“Why is it that a great deal of injury can be sustained by the driver or passengers of a struck vehicle, when only a relatively small amount of damage was done to the vehicle itself?”
This question makes reference to a common misconception that vehicle damage and occupant injuries are proportional. To truly understand this complex relationship, a review of the mechanics of the collision is necessary, since the physical forces involved in a collision must be understood to provide insight as to how injury can exist in the absence of a large amount of damage to the struck vehicle itself.
In a rear end accident we are dealing with a unique phenomenon termed the magnification of accelerations. (2)
Martinez(3) reports that an eight mile per hour rear end collision of two vehicles of the same mass produces a 2g (gravity) acceleration of the vehicle and a 5g acceleration of the occupants head. McNab(
Those factors that limit or minimize the damage to the vehicle are precisely the same factors that result in greater damage to its occupants. McNab (4) describes this paradox in the following fashion: “the amount of damage sustained by the car bears little relationship to the forces applied.”
Headrests are the best protection in rear end collisions. However if the headrest is set too low, the head is able to roll over the top of the headrest, producing even more hyperextension. (5)
A surprise collision will usually cause more injury because the ligaments will be injured more than the muscles. When a person knows they are going to be struck, they will tense up the muscles and therefore injure the muscles first.
Common predisposing factors include degenerative joint disease and spinal stenosis. The potential for injury is increased because the neck is less resilient.
Jackson states: “The seat belt has very little if any deterring effect on the cervical spine as the head and neck continue forward motion. Even the addition of a shoulder harness will not relieve but will only increase the forces which must be absorbed by the head and neck, although such a harness may prevent contact injuries.”(6)
(2) McKenzie, J.; Williams, J. “The Dynamic Behavior of the Head and Cervical Spine During ‘whiplash’
(3) Martinez, J.; Garcia, D. “A Model for Whiplash.” J. Biomech. 1968; 1:23.
(4) McNab, I. “Acceleration Extension Injuries of the Cervical Spine.” The Spine, vol. II. Rothman, Richard, and Simeone 1975.
(5) Severy DM, Mathewson JH, Bechtol CO. Controlled automobile rear-end collisions, an investigation of related enginerring and medical phenomena. Can Serv Med J, 1995; 11:727.
(6) Jackson R. The Cervical Syndrome. Springfield, IL. Charles Thomas Co., 1977
If you've been in a car accident you can learn about my specialized care by visiting www.AskDrNed.com. If you would like to schedule an appointment you can call my office at 801-225-1311.
