RISK FACTORS FOR ONSET OF WAD
A risk factor for an outcome (i.e. disease/injury) is a factor that is independently associated with the outcome or condition in question. Knowledge of the etiology (cause) of WAD is limited. One reason for this is the difficulty in obtaining accurate and appropriate denominators to calculate risks. Rather than using persons exposed to collisions as the denominator, researchers have used proxies, such as registered licensed drivers, population censuses, or persons involved in collisions where at least one person was injured. Some studies have adjusted for possible confounding factors, while others have not. A confounding factor is an independent risk factor for the outcome and is also associated with the exposure/risk factor of interest. Examples of possible confounding factors include gender, age, pre-collision physical and mental health, and severity and direction of crash impact.
Risk factors for WAD reported in published studies include presence of neck pain prior to the collision, being the driver or the front-seat passenger (compared to the rear-seat passenger), and being exposed to a rear-end collision or frontal collision rather than a side collision. Female gender has been suggested to be associated with a slightly higher incidence of WAD in some studies, but other studies have found no gender differences. All these studies have weaknesses, primarily, the lack of ‘true’ denominators and/or the limited possibility to control for potential confounding factors.
One possible risk factor for WAD is the severity of the crash (impact). The biomechanical research on WAD is mainly based on experimental studies using cadavers, volunteers and simulation experiments. So far, the injury mechanism has not been established as a known risk factor. Reasons for this may be that there are different injury mechanisms occurring with different crash types. Car occupant acceleration, velocity and rebound are all factors that should be considered. In much of the research, a major focus is on rear-end injury mechanisms despite consistent findings that rear-end collisions are only responsible for 40-55% of all cases of WAD in MVCs. However, there are some promising results from actual rear-end collisions in that the redesign of headrests and seats so that head/neck extension is limited in rear-end collisions has reduced the incidence of WAD. Before firm conclusions about the magnitude of such preventive interventions can be drawn, larger studies with well-defined outcome measures and controls for potential confounding factors are needed.