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ARTICLES
Jonathan P. Gough
SUMMARY. Over the past 50 years an extensive body of literature has been published on the topic of whiplash injuries resulting from rear end collisions. The hypothesized injury mechanisms have been varied and conflicting, suggesting a misunderstanding of the human occupant dynamics which occur in such collisions. Much of this misunderstanding may result from the use of anthropomorphic dummies in staged collision tests. Without knowledge of the mechanisms by which injuries are sustained, automobile design will be lacking in appropriate safety features. Without an understanding of injury mechanisms,
health professionals may make incorrect assumptions about trauma severity and may render inappropriate treatment for sufferers.
[Article copies available from The Haworth Document Delivery Seivice: 1-800-342-9678. E-mail address: [email protected]] KEYWORDS. Whiplash, occupant dynamics, motor vehicle accidents
INTRODUCTION
The term whiplash was coined in the mid 1940's to describe acute injury to the cervical spine resulting from motor vehicle accidents (1). Whiplash described the motion, and therefore an assumed mechanism of injury to the head/neck complex, rather than the injury itself. The term fell into common usage and is still used to describe apparent soft tissue injuries of the neck resulting most commonly from rear end motor vehicle collisions. The early design of vehicles gave little consideration to protecting the occupants from trauma. Seats designed to control excessive movement of the head and neck in rear end collisions were virtually unknown. In the past 50 years, the introduction of lap and shoulder seat belts, air bags, crumple zones, energy absorbing interior surfaces, and head restraints have all contributed to the reduction of injuries in motor vehicle collisions. However, complaints of soft tissue neck injuries in low-speed rear end collisions remain a significant problem.
HUMAN SUBJECTS vs. ANTHROPOMORPHIC DUMMIES
The earliest study of occupant dynamics in low-speed rear impacts was published by Severy et al. (2). This study of five staged rear-end collisions resulted in speed changes to the struck vehicle of 8.4 and 8.8 km/h with human subjects. The other three tests, resulting in speed changes of 5.7, 9.5 and 16.2 km/h, involved anthropomorphic dummies. Comparison of the head acceleration curves for the human and dummy at similar impact severities revealed significant differences in the induced response. The dummy response curves tended to have higher and more abrupt peaks than the human response curves. This lack of biofidelity of the anthropomorphic dummy neck has continued to plague research into the possible mechanisms of injury resulting from low-speed rear impacts (3, 4). Even the newer Hybrid III test dummy lacks sensitivity to the ∆V [change of velocity] both in angular excursion and rotational accelerations, and appears to be a poor human surrogate for predicting whiplash injury (3). In the field of human testing, utmost caution is the order to prevent injury to human test subjects, usually by conducting impacts at velocities below presumed injury thresholds.
EARLY INVESTIGATIONS OF WHIPLASH MECHANICS
The early studies by Severy (2) noted that forces acting on the struck vehicle caused it to be accelerated to its maximum speed within approximately 200 milliseconds. Movement of the test subject's head did not begin until the acceleration of the vehicle was almost complete. However, the subject's torso, which was in integral contact with the seat back, began its forward movement shortly after the initial contact between the vehicles. In a rear-end “whiplash” event, the mass of the head atop a flexible neck allows the head to be whipped first rearward and then forward relative to the torso. Injuries were postulated to occur either as a consequence of hyperextension and/or hyperflexion of the neck.
In 1967, Mertz and Patrick (5) tested a human [only one], cadavers, and anthropomorphic dummies in order to establish an injury mechanism in low-speed rear impacts. An acceleration/deceleration sled was utilized for the impact tests, which assessed impact severity, seat back rigidity, and head restraints effectiveness. However, the response of the sled's seat back under impact conditions would not have necessarily simulated the response of an automobile seat, nor of a real world vehicle-to-vehicle collision. The dynamics experienced by the two anthropomorphic dummies used by Mertz and Patrick were observed to correspond poorly with each other, with the cadaver test subjects, and with the single human test subject. The cadavers were observed to have similar response characteristics to each other, although again their motion corresponded poorly with that of the human subject. They did note that head-restraints were effective in reducing injury exposure, and that higher head accelerations were produced when the head was farther from the head-restraint. They concluded that the critical factor in the causation of neck injury in low-speed rear impacts was cervical torque rather than shear or axial forces acting on the cervical spine. Work of this nature lead to the introduction of mandatory head restraints in North American vehicles and has encouraged the use of properly adjusted head restraints.
A subsequent study (6) reported on the flexion and extension characteristics of the human cervical spine. They proposed a voluntary response envelope for torque, measured at the occipital condyles, as a function of angle of cervical extension. It was concluded that cervical torque, rather than angle of cervical extension was a better measure of injury exposure. They found that once the limits of the normal range of motion of the neck were reached, large increases in torque occurred for small increases in the angle of extension.
Despite the finding that properly adjusted head-restraints would prevent or reduce cervical hyperextension from occurring in low-speed rear impacts, whiplash injuries continued to be reported based on tests conducted with anthropomorphic dummies. A differential rebound theory proposed that the occupant's torso began to rebound from the seat back while the head was still moving rearward. This forward movement of the torso would then allow the neck to move into hyperextension, regardless of the height of the headrest. However, recent tests conducted using human test subjects have revealed that this differential rebound phenomenon does not occur.
RECENT INVESTIGATIONS OF WHIPLASH
Recent investigations have involved a variety of male and female subjects ranging in age from 22 to 63 years who experienced impact speed changes ranging from 2 to 16 km/h (3, 7–14). These studies considered subjective evaluation of the test subjects along with high speed cinematography, videography, accelerometers, magnetic resonance imaging, cineradiography and electromyography [Table 1].
These studies have provided a good understanding of rear-end occupant dynamics, but still failed to answer the critical question on exact injury mechanismfs]. However, they revealed a possible threshold above which normal human test subjects may experience some minor transient whiplash associated symptoms. Although cervical hyperextension is a probable injury mechanism which may occur in higher severity impacts where adequate head support is not available, it does not appear to be the sole mechanism of injury. In impacts where the struck vehicle's speed change is less than 8 km/h, the range of cervical motion is generally confined within the subject's normal range, regardless of the degree of support afforded by the headrest. However, in all of the tests for speed changes up to and, in some cases, significantly in excess of 8 km/h, if symptoms were experienced, they were minor and resolved spontaneously within hours to days. A speed change of 8 km/h may be a threshold below which injury is not expected.
In a low-speed rear impact, the collision forces cause the struck vehicle to be rapidly accelerated, usually from a stopped position. Under the idealized conditions used for testing with impact sleds, the sled is accelerated entirely along the longitudinal axis. However, in vehicle-to-vehicle impacts, the contacting surfaces of the two vehicles are not always flat and perfectly aligned. This can give rise to additional accelerations in the vertical plane which may effect occupant dynamics.
TABLE 1. Studies of Low-Speed Rear End Collisions Using Human Subjects | Author | Test Subjects ∆V km/h* | Impact | Assessment |
West
(7) | 4 males | 2to16 | Subjective, accelerometers, videography. |
| ages 25 to 43 years | | |
McConnell
(8) | 4 males | 3.0 to 8.1 | Subjective, accelerometers, high speed |
| ages 45 to 56 years | | cinematography. |
Siegmund
(13) | males and females | 1.7 to 8.8 | Subjective. |
| ages unknown | | |
Szabo
(9) | 3 males, 28 to 48 yrs | approx. 8 | Subjective, accelerometers, pre-post-test MRI.** |
| 2 females, 27 to 58 yrs | | |
Segmund
(14) | 2 males, | 5.8 to 7.7 | Subjective, amusement bumper cars. |
| 25 and 32 years | | |
Ono
(10) | 3 males, 22 to 43 yrs | 4 to 8 | Subjective, impact sled, electromyography, |
| | | accelerometers, videography. |
Rosenbluth
(11) | 1 male, 63 yrs | 3.3 to 7.8 | Subjective, accelerometers. |
| 1 female, 55 yrs | | |
Matsushita
(12) | 15 males, 22 to 61 yrs | 2.5 to 5.0 | Subjective, electromyography, accelerometers, |
| 3 females, 24 to 57 yrs | | high speed cinematography and cineradiography. |
Scott
(3) | 1 male, 50 years | 3.9 to 7.8 | Subjective, accelerometers, high speed cinematography. |
* ∆V change o...
