The idea that a bullet can move within the body after the initial injury is often sensationalized in fiction, leading to the assumption that projectiles become static upon entry. While a bullet’s kinetic energy is spent during penetration, the object does not always remain in the precise location where it first comes to rest. Medical records confirm that a retained bullet or fragment can travel from its original resting place to a new site within the body. This phenomenon, known as projectile migration, depends highly on the location of the retained fragment and the surrounding physiological environment.
Defining Projectile Movement
When a foreign object like a bullet enters the body and loses momentum, it either lodges in tissue or passes completely through. If retained, its subsequent movement falls into two categories: minor shifting or major migration. Minor shifting is common, involving small, localized movements within soft tissues like muscle or fat. This slight displacement occurs due to normal body mechanics, including breathing, posture changes, or muscle contractions.
The body typically encapsulates the foreign object within fibrous scar tissue, which largely prevents significant movement over time. Major migration is a distinct and medically significant event, defined as measurable movement across anatomical barriers or through bodily systems to a distant location. This substantial travel is rare, estimated to occur in less than 5% of retained projectile cases, but its consequences can be severe. Major migration changes the clinical problem, as the projectile’s new location may affect structures far from the original wound path.
How Bullets Migrate Through the Body
The process of major migration is driven by specific physiological and physical forces acting on the retained projectile. One dynamic form of movement is vascular migration, or bullet embolism, which occurs when the projectile enters a major blood vessel. If the bullet penetrates an artery or vein without exiting, it loses energy and is carried along by the bloodstream, acting as a ballistic embolus.
The direction of flow dictates the bullet’s ultimate destination within the circulatory system. Venous emboli, originating in the veins, are carried toward the heart’s right side and often lodge in the right ventricle or the pulmonary arteries. Arterial emboli occur less often, traveling away from the heart to lodge in smaller peripheral arteries, such as those in the lower limbs or the axillary artery. In rare instances, venous emboli have been observed to travel in a retrograde fashion, moving against the direction of blood flow.
Migration can also occur due to gravity when a bullet is retained in loose tissue or an open space. For example, a projectile lodged in the upper chest or neck may slowly descend over time into the abdomen or retroperitoneum if it is not firmly encased in scar tissue. Furthermore, the constant, rhythmic movement of internal organs can facilitate displacement, a process sometimes called visceral movement. A fragment resting near a constantly moving structure, such as the diaphragm or a major airway, may erode into the space over months or years, driven by the tissue’s pressure and motion.
Medical Consequences of Migration
The migration of a retained projectile transforms a stable condition into a high-risk situation. Acute complications arise when the migrating projectile causes mechanical obstruction or damage along its new path. A bullet traveling through the vascular system can obstruct blood flow, leading to tissue ischemia, or damage heart valves or muscle, potentially triggering cardiac arrhythmia.
If the projectile migrates into a visceral structure, like the bowel or airway, it can cause perforation, bleeding, or chronic infection. Even after the projectile settles, delayed complications can develop, including chronic pain, nerve compression, or nerve irritation if the new location is near a major nerve bundle.
Lead toxicity is another long-term concern, especially if the fragment is lodged in a joint space where synovial fluid accelerates the metal’s dissolution. Physicians rely on serial imaging, such as X-rays, to monitor the projectile’s location over time. Surgical removal is complex, requiring a balance between the risk posed by the current location and the risks of the procedure itself, including neurovascular injury, infection, and bleeding.