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
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
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).
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
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 147:831-835. 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 Supplementum 489:1-30. 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