Deceleration during ‘real life’ motor vehicle collisions – a sensitive predictor for the risk of sustaining a cervical spine injury?
From: Patient Saf Surg. 2009 Mar 8;3(1):5. [Epub ahead of print]
Whiplash injuries remain a barely understood phenomenon. The economic damage caused by whiplash amounts to some 10 billion Euros a year in Europe and 29 billion US Dollars a year in the USA. As whiplash occurs as a result of motor vehicle accidents, questions inevitably arise regarding who is liable for these costs.
Biomechanical considerations have been based on the assumption that damage to a given material only occurs when the energy that acts on this material is high enough. Thus, energy doses below a defined threshold have been considered harmless. In this context, the parameter delta v (DV), which describes the velocity change of a motor vehicle during a collision with another vehicle, has become a widely accepted criterion for the energy that acts on the vehicle during a collision.
In numerous sled or car crash test studies, volunteers were subjected to acceleration forces in order to define a threshold below which a cervical spine injury could be excluded. The results of these studies are rather inconclusive and sometimes contradictory. Thus the scientific community has not yet reached consensus regarding the threshold value for cervical spine injuries after whiplash. Nonetheless, DV threshold values were adopted very early in the history of insurance law as a criterion to accept or deny the claim settlement for whiplash associated disorders.
Up until now, all volunteer crash test studies precisely defined the subject’s sitting position. While waiting for the collision, the subjects maintained an upright body and head position, with an optimally adjusted headrest. It is obvious that the reallife sitting position in traffic may significantly differ from this laboratory position in one or several points. Furthermore, an increased risk of injury has been observed for various factors such as the seat and headrest settings, the distance between head and headrest, the head rotation, and the collision type. The inherent variability of these factors makes it unclear how easily the results from laboratory crash tests can be transferred to real-life accident situations. In order to elucidate these issues, this study analyzes the correlation between DV and cervical spine injuries in real life accidents and questions whether DV is a valid predictor for cervical spine injuries following whiplash.
This study provides evidence that, in real life accidents, cervical spine injuries may occur at low DV values, while it is possible to escape unscathed from collisions with high DV values. In particular, the correlation between DV and the occurrence of whiplash associated disorders was very low for any of the collision types. Therefore it is impossible to make meaningful statements about the existence of whiplash associated disorders based solely on assessment of the DV value. This finding might be of importance for the surgeon’s assessment and patient’s safety after a car accident. Diagnostic and therapeutic management should not be based solely on information related to trauma impact.
The results of the present study support the findings of numerous sled and carcrash experiments. In those experiments, neck problems were noted after rear end collisions with DVs as low as 7 km/h. In four other studies, neck problems occurred at a DV < 10 km/h. The neck problems were defined as QTF grade I and QTF grade II whiplash associated disorder, persisting from hours to several weeks in all studies. In contrast, four studies reported rear end collisions with DV values of 13.1 km/h to 50 km/h where the occupants escaped without any signs of injury. In other crash test studies, frontal impacts at DV less than 12 km/h caused no injuries. However, different findings were obtained in our study and in a study that performed a retrospective analysis of 24 real-life frontal collisions. In that study, 18 of the 24 subjects were classified as QTF grade II whiplash associated disorder. It is noteworthy that 8 of these had neck problems for more than one year. The DVs in these cases ranged from 3 km/h to 23 km/h. The authors also reported that one subject suffered a prolapsed disk at C5/6 at a DV of 11-15 km/h. The occupant had not been wearing his seat belt and the airbag had deployed. He also had a frontal laceration as a sign of direct head impact. It was assumed that these factors caused the structural injury of the cervical spine at a low DV. The occurrence of structural injuries at DV values of less than 20 km/h had been considered improbable in expert discussions. However, we also observed a luxation fracture at C5/6 resulting from a frontal collision at a DV of 15 km/h and a facet joint fracture at C4 due to a side collision at a DV of 10 km/h. Both occupants had been wearing their seat belts, there had been no head contact, and the airbag had not deployed. In both cases, it is unclear which factors, either alone or in combination, were responsible for these structural injuries at considerably low DV.
In accordance with other studies, these results are indicative that multiple factors may influence the risk of injury in each individual case. Due to the additive effects of various protective factors, high-energy impacts may be absorbed without injury, while the additive effects of unfavorable factors could explain injuries sustained in low-energy impacts. Some factors have been described to influence the risk of injury, such as sex, head position, sitting position, distance between head and headrest and seat construction. The duration of the crash pulse is also thought to significantly contribute to the risk of cervical spine injury. These authors stated that an earlier acceleration peak during deformation of the colliding cars was correlated with a higher probability of cervical spine injury. However, it remains unclear to what extent each one of these factors influences the risk of cervical spine injury.
The current data exclude the assumption of a linear correlation between DV and the risk of suffering a whiplash injury. It is tempting to speculate that the development of a cervical spine injury after whiplash is more like a complex system such as those described in chaos theory. Complex systems cannot be simplified into linear correlations. Even small variations of the initial conditions can affect the end result so that it is no longer predictable, such as in the case of the “butterfly effect”: the flapping of a butterfly’s wings can ultimately result in a different weather pattern. Taken together, it can be concluded that DV is an irrelevant predictive value for cervical spine injury after a motor vehicle accidents. Nevertheless further studies will be necessary to evaluate the development of pain chronification in dependence of the DV to investigate its possible predictive value as “long-term” parameter.
The DV value as measured in the trauma impact does not represent a conclusive predictor for cervical spine injury in real life motor vehicle accidents. This could be important for surgeons and patients in their medicolegal assessment of whiplash associated disorders .
