THE KINEMATICS OF MOTOR VEHICLE COLLISIONS ARE CHANGING

In 2010, the total number of motor vehicle fatalities dropped to the lowest level since 1949.  Watch crash test videos for the large number of Insurance Institute for Highway Safety “top safety pick” vehicle models and you will see no cabin intrusion. That means crush injuries are less and less common each year. The kinematics of motor vehicle trauma in the emergency department are changing.

Frontal airbags prevent most of the facial fractures we used to see, side airbags prevent the rib, kidney, and skull injuries we used to see, and head restraints prevent whiplash.  Electronic stability control prevents about 1/3 of accidents from happening at all.

From a physics standpoint, momentum might be conserved but kinetic energy is not. Collisions are considered “inelastic,” meaning that the kinetic energy of the collision is converted into the “work” of deforming materials. The more the energy is absorbed by the car, the better. Otherwise the work of deformation works on the patient, whose tissues have different densities and physical properties. Here are some of the forms this can take:

-Tensile strain essentially means things are being pulled apart, like the mesentery shearing away from its origin

-Shear strain means things are separating along a line, like the sacroiliac joints shearing in the patient who jumps from a great height

-Compressive strain is direct crush

-Overpressure strain is from compression of a gas or fluid filled cavity, for example, bladder or viscus rupture

In the future we see will fewer of all types of injuries. Clinical studies should tell us what sorts of injuries we will see in the future. For now, I have seen a dramatic decrease in crush injuries to the side (rib fractures, renal contusion, splenic/liver injuries), apparently related to improved engineering of the cage of automobiles as well as because of side airbags;

Take Home Points:

Motor vehicle collision injuries are becoming less severe as cars are engineered to absorb kinetic energy.

Advertisements