Mechanisms of Injury and Kinematics

Mechanisms of Injury and Kinematics

Kinematics is the science of motion. In emergency medicine, it describes how forces act on the body to produce injury. Understanding kinematics allows you to predict which injuries a patient is likely to have—even before physical signs appear—so you can prioritize your assessment and avoid missing life threats.

Energy and the Physics Behind Injury

All trauma begins with energy transfer. Newton's First Law tells us that a body in motion stays in motion until acted on by an outside force. In a motor vehicle collision (MVC), the car stops but the unrestrained occupant continues moving until something stops them—the steering wheel, windshield, or dashboard.

Kinetic energy (KE) governs how severe that transfer can be. The formula is:

KE = ½ × mass × velocity²

Two points are critical here. First, doubling a patient's mass doubles the kinetic energy. Second, doubling velocity quadruples it. This is why high-speed crashes and falls from significant height produce far more devastating injuries than low-speed events. A small increase in speed creates a large increase in destructive energy.

Newton's Third Law—every action has an equal and opposite reaction—explains why occupants rebound after striking interior surfaces, often sustaining secondary impacts.

Blunt Trauma

Blunt trauma results from forces applied over a wide area without penetrating the skin. The body undergoes three collisions in a typical MVC:

1. The vehicle hits an object and decelerates rapidly.

2. The occupant hits the interior of the vehicle (steering column, dash, A-pillar).

3. Internal organs hit the chest wall or abdominal wall—the heart can lacerate against the sternum; the liver and spleen can tear at their ligamentous attachments.

Predictable patterns by restraint status matter clinically. Unrestrained drivers commonly sustain head, face, cervical spine, chest, and knee injuries. Lap-belt-only occupants are at risk for lumbar spine fractures (Chance fracture) and small-bowel injuries. Properly restrained occupants still risk sternal fractures and clavicle injuries from the shoulder belt.

Falls from height follow similar physics. The greater the height, the greater the velocity at impact. Adults who fall and land feet-first often sustain calcaneal fractures, tibial plateau fractures, and lumbar compression fractures in a predictable axial-load pattern. Falls in pediatric patients tend to produce head injuries because children's heads are proportionally larger.

Rollover crashes increase injury risk because the occupant may be subjected to multiple impacts in multiple directions.

Penetrating Trauma

Penetrating injuries are caused by an object that breaks the skin and transfers energy along a tract. Three factors determine severity:

• Profile: the surface area of the projectile contacting tissue
• Fragmentation: a bullet or fragment that breaks apart creates multiple wound tracts
• Velocity: low-velocity weapons (knives, most handguns below a threshold) produce injury primarily along the direct tract; high-velocity rifle rounds create a temporary cavity much wider than the bullet itself

The temporary cavity is a key concept. High-velocity rounds compress and stretch surrounding tissue radially outward in milliseconds, tearing structures that are not directly in the bullet's path. Organs with high fluid content (liver, spleen, bladder) and low elasticity are most susceptible. Lung tissue, being elastic and air-filled, tolerates temporary cavitation better.

Always consider the trajectory. A stab wound or gunshot wound to the upper abdomen may involve the chest; thoracoabdominal injuries are common and easy to underestimate.

Blast Injuries

Explosions produce a complex injury pattern that is categorized into four phases:

• Primary: the overpressure wave strikes the body. Air-filled organs (lungs, bowel, middle ear) are most vulnerable. Pulmonary blast injury is a leading cause of early death in survivors who reach care.
• Secondary: fragmentation and shrapnel cause penetrating wounds.
• Tertiary: the victim is thrown by the blast wind and sustains blunt trauma on impact.
• Quaternary: burns, crush injuries from structural collapse, and toxic gas inhalation.

Patients near an explosion should be assessed for all four mechanisms simultaneously.

Clinical Application and Safety

Kinematics is a guide, not a guarantee. A high-energy mechanism should raise your index of suspicion and prompt a thorough head-to-toe assessment even when the patient appears stable. Conversely, a seemingly minor mechanism in an older adult or a patient on anticoagulants can produce serious injury.

Scene safety always comes first. Do not enter an active shooting scene, a blast site with structural instability, or an MVC with uncontrolled traffic until law enforcement and safety officers clear the scene. Gathering mechanism information must never delay your own safety or basic life-saving interventions.

Spinal motion restriction decisions, hemorrhage control priorities, and transport destination choices are all informed by mechanism of injury. Document the mechanism clearly in your patient care report—this information guides emergency department physicians even after you transfer care.