Within the single exposure data, the majority of the volunteers were unaware of the impending impact (n = 422). A smaller portion indicated that the volunteer was braced (n = 28) at the time of the rear impact. The proportion of volunteers reporting symptoms was greater for those who were braced (25%) than those who were not braced (7.6%). This difference was statistically significant using Fisher exact test (P = 0.0065). The difference between impact severity for the braced (7.2, Std. Dev = 2.3 kph) and relaxed (6.3, Std. Dev = 3.3 kph) volunteers was 1 kph and was not statistically significant (P = 0.51).
The OC curve was calculated on the basis of the 480 nonrepeated tests within the volunteer dataset. The level of certainty based on the entire single exposure dataset (Figure 5) showed that there would be a 95% chance of having a volunteer within the dataset that did sustain an injury if the underlying risk of injury was about 0.6%. For impacts with a change in speed less than and greater than 8 kph, the certainty was 84% and 64%, respectively. Therefore, the large number of single exposure volunteer tests provide a strong indicator that the likelihood of injury during a minor rear impact is remote. This is consistent with the general acceptance that human volunteers can be exposed to these events without a risk of injury.
Neck moments experienced by human volunteers have also been reported within previous studies (Figure 7). The values for neck extension were reported for volunteers with and without head restraints.64,74,75 These studies demonstrate the primary loading condition for the cervical spine in rear impacts is comprised of tension and extension forces. The magnitude of the extension moment experienced by the human volunteers during rear impacts is similar to those experienced by volunteer during everyday and vigorous activities.64,76,77
Within real-world rear impacts with a change in speed below 8 kph, females had a higher rate of symptoms; however, this difference was not statistically significant (P = 0.39). For impacts with a change in speed between 8 and 16 kph, the incidence of neck strain was greater for males, with no statistically significance from females (P = 0.60) (Figure 9). The rate of neck strain for rear impacts with a change in speed between 0 and 16 kph was 19% and 22% for the females and males, respectively.
Overall, the real-world data showed that the risk of any neck injury (AIS 2+) beyond muscle strain is small, even for high levels of change in speed (Figure 10). The likelihood of neck strain was significantly greater than more consequential, AIS 2+ injuries.
Injury risk was also assessed using logistic regression analyses. For neck strain and AIS 2+ injuries, only a change in speed was found to be statistically significant. Age, gender, belt use, head restraint, and vehicle type were not statistically significant in predicting neck strain or AIS 2+ neck injuries.
The results of this study, using both experiment-based and real-world data, reinforce the general acceptance that human volunteers can be safely exposed to rear impacts of less than 18 kph without a meaningful risk of injury. Interestingly, both data sources produced similar rates of neck strain by gender and impact severity. This indicates that there is no reporting bias by the volunteers, some of whom are the authors of the articles. In addition, the similarity between the volunteer and real-world data indicate that any minor variation in head or body position that may be present in real-world collisions did not result in an increased rate of injury. Contrary to common belief, the volunteers that were reported as being braced had a higher rate of reporting symptoms than those that were not. The small underlying risk of injury indicated by the operating characteristic curve for the volunteer studies is consistent with the small risk of neck injury predicted by the real-world analysis.
The lack of injury to the volunteers results from the minor nature of the motion and forces they are exposed to as well as the direction of the forces applied to their bodies. Biomechanical research has shown that in order to produce a specific injury, a specific set of forces are required at the proper location and orientation. Experimental studies of the cervical and lumbar spine have demonstrated these mechanisms and have shown that spinal injury cannot be produced simply by being exposed to a motor vehicle collision. The lack of spinal injuries as a result of rear impacts is due to the lack of the specific mechanism to produce such injuries.
This study has limitations with respect to the conclusions that can be drawn. The volunteer dataset is limited to the information provided within the published work. The pre-impact screening of volunteers was not always addressed and when it was, there was only a brief discussion. Therefore, the health of the volunteers before the testing is not known at all, or to a limited extent for most of the volunteers.
The results of this study demonstrate that the incidence of symptoms of neck pain is directly related to the change in velocity experienced by the vehicle. This trend was similar for staged volunteer testing and real-world motor vehicle collisions. The number of volunteers and exposures within the volunteer testing is sufficient to provide a high degree of certainty that injury is not expected at the delta-V levels studied, even though subjective pain may occur for a short duration. This is not a surprising outcome given the lack of reported injuries, the minor nature of the motion and forces measured during these studies, and the ongoing use of research volunteers around the world. The similarities between real-world outcomes and staged collisions demonstrate that any preparation by the volunteers does not influence the risk of neck pain. Statistical analyses of real-world rear impacts confirm the low risk of more severe injuries at impacts of similar severity to the volunteer research.
The authors would like to acknowledge Dr. Robert Banks of Biodynamic Research Corporation (BRC) for his initial work in developing the volunteer database and Dr. James Funk of Biomechanics Consulting and Research (BIOCORE) for his technical assistance.
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