At the Spine Research Institute of San Diego's proving grounds (CRASH 2004), we conducted an experimental study subjecting passengers to frontal collisions with speed changes of under 4 mph, and utilizing posttest structured interviews immediately after and four weeks after impact to quantify subjects' clinical responses. Our goal was to determine whether healthy adult volunteers report symptoms following exposure to low-speed frontal crashes at speeds below those hypothesized to represent a threshold for injury risk; and whether experimental frontal crashes can be conducted safely at the levels studied.
Recent studies of frontal crashes, in which the authors had access to on-board crash pulse recorders and in which formal engineering examinations of the vehicles were conducted, have shown that neither speed change (delta V) nor structural damage are strong correlates of injury risk. No standards currently exist for the level at which frontal crash tests can be safely conducted with human volunteer passengers, particularly those seated in rear seats.
In our experimental study, 17 medically screened, healthy, informed, and willing volunteers were exposed to low-speed frontal crashes. All volunteers were seated in the rear seat position of the bullet vehicle, with seat belts and shoulder harnesses fastened. Nine were seated in the inboard (middle seat) position and had only a lap belt. Closing velocities ranged from 4.1 mph to 8.3 mph. For the bullet vehicle, the delta V ranged from 1.4 mph to 3.9 mph.
Somewhat unexpectedly, 88 percent of the volunteers attributed symptoms of discomfort to their crash exposure. All reported symptoms were transient and none required medical treatment. All symptoms were characterized as either minimal or slight in severity, and none had a duration exceeding one week. The mean duration was one day. The predominant symptoms were posterior neck discomfort, followed (in order of reported frequency) by shoulder and thoracic spine discomfort. Several of the volunteers had symptoms lasting two to three days, although these were characterized as minimal or slight in severity only. One subject had slight neck discomfort for one week. In all cases, the subjects were well-aware of the impending crash, and were belted and positioned in normal postures. None of the crashes resulted in structural damage to either the bullet or the target vehicles.
One of the surprising findings in the present study was the high rate of minor injury in the volunteers relative to that seen in rear-impact studies. It is well-established that low-speed, rear-impact collisions are more likely to produce injury in comparison with frontal collisions of the same severity. In one recent report, a group of authors examined crash-related injuries reported to a hospital emergency room, and then reconstructed the crashes.1 They found that the mean delta V of injury for rear impacts was 5 mph, whereas the mean delta V for frontal-impact injuries was 8 mph. The explanation for this finding has been explored experimentally in a study in which frontal and rear-impact crashes were compared using identical crash speeds, vehicles, instruments, and subjects. Head accelerations in the rear-impact crashes were a multiple of those in frontal crashes, and the kinematic response was markedly more complicated. Moreover, volunteers rated them markedly less tolerable than the frontal type.2
Although the results of the present study are intended to be generalized in a very narrow manner, there are important limitations to the present study that tended to reduce injury likelihood, relative to the at-risk population. Primarily, this includes the fact that the subjects were mostly male, and were relatively young and healthy, with an absence of other known risk factors. Positioning was also optimized in these tests.
The results of the present study indicate that while human-subject crash testing for rear-seat passengers in frontal collisions is unlikely to result in serious or permanent injury, it is highly likely to result in some degree of minor and short-lived injury. While the reported mean delta V in one study of people injured in frontal crashes was reported to be as high as 8 mph, this finding does not offer any insight into the minimum threshold for such injuries among the universe of exposed vehicle occupants, and the results of the present study suggest that it could be significantly lower, depending on the position of the occupant in the vehicle.
- Bunketorp O, Jakobsson L, Norin H. Comparison of frontal and rear-end impacts for car occupants with whiplash-associated disorders: symptoms and clinical findings. Proceedings of the International IRCOBI Conference, Graz, Austria, 2004:245-56.
- Croft AC, Haneline MT, Freeman MD. Differential Occupant Kinematics and Head Linear Acceleration Between Frontal and Rear Automobile Impacts at Low Speed: Evidence for a Differential Injury Risk. International Congress on Whiplash-Associated Disorders, Berne, Switzerland, 2001.
Arthur Croft, DC, MS, MPH, FACO
Director, Spine Research Institute of San Diego
San Diego, California
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