Report investigating the importance of head restraint positioning in reducing neck injury in rear impact

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Abstract

Neck injury resulting from rear impact (often known as whiplash) is a serious cause of road trauma. It is often underestimated or overlooked because such injuries are minor on traditional injury scales but can result in long term pain and disability. The paper begins with a brief review of research into head restraints and whiplash done so far. A review of international head restraint regulations revealed the absence of any horizontal offset requirements. A review of seat strength requirements and testing procedures showed that a regulation that required a collapsible seat would involve significant compliance testing. This paper concludes a preliminary project conducted by the Federal Office of Road Safety (FORS) where the head restraints for twenty Australian market vehicles were assessed using known performance criteria. A key finding of the report was that most of the vehicles allowed for vertical adjustment of the head restraint. Also important was that none of the vehicles measured allowed horizontal adjustment and on some of the head restraints the horizontal displacement increased as the vertical height increased. As the understanding of neck injury mechanisms in rear impact develops, there may be some scope for FORS to facilitate the improvement of these standards. Further research into neck injury mechanisms may reveal yielding seat backs or new ‘active’ head restraint technology as a more effective countermeasure. In the meantime, educating occupants to correctly adjust their head restraints seems to be an effective way to reduce injuries in existing vehicles.

Section snippets

Head restraint history

Whiplash or Whiplash Associated Disorders may go unreported or not gestate until well after a crash. They do not show up in police/coroner reports because they are frequently not the cause of death and cannot be proven without dissection of the neck. The severity of neck injury may be reduced by head contact with the head restraint, making head restraint position and adjustment an important safety issue.

The installation of head restraints in the front seats of Australian vehicles has been

Head restraint measurement device

It has been widely accepted that the sooner the head contacts the head restraint the less chance there is of serious neck injury (States, 1972, Cameron, 1980, Meeting of Experts of Passive Safety, 1995).

This was the basis for the head restraint performance criterion, namely the relative vertical and horizontal distances from the standard head position of a 50th percentile adult male. Two performance criteria suggested for evaluation of vehicles with the Head Restraint Measurement Device have

Results and analysis

The results of the head restraint measurements are summarised in the Table 1 below.

The first column specifies the make and model of the vehicle. The second column specifies the head restraint type, either fixed or adjustable.

The next four columns assess the head restraints in their lowest rearmost position. First, the vertical and horizontal measurements taken using the Head Restraint Measurement Device are listed, and then the IIHS and ICBC criterion resulting from the measurements are listed.

Discussion

The measured vehicles represent a majority of the vehicles sold in Australia. The head restraint types also represent the majority available across the Australian fleet, i.e. fixed or adjustable in the vertical direction only. All the vehicles showed a significant improvement in position if adjusted. However, any possible benefit offered is not achieved if head restraints are maintained in the lowest rearmost position.

In interpreting the results, several important points need to be reinforced.

Conclusion

A properly adjusted head restraint will prevent excessive extension of the neck of a properly positioned occupant in a rear impact collision. However, head restraints are not as effective at reducing soft tissue injuries in the neck as seat back yielding. Seat back yielding appears to be the most effective means of reducing AIS 1 neck injuries. Further investigation is needed to explore the implementation of these injury-mitigating technologies.

In the study undertaken, adjusting head restraints

Future work

FORS intends to continue research into opportunities for improving head restraint effectiveness especially in areas such as horizontal offset and seat back yielding. There is also still a lack of knowledge and understanding of neck injury mechanisms, this is the major obstacle in producing an effective countermeasure.

FORS will participate in the international debate on the direction neck injury mitigation should take. ADR 22 is currently being reviewed with a view to aligning it fully with

Acknowledgements

This project would not have been completed without the assistance of several people. Keith Seyer and Craig Newland for their assistance in editing the paper. Also Mark Terrell, Matt O’Keefe and Stephen Stratton for their assistance with the head restraint measurements.

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