Copyright Ann H. Ross
I. Basic ballistic principles
Ballistics is the area of study dealing with the motion
of projectiles, i.e., bullets, and is further divided into
internal ballistics, the study of projectiles in the weapon;
external ballistics, the behavior of the projectile through
air; and terminal ballistics, the study of the penetration
of a medium denser than air by projectiles (Barach et al.,
1986; Belkin, 1978; Di Maio, 1985; Ordog et al., 1984). One
area of terminal ballistics, wound ballistics, is primarily
concerned with the "...penetration, motion, and effects of
missiles on animals" (Collins and Lantz, 1994:97).
The amount of tissue damage is determined by the amount
of kinetic energy lost by the projectile in the body
(Callender and French, 1935; Coates and Beyer, 1962; Di Maio
et al., 1974; Harvey et al., 1945). Kinetic energy is
illustrated as KE = WV2/2g, where: W=bullet weight,
V=velocity, g=gravitational acceleration. Bullet weight and
velocity determine the kinetic energy possessed by a
projectile with velocity being the most critical component
(Berlin, 1976; DeMuth, 1966; Hopkinson and Marshall, 1967;
Ordog et al., 1984). A variety of factors are responsible
for the amount of kinetic energy lost in the body:
"...amount of kinetic energy possessed by the bullet at the
time of impact..." (Di Maio, 1985:46), mass, yaw (deviation
of the missile from its flight path), caliber or size of
bullet, shape, deformation, and density of the tissue being
struck (Callender, 1943; Fatteh, 1976; Ragsdale, 1984).
Some, such as Lindsey (1980), reject the concept that
velocity is the primary mechanism in the wounding force and
suggest that the kinetic energy formula is solely a formula
for kinetic energy and not of wounding capacity. Others,
notably Barach and coworkers (1986), maintain that mass or
weight is as critical in wound production as velocity since
KE is a product of both weight and velocity, and not merely
velocity.
Principally, there are three mechanisms of tissue
damage due to bullets: laceration and crushing, shock
waves, and cavitation (Adams, 1982; Hopkinson and Marshall,
1967; Ordog et al., 1984). Laceration and crushing are
generated by the projectile displacing the tissues in its
track and are recognized as the primary wounding mechanism
produced by handguns (Fackler, 1986; Hopkinson and Marshall,
1967). The degree and amount of laceration and crushing are
dependent upon missile velocity, shape, angle of impact,
yaw, and tumbling (Adams, 1982; Ordog et al., 1984).
Fackler (1986), however, adds that the shape and
construction of a bullet are not significant factors at such
low-velocities as observed in handguns. Shock waves, the
second mechanism often cited as significant in wounding,
occur by the compression of tissues that lay ahead of the
bullet, are generated by high velocity missiles generally
exceeding 2,500 feet per second (Hopkinson and Marshall,
1967; Ordog et al., 1984), and thus not a major factor in
most handgun wounds.
A missile's ability to produce a temporary cavity is
considered an important component in wound production and
degree of destruction (Barach et al., 1986). Most
researchers agree that the wounding effect of the cavitation
phenomenon is only significant in velocities surpassing
1,000 feet per second (Amato et al., 1974; DeMuth, 1966).
When a missile enters the body, the kinetic energy imparted
on the surrounding tissues forces them forward and radially
producing a temporary cavity or temporary displacement of
tissues (Belkin, 1978; DeMuth, 1966; Ragsdale 1984). The
temporary cavity may be considerably larger than the
diameter of the bullet, and rarely lasts longer than a few
milliseconds before collapsing into the permanent cavity or
wound (bullet) track (Kirkpatrick, 1988).
The permanent cavity, or wound track, is the defect
generated when the tissues in the projectile's path
are expelled from the body (Huelke and Darling, 1964). The
cavitation phenomenon has been used to explain the
fracturing of bone not in the direct path of a missile
(Figure 1).
Furthermore, the bone fragments will often function as
secondary projectiles, which thereby will often increase
tissue disruption (Fackler, 1987; Hopkinson and Marshall,
1967; Kirkpatrick and Di Maio, 1978). Nonetheless, Barach
et al. (1986), Fackler (1988), Ragsdale (1984), Ragsdale and
Josselson (1988), argue that handguns also generate some
proportion of cavitation. Similarly, skeptics contest that
the temporary cavity phenomenon is nothing more than the
simple displacement of tissues akin to blunt trauma
(Fackler, 1988; Lindsey, 1980).
Once the missile strikes the body, not only is the amount of kinetic energy displaced into the surrounding tissues important, but also the density of the tissue being penetrated. Consequently, the wounding capacity of a missile striking bone will be greater than in soft tissues, as bone acts as a superior retardant force that is more effective at decelerating a projectile and increasing the energy transfer than less compact substances (Adams, 1982; Ordog et al., 1984). In addition, cancellous bone, the spongy bone found on the epiphyses of long bones, will experience less damage than the more compact cortical bone, because the KE can more readily dissipate within the honeycomb structures of the cancellous bone (Belkin, 1978; Fatteh, 1976; Huelke and Darling, 1964; La Garde, 1916).
II. Cranial entrance and exit sites
Gunshot wounds can be identified as either penetrating, when
a bullet enters a substance but does not exit, or
perforating, a through-and-through passage of an object by a
bullet (Di Maio, 1985). Because the skull is formed of an
inner and outer table, entrance and exit sites are usually
easily determined. When a bullet enters the skull it
produces a sharp-edged "punched-out" hole in the outer
table, with a larger corresponding "beveled-out" hole on the
inner table (Figure 2).
The mechanism of injury used to explain keyhole lesions is
that as the bullet enters the skull tangentially, the bullet
is split, one portion entering the cranial cavity while the
other is expelled producing the exit defect (Coe, 1982).
However, as demonstrated by Dixon (1982) this is not always
the case, the keyhole defect may be produced by a bullet
that remains virtually intact. Keyhole defects, although,
are not exclusive to entrance sites and have also been
observed in exit sites (Dixon, 1984a).
External beveling of entrance sites produced when a
bullet enters the skull perpendicularly is not well
understood (Coe, 1982; Peterson, 1991)(Figure 5).
Figure 5. External beveling of an entrance site.
According to Coe (1982), the mechanism responsible in the
majority of the cases is due to contact wounds, where the
handgun is held against the head. "In such cases it seems
plausible that the gases expanding in the subcutaneous
tissues penetrate the marrow cavity of the bone and lift the
outer table of the skull" (Coe, 1982:218). Although in
cases of distant range, Spitz and Fisher (1993) attribute
this phenomenon to bullet rotation. Peterson (1991), per
contra, argues that the blowback from the pressure buildup
associated with temporary cavity formation is a more
plausible explanation.
Smith et al. (1993) have observed atypical exit defects
to the cranial vault mimicking blunt(closed head) trauma.
Rather than the typical central defect with external
beveling, they observed an epicenter of curvilinear radial
cracking with plastic deformation or warping "...of bone due
to slow loading and blunt trauma" (Smith et al., 1993).
They ascribed this anomaly to slow-moving projectiles.
III. Fracture patterns on the skull
Spitz and Fisher (1993) used fracture patterns to
determine the sequence of fire or which of the entrance
defects occurred first. They claim that the fractures that
originate from the second entrance defect are arrested by
the radiating linear fractures from the first hole.
Similarly, Dixon (1984b) has used fracture patterns to
determine direction of fire. He maintains that the linear
fractures associated with typical exit sites terminate at
the preexisting linear fractures produced by the entering
bullet, supporting an earlier premise of Gonzales et al.
(1954) that fracture patterns produced by the passage of a
missile travel faster than the bullet. In addition, Smith
et al. (1987) assert that radiating linear fractures as well
as concentric heaving fractures can be used to determine
direction of fire. They argue that radiating fractures
associated with entrance defects are longer and are not
arrested by preexisting fractures. Likewise,
heaving fractures, if present, have more
generations and longer radii than exit
associated fractures...Exit fractures show
radial and heaving fractures of lesser magnitude,
and may be arrested by preexisting fractures...
(Smith et al. 1987:1421)
generated by the entrance wound.
IV. Entrance and exit defects to extremities
Detection of gunshot trauma on long bones and
especially irregular bones can be a much more difficult
process. Smaller bones, cancellous bone, and bone affected
by degenerative diseases can shatter on impact, bearing
little resemblance to the typical trauma site. In such
cases where damage from a bullet is suspected, radiographs
taken of the area can confirm the existence of radio-opaque
particles left by the slug's path.
Entrance defects on the distal end of bones are smooth and
clean or "drill hole" in appearance while those on the
shafts are generally comminuted (Belkin,1978; Huelke and
Darling, 1964; La Garde, 1916) (Figure 6).
Huelke and Darling (1964) conducted a study of bone
fractures produced by bullets of the femur and tibia in both
dry and cadaver bones. They observed that metaphyseal and
diaphyseal fracture patterns differed greatly. Projectile
trauma to the diaphyses in cadaver specimens differed from
dry bone specimens in that there were numerous fractures
surrounding the exit with little or no fragmentation around
the exit in dry bone. The "drill hole" defect was apparent
on both cadaver and dry bone entrance sites.
Shaft impacts of both dry and cadaver specimens were
comminuted with "butterfly" fragments produced bilaterally
(Figure 7).
Huelke and Darling (1964) attribute the variation in
fracture patterns between bone metaphyses and diaphyses to
the different types of bone found in these two areas.
Because the distal end is mostly composed of cancellous bone
with only a thin layer of cortical bone the kinetic energy
is better able to dissipate within the spongy area. This
produces less destruction than in more compact cortical bone
found in diaphyses which generate more deformation. La
Garde (1916) was the first to document "butterfly" fractures
in the diaphyses of cadaver specimens.
Adams, D.B.
1982 Wound ballistics: A Review. Military Medicine
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Amato, J.J., L.J. Billy, N.S. Lawson, N.M. Rich
1974 High-Velocity missile injury: An experimental
study of the retentive forces of tissue.
American Journal of Surgery 127:454-458.
Barach, E., M. Tomlanovich, and R. Nowak
1986 Ballistics: A pathophysiologic examination of the
wounding mechanism of firearms: Part I. The
Journal of Trauma 26:225-235.
Belkin,M.
1978 Wound ballistics. Progress in Surgery 16:7-24.
Berlin, R., B. Janzon, Kokinakis, W., and D. cepanovic
1976 Various technical parameters influencing wound
production. Acta Chirurgica Scandinavica
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Callender, G.R.
1943 Wound ballistics. War Medicine 3:337-350.
Callender, G.R., and R.W. French
1935 Wound ballistics. The Military Surgeon 4:177-
201.
Coates, J.B., and J.C. Beyer
1962 Wound Ballistics. Office of The Surgeon General
Department of the Army, Washington.
Coe, J.I.
1982 External beveling of entrance wounds by handguns.
The American Journal of Forensic Medicine and
Pathology 3:215-219.
Collins, K.A. and P.E. Lantz