Whiplash injury — are current head restraints doing their job?

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Abstract

It is generally accepted that the incidence of whiplash associated disorders is increasing in all industrialised countries, despite the almost universal fitment of head restraints in at least the front seats of cars. This is usually attributed to the fact that few people can be observed to follow the standard recommendations as regards head restraint positioning, that is, level with the head vertically and as close to the head as possible horizontally. This study set out to determine whether any other factors, in addition to head restraint adjustment, could be found which would influence the severity of whiplash injury. This was done by linking medical assessment of real-world accident victims with engineering assessment of the accident vehicles. A random sample of road accident victims suffering from whiplash associated disorder was studied. The vehicles they had been travelling in were examined to assess impact severity and, where possible, measurements were made of seat and head restraint adjustment with the subject sitting in the vehicle. All subjects were interviewed to assess the disability resulting from their injuries, and their progress was followed for 12 months. The results were subjected to statistical analysis to try to determine relationships between severity of injury (as measured by resultant disability) and a number of occupant- and vehicle-related factors. A significant proportion of the sample had suffered lumbar strain injury in addition to whiplash, and these were excluded from the present analysis. Frontal impact victims suffered symptoms indistinguishable from those of rear impact victims. The beneficial effects of good head restraint adjustment could not be clearly demonstrated, and some trends, especially in rear impacts, where the benefits of a well-adjusted restraint should have been very clear, indicated that larger distances from head to restraint were associated with lower disability. The paper discusses these counter-intuitive results and their implications.

Introduction

Soft-tissue cervical sprain injuries (frequently referred to as ‘whiplash’ injuries), though generally only classified as 1 on the Abbreviated Injury Scale, have been highlighted as having long-lasting and disabling effects. Spitzer et al. (1995) have proposed the term ‘Whiplash Associated Disorder’ (WAD) for the range of symptoms associated with this injury, which can include pain in the neck or shoulders, headaches, blurred vision, tinnitus, dizziness and numbness in the upper limbs (Bogduk, 1986). In some cases these can persist for years (Murray et al., 1993). Hopkin et al. (1993) showed that over half of all car occupants involved in road accidents had these injuries. Nygren (1984) showed that, despite their low AIS rating, neck strain injuries lead to permanent disability in some 10% of cases. By comparison, the risk of permanent disability associated with other AIS 1 injuries is only about 0.1% (Nygren et al., 1985).

In a rear impact, involving a rapid flexion–extension motion of the neck, it is the extension phase which is usually taken to be the most injurious. Hence, it has been postulated, and widely accepted, that the provision of a head restraint which will prevent rearward hyperextension of the neck will prevent the occurrence of whiplash injuries. However, it has been widely reported (Maag et al., 1990, Foret-Bruno et al., 1991, von Koch et al., 1995, Morris and Thomas, 1996) that whiplash-type injuries can also occur in frontal and side impacts, where rearward hyperextension of the neck is presumed not to be a major factor, if it occurs at all. Lövsund et al. (1988) have shown that the risk of incurring WAD is higher in rear impacts. However, Morris and Thomas (1996) have shown that frontal impacts actually produce greater absolute numbers of WAD victims because the number of frontal impacts which occur is much greater.

Previous prospective studies involving long-term medical follow-up coupled with detailed examination of the damaged vehicle include that of Olsson et al. (1990). In a study of rear-end collisions in Volvo cars, they found no correlation between impact speed and either the initial spectrum of, or the duration of, symptoms. However, a significant correlation was found between duration of symptoms and horizontal distance between head and restraint, with a distance greater than 10 cm being associated with symptoms lasting at least a year, as opposed to less than a year.

Ryan et al. (1994) followed the progress of 32 WAD victims for 6 months. Vehicles were examined to assess crash severity but seat/head restraint parameters for the occupants were not measured. Correlations were found between injury severity and both velocity change and maximum vehicle deformation, particularly for rear impacts. Victims who had been aware of the impending impact had significantly better outcomes than those who had been unaware, and awareness was the only factor to show any effect on long-term (6 months) outcome.

Section snippets

Experimental procedure

Any patient presenting at the Accident and Emergency department of a large hospital in the Manchester area with a ‘whiplash’ injury as a result of a road traffic accident was considered for inclusion in our Whiplash/Vehicle Study (WVS). Other injuries at the level of cuts and bruises were allowed, but any injury with an AIS>1 resulted in exclusion from the study. Casualty records at the hospital were examined on a daily basis to identify possible recruits, who were then invited to join the

Results and analysis

Of those whose restraint distance measurements were known, two were leaning forward, out of their normal seating positions, and had very large horizontal head to restraint distances, but very low disabilities. Unusually low disabilities were also found amongst such people in the lumbar sub-group and amongst those who did not complete the full 12 month follow-up. They were excluded from analysis, leaving sample sizes at the three assessments of 94, 113 and 132. In the initial sample of 94

Conclusions

The well-documented increasing incidence of neck strain injuries shows that current seat and head restraint designs are failing to have the desired effect in reducing WAD incidence, and the conventional wisdom holds that this is, to a large extent, due to incorrect use of those items by vehicle occupants. Our study did not address WAD incidence, only the severity of a whiplash injured population. However, measures found to reduce injury severity normally also have a beneficial effect on

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