Effects of Head Restraint Position on Neck Injury in Rear Impact
This paper was presented by Emily Sun at the Traffic Safety and Auto Engineering Stream of the Whiplash-Associated Disorders World Congress
7-l 1 February 1999, Vancouver, Canada

Introduction
Whiplash injuries of the neck are one of the most common injuries reported from automotive rear impacts. Although these injuries are classified as minor (AIS I), their high incidence rate and often longterm consequences lead to significant societal costs. Based on data collected in the National Automotive Sampling System Crashworthiness Data System (NASS CDS), it has been estimated that more than 740,000 whiplash injuries occur annually[ I]. Assuming an average cost of more th& $6,000 per injury (including medical, legal, insurance, loss of productivity, and loss of work), the total annual monetary cost of whiplash in the United States is roughly $4.5 billion.

Since January 1, 1969, passenger cars have been required by Federal Motor Vehicle Safety Standard (FMVSS) No. 202 to provide head restraints that meet specified requirements for each designated front-outboard seating position. In 199 1 this standard was extended to light trucks and vans, multipurpose passenger vehicles (MPVs), and buses with a gross vehicle weight rating (GVWR) of 10,000 pounds or less. The standard requires that either of two conditions be met:

1. During a forward acceleration with a maximum value of 8.0 to 9.6 g over a duration of80 to 96 msec on the seat supporting structure, the rearward angular displacement of the head reference line shall be limited to 45E from the torso reference line; or
2. The head restraint must measure at least 27.5 inches above the seating reference point, with the head restraint in its fully extended position. The width of the head restraint, at a point 2.5 inches From the top of the head restraint or at 25 inches above the seating reference point, must not be less than 10 inches for use with bench seats and 6.75 inches for use with individual seats. The head restraint must withstand an increasing rearward load until there is a failure of the seat or seat back, or until a load of 200 pounds is applied. When the load is such that the applied moment is 3300 inch-pounds, the portion of the head form in contact with the restraint must not exceed a rearward displacement (perpendicular to the extended torso reference line) of 4 inches.

Condition 2 is almost universally used by the automotive industry, leading to a head restraint design based on geometric position rather than dynamic performance.

In 1982, NHTSA estimated the effectiveness of head restraints in reducing the overall risk of injury in rear impacts at 17% for integral head restraints and 10% for adjustable head restraints [2]. The effectiveness of adjustable head restraints may be lower most likely because they are frequently left in the down position.

Whiplash injuries were originally thought to be caused by hyperextension of the neck as the head rotated rearward over the seat back. However, recent studies by McConnell ef al. [3] reported that some healthy middle-aged male subjects exposed to low speed rear impact of 4-8 kph experienced transient, mild cervical strain without exceeding the normal voluntary range of motion. Despite numerous studies being conducted on human volunteers, cadavers, and animals, no consensus has been reached on this difficult issue although several new theories have been proposed. Bogduk er al. [4] have isolated pain from whiplash to the facet capsules. Ono et al. [5] have observed that torso ramping causes compressive loading on the cervical spine, causing the lower vertebral segments to undergo motions beyond the normal physiological range. Svensson et al. [6] have investigated the effects of localized flexion and extension on the fluid pressure within the spinal canal. Common symptoms of whiplash injury include neck pain, headaches, blurred vision, tinnitus, dizziness, concussion and numbness[7]. Some of these symptoms are consistent with damage to the cervical muscles, ligaments and vertebrae while others are more difficult to explain since there are no lesions present on x-ray, CT scan or MRI.

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• Case Study: Pedestrian Accident Analysis
• Effects of Head Restraint Position on Neck Injury in Rear Impact
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