The classic rear-end collision(“rear-ender”) in car wrecks is a major cause of mild traumatic brain injuries (MTBI). Studies have shown that people can suffer mild traumatic brain injuries even at low or minimal speeds. Rear-end impacts are usually more violent than front or side impacts to occupants of the automobile, as described below.
To help understand why rear-impacts are more violent, we must look to how a person’s head moves during a rear-end crash. First, the seat back usually hits a person’s spine, causing it to flatten against the seat. After the spine and car seat are thrown forward, the person’s head will roll back and hit the headrest. After hitting the headrest, her head accelerates forward with great speed and momentum. In other words, a person’s head will accelerate with greater speed than the rest of her body and torso. As her torso moves forward, it is then caught and restrained by the seat-belt, causing the head then to decelerate abruptly. What happens to a person’s brain inside the skull is what can cause injury to it. In order to understand what happens to the brain, it may be useful to imagine the brain as an egg.
An egg has a protective covering (shell) and a clear liquid surrounding the inner yolk. The shell and the liquid protect the inner yolk. However, if the egg was thrown forward by an object traveling at 20 miles per hour and then suddenly decelerates from 20 miles per hour to zero miles per hour in a split second, the yolk continues to travel inside the shell and will strike the inside of the egg shell. It is this striking of the yolk against the hard shell that can cause damage to the yolk.
The human skull is no different than an egg for the purposes of this illustration. The human brain is protected by the skull and the fluid that surrounds the brain. The fluid provides a cushion to the brain when a person’s head is jarred. However, it was not designed to be able to absorb the great forces that can act on it in a rear-end collision. The brain can move forward inside the skull, striking the inside of the skull and the indentations on the skull wall. The illustration below shows how a human brain can move within the skull and strike the skull walls.
This force on the brain is called a shearing force and it can have profound effects on the the function of the brain, which we will explore in the next posts.