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Thoughts From A Clinician

Trauma in Motion

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Many years ago a man took his afternoon tea underneath an apple tree.  As he was enjoying this most delicious tea, he was struck on the head by a falling apple.  What does this have to do with trauma?.  This incident gave birth to modern day physics and most importantly to Sir Isaac Newton’s three laws of motion.  The three laws of motion have been around since 1686 and have given rise to things like aeronautics and eventually spaceflight.  These laws are ever present in our daily lives, but more importantly with the patients we care for and the scenes we respond to.  Let’s take a closer look at the laws of motion and what they entail.

Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. If an external force is applied, the velocity will change because of the force (1).

The law of inertia is the basis of all traumatic injury; and can be utilized to read the scene of a motor vehicle accident or act of violence.  Each traumatic injury is going to be approached differently, however, we must maintain situationally aware and keep this law of motion in our mind.  An example of this in a trauma scenario goes like this – a car is traveling down the interstate and it hits a bridge abutment or another vehicle, forcing it to stop suddenly.  We understand this from kinematics of trauma from our standard trauma courses.  However, what about potential patient injuries from this phenomenon?  Obviously, reading the scene as mentioned previously will tell us more, but understanding this first law will explain the patient’s presentation.  This law will give us a patient with head injuries, pneumothoracies, and a myriad of potential tissue damage. 

The second law explains how the velocity of an object changes when it is subjected to an external force. The law defines a force to be equal to change in momentum (mass times velocity) per change in time. Newton also developed the calculus of mathematics, and the "changes" expressed in the second law are most accurately defined in differential forms. (Calculus can also be used to determine the velocity and location variations experienced by an object subjected to an external force.) For an object with a constant mass m, the second law states that the force F is the product of an object's mass and its acceleration a:

F = m * a

For an external applied force, the change in velocity depends on the mass of the object. A force will cause a change in velocity; and likewise, a change in velocity will generate a force. The equation works both ways (1).

This law outlines the fact that velocity is the king when it comes to traumatic injury.  The size of an object makes an impact upon the trauma sustained to the patient, however, velocity is king.  An example that would illustrate this particular law would be a gunshot wound.  Gunshot wounds are deadly, but the size and caliber of the bullet make the difference.  GSW’s are handled differently depending on the injury sustained.  We can anticipate a myriad of injuries with a gunshot wound and anticipate our care based on the scene.  Velocity is not just with use of firearms.  We can also talk about low velocity weapons such as knives and other objects used to strike or injure a patient. 

The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. Notice that the forces are exerted on different objects. The third law can be used to explain the generation of lift by a wing and the production of thrust by a jet engine (1).

When we look at the third law of motion one thing that comes to mind is a patient being struck in a motor vehicle from behind.  There is an action which pushes the car and patient forward.  This would explain a traumatic brain injury or other injuries sustained from being struck from the rear of the vehicle.  No matter the speed, this law is still going to apply to this situation.  Speed plays a role in the severity of injury, related to the other laws, however, it still applies that for every action there is an equal and opposite reaction.  This third law of motion would also give us explanation as to what happens to internal organs and why injuries are sustained so often (2).

Finally, the purpose of this article is to take patient presentation and scene size up into consideration.  The more we consider Newton’s Laws of Motion, the better we understand the scene in front of us and patient presentation.  While these laws may not be the first thing on your mind when doing a scene size up and scene survey, they can tell you a lot about anticipated injuries.  Always remember that the scene can explain a lot of what you see within the patient.  You have so many resources within these three simple laws to explain patient presentation.  Our greatest ally is anticipation of injuries so we can better treat the patient.  Understanding the Laws of motion are just a small part of that.  Thank you all for taking the time out of your day to read this quick article on the laws of motion.  I hope to see you all at FAST18 in Nashville.  Remember, be safe out there and to always think like a clinician.


  2. PHTLS 8th Edition: Kinematics of Trauma

Peer Review #1:

Thanks to Klint for awakening our inner Newtons!  The timing of this essay is good, as advances in automotive technology have changed injury patterns dramatically.  Through the use of seat belts, air bags, and crumple zones, the energy transfer in a crash is increasingly being absorbed by the vehicle and/or distributed more evenly across the human torso.  However, we cannot escape the basic laws of physics and must appreciate that an object (i.e., our patient) cannot decelerate from 80 mph to a complete stop (or vice versa) over a relatively short distance without absorbing incredible amounts of force.  Rather than the force being applied to the face as our patient exits the vehicle through the windshield, the forces are absorbed internally.  This is where it gets interesting - the differences in density across various tissues and organs creates internal havoc with these acceleration/deceleration injuries.  For example, the brain may continue forward even after the skull has stopped, tearing bridging veins and resulting in a subdural hematoma.  Or the heart may move forward a bit further than the aorta, which is tethered to the spine, tearing a hole at the attachment of the ligamentum.  All of these life-threatening injuries are occurring without the traditional external hematoma, abrasions, lacerations, and deformities - essentially, we're seeing much better looking corpses!  So take a lesson from Newton and don't be fooled by the absence of external injuries:  The forces went somewhere, and it's our job to figure out where and treat accordingly.

Dr. Dan Davis, MD

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