Introduction

In today’s rigorous and highly contentious medicolegal climate, risk assessment can add substantially to the overall effectiveness of your overall case management and to the credibility of your opinions. When it comes to assessing the risk for injury of any individual occupant exposed to a low velocity rear impact collision, probably the most deterministic factor related to the vehicle itself is the head restraint geometry. This term considers two fundamental dimensions: topset and backset, Figure 1. When the backset exceeds 2 inches, the shear loads which occur during the initial phase of whiplash—which is sometimes referred to as the retraction phase—increase. It takes only about 80-100 milliseconds (about a tenth of a second) for the head to make contact with the head restraint and, by that time head contact occurs, an injury may have already occurred. Thus, head restraint geometry is one of the most critical issues for researchers today.

Figure 1. Head restraint geometry is a function of two dimensions: topset and backset.

As evidence of this, consider that the newly proposed neck injury criterion (NIC) is chiefly affected by head restraint geometry. It is a measurement of the relative acceleration and relative velocity of the head and T1 segment. When the head and torso move as a single unit, the NIC is very low and that’s a good thing. When there is a lot of head lag during the retraction phase, the differential acceleration and velocity between head and torso can be quite high and that’s a problem. Despite some drawbacks, the NIC is currently a top contender for a new crash criteria for whiplash protection 1. Also, beginning in 2008, a new regulation will go into effect requiring better overall head restraint geometry in cars sold in the U.S., including a backset not to exceed 2 inches.

As further evidence of the huge importance of head restraint geometry, the Insurance Institute for Highway Safety (IIHS) currently tests head restraint and seat back geometry and function as an indicator of potential injury risk. They started this program back in 1995 using a seat phantom which had a dummy-like structure and was designed to allow the institute to accurately measure head restraint geometry. Part of their “name and shame game,” the idea was to shame manufacturers who did not take safety as seriously as they should have. The reason these restraints are still a problem today is that when head restraints became mandatory in 1969 as part of a new Federal Motor Vehicle Safety Standard, there was no provision for their actual performance. They simply had to be there. Translation? You could literally fill a casing of seat cloth with cotton balls and call it a head restraint. American manufacturers are the most blatant in this (dis)regard and, believe me, it is not simply a matter of ignorance. So the IIHS’ idea was to report the head restraint geometry as being either “good,” “acceptable,” “marginal,” or “poor,” and post it on their website with the hope that manufacturers would see the light and take head restraints more seriously. In the program’s inaugural year, 1995, only 3% of the cars tested by IIHS got “good” ratings. Even today it’s risen only to about 50%. And, of course, because of the higher speed crash tests cars are being subjected to with the government’s 35 mph New Car Assessment Program (NCAP) and IIHS’ 40% deformable barrier frontal crash at 40 mph, manufacturers have had to make their cars stiffer than in years past. That makes them stiffer also in low velocity rear impacts. The result? An overall stiffness increase of 34% and a significantly higher incidence of whiplash injury.

This concern for better crashworthiness in 35-40 mph frontal crashes has thus had the unintended consequence of increasing whiplash injuries all over the world because all major countries now use a version of our 1979 NCAP. Fortunately, many of the European and Japanese manufacturers are more responsible than their American counterparts in their efforts to curb neck injuries. They have actually broken new ethical ground by implementing design changes even though the regulations didn’t require it.

Meanwhile, delving under the surface, you might sense a very interesting sort of schizophrenia within the automotive insurance industry. Specifically, I needn’t tell any practicing physician that all auto insurers appear to subscribe to the notion that whiplash injuries are not much more than a clinical mirage and an opportunity for priggish third parties to exploit the vulnerable insurer. But, on the research end of things, insurers are quite literally at the vanguard of whiplash research today. The IIHS, which is itself a consortium of American auto insurers, is part of a larger multinational consortium of groups known as the International Insurance Whiplash Protection Group (IIWPG). One might then well inquire why, if whiplash were truly a purely fictional injury, there would be so much interest in research on the part of insurers? Before we go any farther, let me describe the static test.

The IIHS Head Restraint Static Test

Using a seat phantom the IIHS measures the backset and topset of the seat and head restraint. In order to give the manufacturer the benefit of the doubt, as it were, they first adjust the restraint into the optimal (read that “up”) position. They then take this composite metric and compare it to their diagram, Figure 2, and give the rating earned. But here we have a couple of potential problems. Number one, your patient is probably not the exact height of the seat phantom which simulates a 50th percentile male (5 ft 10 inches tall). Your patient may also have a forward posture, may have been out of position, or may have had his/her seat and/or restraint adjusted differently. Observational studies have demonstrated that about 80% of persons keep their head restraints in the down (not optimal) position.

Figure 2. Adaptation of the head restraint rating diagram of the IIHS. I reversed the diagram so that it would be concordant with a photo of the patient taken from the passenger side of the vehicle. This subject has poor head restraint geometry. The largest (leftmost) area corresponds to “poor;” the rectangular (top right) area corresponds to “good.” “Marginal” and “adequate” are in between. See Summary for how to get a copy of this diagram.

What It Means To Practitioners

Perhaps I should finish the IIHS story first. In recent years the IIHS added a new twist to their head restraint static geometry test. They installed a state-of-the-art hydraulic sled in their Virginia testing facility which was capable of simulating a low velocity (10 mph) rear impact collision. To this sled they attach the vehicle’s seat and, riding in it, the latest iteration of the biofidelic rear impact dummy, the BioRID II. The way it works now is this. First they do the static test using the seat phantom. If the geometry is either good or acceptable, they run the dynamic sled test. If the geometry is marginal or poor, no sled test is conducted. The rating for some current year models can be found on their website at www.iihs.org.

So, do you still think the insurance industry is really in the dark about head restraints; that they don’t know what head restraint geometry means in terms of risk? Do you believe they really don’t consider whiplash a problem? Well consider this. As of this year, in order to get the highly coveted “Best Pick” award from the IIHS, manufacturers have to not only do well in the institute’s frontal and side impact crash tests, they must also have side impact airbags, electronic stability control (ESC) AND a “good” head restraint rating!

The interesting thing is that practitioners can use head restraint geometry to assess injury risk in individual patients and that risk assessment can be even more deterministic than that of the IIHS. Here is how you can accomplish this. Take the subject out to their vehicle and have them positioned in the vehicle exactly as they were at the time of the crash. They should have the seat and head restraints adjusted exactly as they were. Take a digital photograph from the passenger side, directly lateral to the subject. Placing a measuring device within the frame is a good idea too. Now, considering both topset and backset, place the subject in the colored area of the IIHS diagram and get the rating. I call this the “Subject-Specific IIHS Head Restraint Geometry.” It is a much better gauge than the one published on the IIHS website because it accounts for the subject’s particular stature, their exact position at the time of the crash, and the actual seat and head restraint adjustment in the crash.

Summary

The IIHS head restraint rating was a great idea. The name and shame game is also played skillfully by the IIHS’ UK counterpart, THATCHAM, with similar success. If you can’t force the manufacturers into ethical and moral rectitude, you can always kick them where it hurts—in the wallet. Don’t think for a second that manufacturers won’t work to improve their head restraints to earn that “Best Pick” rating from IIHS. I have colleagues in both centers and have known these people for many years. In fact, the former president of the IIHS, Brian O’Neill, helped me way back when I was writing my first whiplash textbook. They also have spoken at SRISD events, and assisted in the development of our Auto Safety Facts program, a PowerPoint, poster, and brochure-based system for physicians. I applaud their efforts and willingness to deal with the potential fallout they face from plaintiff’s quoting “poor IIHS ratings” in lawsuits. So, while the corporate face of insurers remains one of dissimulation, the countenance of the research branch is one of determination. The goal is to make the problem go away by actually learning how to prevent these injuries. Clinicians and forensic experts, meanwhile, can develop subject-specific IIHS head restraint geometry ratings and use them as an important part of their overall risk assessment.

As one final caveat, because of suspension compliance or bumper override or underride conditions, the rear-ended car may be pushed down or up as a result of the collision. And seats will bend rearward because of the inertia of the occupant. These conditions can affect head restraint geometry dynamically, usually making it worse in my experience. These factors should always be borne in mind.

If you would like an Adobe Illustrator file or JPG image of the IIHS diagram (inset in Figure 2), please email your request to me at drcroft@srisd.comand specify which file type you prefer.

References

1. Croft AC, Herring P, Freeman MD, et al. The neck injury criterion: future considerations. Accid Anal Prev 2002;34:247-55.