Several recent studies have failed to find a correlation between crash velocity and risk of sustaining a whiplash injury. Let us explore both vehicular and human factors that govern the risk of sustaining a whiplash injury.
Vehicular factors in whiplash injury
Impact Velocity and delta V
Delta V or change in the velocity is a relatively easy parameter to estimate and hence it is more popular for injury probability assessment. Delta V is only part of the equation & acceleration is more important parameter. Both acceleration and speed change affect occupant kinematics.
It is also important to understand that the delta V of the vehicle and delta V of the occupant are not always the same.If the occupants vehicle is struck on the drivers side door – a relatively soft part of the car – the driver may be subjected to a delta V much larger than that of his car.
Further it is important to ascertain the occupants seated position at the time of the crash because if the occupant is leaning forward with partially unsupported back then this can lead to injuries.
A large proportion of injuries occur at the speeds below those needed to cause permanent damage to vehicles. There is a paradoxic relationship between the vehicle damage and the risk of the injury. Outcome studies have shown that greater the vehicular damage , the less the biomechanical loading ( and the inverse).
In a LOSRIC study ( Low Speed Rear Impact Crash ), the largest category of injury crashes were graded as having no damage . In these 38% females and 19% males had symptoms. When damage was rated as minor, these percentages were 54% and 34%.
However , there may be subtle signs of damage in the form of damaged or sprung seat backs, witness marks ( stretch marks) on restraint webbing, scrap marks on bumper isolators , and damage to frame and bumper systems that is not visible from the exterior of the vehicle.( Just checking the photographs of the damage not helpful)
Many vehicles will be undamaged at crash severities that are above the range where human volunteers have reported neck and back symptoms.
Vehicle size and stiffness
Severity of injury from a rear impact collision is likely to be proportional to the mass of striking vehicle and inversely proportional to the mass of struck vehicle.
A larger car has more room to crush than smaller car. If both large and small cars crash into a fixed barrier at the same speed, the duration will be much shorter for a smaller, stiffer vehicle’s occupant. This results in much higher acceleration to the head, neck, torso and other body parts, and can make a difference between the injury pattern.
Other Vehicle Factors to consider:
- Head restraints
- Seat Belts and shoulder harness
- Seat Backs
- Bumper systems
- Collision Vectors – Rear Impacts vs Frontal vs Side
Human factors in whiplash injury
The researchers have looked at engineering and biomedical literature and from this meta-analysis the final answer was that there have not been any studies published in the peer reviewed , scientific literature that have demonstrated a significant relationship between crash severity and (a) acute injury risk (b) injury severity (C) probability of chronic symptoms.
Human factors should be given more important consideration when looking at the probability of the injury risk:
- Awareness of impending impact
- Bracing for impact
- Sex – Females more than males
- Position in the vehicle
- Ramping – upward motion of the occupant of a struck vehicle which occurs just after the impact. When this happens the head-neck complex may no longer be protected by the head restraint and restraint may act as a fulcrum
Acknowledgment – I would like to thank Arthur C Croft PHD. DC, MSc, MPH of Spine Research Institute of San Diego for providing the coursework that enabled me to put out together this article. Visit www.srisd.com. Please email me if you need any references.