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Patterns of Tissue Injury

Classification

One of the commonest determinations of the forensic pathologist is the range of fire. Gunshot wounds are typically classified as:

  1. Contact
  2. Intermediate range
  3. Distant range

Example images demonstrating gross and microscopic appearances of gunshot wounds:

  1. Sooting of hand, gross
  2. Contact range gunshot wound, gross
  3. Contact range gunshot wound, gross
  4. Contact range gunshot wound, gross
  5. Contact range gunshot wound, gross
  6. Blood spatter on hand, gross
  7. Gunshot entrance wound with GSR, microsopic
  8. Intermediate range gunshot wound, gross
  9. Intermediate range gunshot wound, gross
  10. Entrance-exit wound, close proximity from low angle of bullet entrance, gross
  11. Exit gunshot wound, gross

Entrance Wounds

Contact wounds characteristically have soot on the outside of the skin, and muzzle imprint, or laceration of the skin from effects of gases. Contact wounds of airguns usually lack these features (Cohle et al, 1987). Intermediate, or close-range, wounds may show a wide zone of powder stippling, but lack a muzzle imprint and laceration. The area of powder stippling will depend upon the distance from the muzzle. (Denton et al, 2006)

Distant range wounds are lacking powder stippling and usually exhibit a hole roughly the caliber of the projectile fired.

The most difficult problem is distinguishing a distant from a contact wound. The factors that can affect the amount and distribution of gunshot residue (GSR) on skin and clothing include: (1) firing distance, (2) length and diameter of the firearm barrel, (3) characteristics of the gunpowder, (4) angle between the firearm barrel and target, (5) characteristics of the cartridge, (6) the environment (moisture, wind, heat), (7) type of clothing, (8) intermediate targets, and (9) characteristics of the target (tissue type, putrefaction, blood marks) (Tugcu et al, 2006).

Scanning electron microscopy of entrance wounds shows gunshot residue within collagen fibrils. The entrance wound appears abraded, with loss of the papillary pattern and laceration of basement membrane (Torre et al, 1986). Computer assisted image analysis may aid detection of GSR (Tugcu et al, 2006).

Entrance wounds into skull bone typically produces bevelling, or coning, of the bone at the surface away from the weapon on the inner table. In thin areas such as the temple, this may not be observed. Sternum, iliac crest, scapula, or rib may show similar features. These observations may permit determination of the direction of fire. A small, dense projectile may "punch out" a rounded portion of cranium, while a larger projectile may produce circumferential fractures that radiate outward from the point of entrance. (Jandial et al, 2008)

The pattern of fractures may permit identification of the sequence and direction of fire. Puppe's rule states that when two or more fracture lines of the skull produced by different blunt forces intersect, it is possible to reconstruct the sequence of injuries. The presence of bone damage from an initial injury causes subsequent injuries to stop in the point of intersection with the previous wounds. In the skull, the fracture lines produced by a second gunshot stops at pre-existing fractures of the skull. (Viel et al, 2009)

Example images demonstrating gunshot wounds to skull:

  1. Skull, contact range gunshot wound, gross
  2. Bullet track through skull, diagram

Tangential entrance wounds into bone may produce "keyhole" defects with entrance and exit side-by-side, so that the arrangement of bevelling can be used to determine the direction of fire. (Dixon, 1982) (Denton et al, 2006)

Dixon (1984a) has described how the direction of fire of a graze gunshot wound of the skin surface can be determined by careful examination of the so-called skin tags located along the lateral margins of the graze wound trough, by use of a dissecting microscope or hand lens. Characteristically, the side of the tag demonstrating a laceration is the side of the projection toward the weapon.

"Shoring" of entrance wounds can occur when firm material is pressed against the skin, such as when a victim is shot through a wooden, glass, or metal door while pressing against it to prevent entry of an assailant. A study by Dixon (1980) showed that such wounds have a greater wound diameter and demonstrate greater marginal abrasion than control wounds produced by the same weapons. The features were directly proportional to the KE of the projectile and the rigidity of the shoring material. Stellate radiating lacerations of some shored wounds could lead to misinterpretation of distant range of fire as a contact wound. (Denton et al, 2006)

Use of silencers (or "muzzle brakes" to deflect gas and recoil) may produce atypical entrance wounds. A silencer is a device, often homemade, fitting over the muzzle that attempts to reduce noise by baffling the rapid escape of gases. Their possession is illegal. Entrance wounds produced when silencers are present lead to muzzle imprints that are erythematous rather than abraded and disproportionately large for the size of the wound. Entrance wounds may appear atypical at close range. (Menzies et al, 1981)

Firearm missile emboli ("wandering bullets") are rare, with only 87 reported through 1984, but may occur in victims that survive for some time and may require surgical intervention. (Chapman and McClain, 1984)

Can a bullet that is fired skyward cause death? Rare cases of fatal injury have occurred. However, a terminal velocity of 200 feet per second (fps) must be reached to penetrate skin, and a bullet fired vertically would have to fall base first, without tumbling, to exceed 200 fps. A bullet fired at a high-arching angle would have to maintain a flight path without tumbling and land nose forward to maintain sufficient velocity to achieve tissue penetration. Such events are possible, but improbable. (Rapkiewwicz et al, 2013) (Hyneman and Savage, 2006)

Shotgun shells contain variable numbers of round metal pellets, and the characteristics of entrance wounds vary with the range of fire. Contact range approximates the barrel diameter. At increasing distances, there is dispersion of the pellets. Dispersion also depends upon the "choke" or constriction of the barrel intended to keep the pellets grouped more closely. Portions of the shotgun wad or casing may also strike a target and leave an impression, but are unlikely to penetrate skin; they are unlikely to go beyond 20 yards. The pellet size and muzzle velocity determine the energy of individual pellets, and their ability to penetrate the target. The mass of pellets also interacts, with pellets impacting each other to diminish their energy and increase their dispersion from the point of aim. At a distance of 5 yards, the pellets are likely to be individually dispersed. Size of an entrance would can approximate that of the size of the pellet (Drake, 1962). Dispersal of pellets can be defined as a "dispersion index" that is proportional to the square root of the ratio of potential strain energy to kinetic energy possessed by the shot mass as the muzzle. Pellets acquire radial velocity from their interaction to produce the spread pattern (Nag and Sinha, 1992).

The ballistic properties of shotgun shells are complex because of multiple projectiles fired simultaneously that interact and spread out to affect their energy relayed to a human target. Intermediate targets such as clothing can affect penetration into tissues. Using a standard 12-gauge shotgun with modified choke and no. 8 shot ammunition, the protection afforded by fabrics to reduce penetration of shotgun pellets into tissues was greater at increasing distance from the muzzle beyond 40 yd (36.6 m). Thicker denim and cotton fabrics provided slightly greater protection than polyester. (Cail and Klatt, 2013)

Shotgun slug entrance wounds approximate the size of the slug but at close ranges the blast effect may create a wound larger than the slug diameter. A "sabot slug" has an hour-glass shape, and yawing of the slug can create a larger entrance wound than the slug diameter. The soft lead of the slug may cause deformation upon impact (Gestring et al, 1996).

Entrance wounds associated with black powder handguns are associated with extensive sooting, a long range of travel of the sooting into the wound, and skin burns. Large pocket-like underminings may be seen even in deeper tissue layers with contact range wounds. (Karger and Teige, 1998)

The skin defect at the entrance site occurs from multiple mechanisms. Most of the defect results from skin fragmentation with fragements carried into the bullet track. The negative pressure of temporary cavitation pulls skin particles into the wound. There is also backspatter of skin particles away from the direction of bullet travel upon entrance. (Perdekamp et al, 2005)

Infection may result from gunshot wounds. Bullets are not sterile objects, either before or after firing. Bacteria are ubiquitous on skin surfaces and clothing. The bullet carries bacteria into the wound track. Skin particles serve as a transport vehicle for the bacteria. (Perdekamp et al, 2006)

Burn injury at the entrance site with close contact range is typically a minor component of the tissue injury, but some coagulative necrosis does occur. (Tschirhart et al, 1991). Toy cap guns with no projectile, may produce injury via burn alone. (Maze and Holland, 2007).

Bullet Tracks

The track made by the diameter of the bullet (caliber plus change in size through deformation) is a permanent cavity lined by crushed tissue and cellular debris. Adjacent to this track is a region in which the pressure wave created by the bullet causes outward stretching and shearing forces that produce tissue contusion, termed a temporary cavity, which lasts just milliseconds. The higher the velocity of the bullet, the more kinetic energy, and the greater the temporary cavity size, which may be more than 10 times the caliber of the bullet. Rifle bullets generally have a high velocity >2000 feet per second (fps) while bullets fired from handguns have lower velocity. Bullets that traverse tissue without deformation or tumbling impart less kinetic energy and are more likely to exit the body. (Bruner et al, 2011).

Deformation of the bullet, fragmentation of the bullet or secondary targets such as bone, and amount of kinetic energy imparted to tissues, as well as tissue characteristics affect patterns of tissue injury. The higher the specific gravity of tissue, the greater the damage. Elasticity reduces damage. Thus, lung tissue of low density and high elasticity is damaged less than muscle with higher density but some elasticity. Liver, spleen, and brain have little tensile strength and elasticity and are easily injured, as is adipose tissue. Fluid-filled organs (bladder, heart, great vessels, bowel) can burst because of pressure waves generated. A bullet striking bone may cause fragmentation of bone and/or bullet, with numerous secondary missiles formed, each producing additional wounding. Fragmentation increases the permanent cavity size (Maiden, 2009; Bruner et al, 2011).

Formation of the temorary cavity exerts pressure waves and shearing forces. These forces can rupture blood vessels to allow blood to escape. The extracellular tissue matrix with collagen, reticular, and elastic fibers can be disrupted. Thus, a contusion of tissue surrounding the bullet track can fill the track with blood and interstitial fluid, as well as cause edema of contused tissue.

Within the cranial cavity, formation of a temporary cavity is restricted, and pressure waves can damage tissues via contusion away from the permanent bullet track. These intracranial pressure effects most immediately affect the brain stem, while edema and neocortical effects may develop over days to weeks. (Jandial et al, 2008)

For lower velocity cartridges, particularly those designed for handguns, bullets that deform and expand, such as hollow-point projectiles, produce the greatest increase in volume of disrupted tissue, along with fragmentation, and are less likely to produce an exit wound. Full metal jacket projectiles typically designed for use with rifles are more likely to exit. Both full and partial metal jacket projectiles may ricochet off bone. At low muzzle velocity the difference between a permanent and temporary cavity is small; at high velocity the temporary cavity is larger. (vonSee et al, 2009)

For frangible bullets designed to fragment upon impact, the wounding capacity depends upon the nature of the surface impacted, the material comprising the bullet, and the velocity. If the compacted material, often copper powder, is very fine (ultra-frangible), then disintegration may occur upon impact or soon after penetration of soft tissues, creating many small tracks similar to an explosive projectile. If the bullet is composed of less fragile particles that are more compact, then disintegration may not occur until impact with harder tissues such as bones, teeth, or fibrous fascia. Fragments less than a gram may penetrate soft tissues to a depth of 10 to 15 cm. If an intermediate target is present, such as clothing, then fragmentation may occur even before tissue entry. If fragmentation does not occur readily, then the bullet may produce cavitation similar to a jacketed projectile of the same caliber. Even though frangible rounds are designed to minimize ricochet and collateral injury to other persons nearby, variability in fragmentation, and impact upon intermediate targets such as glass, may produce a shower of secondary fragments with enough energy to cause injury. (Komenda et al, 2013)

For shotgun slugs, a large amount of energy is transmitted to the tissues. The slug has a large mass and large diameter, deforming ("pancaking") upon impact, or breaking into fragments, so that most of the kinetic energy is absorbed by tissues. The "sabot slug" has an hour-glass shape with hollow base and is designed for use with a rifled barrel for more accuracy at greater distance because of its smaller mass than the standard rifled slug. Its shape causes it to tumble upon impact to produce a larger wound (Gestring et al, 1996).

Wounding is an extremely complex situation with variables of bullet size, velocity, shape, spin, distance from muzzle to target, and nature of tissue. These factors are interrelated, and the wounding potential may be difficult to predict even under controlled test conditions. In an actual forensic case, few of the variables may be known, and it is up to the medical examiner to determine what can be known from examination of the evidence.

Exit Wounds

Most bullets are designed to hit the target without exiting, for this imparts all the bullet's KE to the target and does the most damage. However, in many situations an exit wound will be present. This may be due to the use of a projectile more powerful than necessary, or the projectile may strike an area (such as an extremity) with minimal tissue.

Exit wounds are generally larger than entrance wounds because the bullet has expanded or tumbled on its axis. Exit wounds either do not exhibit gunshot residues or far less residues than associated entrance wounds. In bone, typical "bevelling" may be present that is oriented away from the entrance wound. (Denton et al, 2006)

Scanning electron microscopy of exit wounds shows irregular lacerations with protruding collagen fibers, but relatively undamaged papillae. (Torre, 1986)

Fragmentation of the bullet may produce secondary missiles, one or more of which may have exit wounds. The bullet path may be altered by striking bone or other firm tissues, such that the bullet track may not be linear, and exit wounds may not appear directly opposite entrance wounds.

It is important to remember that the orientation of the bullet track may be positional. The victim may have been shot while standing or sitting, but when the body is typically examined at autopsy, it is lying down, so that soft tissues may shift position. This must be remembered when rendering opinions as to the angle, or direction, of fire.

If the exit wound is "shored" or abutted by a firm support such as clothing, furniture, or building materials, then the exit wound may take on appearances of an entrance wound, such as a circular defect with an abraded margin. This can occur with contact, close range, or distant shots. 92% of shored exit wounds in one study had a round or ovoid defect, and all had some degree of abrasion. The degree of shoring abrasion increased directly with the KE of the projectile and the rigidity of the shoring material. (Dixon, 1981)

A keyhole lesion, typically identified with entrance wounds, has been described with an exit wound. (Dixon, 1984b)

Sequence of Fire

In some situations, pathologic findings may help to establish in what sequence the bullets were fired that caused the injuries. For example, multiple gunshot wounds to the head may produce fracture lines, and a subsequent fracture line typically does not cross a pre-existing fracture line (Viel et al, 2009).

Subjective reasoning would suggest that the first shot may be horizontal (victim upright) but subsequent shots would be oriented down or to the back of the victim as he fell or fled. Without witnesses and scene investigation, such opinions would be conjectural.

Sexton and Hennigar (1979) have reported cases in which examination of projectile collisions have aided in determining the sequence of fire.

The management of gunshot wounds may require accounting for all bullets and bullet fragments to determine the need for surgery. A simple rule of accounting for bullets is as follows: the number of entrance wounds must equal the number of exit wounds plus bullets retained. An unequal number may result from bullet fragmentation or from embolization, migration, or ricochet to unsuspected tissue sites.

Radiologic Imaging

Radiographic imaging may be needed to account for retained bullets and fragments and to help determine the bullet track. This has been accomplished for decades with plain film radiography, utilizing multiple projections. Bullets and fragments, including primer and jacket, are radiopague, improving ease of detection. (Folio, McHugh, and Hoffman, 2007).

Computed tomographic (CT) imaging has been applied to forensic investigations. CT provides multiple views with higher resolution than plain film radiography. In addition, radiography post-mortem is not limited by potential hazards of cumulative radiation exposure as would be the case in a living person. Thus, higher amounts of radiation energy, and unlimited dosages, can be utilized. With CT, cross sections can be computationally arranged into three dimensional images. (Jeffery et al, 2008)

MDCT imaging can be superior to conventional radiology for detection of soft tissue injuries. A three dimensional display improves localization of bullets and bullet fragments as well as determining details of fractures present. (Harcke HT et al, 2008).

Use of magnetic resonance imaging (MRI) has been shown to be safe in patients with projectiles composed of lead and/or copper. However, movement and heating of projectiles containing steel may be unsafe for patients, as well as produce artefact interfering with interpretation of the imaging. (Dedini et al, 2013)

Manner (Mode) of Death

The manner of death from firearms injuries can be classified as homicide, suicide, accident, or undetermined. There is no single characteristic appearance of a gunshot wound that defines the manner of death. Such a determination requires analysis of multiple pieces of evidence, including the scene investigation, the examination of the body, ballistics evidence, analysis for gunshot residue, and interviews of persons involved with the decedent and the scene of death.

In many cases, the distinction between death from homicide and suicide must be determined. The presence of multiple entrance wounds may not exclude suicide. Hejna and Safr (2010) determined that only 9% of suicide victims removed clothing from the area of a self-infllicted gunshot wound, and therefore defects present on the clothing are not an absolute criterion for disproving the possibility of suicide. However, if a suicide victim removes the clothing from the area of the future wound, then this is almost always an indication of suicide.

Kohlmeier et al (2001) have analyzed a large series of 1704 suicidal firearms deaths and determined characteristics of those injuries. The type of weapon used was a revolver in 49.8%, an automatic pistol in 19.5%, a rifle in 30.0%, and some other firearm in 0.7%. The site of the entrance wound involved the head in 83.7% of cases, the chest in 14.0%, the abdomen in 1.9%, and a combination of sites in 0.4%. The table below identifies the site of the entrance wound by type of weapon used in suicidal firearms deaths:


Suicidal Firearms Deaths
SiteHandgun (%)Rifle (%)Shotgun (%)
Right temple50.022.99.3
Left temple5.83.33.7
Mouth14.524.331.7
Forehead5.915.78.1
Under chin2.49.110.6
Back of head3.63.81.2
Chest13.215.719.9
Abdomen1.41.95.6
Other3.23.39.9


In the above series, contact wounds were found in 97.9%, intermediate in 2.0%, and a combination of these or an unknown range in the remainder.

In a study of rifle wounds, Molina and DiMaio (2008) found that 96% were contact in nature with suicides, while only 5% were contact with homicides.

Shotgun wound suicides are contact range in 96% of cases, while shotgun wound homicides are contact range in 8% and distant range in 59% (Molina, Wood and DiMaio, 2007)

Injury Patterns and Wound Care

Morbidity and mortality is high with ballistic injuries. Three-fourths of persons with a penetrating gunshot wound to the head die within 48 hours. Criteria for evaluation of injury and management have been devised. (Bruner et al, 2011)

Blood loss depends upon the size of the wound, the number and size of blood vessels damaged, and total body blood volume. A healthy 80 kg man has a blood volume of 4800 mL, and loss of 25% of this volume leads to incapacitation through diminished cardiac output and oxygenation (Maiden, 2009).

The best approach to wound care is conservative. With simple punctures and no apparent tissue disruption, just irrigation and application of a dressing may suffice. So-called "high velocity" rounds are not necessarily more damaging because they are jacketed and the bullet is smaller in size. Variability in wounding from such rounds is potentially, but not often practically, a function of bullet yaw. A fully jacketed 7.62 mm military round creates a much smaller temporary and permanent cavity in tissue than a 7.62 mm civilian "hunting" round with a soft point tip, despite the fact that both are "high velocity" rounds. Treatment guidelines include the use of antibiotics if necessary, and debridement of devitalized tissues when greater tissue disruption is apparent. It can be difficult to determine the extent of disruption and the amount of non-viable tissue, so reassessment of more disruptive wounds left open for 48 hours can be done. In short, "treat the wound, not the weapon." (Santucci and Chang, 2004) (Fackler, 1998)

Antibiotic prophylaxis is recommended in high-velocity, shotgun, and intraarticular gunshot fractures. Bullets are not sterile and may have encountered intermediate targets such as clothing prior to entering the body. The pressure difference from atmospheric pressure to a temporary cavity through a bullet track may allow air to sweep debris inward, causing contamination of the wound. (Jandial et al, 2008) The recommendation for high-velocity gunshot or shotgun injuries is intravenous administration of at least 48 hours of a first-generation cephalosporin, with addition of gentamicin in cases of soft tissue defects or cavitary lesions. Penicillin must be added in patients with gross wound contamination. (Simpson et al, 2003) (Santucci and Chang, 2004)

In a study of over 12,000 abdominal gunshot wound patients, selective non-operative management was selected in a fifth of cases, and failed in a fifth of those, predicited by a need for blood transfusion and higher injury score on initial assessment. Those patient who failed the non-operative management had a higher mortality rate. The availability and usage of abdominal radiologic CT imaging along with expertise in the application of criteria for treatment aids in patient management (Zafar et al, 2012).

The use of armor or shielding may modify injury patterns from bullets. Body armor may reduce or eliminate penetration of tissues by a projectile, but not eliminate tissue damage. The term 'behind armor blunt trauma' defines the tissue damage from a pressure wave generated by a high velocity projectile striking body armor. One study showed that such trauma may affect neuronal function in the spinal cord. (Zhang et al, 2011)


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