BALLISTICS

Low velocity (<2,000 ft/sec): This includes all handguns.

High velocity (>2,000 ft/sec): This includes all military rifles and most hunting rifles.

Shotgun wounding potential is dependent on:

1. Chote (shot pattern)

2. Load (size of the individual pellet)

3. Distance from the target

ENERGY

The kinetic energy (KE) of any moving object is directly proportional to its mass (m) and the square of its velocity (v2) and is defined by the equation:

The energy delivered by a missile to a target is dependent on:

1. The energy of the missile on impact (striking energy)

2. The energy of the missile on exiting the tissue (exit energy)

3. The behavior of the missile while traversing the target: tumbling, deformation, fragmentation

TISSUE PARAMETERS

The wounding potential of a bullet depends on the missile parameters, including caliber, mass, velocity, range, composition, and design, as well as those of the target tissue.

The degree of injury created by the missile is generally dependent on the specific gravity of the traversed tissue: higher specific gravity = greater tissue damage.

A missile projectile achieves a high kinetic energy because of its relatively high velocity. The impact area is relatively small, resulting in a small area of entry with a momentary vacuum created by the soft tissue shock wave. This can draw adjacent material, such as clothing and skin, into the

wound.

The direct passage of the missile through the target tissue becomes the permanent cavity. The permanent cavity is small, and its tissues are subjected to crush (Fig. 4.1).

The temporary cavity (cone of cavitation) is the result of a stretch-type injury from the dissipation of imparted kinetic energy (i.e., shock wave). It is large and its size distinguishes high-energy from low-energy wounds.

Gases are compressible, whereas liquids are not; therefore, penetrating missile injuries to the chest may produce destructive patterns only along the direct path of impact as a result of air-filled structures, whereas similar injuries to fluid-filled structures (e.g., liver, muscle) produce considerable displacement of the incompressible liquid with shock-wave dissipation, resulting in significant momentary cavities. This may lead to regions of destruction apparently distant to the immediate path of the missile with resultant soft tissue compromise.

CLINICAL EVALUATION

Following initial trauma survey and management (see Chapter 2), specific evaluation of the gunshot injury will vary based on the location of injury and patient presentation. Careful neurovascular examination must be undertaken to rule out the possibility of disruption to vascular or neural elements.

Entrance wounds are characterized by a reddish zone of abraded (“rubbed raw”) skin classically referred to as “the abrasion ring.” Muzzle imprinting, soot deposition, and powder tattooing may also be visible.

Exit wounds are larger in size than entrance wounds and more irregular in shape (e.g., stellate, slit-

like, or crescent).

RADIOGRAPHIC EVALUATION

Standard anteroposterior (AP) and lateral radiographs of the injured sites should be obtained.

Fractures caused by low-velocity missiles tend to have multiple nondisplaced fracture lines that can potentially displace.

TREATMENT OF ORTHOPAEDIC GUNSHOT INJURIES

Low-Velocity Wounds

Steps in treatment include:

1. Administration of antibiotics (first-generation cephalosporin), tetanus toxoid, and antitoxin.

2. Irrigation and debridement of the entrance and exit skin edges. Furthermore, one must understand that nonvital tissue and detritus may have been introduced to the fracture site. If so, operative debridement in the operating room may be required.

3. Indications for operative debridement in the absence of an operative fracture pattern:

   • Retention in the subarachnoid space

   • Articular involvement (intra-articular bone or missile fragments)

   • Vascular disruption

   • Gross contamination

   • Massive hematoma

   • Severe tissue damage

   • Compartment syndrome

   • Gastrointestinal contamination

4. Fracture treatment: Unstable fracture patterns are treated surgically, whereas stable patterns may be treated functionally following debridement.

High-Velocity and Shotgun Wounds

These should be treated as high-energy injuries with significant soft tissue damage.

Steps in treatment include:

1. Administration of antibiotics (first-generation cephalosporin), tetanus toxoid, and antitoxin

2. Extensive and often multiple operative debridements

3. Fracture stabilization

4. Delayed wound closure with possible skin grafts or flaps for extensive soft tissue loss

Important: Gunshot wounds that pass through the abdomen and exit through the soft tissues with bowel contamination deserve special attention. These require debridement of the intra-abdominal and extra-abdominal missile paths, along with administration of broad-spectrum antibiotics covering gram-negative and anaerobic pathogens.

COMPLICATIONS

Retained missile fragments: These are generally tolerated well by the patient and do not warrant a specific indication for surgery or a hunt for fragments at the time of surgery unless they cause symptoms (pain, loss of function), are superficial in location especially on the palms or soles, are involved in an infected wound, or are intra-articular in location. Occasionally, the patient will develop a draining sinus through which fragments will be expressed.

Infection: Studies have demonstrated that gunshot injuries are not necessarily “sterile injuries” as was once thought. This is secondary to skin flora, clothing, and other foreign bodies that are drawn into the wound at the time of injury. In addition, missiles that pass through the mouth or abdomen are seeded with pathogens that are then dispersed along the missile path. Meticulous debridement and copious irrigation will minimize the possibility of wound infection, abscess formation, and osteomyelitis.

Neurovascular disruption: The incidence of damage to neurovascular structures is much higher in high-velocity injuries (military weapons, hunting rifles), owing to the energy dissipation through tissues created by the shock wave. Temporary cavitation may produce traction or avulsion injuries to structures remote from the immediate path of the missile. These may result in injuries ranging from neuropraxia and thrombosis to frank disruption of neural and vascular structures.

Lead poisoning: Synovial or cerebrospinal fluid is caustic to lead components of bullet missiles, resulting in lead breakdown products that may produce severe synovitis and low-grade lead poisoning. Intra-articular or subarachnoid retention of missiles or missile fragments is thus an indication for exploration and removal.