Relationship of head restraint positioning to driver neck injury in rear-end crashes

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Abstract

Insurance claims were examined for evidence of neck injuries to drivers of passenger cars struck in the rear. Neck injury rates were significantly lower for male drivers, elderly drivers, and drivers in less severe crashes. Even after accounting for differences in driver demographics and crash severity, neck injury rates were significantly lower for drivers of cars with head restraints that were more likely to be behind the heads of motorists.

Introduction

According to data from the National Highway Traffic Safety Administration (NHTSA), there are more than 3 million rear-end crashes in the United States each year, less than half of which are reported to the police (Knipling et al., 1993). Among the police-reported rear-end crashes, approximately 35% involve bodily injuries (NHTSA, 1997). Crashes not reported to the police are less likely to involve injuries. However, it is not unusual for injuries from rear-end crashes, especially soft-tissue neck injuries, to go unnoticed until several hours or days after the crash.

The term whiplash was first used by Dr Harold E. Crow in 1928 to describe the sudden jolting of the head and neck in automobile crashes. deGravelles and Kelley (1969) described the mechanism as a hyperextension of the neck due to the head and neck lagging behind when the body is suddenly pushed forward by the seat back, followed by hyperflexion as the body surpasses the speed of the decelerating car and the head snaps forward. The designation of ‘whiplash-associated disorders’ encompasses a group of complaints characterized by a variety of symptoms, such as pain and stiffness in the neck, that often lacks any external or radiological evidence (Spitzer, 1995). The pathology accounting for whiplash symptoms has been and continues to be the subject of considerable medical interest, investigation and debate.

Braunstein and Moore (1959) reported that 8% of all occupants exposed in rural highway rear-end collisions sustained whiplash injuries—8% of drivers, 14% of right-front passengers, and 6% of rear-seat passengers. Kihlberg (1969) reported that the risk of flexion-torsion neck injury was significantly higher among front-seat passengers than among rear-seat passengers, and twice as high among women as men.

In the mid-1950s, padded head restraints attached to the top of automobile seat backs were proposed as countermeasures to the hyperextension phase of whiplash (Ruedemann, 1957). Subsequent research led to federal mandates for automobile head restraints in the 1960s (Severy et al., 1968). Federal Motor Vehicle Safety Standard (FMVSS) 202 requires that all passenger cars manufactured for sale in the United States after December 31, 1968 include head restraints in the front outboard seating positions. The head restraints must extend at least 27.5 inches above the seating reference point in their highest position (measured parallel to the plane of the torso) and not deflect more than 4 inches under a 120-pound load (see Fig. 1). FMVSS 202 also applies to light trucks manufactured after August 31, 1991.

Evaluations of the effectiveness of head restraints subsequent to FMVSS 202 have ranged from a 13% reduction in injuries to an 18% reduction. In a study of 5663 drivers of passenger cars struck in the rear, O’Neill et al. (1972) reported neck injury incidence rates of 29% in cars without head restraints and 24% in head restraint-equipped cars. States and Balcerak (1973) interviewed 439 drivers and right-front passengers of cars struck in the rear in Rochester, New York, during the first 3 months of 1972. A total of 43% of those in cars without head restraints reported neck injuries compared with 37% in cars with head restraints. Kahane (1982) analyzed Texas crash files from 1972, 1974 and 1977 and compared rear-impact driver injury rates for cars of model year 1968 with those of model year 1969. Of the more than 20 000 drivers in each group, 7.6% in model year 1968 cars were injured compared with 7.0% in 1969 models, an 8% reduction. After adjusting for the fact that ≈12% of 1968 models had head restraints and ≈12% of 1969 models did not, the effectiveness of head restraints was estimated to be a 13% reduction in injuries—17% for fixed head restraints and 10% for adjustable head restraints.

The 27.5 inch height required by FMVSS 202 is based on the conclusion of Severy et al. (1968) that front-seat occupants ‘are adequately protected from rear-end collision injuries for striking car speeds through 30 mph if they have an adequately structured 28-inch seatback…’ However, adjustable head restraints in the lowest position may be several inches below this optimal height (Kahane, 1982). A number of studies have reported that more than 70% of drivers leave their adjustable head restraints in the down position (O’Neill et al., 1972, States and Balcerak, 1973, Viano and Gargan, 1995).

Head restraint regulations in Europe are more stringent. According to Directive 96/37 of the Council of the European Communities (1996), head restraints must extend at least 75 cm (29.5 inches) above the seating reference point in their lowest position and at least 80 cm (31.5 inches) above the seating reference point in their highest position. These requirements are consistent with conclusions of an international group of neck injury experts that met in Lyon, France in 1994—that head restraints should be as high as the top of the head of an average-size male (Tarrière, 1995).

Even a head restraint that is sufficiently high may not prevent whiplash injuries, if the horizontal distance between the head and the head restraint is too great. A study in Sweden of 33 front-seat occupants of rear-struck Volvo cars reported more serious neck injuries among those more than 10 cm (4 inches) in front of the head restraint (Olsson et al., 1990). Svensson (1993), after a series of rear-end collision sled tests, reported much smaller angular displacements of the dummy’s head when the horizontal distance was 4 versus 10 cm. Consistent with this finding, the experts meeting in Lyon also recommended that head restraints should be as close to the back of the head as possible (Tarrière, 1995).

A number of studies have reported that adjustable head restraints are often improperly positioned by drivers (O’Neill et al., 1972, Nygren et al., 1985, Viano and Gargan, 1995, Cullen et al., 1996). Therefore, a comparison of adjustable head restraint designs must take into account the positioning of the head restraint relative to the driver’s head both in the adjusted and unadjusted situations.

Estep et al. (1995) evaluated 164 model year 1995 passenger cars and passenger vans according to the vertical and horizontal proximity of the head restraint to the head of an average-size male. Measurements were obtained using a specially designed head form mounted on a standard H-point machine. Head restraints were rated as poor if, in the lowest position, they were more than 10 cm below the top of the head or 12 cm or more behind the head and, in the highest position, were more than 8 cm below the top of the head or 10 cm or more behind the head. More than 70% (117) of the vehicles received poor evaluations. It is important to note that all these head restraints complied with current FMVSS 202 requirements. However, their geometric relationships to the head of an average-size male were far below the recommendations made at the Lyon meeting (Tarrière, 1995).

The purpose of the present study was to determine the relationship between head restraint positions and driver neck injury rates in a large sample of rear-end crashes. Unlike many studies, this study was not based on a sample of injured drivers, but rather on a sample from all rear-end property damage liability claims involving a selected group of cars. Information was taken from the files of State Farm Insurance Companies, which account for over 20% of all personal automobile insurance premiums paid in the United States. The present study also controlled for other factors that may contribute to the likelihood of neck injury, such as crash severity and driver age and gender.

Section snippets

Method

In April 1997, identifying information for all rear-damage property damage liability claims (in tort and add-on tort liability states) and rear-damage collision coverage claims (in Michigan) dated January 1, 1993 through September 30, 1996 was extracted from the electronic database of the State Farm Mutual Automobile Insurance Company. The database included information on the state in which the claim was filed and the vehicle identification number (VIN) of the rear-struck vehicle. The study was

Results

Neck injury claim rates were significantly lower in Michigan than in tort and add-on states, 13 versus 26%. This difference held for both male and female drivers (Table 1). Also, female drivers were significantly more likely to claim neck injuries than male drivers, 18 versus 7% in Michigan, and 30 versus 23% in tort and add-on states.

In the tort and add-on states, drivers age 65 or older were significantly less likely to claim neck injuries than drivers younger than age 65 (Table 2). Only 13%

Discussion

Neck injury claims were only half as prevalent among the Michigan property-damage claims as among the property-damage claims filed in tort and add-on states. This was true even though the rear-struck cars in Michigan were more likely to have been severely damaged (27%) than the rear-struck cars in the tort and add-on states (19%).

In the tort and add-on states, 30% of female drivers and 23% of male drivers reported neck injuries. Thus female drivers in the study were approximately 30% more

Acknowledgements

The authors wish to acknowledge the cooperation of the State Farm Mutual Automobile Insurance Company throughout the study. Special thanks are due to the State Farm employees, especially Steve Roberson, Karon Bakel, Dave Eaton, and Ken Holcomb, and to Mike Cammisa of the Insurance Institute for Highway Safety and Patrick Creegan of the Highway Loss Data Institute. This work was supported by the Insurance Institute for Highway safety.

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