Mechanisms of Injury Help Determine Severity of Trauma

When someone falls off a five-foot deck and walks away with ease, it would make a funny video on YouTube. But if someone falls off the top of a five-story building and walks away unscathed, this would make the evening news. Why? Because we all instinctively know that one can't possibly survive (or barely survive) such a long fall.

Long falls are just one type of mechanism of injury that is seen in emergency medicine.

Two men dealing with a car crash
Pattanaphong Khuankaew / EyeEm / Getty Images

Mechanism of injury, or MOI, refers to the method by which damage (trauma) to skin, muscles, organs, and bones occurs. Healthcare providers use MOI to help determine how likely it is that a serious injury has occurred.

But the term is not used only by healthcare providers. We all know what it is even if we don't know what to call it. There's an old joke that says, "It's not the fall that kills you, it's the sudden stop at the end." In MOI terms, that's known as "sudden deceleration."

Besides a fall, other examples of "sudden deceleration" are a low-speed fender-bender in a parking lot and a rollover accident on the freeway. It is obvious which one would lead to life-threatening injuries.

Similarly, we can all imagine how a gunshot wound has more potential for serious injuries than a fistfight.

One important thing to remember is that MOI is not the same for everybody. A lot depends on the physical condition of the person.

A good rule of thumb is: For a young and healthy adult, a fall from a distance more than three times the height is considered significant. By contrast, an elderly person (usually with brittle bones) would be likely to be injured in a ground-level fall, or tripping.

Complications (Co-Morbid Factors)

Not everyone is young and healthy. As we age, our skin gets more delicate and our bones, more brittle. Sudden decelerations like in a fall, a car accident, and the like would affect the very old and the very young more severely than the average young, healthy adult.

Differences in factors such as age—and a lot more—are known as "co-morbid factors." Heart disease, for example, may affect the patient's ability to compensate for shock. Liver or kidney disease can lead to thinner blood that doesn't clot as well as in someone without the disease. Residual weakness from a stroke or other neurological diseases can turn a minor trip and fall (known as a ground-level fall) into a life-threatening event. That's why falls in the elderly are so concerning.

Alcohol and Substances

Anything that gets you high, drunk, or stoned changes your behavior. Injuries to the brain often cause similar changes in behavior. As a result, it is more difficult to assess an intoxicated patient for significant injuries. In addition, alcohol specifically changes the chemistry in the blood, making it thinner and less likely to clot. Under such circumstances, what would otherwise be a generally safe, low-mechanism injury like a ground-level fall would become a serious, life-threatening event.

A Word From Verywell

Mechanism of injury is a moving target, which varies from person to person. Use your gut instinct if you are in a position to handle a likely emergency. If the incident seems like it would be life-threatening, you're probably right. If the patient is old, pregnant, an infant, sick, drunk or otherwise compromised, and it makes you more concerned than normal, you're probably right. Trust your gut to take the proper action.

2 Sources
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  2. Salem RO, Laposata M. Effects of alcohol on hemostasis. Am J Clin Pathol. 2005;123 Suppl:S96-105. doi:10.1309/113N8EUFXYUECCNA

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