The effect of crash pulse shape on AISl neck injuries in frontal impacts

Crash data from real-life frontal car collisions, where the crash pulses have been measured with crash pulse recorders and where the influence of pulse shape on the risk of both shortand long-term disability from AIS 1 neck injuries, have been studied. The risk of long-term consequences was especially influenced by the shape of the crash pulse. To understand how the shape of the crash pulse affected occupant motion, a series of computer simulations of frontal impacts were conducted where the inforrnation from the crash pulse recorder data has been used in the simulations. Several dummy response parameters, such as neck loads and accelerations, were compared with the injuries in 143 real-life collisions using the recorded crash pulses and the injury records. The results showed that for a specific change of velocity the pulse shape could significantly influence some of the dummy response parameters, such as angular head acceleration and neck bending moments. lt was also found that there was a correlation between these dummy response parameters and the long-term consequences. The results may help to explain the injury mechanism of the AIS 1 neck injury in frontal impacts and may have implications on the design of the seat belt system. THERE IS ONE INJURY that has increased substantially, both in terms of risk and in number (Morris and Thomas 1996, Krafft 1998). In Sweden between 1990 and 1995, it has been shown that approxirnately 60% of the injuries causing a disability were AISl neck injuries (Krafft 1998). Nygren (1984) found that 10% of the occupants in rear impacts and 5% of the occupants in frontal impacts that reported initial whiplash symptoms suffered disability at least one year after the collision. In most research on AIS 1 neck injuries, only the initial symptoms have been studied. lt is, however, important to separate the analyses for initial and residual symptoms since there may be different injury types and different injury mechanisms (Krafft 1998, Kullgren 1998). · Injuries to the neck are often regarded as a problem in rear-end impacts. However, about 113 of the AIS l neck injuries occur in frontal impacts (Galasko et al. 1993, Krafft 1998). The AIS l neck injury mechanisms in different collision modes are still not known. Different hypotheses exist concerning injury mechanisms in rear-end irnpacts, attributing the injury to the flexion (v Koch et al. 1995) or the extension (McConnell et al. 1995) motions of the neck. In frontal collisions, Larder et al. (1985) found that no head contact with the interior of the vehicle compartment had been noticed, thus the forward flexion of the neck was assumed to be the injury-causing motion. Walz and Muser (1995) and Ewing et al. (1975) described the motion of the head relative the neck in a frontal collision with no head contact. For a restrained occupant, the initial phase of a collision, results in a purely translational head motion producing a S-shape of the cervical spine followed by flexion of the neck. Walz and Muser (1995) suggested that neck injury rnay occur in this predominantly inertial loading of the head and neck. IRCOBI Conference Sitges (Spain), September 1999 231 Several studies show that the neck injury risk is associated with seat-belt use (Larder et al. 1995; Otremski et al. 1989). Galasko et al. ( 1993) found an increase in neck injuries from 8% to 21 % associated with an increase in belt wearing rates in the UK. In the studies mentioned above, it is unknown if the increase occurred for all impact directions. Regarding impact severity, studies ha ve shown a correlation between change of velocity and initial neck injury symptoms in rear impacts (v Koch et al. 1995, Ryan et al. 1994). However, Krafft ( 1998) found that a higher change of velocity of the struck cars did not increase the risk of long-term consequences compared to short-term consequences. The results indicated that the acceleration levels seemed to influence the risk of long-term consequences to a higher extent. Olsson et al. ( 1990) also found sirnilar results. Furthermore, results from real-life rear-end impacts where the crash pulse has been measured with a crash pulse recorder indicate that acceleration levels seem to better explain the neck injury risk than change of velocity (Krafft et al. 1998, Krafft 1998). In frontal impacts, Kullgren et al. ( 1999) has found that the shape of the crash pulse particularly influences the risk of long-term consequences to the neck. The study by Kullgren et al. was based on results from real-life impacts where the crash pulse was measured with a crash pulse recorder. Crash pulse characteristics were related to the risk of both shortand long-term consequences to the neck. The crash pulses were divided in intervals of 33 ms: LlvJ 1-33 ms, Llv2 34-66 ms, Llv3 67-99 ms, etc. lt was found that a !arge change of velocity in the second part of the pulse followed by a smaller velocity change in the third part, resulted in a high risk of long-term consequences. In Figure 1 it can be seen that the mean crash pulse for the impacts with occupants that sustained long-term consequences differed significantly from the impacts with occupants that sustained short-term consequences and the occupants that did not report an AIS 1 neck injury (Kullgren et al. 1 999). lt is also important to notice that the mean pulse for the impacts with occupants that sustained short-term consequences was almost the same as for the occupants that did not report an AIS 1 neck injury.