Brain Injury Despite Seatbelt
Seat belts save many lives, especially in combination with airbags. But while the may prevent the head from being slammed into a windshield they do not eliminate all acceleration on the brain. The seat belt, especially the three point seat belt, converts largely linear force to centrifugal (rotational) force, which can involve greater acceleration and deceleration than straight line force. Brain Injury despite seatbelt may occur.
This rotational force increases the exposure to shear forces discussed previously which can result in diffuse axonal injury( DAI); diffuse, meaning throughout the brain and axonal, involving axons, the long fibrous parts of the neuron which stretch across layers of different density in the internal portions of brain. You can find more information on the cells work and what happens when they are damaged by clicking here and going to the page Axons are Similar to Electrical Wires.
Caveat: While rotational forces cause more Diffuse Axonal Injury than linear forces, the extent of acceleration/deceleration forces would be so much greater from the head hitting some hard part of the inside of the car, that seat belts do infinitely more good than harm. In essence, those with seat belts are exposed to some risk of Mild Traumatic Brain Injury where those without are at greater risk of death and severe brain injury. With this being said, brain injury despite seatbelts is very common. It may be much harder to diagnose or even be missed because there is no obvious trauma or any sign as to a blow to the head. Most brain injury is diagnosed because a blow to the head and the emergency room personnel may not initially order the tests to detect traumatic brain damage.
Rotational force still exists with a seat belt.