavity of 5 to 6 times the diameter
of the bullet. Knife injuries, on the other hand, result
in little or no cavitation.
Tissue damage from a high-velocity missile can occur
at some distance from the bullet track itself. Sharp
missiles with small, cross-sectional fronts slow with
tissue impact, resulting in little injury or cavitation.
Missiles with large, cross-sectional fronts, such as
hollow-point bullets that spread or mushroom on
impact, cause more injury or cavitation.
Bullets
Some bullets are specifically designed to increase the
amount of damage they cause. Recall that it is the
transfer of energy to the tissue, the time over which the
energy transfer occurs, and the surface area over which
the energy exchange is distributed that determine the
degree of tissue damage. Bullets with hollow noses
or semijacketed coverings are designed to flatten
on impact, thereby increasing their cross-sectional
area and resulting in more rapid deceleration and
consequentially a greater transfer of kinetic energy.
Some bullets are specially designed to fragment on
impact or even explode, which extends tissue damage.
Magnum rounds, or cartridges with a greater amount
of gunpowder than normal rounds, are designed to
increase the muzzle velocity of the missile.
The wound at the point of bullet impact is determined
by
••
The shape of the missile (“mushroom”)
••
The position of the missile relative to the
impact site (tumble, yaw)
••
Fragmentation (shotgun, bullet fragments,
special bullets)
Yaw (the orientation of the longitudinal axis of
the missile to its trajectory) and tumble increase the
surface area of the bullet with respect to the tissue it
contacts and, therefore, increase the amount of energy
transferred. Bullets do not tumble in flight but will
tumble as they lose kinetic energy in tissue (n FIGURE 3).
In general, the later the bullet begins to yaw after
penetrating tissue, the deeper the maximum injury.
Bullet deformation and fragmentation of semijacketed
ammunition increase surface area relative to the tissue
and the dissipation of kinetic energy.
Shotgun Wounds
Wounds inflicted by shotguns require special
considerations. The muzzle velocity of most of these
weapons is generally 1200 ft/sec (360 m/sec), but
the mass is high. After firing, the shot radiates in a
conical distribution from the muzzle. With a choked
or narrowed muzzle, 70% of the pellets are deposited
n FIGURE 2 Sharp missiles with small cross-sectional fronts slow
with tissue impact, resulting in little injury or cavitation. Missiles
with large cross-sectional fronts, such as hollow-point bullets that
spread or “mushroom” on impact, cause more injury and cavitation.
n FIGURE 3 Yaw (the orientation of the longitudinal axis of the
missile to its trajectory) and tumble increase the surface area of
the bullet with respect to the tissue it contacts and, therefore,
increase the amount of energy transferred. In general, the later
the bullet begins to yaw after penetrating tissue, the deeper the
maximum injury.
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