Copyright Ann H. Ross
Author: Ann H. Ross ( M.A. awarded Dec. 1995) Address: The University of Tennessee, Department of Anthropology, 250 South Stadium Hall, Knoxville, TN 37996

Law enforcement agencies and medical examiner facilities are increasingly using the knowledge developed by forensic anthropologists in the identification of skeletal remains. Identity, manner of death, as well as osteopathology, are often difficult to determine by traditionally trained medical specialists because their focus is usually upon soft tissue. Therefore, a role exists for the forensic anthropologist.

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).

Figure 1.

a. Radiograph of sheep femur showing the temporary cavity and fracture b. reconstructed femur. Source: D. Hopkinson, T. Marshall Firearm Injuries. British Journal of Surgery 54:350, 1967.

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).

Figure 2. Typical outer table entrance site.
Similarly, as the bullet exits the cranial cavity, the inner table appears "punched-out" with beveling on the outer table (Di Maio, 1985; Spitz and Fisher, 1993) (Figure 3).

Figure 3. Typical exit site with outer table beveling.

External beveling of an entrance site, however, may be observed when a bullet strikes the skull tangentially or perpendicularly to the bony surface (Coe, 1982; Peterson 1991). A missile striking the skull tangentially, as may occur in graze wounds, produces a keyhole defect where entrance and exit defects overlap (Coe, 1982; Dixon, 1982; Peterson, 1991; Spitz and Fisher, 1993). In the keyhole lesion, one end of the perforation will resemble a typical entrance defect, while the other end will show external beveling consistent with exit holes (Figure 4).

Figure 4. Keyhole wound resulting from a tangential source.

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).

Figure 6. Entrance defect in a distal femur resembling a drill hole. Source: B. Ragsdale Gunshot Wounds: A Historical Perspective. Military Medicine 149:307, 1984.

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).

Figure 7. "Butterfly" fracture of tibia. Source: L. La Garde Gunshot Injuries, 2nd ed.. New York: William Wood and Company, 1916.

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.

References Cited

     Adams, D.B.
       1982    Wound ballistics: A Review.  Military Medicine     
     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.
       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-     
     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

Updated 20th October 1995 by James Batchelor