3.1 Mechanism of Injury & Scene Assessment
Key Takeaways
- Kinematics is the study of energy transfer during a traumatic event; kinetic energy = 1/2 mass x velocity squared, meaning speed has a greater impact on injury severity than weight
- Newton's First Law (inertia) explains why unrestrained occupants continue moving at vehicle speed during a sudden stop
- Blunt trauma causes injury through compression and deceleration forces, while penetrating trauma causes injury along the path of the projectile or object
- The five types of motor vehicle collisions are frontal (head-on), lateral (T-bone), rear-end, rollover, and rotational; each produces a predictable injury pattern
- Falls are assessed by three factors: height of fall, surface landed on, and body part that struck first
- Scene size-up follows the sequence: BSI/PPE, scene safety, mechanism of injury/nature of illness, number of patients, and need for additional resources
- The Golden Hour refers to the critical first 60 minutes after major trauma during which definitive surgical care should ideally begin; the Platinum 10 Minutes is the maximum on-scene time for critical trauma patients
Understanding how injuries occur is just as important as recognizing the injuries themselves. By evaluating the mechanism of injury (MOI), EMTs predict injury patterns and prioritize care before a full assessment is complete. On the NREMT exam, MOI questions test whether you can connect a described event to the organs and structures most likely damaged, and whether you correctly classify the MOI as significant (high-energy) or non-significant.
Kinematics of Trauma
Kinematics is the study of motion and energy transfer as it relates to traumatic injury. The governing principle is the law of conservation of energy: energy is neither created nor destroyed, only changed in form. When a moving body suddenly stops, its kinetic energy is transferred into the tissues, and that energy does the damage.
Kinetic Energy Formula:
KE = 1/2 x mass x velocity^2
Because velocity is squared, doubling the speed quadruples the kinetic energy. A car at 60 mph carries four times the energy of one at 30 mph, which is why speed predicts injury severity far more strongly than weight. A second principle, the change-of-speed concept, explains that the more abruptly an object decelerates (a windshield versus an airbag), the greater the force transmitted, because force equals mass times deceleration.
Newton's Laws Applied to Trauma
| Law | Principle | Trauma Application |
|---|---|---|
| First Law (Inertia) | A body in motion stays in motion unless acted on by an outside force | An unrestrained occupant keeps moving at vehicle speed during a sudden stop |
| Second Law (Force) | Force = Mass x Acceleration | Greater mass or sharper deceleration produces greater force on tissue |
| Third Law (Action-Reaction) | Every action has an equal and opposite reaction | When the body strikes an object, the object pushes back with equal force |
A useful teaching point is the concept of three collisions in a motor vehicle crash: (1) the vehicle strikes an object, (2) the occupant strikes the interior of the vehicle, and (3) the internal organs strike the inside of the body wall. The third collision causes the deceleration injuries (aortic tears, liver and spleen lacerations) that are invisible on the outside.
Blunt vs. Penetrating Trauma
Blunt trauma results from an object striking the body, or the body striking an object, without breaking the skin. Damage occurs through compression (tissues crushed between the impacting surface and internal structures) and deceleration (organs keep moving and tear at their fixed points, such as the aorta at the ligamentum arteriosum, or the kidneys at the renal pedicle). Blunt trauma is treacherous because severe internal injury can exist with little external sign.
Penetrating trauma involves an object breaking the skin and entering the body. Severity depends on velocity of the projectile (low, medium, or high), profile (the size of the frontal surface, which expands if a bullet tumbles or mushrooms), fragmentation (whether the object breaks apart and creates multiple wound tracks), and the path through the body. With high-velocity rounds, cavitation creates a temporary cavity many times the diameter of the projectile, damaging tissue well beyond the visible track.
Motor Vehicle Collision Patterns
Frontal (Head-On) Impact: Up-and-over pathway sends the head into the windshield (cervical compression) and the chest/abdomen into the steering wheel. Down-and-under pathway drives the knees into the dashboard, causing femur fractures and posterior hip dislocations. Suspect cervical spine injury, traumatic brain injury, flail chest, myocardial contusion, aortic tears, and spleen/liver lacerations.
Lateral (T-Bone) Impact: The near-side occupant absorbs the most energy. Suspect lateral cervical fractures, clavicle and lateral rib fractures, splenic injury (left-side impact) or liver injury (right-side impact), and pelvic fractures.
Rear-End Impact: The vehicle and occupant accelerate forward while the head hyperextends over the headrest, producing cervical hyperextension (whiplash), especially when the headrest is set too low.
Rollover: Multiple impacts strike in unpredictable directions; the unrestrained occupant faces the highest ejection risk and may sustain multi-system trauma anywhere on the body.
Ejection: Being ejected increases the risk of death roughly 25-fold; the body strikes the ground or other objects at vehicle speed with no protective restraint.
Fall Assessment
Evaluate falls using three factors: height (a fall greater than three times the patient's height, or more than 15-20 feet for an adult, is significant), surface struck (concrete transfers far more energy than grass), and body part that landed first (feet-first landings produce calcaneal fractures and lumbar compression in a pattern called Don Juan syndrome; head-first landings threaten the cervical spine and brain).
Scene Size-Up
Every trauma call begins with a structured size-up performed before patient contact: (1) BSI/PPE, (2) scene safety, (3) MOI/NOI, (4) number of patients, and (5) additional resources (ALS, fire, law enforcement, aeromedical). If the scene is unsafe, stage at a distance and request resources rather than entering.
The Golden Hour & Platinum 10 Minutes
The Golden Hour describes the principle that critically injured trauma patients have the best outcomes when definitive surgical care begins within roughly 60 minutes of injury. The Platinum 10 Minutes is the goal of limiting on-scene time to 10 minutes or less for critical patients, reserving on-scene effort for life-saving interventions and expediting transport to an appropriate trauma center. Modern trauma systems treat these figures as targets that emphasize speed, not rigid clocks.
The practical takeaway for the EMT is to read the mechanism early, perform only the interventions that cannot wait, package the patient efficiently, and choose a destination matched to the injuries - because for major trauma, the clock is the enemy and the operating room is the cure.
According to the kinetic energy formula (KE = 1/2mv^2), what happens to kinetic energy when the speed of a vehicle doubles?
Which motor vehicle collision type is MOST commonly associated with cervical spine hyperextension injuries?
An EMT arrives at a motor vehicle crash scene. What is the FIRST step in the scene size-up?
A patient was ejected from a vehicle during a rollover crash. Being ejected increases the risk of death by approximately how many times?
The "Platinum 10 Minutes" concept refers to:
Match each motor vehicle collision type to its expected injury pattern:
Match each item on the left with the correct item on the right