MOI stands for “mechanism of injury,” a term used in emergency medicine to describe how a person was hurt. It captures the physical event that caused the trauma: a car crash, a fall, a stabbing, a blast. Paramedics, emergency physicians, and trauma surgeons use the MOI to predict what internal injuries a patient might have, even before any scans or lab results come back.
The concept matters because the forces involved in an injury often tell medical teams more than what’s visible on the surface. A person who fell 15 feet may look relatively fine on arrival but could have internal bleeding or spinal injuries. Knowing the MOI helps responders decide how urgently someone needs a trauma center and which body regions to examine first.
How MOI Works as a Predictive Tool
The core idea behind MOI comes from basic physics: energy cannot be created or destroyed, only transferred. When two objects collide, the energy exchanged follows the equation where kinetic energy equals half the mass multiplied by the velocity squared. That’s why speed matters so much in trauma. Doubling the velocity of a crash quadruples the energy transferred to the body. A low-speed fender bender and a highway-speed collision involve the same basic event, but the energy absorbed by the occupant is dramatically different.
This principle lets medical teams work backward from what happened to predict what they’ll find. A side-impact car crash at high speed, for instance, is strongly associated with aortic tears, pelvic fractures, and skull fractures on the impact side. A rollover crash can cause roof intrusion into the passenger compartment, making head injuries likely. Someone ejected from a vehicle faces impacts with the windshield, the road surface, and whatever objects are nearby. Each scenario creates a distinct pattern of expected injuries, and the MOI gives the trauma team a head start on finding them.
Major Categories of MOI
Mechanisms of injury fall into a few broad categories, each producing different damage patterns in the body.
- Blunt force trauma includes car crashes, falls, assaults with fists or objects, and sports collisions. Energy spreads across a wider area, often damaging internal organs without breaking the skin.
- Penetrating trauma involves an object piercing the body, such as a knife wound or gunshot. The damage track is narrower but can reach deep structures.
- Blast injuries are the most complex. Explosions cause five distinct types of harm: the pressure wave itself damages air-filled organs like the lungs, ears, and intestines; flying debris causes penetrating wounds; the blast wind throws the body into objects, causing blunt trauma; heat and toxic fumes cause burns and inhalation injuries; and chemical additives can trigger widespread inflammatory reactions throughout the body. Secondary injuries from flying debris actually account for the majority of explosion-related trauma.
- Thermal and chemical injuries include burns, frostbite, and exposure to caustic substances, where energy transfer happens through heat or chemical reactions rather than physical impact.
Specific Scenarios That Trigger Trauma Alerts
Not every injury requires a trauma center, so national guidelines lay out specific MOI criteria that help paramedics decide where to take a patient. The 2021 National Guideline for the Field Triage of Injured Patients, widely used across the United States, lists these mechanism-based triggers:
- Ejection (partial or complete) from a vehicle
- A death in the same passenger compartment
- Significant vehicle intrusion: more than 12 inches at the occupant’s seat or more than 18 inches anywhere in the vehicle
- A rider separated from a motorcycle, ATV, or horse with significant impact
- A pedestrian or cyclist thrown, run over, or struck with significant force
- A fall from more than 10 feet for any age
- An unrestrained child (age 0 to 9) or one in an unsecured car seat
- Vehicle telemetry data consistent with severe injury
These criteria classify as moderate-risk, meaning the patient should be taken to a trauma center when one is reasonably available, even if they don’t have obvious severe injuries yet. Higher-risk criteria focus on what’s already visible: abnormal vital signs or clear anatomical injuries like open skull fractures or amputations.
How Accurate MOI Is at Predicting Serious Injury
MOI-based triage criteria are very good at ruling out serious injury but less reliable at catching every case. In a large analysis of motor vehicle crashes, the current CDC triage criteria had a specificity of 97%, meaning when someone didn’t meet any MOI criteria, they almost certainly didn’t have a severe injury. But the sensitivity was only 55%, meaning roughly half of patients with serious injuries weren’t flagged by mechanism alone.
That gap widens with age. For patients 18 and under, the sensitivity was 62%. For adults 19 to 54, it was 59%. But for adults 55 and older, it dropped to 37%, and for those 65 and older, just 36%. In other words, MOI criteria miss about two-thirds of seriously injured older adults. Among the individual criteria, ejection from a vehicle was the best at catching serious injuries, while a death in the same passenger compartment was the most reliable indicator that an injury, when present, was truly severe.
Why MOI Can Be Misleading in Older Adults
Traditional MOI assessment assumes a certain relationship between the force of an event and the severity of injury. That relationship breaks down in older patients. Aging brings increased bone fragility, loss of muscle mass, and broader physical frailty, all of which mean that relatively minor events can cause disproportionately severe injuries.
The numbers are striking. In one Australian study examining trauma deaths, low-energy injuries (falls from standing height or just above) were responsible for 41% of all trauma fatalities, and every one of those deaths occurred in patients who made it to the hospital. The average age in this group was 83, with a mean injury severity score of 14. By contrast, high-energy trauma deaths occurred mostly before patients reached the hospital, in patients averaging age 43 with much higher injury severity scores. The takeaway: older adults can die from injuries that wouldn’t meet any standard MOI threshold, because the threshold was designed around younger, more resilient bodies.
This is why emergency teams increasingly look beyond the mechanism itself when evaluating older patients. Blood-thinning medications, pre-existing heart conditions, and reduced physiological reserve all amplify the consequences of even a ground-level fall.
How MOI Gets Communicated Between Teams
When paramedics arrive at a hospital with a trauma patient, they need to relay critical information quickly and consistently. Many EMS systems use the MIST format: Mechanism of injury, Injuries found or suspected, vital Signs, and Treatments given. This structured handoff ensures the receiving team immediately knows what forces were involved and can begin planning their assessment.
MOI is the most reliably communicated element in these handoffs. Studies of MIST implementation at trauma centers found that agreement between paramedic reports and hospital records for mechanism of injury was 96% or higher, both before and after the standardized tool was introduced. Vital signs and treatments were communicated less consistently, but the mechanism itself transferred well, likely because it’s concrete and narrative: “unrestrained driver, rollover at highway speed, ejected” paints a clear and memorable picture.
Falls and Height Thresholds
Falls are one of the most common injury mechanisms, and height is the primary variable that determines severity. The U.S. triage guideline uses 10 feet as the threshold for all ages, but research suggests the relationship between height and injury is more of a continuum than a single cutoff.
Falls from roughly 20 feet (about 6 meters) are a reliable predictor of major trauma, with injury severity scores consistently above 15. At around 25 feet (7.6 meters), the need for blood transfusions and worse neurological outcomes increase significantly. Mortality climbs sharply above 26 feet (8 meters), and falls from approximately 48 feet (14.6 meters) carry a mortality rate above 50%. Falls above 61 feet (18.6 meters) are considered almost universally fatal.
These thresholds apply broadly, but the same caveat about older adults holds: a 75-year-old falling from a step stool can sustain injuries that a 25-year-old might walk away from after a 10-foot fall. Context always matters alongside the raw numbers.