Kinematics Part 2

By James Beauchamp, D.C.

When cars collide at commonly experienced speeds, the entire event is over in around a third of a second.  In our culture this happens routinely, but from an evolutionary perspective it is a freak occurrence.  The human body evolved to run and jump and lift and carry in a completely natural environment.  Feet were never designed to walk on completely flat, level surfaces, for where can such be found in nature?  In an artificial machine based situation such as a car accident, the forces involved are grossly excessive to the design tolerances of our bodies.  A third of a second impact happens so fast that your brain remains uninformed of its occurrence until it is already over.  We therefore have no ability to meaningfully react to an unexpected car accident once the collision occurs.

Since human reactions aren’t a factor, let us consider how the energy of a collision affects the human frame.  Physical blows to the body cause cavitation of the tissues in much the same way that water splashes away from your hand when you slap its surface.  The striking object gives up its energy and decelerates just as the struck object absorbs the energy and accelerates.  Take a look at this video of a punch to the body:

Did you see how the flesh rippled away from the blow?  That was energy spreading through cavitation, as the molecules and even organs of the body are accelerated away from the direction of the blow.  The impact of the driver’s seat into the low back and pelvis of an unexpecting driver acts much like the hand of a giant slapping him from behind.  His flesh will, in effect, splash away from the impact.  This is called cavitation.

There are two types of cavitation: permanent and temporary.  In permanent cavitation such as occurs in a gunshot wound, a visible hole or tear is generated somewhere in the body.  The aorta, for example, can be severed, the spleen ruptured or the liver lacerated, all without body being pierced.  In a garden variety car accident such as the one we have been discussing, the cavitation is temporary.  Tissue is torn to be sure, but it is at the microscopic level and is laced throughout the involved muscles, tendons and fascia.  There is generally no sign of this type of injury initially, and many times the hospital records show a paucity of examination findings.  But as I noted in a previous post dealing with the concept of delayed soreness after injury, the true extent of injury may not be apparent for several days.

The force of a rear impact accident will cause the drivers seat to rapidly accelerate the bony pelvis forward.  The pelvis will in turn slam into the rearmost abdominal and pelvic organs, causing them to strike other tissues.  This expanding field of energy causes mostly compressive cavitation.  It is also common for the posterior elements of the cervical spine, upon initial movement, to be compressed.  Facet fractures, cartilage damage, capsular tearing and posterior disc crushing are potential sequellae for posterior cervical compression.  However, in the neck most of the cavitation will be distractive in nature.  The human head has a weight roughly equivalent to a bowling ball and, after striking the headrest, the combination of mass and acceleration will rapidly overstretch the tissues of the neck.  This rapid forward acceleration will cause distractive cavitation (tearing) of soft tissues as they absorb the head’s newfound kinetic energy.

At this point in the collision, the body has absorbed a tremendous amount of the energy of the accident.  The problem now is the differing velocities of the car vs. the driver.  The lighter driver usually accelerates rapidly even as the vehicle slows.  Please reference the following video:

Forward movement will be stopped by the structures of the interior of the vehicle.  Most people wisely use seatbelts with a shoulder harness to stop their forward movement in the event of an accident.  Since seatbelts have been mandatory in some form since the 1960’s there has been a virtual invasion of these body catchers within the automobile industry.  Seatbelts exact their own toll because they cause crush the anterior portions of the trunk that they come into contact with.  Seatbelts also increase injury to the neck by stopping forward trunk motion before the head slows down, thereby adding a further rotational vector to the not inconsiderable load that the head exerts upon the tissues of the neck.   It’s better than planting your face into the windshield, but not ideal, which is why airbags were invented.  In a rear end collision the airbags do not usually deploy, leaving us with only the seatbelt to stop the body’s forward motion.  Compressive anterior thorax cavitation and aggravated distractive cervical cavitational tearing are the result.

To summarize, a moment of inattention while driving can cause an accident that can not only cause significant property damage, but a whole host of injuries ranging from mild to lethal and everywhere in between.  Soft tissue injuries are damaged in blunt force trauma via cavitation as tissues are accelerated into and away from each other, up to and including fractures and internal injuries which can be lethal.  In general, the common rear-end motor vehicle impact causes soft tissue injuries which are often either undetectable at the Emergency Room or appear far less severe than they actually are.

Dr. James Beauchamp is a Multi-Specialty HealthCare provider specializing in Chiropractic Care. He is certified in spinal trauma, manipulation under anesthesia, and as an automobile accident reconstructionist. He is also a member of the advisory board for Operation Backbone.

References available by request. Copyright James W. Beauchamp, DC

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