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MOI - Mechanism of Injury

Mechanism of injury is a term often bandied around on medical courses, so most people who have ever done a course will be familiar with it. But how much do you really know about it and what does it really mean? 

Quite simply MOI is used to estimate the force (or forces) that cause injury when they are applied to the body, in relation to traumatic injuries. 

This might sound like a bad thing, but MOI has very practical uses (especially if your casualty is unconscious) in working out what potential injuries have occurred. MOI can give us really valuable information as to what areas we should focus on once we’re happy that our Airway, Breathing and Circulation are in good working order!. 

As we approach our casualty, we are already assessing the mechanism of injury. In checking for danger, you will be surveying the scene, which will provide vital clues as to what has happened, and therefore what injuries your casualty might have.

The biggest killers in trauma are catastrophic haemorrhage, followed by obstructed airways. So we would aim to stabilise these as the priority. Of course we can’t save everyone, but simple things done at the scene can make a huge difference.

Mechanism of injury does involve a bit of science. Kinematics to be precise. But stick with us, and let's engage our inner science geek because this bit could be really useful!

Kinetic energy is formed by two factors - velocity and mass. Or speed and weight to the rest of us. Speed is generally the bigger risk factor, but a slow moving heavy object can do untold damage too. As an example, a high speed power boat collision could obviously cause serious injuries, but a ship touching the dockside would be slow and catastrophic if someone fell into the gap. 

Simply put, the transfer of massive energy will cause massive injuries.

Working out the mechanism of injury can direct your attention to less obvious injuries, or even previously unidentified casualties. 

Take a look at this UK commercial to promote the wearing of seatbelts. It talks about the different forces involved in an impact (not just cars!). In this case the vehicle stops with a violent decelerating force. Next the casualty stops, but the internal organs are still moving until they hit something solid. The result is catastrophic. But it’s a good illustration of how energy transfer works. (NOTE: it’s a bit gory if you’re of a squeamish nature!)


Let’s look at a couple of possible scenarios and think about what could have happened…
(We will assume there is no catastrophic haemorrhage in each case.)

Superyacht deckhand found unconscious at the bottom of a stairway.
  • They’ve fainted and fallen from standing where they are.
  • They’ve slipped on the stairs and fallen, knocking themselves out.
  • They’ve fainted and fallen down the stairs from the top.
  • They’re having a snooze.
If they’ve fallen from standing and were unconscious when they fell, their injuries should be relatively minor. But you need to consider the cause. Why have they fainted? Also, is there anything around that they could have hit their head on, or that could cause other injuries.

If they’ve sipped and fallen, initially they might have been able to help control the descent and protect their head and neck. But at the point of unconsciousness, this stops. Descent becomes rapid and uncontrolled, there is a risk of impact with the stairs, bulkheads, deck and anything else protruding. We would certainly consider possible head and cspine trauma, with an in depth top to toe survey (part of your secondary survey) to investigate for other injuries. 

If they’ve fainted and fallen down the stairs, they are unlikely to have been able to protect themselves in any way. Similarly to the above, we would be considering head and cspine injuries as a priority, then using a top to toe survey to look for chest, back, abdominal, pelvic and other musculo-skeletal (MSK) injuries. We would also be investigating the cause of the faint.

If they’ve decided on an unusual location for a snooze the worst that would happen is they’d probably be in for a bit of mickey taking! 

Man overboard from a middle deck. 
  • Slipped and fell into the water.
  • Fainted and fell overboard.
  • Injured during a task and fell into the water.
  • Jumped in on purpose.

In this case we have additional complications of a fall from height, submersion in water and possible hypothermia, plus getting the casualty out of the water into a safe area in order to conduct our assessment. 

If the casualty is conscious and slipped, they will try to protect themselves, but any unexpected fall means they aren’t in control. In this case we would suspect a cspine injury and investigate the possibility of multiple other injuries. Conscious casualties will try to look after their own airway, but the risk of inhaling or swallowing water is high.

If they are unconscious before they hit the water we have to consider head and cspine injuries, plus the possibility of multiple other injuries. The impact of an unconscious body into water from height is an extremely violent mechanism. We also have the risk of drowning as an unconscious casualty is unable to protect their airway. 

If an injury preceeds the fall, does our assessment change? The answer is no. We would consider head and cspine injuries, then carry out our top to toe survey to uncover other injuries, plus investigate the injury that caused the fall. 

If the person has jumped in on purpose, the chances are that they have planned the event and controlled the fall. The risk of injury is lower if they enter the water in a controlled manner. But there’s always the risk of a cspine/head injury that could cause paralysis if a dive causes them to hit the bottom head first, or a submerged obstruction. Or, if they go in feet first, the shock can travel up the legs and spine, resulting in a cspine injury. You should also ask the question why have they jumped in? It could simply be off duty fun with colleagues, but could also be psychologically motivated and an attempt to end their life.

So, in summary, understanding the mechanism of injury can be a really valuable tool in our tool box of casualty assessment. Thinking about what’s actually happened and the potential impact on a human body can provide big clues about the potential injuries and therefore help you to treat your casualty more efficiently and effectively.

If you’d like to try out one of the Red Square Medical onboard drills involving mechanism of injury, please get in touch.












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