A pedestrian-vehicle collision is one of the most violent forms of blunt force trauma the human body can sustain. Unlike occupants protected by a car’s structural cage, a person on foot absorbs the entirety of a vehicle’s kinetic energy during impact. This sudden, massive transfer of energy, often involving rapid deceleration and multiple strikes, creates a distinct pattern of injury. The resulting trauma is categorized by the specific mechanical forces and subsequent physiological collapse that follow the initial contact.
The Physics and Phases of Impact
The severity of injuries in a pedestrian strike is governed by the laws of physics, particularly the concept of kinetic energy, which is proportional to the vehicle’s mass and the square of its speed. A small increase in velocity results in a disproportionately large increase in the energy transferred to the pedestrian, drastically increasing the risk of serious injury or death. The entire event unfolds in a sequence of mechanical phases that dictate the location and type of trauma sustained.
The first phase is the Initial Contact, where the vehicle’s bumper strikes the pedestrian, typically in the lower extremities. This interaction often occurs below the person’s center of gravity, accelerating the body forward and upward. This strike commonly leads to specific lower-leg fractures, which can provide clues about the vehicle’s height and speed.
The second phase is the Impact or Winding-Up stage, where the pedestrian’s torso and head rotate onto the hood and windshield. The upper body absorbs a secondary, high-velocity impact, leading to trauma in the chest, abdomen, and head. The force of the strike, especially to the head against the windshield, is a major contributor to severe brain injuries.
The final phase is the Secondary Impact, which occurs when the body separates from the vehicle and strikes the ground, curb, or other fixed objects. This third strike often involves rotational forces, resulting in additional, complex injuries to the head, neck, and spine.
Immediate Skeletal and Muscular Trauma
The musculoskeletal system is the first to suffer structural damage upon impact, absorbing and transmitting the forces from the vehicle. Fractures of long bones, such as the femur and tibia, are common, especially in the lower extremities where the bumper first makes contact. The classic “bumper fracture” involves the lateral tibial plateau near the knee joint, resulting from a direct blow and a strong valgus (outward) force.
These impacts frequently cause complex breaks, including comminuted fractures, where the bone shatters into multiple pieces, or open (compound) fractures, where bone fragments pierce the skin. Soft tissues suffer massive injury, including lacerations, abrasions often referred to as “road rash,” and crush injuries. Joint capsules and ligaments are also damaged, leading to dislocations and tears that destabilize major weight-bearing structures. These widespread injuries compromise the integrity of the limbs, sometimes requiring surgical amputation due to extensive damage to the bone, muscle, and blood vessels.
Life-Threatening Internal and Neurological Damage
While external injuries are apparent, the greatest danger lies in the internal trauma caused by rapid deceleration forces. The sudden change in velocity can cause internal organs to tear away from their fixed points of attachment, resulting in organ rupture. Solid organs, such as the liver and spleen, are vulnerable to blunt force trauma and shearing forces, leading to immediate internal bleeding.
Damage to the abdominal aorta or vena cava, the body’s largest blood vessels, can lead to rapid and uncontrolled hemorrhage, which is often fatal. Hollow organs like the bowel or bladder can also be ruptured, spilling their contents into the sterile body cavity and leading to severe infection. These internal injuries may not present with obvious external symptoms, requiring immediate, specialized medical intervention.
Neurological damage occurs from both the direct impact of the head hitting the vehicle or ground and the violent acceleration-deceleration forces. Traumatic Brain Injury (TBI) can range from a concussion to a severe contusion or hematoma, where blood clots form within or around the brain tissue. The brain’s soft tissue can slam against the inside of the skull, causing a coup-contrecoup injury or a Diffuse Axonal Injury (DAI), where nerve fibers are microscopically torn. Furthermore, the violent twisting and bending of the body can fracture vertebrae, resulting in a Spinal Cord Injury (SCI) that can cause varying degrees of paralysis or sensory loss.
The Body’s Response: Hemorrhagic Shock and Systemic Collapse
The physical trauma rapidly initiates a physiological cascade, primarily driven by blood loss. Hemorrhagic shock is the systemic reaction to the rapid loss of circulating blood volume, which severely compromises the body’s ability to deliver oxygen to tissues and organs. As blood volume drops, the heart rate increases in an attempt to compensate and maintain blood pressure.
The body attempts to shunt blood away from non-essential areas like the skin, causing extremities to become cold and pale, prioritizing flow to the brain and heart. If bleeding remains unchecked, tissue perfusion decreases, leading to cellular hypoxia. This forces cells to switch to anaerobic metabolism, generating lactic acid and leading to systemic acidosis. Acidosis, combined with hypothermia and the trauma itself, can impair the blood’s ability to clot. This vicious cycle of acidosis, hypothermia, and coagulopathy, often referred to as the “lethal triad,” drives the body toward multi-organ failure.