Patterns of Penetrating Injury

Gunshot Wounds

I. Introduction

To understand the mechanisms of gunshot injuries, it is important to understand the nature of firearms and their projectiles. Ballistics is defined as the scientific study of projectile motion and is divided into three categories: internal, external, and terminal ballistics. Internal ballistics has to do with the projectile within the firearm. External ballistics describes the projectile in the air. Terminal ballistics relates to actions of the projectile in its target. Wound ballistics is a subset of terminal ballistics and is the most important aspect of ballistics for physicians to understand. However, to completely understand the wounding process some knowledge of all aspects of ballistics is necessary.

II. Types of Firearms

• Handguns, rifles, airguns, and shotguns are the major firearms encountered in civilian injuries in the United States. The wounding potential of each is different; it is important to be aware of these differences. Fully automatic weapons are illegal in the United States and injuries from these weapons are infrequent in civilian practice. Fully automatic weapons differ only from rifles and handguns in their ability to autoload and their resultant rapidity of fire. The injuries caused by the individual projectiles are essentially the same as those of handgun and rifle injuries. Handguns and rifles have many characteristics in common and will be discussed together. Airguns and shotguns, on the other hand, differ significantly in their wounding characteristics from other weapons and will be discussed separately.
• Handguns and rifles Handguns and rifles fire bullets. Before firing, the lead bullet is held firmly in the end of a brass cartridge case. This cartridge case contains a flammable propellant (the charge) and has a primer at its base. When the firing pin of the gun strikes the primer, the primer is detonated, igniting the charge within the cartridge case. The burning gases expand and propel the bullet from the cartridge case and along the barrel of the gun. Spiral grooves within the barrel of the gun (rifling) grip the bullet causing it to spin around its long axis. This spinning creates a gyroscopic effect, which prevents yaw, or the deviation of the longitudinal axis of the bullet from its line of flight. This gives the bullet directional stability in the air, enabling it to travel more accurately than a nonspinning projectile, analogous to the stable flight of a football when a long pass is thrown with a “perfect” spiral, as compared with the wobble when thrown imperfectly. The longer the barrel, the more time the bullet has to accelerate and the faster it will be going when it leaves the gun. Because rifles have longer barrels than handguns, rifle bullets leave the gun with much higher velocities than handgun bullets (Tables 3-1, 3-2).
  • Since the kinetic energy of the bullet is equal to half its mass multiplied by the square of its velocity, high-velocity bullets have much higher kinetic energy than low-velocity bullets.
    KE = 1/2 mv²
    (KE = kinetic energy in joules; m = mass in grams; v = velocity in feet/second.) This higher kinetic energy gives rifle bullets greater wounding potential than the handgun bullets.
    P.17
    
    

    TABLE 3-1 Ballistic Data for Four Handguns
    Caliber (in) Weapon type Bullet weight (grains) Muzzle velocity (ft/s) Kinetic energy (ft-lb) 0.25 25 automatic 50 810 73 0.354 9-mm Luger 115 1,155 341 0.357 357 magnum 158 1,410 696 0.44 44 magnum 240 1,470 1,150 In, inches; ft/s, feet per second; ft-lb, foot-pounds; mm, millimeters.

    TABLE 3-2 Ballistic Data for Four Rifles
    Caliber (in) Weapon type Bullet weight (grains) Muzzle velocity (ft/s) Kinetic energy (ft-lb) 0.22 Remington 22 40 1,180 124 0.223 M-16 55 3,200 1,248 0.270 270 Winchester 150 2,900 2,810 0.308 30–0 150 2,910 2,820 In, inches; ft/s, feet per second; ft-lb, foot-pounds.

  • Wounding potential is only part of the equation. The type of projectile, type of tissue injured, and the distance (range) between the weapon and the victim all have major effect on these injuries. In direct contact injuries, not only does the energy of the bullet enter the victim but also most of the combustion gases, causing considerable tissue expansion and much more severe injuries than noncontact injuries.
  • The same handgun or rifle can often fire several different types of projectiles. The construction of these projectiles has a major effect on wounding. It is important to be aware not only of the behavior of projectiles before and after impact, but also of the manner in which various tissues respond to gunshot injuries. More elastic tissues, such as lung or fat, dissipate energy well. Less elastic tissues, such as brain, liver, or spleen (solid organs), do not dissipate energy well, with more tissue damage resulting from similar kinetic energy of the missile.
  • Although rifle bullets have greater energy available for wounding than handgun bullets, the wounding mechanisms of the two are similar in many respects. When the bullet strikes human tissue, it ceases spinning. Having lost its directional stability, it is now able to “tumble” (rotate around its short axis). A nondeformed bullet will usually be tapered at its tip and hence have more mass concentrated at its base. Momentum will cause the bullet to tumble through 180 degrees and continue through the tissue with its heavier base leading. If the bullet is deformed by impact with the tissue, this tendency to tumble will be modified and may even be eliminated completely. If there is sufficient width of tissue for the bullet to complete its 180-degree tumble, it will carve an elliptical-shaped tunnel of tissue damage, known as the permanent cavity. A shockwave is generated by this damage, compressing the adjacent tissue. This is known as the temporary cavity and is also elliptical in

    P.18

    
    shape. Damage in the temporary cavity varies from one tissue to another and generally increases with increasing tissue specific gravity.
  • Bullets are usually classified by caliber, which is a measurement of the diameter of the bullet, most commonly in decimals of an inch (e.g., 0.357) or in millimeters (e.g., 9 mm). The measurement of caliber does not address the weight of the bullet, construction of the bullet, or the size of the charge, all of which are important factors in determining the wounding potential (Tables 3-1, 3-2). Most bullets are made of lead. In some low-velocity weapons, the bullet is made entirely of lead. In medium- and high-velocity weapons, bullets usually have a metal jacket surrounding the lead to protect it from deformity, while the bullet is in the gun barrel. A copper jacket is most common, but occasionally other metals are used. If the bullet is entirely encased, it is said to have a full metal jacket; if the bullet is partially encased, it is said to be semi-jacketed. Semi-jacketed bullets typically have exposed lead at the tip, are also referred to as soft-point bullets, and are designed to deform on impact. Some bullets have an open cavity in their tips and are referred to as hollow-point bullets. Hollow-point bullets are also designed to deform on impact and typically transform into a mushroom shape. When bullets deform on impact, they decelerate faster, delivering more of their energy to a smaller volume of tissue. This is intended to increase tissue damage, making deforming bullets more effective at wounding than nondeforming bullets. A deformed bullet, particularly one with a mushroomed tip, tends not to tumble. High-velocity rifle bullets, particularly the soft- and hollow-point varieties, tend to fragment on impact with tissue. This fragmentation leads to an expanding conical pattern of permanent injury, as the fragments separate from one another.
  • Bullet injuries are most severe in friable solid organs (e.g., the liver and brain), where damage may be caused by temporary cavitation remote from the actual bullet track. Dense tissues (e.g., bone) and loose tissues (e.g., subcutaneous fat) are more resistant to bullet injury. Bones modify the behavior of bullets markedly, altering their course, slowing them down, and increasing their deformity and fragmentation.
  • In general, bullets and bullet fragments follow a straight trajectory, even after entering tissue. Bullets that strike rigid changes in tissue density, such as bone cortices and dense fascia, will often be deflected from their initial course. After this deflection the fragments will continue in a relatively straight trajectory unless they encounter another solid interface. Bullets that pass through intermediate targets (e.g., a door or wall) may already be deformed before they strike the victim and will always have reduced energy.
• Airguns Airguns do not use a flammable charge to propel their projectiles down the barrel but rely simply on air pressure from pumps, springs, or gas canisters. These weapons usually fire pellets in the form of round BBs or waisted lead slugs. In general these weapons have low muzzle velocity and therefore low wounding potential. The construction of airguns may resemble either rifles or handguns and once again, longer barreled weapons will impart more kinetic energy to their projectiles.
  • Low-velocity weapons, including both airguns and small-caliber handguns, have much lower wounding potential than high-velocity weapons. Whereas close-range injuries with low-velocity projectiles can be fatal, medium- and long-range injuries are often superficial. The subcutaneous tissue offers the path of least resistance for low-velocity projectiles, which may travel long distances through the subcutaneous tissue but fail to penetrate the fascia. This phenomenon is most common at medium to long range and when the entry wound is at a shallow angle to the skin surface.
• Shotguns
  • Shotgun injuries differ substantially from rifle and handgun wounds. Unlike the single bullet of a rifle or handgun cartridge, shotgun shells usually contain multiple metal pellets, also known as shot. Typically, the shotgun shell is made

    P.19

    
    of plastic with a brass cap at the base, containing the primer. The charge is separated from the pellets by a wadding material that may be either paper or plastic. In contact or very close range injuries the wadding will be projected into the victim, along with the pellets and expanding gases. Shotguns do not have rifled barrels, and their pellets do not spin.
  • The size of shotgun cartridges is not measured in caliber but in gauge. The higher the gauge, the smaller is the diameter. Shotgun shells are usually much larger than rifle or handgun cartridges, containing a much bigger charge and a greater total mass of projectiles. Shotgun pellets separate after leaving the barrel of the gun and their velocity rapidly decreases. As the pellets spread with increasing range, their area of distribution increases and the energy in each pellet decreases. Thus, range differences affect the wounding potential of shotgun pellets far more than they affect the wounding potential of bullets. At close range (less than 15 feet) shotgun injuries are usually far more severe than bullet injuries because the total energy available is much greater.
  • The combined mass of multiple pellets spread over a small area can produce massive destruction of soft tissue and bone. At long range, the wider spread and lower velocity of the pellets produce multiple, widely separated, superficial injuries that are often painful but rarely life threatening. At intermediate range, shotgun injuries are less predictable, with severity being mainly a function of anatomic location and pellet density. Pellets come in many sizes, but each individual cartridge usually contains only one size. Larger pellets are known as buckshot, and smaller pellets are called birdshot. Most injuries encountered in clinical practice involve birdshot.
  • As with bullet injuries, the severity of shotgun injuries varies with tissue type and local anatomy. Vascular injuries are of particular concern because the smaller size of the pellets makes embolization more likely than with bullets. Such emboli can result in tissue infarction.
  • In the past, all shotgun pellets were made of lead. However, recent wildlife regulations require shotgun pellets to be made of steel, when they are used on waterfowl. Steel pellets are ferromagnetic and can move if the patient is exposed to a strong magnetic field, thus causing additional damage. Therefore, magnetic resonance (MR) imaging may be contraindicated in such patients. Fortunately, steel and lead pellets can usually be distinguished from one another at radiography. Lead pellets tend to be deformed and fragmented by impact with soft tissues and bone, whereas steel shot usually remains round. Simple analysis of a radiograph is all that is needed to determine if a patient with a shotgun injury can be safely placed in the MR imaging magnet.

III. Gunshot Injury Assessment

• Prompt and accurate assessment of the injuries is essential, both clinically and radi-ographically. While entrance and exit wounds have differing characteristics, distinguishing one from the other is unreliable. It is therefore best to refer to both simply as surface wounds, and characterize their appearance and location carefully. Both the surgical approach and appropriate planning of additional imaging are aided by the prompt acquisition of appropriate radiographs. These radiographs can often provide an accurate determination of the paths of the projectiles. Metallic markers should be put beside each surface wound, prior to obtaining the radiographs. Two perpendicular projections of the injured area are essential. If the projectiles are a long distance from the entry site, they may not be included in the field of view of the initial radiographs. If a low-velocity projectile is not found on the initial radiographs and there is no exit wound, additional radiographs over a wider field of view should be obtained.
• With good imaging data, the organs at risk can be determined and the best possible action plan formulated. In hemodynamically unstable patients, there is often time only for conventional radiography before the patient must be taken to

  P.20

  
  the operating room, but these essential initial films can provide important information as to bullet quality, fragmentation, pathway, or other unsuspected foreign bodies. However, in stable patients computed tomography (CT) offers a far more accurate road map of the injury. Careful evaluation of radiographs and CT images is generally more reliable than clinical examination for determining the direction of projectile travel and the tissue or tissues injured. Rapid acquisition of adequate images is essential in all patients. A rapid and accurate assessment of the path of the projectile and its direction of travel often aids the planning of surgical approach, particularly if the fragment has crossed body cavities, such as trans-mediastinal, transdiaphragmatic, or transpelvic. Any time the bullet or pellets are close to major vessels, conventional or CT angiography should be considered. Significant vascular injuries may be present, even when peripheral pulses are normal. To emphasize, hemodynamic instability from presumed hemorrhagic shock precludes detailed imaging.
• In general, bullets that are not causing mechanical problems can safely be left in the tissues. There is one exception, however. Bullets left within synovial joints result in slow leeching of lead by the synovial fluid. This in turn leads to chronic inflammatory changes within the synovium and to a gradual increase in serum lead levels. After many years, the victim will develop not only a chronic, debilitating lead arthropathy but also systemic lead poisoning. Therefore, bullets and pellets within synovial joints should always be removed.
• Imaging details Evaluation of bone injuries and the distribution of bone and bullet fragments on radiographs can be helpful in determining the direction of travel, which is important not only for clinical assessment but also for forensic evaluation of the incident. Bone and bullet fragments are usually distributed along the bullet track within the soft tissues, beyond the defect in the bone. Careful examination of the images should reveal beveling of the bone toward the direction of travel.
  • The degree of bullet fragmentation is also readily visible on radiographs. Bullets with full metal jackets often remain in one piece and usually do not deform much. These projectiles typically do not leave a trail of lead fragments along their path. On the other hand, hollow-point, nonjacketed, and soft-point bullets tend to deform on impact or break apart, leaving a trail of metal fragments through the soft tissues. Hollow-point handgun bullets usually deform by simply mushrooming with minimal fragmentation, whereas high-velocity soft-point rifle bullets usually undergo marked fragmentation. This fragmentation of high-velocity bullets creates a “lead snowstorm” appearance on radiographs. The area over which the lead snowstorm fragments are deposited in the soft tissues widens as the distance from the entry site increases. Thus, a conical distribution of lead fragments is seen on radiographs, with the apex of the cone pointing toward the entry site.
  • While the makeup of shotgun pellets (lead vs. steel) can usually be determined based on their radiographic image as stated previously, the same is not true for jacketed bullets. The type of metal used for the jacket cannot be determined from radiographs. Because bullet jackets are occasionally made of steel, it may not be safe to place a bullet wound victim into an MR imager when the nature of the bullet construction is unknown.

IV. Stab Wounds

• Stab wounds result from “hand-driven” weapons, such as knives, but also include more unusual weapons or offending agents such as ice picks, glass shards, sharp edges of metal, or even wooden posts. A description of the stab wound includes the length, width, and the depth of penetration of the offending agent, although this last dimension is seldom known at the time of initial evaluation. It is helpful to have direct examination of the weapon, as the victim's or witnesses' perceptions may not be accurate given the heightened emotional states at the time of injury. Wound size and history of type of weapon do not necessarily correlate to depth of wound or wound trajectory.
  P.21
  
  
• Slash wounds are usually long lacerations of relatively shallow depth. These wounds tend to gape, allowing easy visual inspection of their depth. Impalement wounds are those in which the offending agent is plunged into the victim along the long axis of the blade, resulting in a small puncture wound of the skin and unknown depth. In common use, “stab” implies the use of a knife, whereas “impalement” connotes a larger object driven into the torso. If the wounding agent is still in the victim on arrival at the treatment facility, it is best removed in the operating room. An impaling object can be providing tamponade of major vessels and therefore should be removed under direct vision. Of stab wounds, 4% mortality rate is primarily from direct injury to the great vessels or the heart.
• Impalement usually occurs secondary to a fall onto a piercing object or sustained from machinery or pneumatic tools (nail guns), but also includes low-velocity missiles such as arrows. The wound can be complicated by blunt deceleration from the fall, by secondary injuries resulting from extraction by untrained personnel, or by unintentional shifts of the impaling object during transport.
• Arrows are fired for hunting and recreational pursuit. Crossbows generate bolt velocity of 61.0–84.4 meters per second (m/s) (200–275 feet per second [ft/s]). Bolts are usually unable to pass through weight-bearing bone, but easily penetrate ribs, sternum, posterior vertebral elements, and calvarium. Archery and hunting bows can generate arrow velocities up to 74 m/s (240 ft/s). Arrow penetration is a function of arrow momentum (weight and velocity) and type of tip (target vs. hunting). These wounds should be treated as an impalement.

Axioms

• Trajectory defines anatomic injury.
• Do not describe bullet wounds as exit or entrance wounds; describe location and appearance of wounds only.
• Objects that are impaled in the victim should be removed in the operating room.