Is Common Sense Non-Sense? (Continued from page 15)
forces to be less evenly distributed and results in greater damage. Awareness of the Crash Occupants caught by surprise are 15 times more likely to develop chronic symptoms. Being aware and braced, increases the duration of the crash and allows the force to be more tolerable. Preparedness for the Crash Awareness is not the same as pre-
paredness. Crash Duration
The occupant of the bullet vehicle undergoes a long duration crash with ride down time. The occupant of the target vehicle undergoes a very short duration crash. Position in Vehicle People do not always sit in a per- fect position. Higher loads have been found when the body was leaned for- ward as the head creates downward pressure on the spine. Head position Occupants are more likely to be in-
jured if their head is rotated at the time of impact. Rotation pre-stresses the facets, capsule and disc and produces less motion before the pliability of muscle bottoms out. A 45 degree ro- tation decreases extension by 50%. Head Restraint Neck stresses are higher with a
lower head restraint than with no re- straint at all. According to the Insurance Institute for Highway Safety, only 3% of head restraints are good and 90% of the time they are improp- erly adjusted.
Seat Belt Seatbelts only prevent acceleration
of the trunk, not the head. The lap belt and shoulder harness cause a rela- tive increase in the head acceleration in relation to the torso. Seatback
A soft seatback is undesirable as a driver may end up in the rear seat when the car is accelerated beneath him. A stiff seatback is undesirable as it may accelerate the torso forward while the head continues to move rearward and amplify the extension phase of injury. An inclined seatback increases cervi- cal compressive loads and causes injury to the facet. Heavy Braking Heavy braking places the occupant
forward in the seatback and increases back set - the distance between the base of the skull and the top of the head restraint. Relative Size of Vehicles Impact speed alone is of little rel-
evance since the consequence of a low speed crash is largely dependent on the relative sizes of the involved vehicles. Size and Speed of the Striking Vehicle The speed and size of the bullet ve- hicle are equally important in determining the acceleration of the struck vehicle. Size and Speed of the Stuck Vehicle Smaller vehicles give more accelera- tion to the occupant and therefore are not as safe. In addition, static friction is greater than motion friction. A mov- ing vehicle will accelerate faster than a stationary one.
Experts in Human Occupant Dy- namics also help elucidate the
misconceptions regarding low speed rear impact crashes (LOSRIC). Some common misconceptions are:
Injuries can be assessed by looking at the damage to a vehicle The repair to a damaged vehicle is likely subjective depending on the ad- equacy of the examination to the vehicle. It is important that the ve- hicle be thoroughly examined.
It is
often difficult to estimate damage without removing the bumper cover and getting underneath to look for signs of structural damage, which can sometimes be subtle. Bumpers are designed to protect occu- pants Bumpers are not designed to pro- tect occupants, but are designed to decrease override and under ride in high speed crashes and to minimize the cost of the repair to the vehicle follow- ing a low speed crash. Photographs of a crash can indicate oc- cupant injury Photographs cannot reveal many of the factors needed to evaluate occupant injury, such as speed, acceleration, force, tissue tolerance, occupant posi- tion at the time of impact, previous medical conditions, etc. The only way to determine if an occupant is injured is to perform a physical examination. The distance a vehicle travels after a crash correlates to occupant injury Target vehicle rollout is a poor in- dicator of impact velocity and does not correlate to occupant injury. Pain is subjective Pain is clearly objective when ac- companied by an abrupt withdrawal from the stimulus or a painful facial expression.
The practice of estimating injury based
on property damage must be abandoned and laid to rest. There is absolutely no correlation between the two. In no other area of medicine are opinions polarized to the point where one party claims in- jury and the other categorically denies the possibility.
In evaluating an injury caused
by a low speed rear impact crash we must consider all of the contributing facts and avoid common misconceptions.
16
Trial Reporter
Fall 2003
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