Whiplash injuries - time to implement the knowledge?

Ted Olsson, Andrew Morris, Niklas Truedsson,
*Astrid Linder, Magda Les, Brian Fildes
Accident Research Centre, Monash University,
*Department of Machine and Vehicle Design,
Chalmers University of Technology, SE 412 96 Göteborg, Sweden

Abstract

Much knowledge has been gained during the past few years about the nature and cause of whiplash injuries. This paper is an attempt to summarise the findings and proposes the directions which future research could take. Reducing the relative intervertebral acceleration and thereby the injury risk in all situations is difficult to achieve. Therefore there should be a focus on reducing the maximum and mean occupant acceleration using the vehicle’s body structure and safety systems. In rear crashes the main issue is that the seat absorbs energy in a controlled manner and gives support for the whole spine. In frontal crashes the air bag, seat belt pretensioner and load limiter probably must work together in reducing the relative acceleration between the vertebrae, the maximum and the mean occupant acceleration.

Introduction

Monash University Accident Research Centre is currently undertaking research into seat design and head restraint effectiveness in rear crashes. This research sets out to develop a design strategy to reduce neck and spinal injuries in low speed rear end crashes. The research includes a literature review, computer simulations and mechanical testing. The literature review is quite extensive and this paper summarises the main points of this review.

Results

Early medical reports refer to whiplash as "railway spine". This term was used in the 19th century to describe the persistent pain and other "subjective symptoms" reported by railway passengers and personnel due to minor railway crashes. Similar injuries can result from all types of motor vehicle crashes, but the typical mechanism involves rear end collisions (Deans et al. 1987).

Other terms have been suggested instead of whiplash. They have not found favour and "whiplash" is still the most popular term in general and in medical literature. The term "Soft Tissue Neck Injuries" (STNI) and Whiplash Associated Disorder (WAD) are also used from time to time. The whiplash injury is impairment that is evidently caused by the whiplash event. Much research into this type of injury following rear end crashes has been conducted during the last decade and many of the myths and concerns have been dispelled (Barnsley et al. 1998). However even now the injury mechanism has not been exactly identified.

Figure 1. A schematic drawing of the initial head-neck motion during rear end impact (Phase b: Retraction motion, Phase c: Extension motion), (Svensson et al. 2000).

Figure 2. Schematic drawing of the head-neck motion during a frontal collision and also the rebound motion in rear crashes (Phase b: Protraction motion, Phase c: Flexion motion), (Svensson et al. 2000).

During a typical rear end collision the struck vehicle is subjected to a forceful forward acceleration. The occupant is pushed forward by the seat back and the head lags behind due to its inertia. The head is subjected to a swift rearward translational motion followed by an extension motion (figure 1). At a later stage the head and torso will rebound forward and hit the seat belt, flexing the neck (figure 2) (Svensson et al. 2000). This motion has also been described by Ono et al. (1993) among others.

The Quebec Task Force (Spitzer et al. 1995) chose the following definition of whiplash:

"Whiplash is an acceleration-deceleration mechanism of energy transfer to the neck. It may result from rear end, side-impact and frontal vehicle collision, but can also occur during diving or other mishaps. The impact may result in bony or soft tissue injuries (whiplash injury), which in turn may lead to a variety of clinical manifestations (Whiplash-Associated Disorders)".

As can be seen in figure 3, the recovery rate after 6 months of chronicity can be quite low (Radanov et al. 1995). Furthermore, research has shown that approximately 60% of the injuries due to vehicle crashes causing disability in Sweden between 1990 and 1995 were whiplash injuries, compared to only 30% during the years 1976-78 (Krafft 1998).

Figure 3. The evolution of symptoms of patients with whiplash injury
(Radanov et al. 1995)

Analysis of police reported tow away crashes from Queensland (figure 4) and Victoria (figure 5) between 1987-1998 show a significant higher whiplash injury percentage risk for the struck vehicle compared to the striking vehicle. This is consistent with other studies (Morris et al. 1996, Krafft 1998, Jakobsson et al. 1999).

The analysis of the Queensland data was based on injuries that are classified according to the International Classification of Disease System (ICD). Using this system, whiplash accounted for 50% of injuries. However, this percentage decreased to 32.7% when other injuries (including those not codeable to the ICD classification) were incorporated in the analysis.

PATHOLOGY OF WHIPLASH The literature suggests that the most common injuries due to whiplash involve the zygapophyseal joints (Yoganadan et al. 1999), the intervertebral discs and the upper cervical ligaments (Barnsley et al. 1998). Other injuries may occur for example nerve damage (Svensson et al. 1993) and muscle injuries (Barnsley et al. 1998). In this respect, headache and neck pain are the dominating symptoms followed by pain in the shoulder girdle. The third most common symptom is paresthesiae and weakness in the upper limbs (Barnsley et al. 1998). The majority of occupants who report neck pain after a crash also report pain elsewhere in the body (Chapline et al. 1999).

INJURY MECHANISMS AND CRITERIONS Several injury mechanisms are proposed to cause whiplash injuries. These include axial compression forces (McConnell et al. 1995), shear forces (Walz et al. 1995), bending moments (Kaneoka et al. 1999) and pressure transients (Svensson 1993). What all the injury mechanisms have in common is that they are caused by relative acceleration between the vertebrae. To evaluate the injury risk under different circumstances several whiplash injury criterions have been developed. The NIC (relative x-acceleration between T1-head) developed by Boström et al. (1996), ?V-NIC (based on intervertebral rotation) was proposed by Panjabi (1999), NIJ criterion (combines moments of torque and shear forces) by Kleinberger (1998), NICprotraction and Myflexion proposed by Boström et al. (2000).

Currently there are no existing dynamic seat test standards, but the ISO/TC22 /SC10/WG1 is in the process of developing criteria for a rear end dynamic test standard (Langweider et al. 2000).

SEAT DESIGN To reduce the risk for neck injuries in low severity rear end crashes, seat design is very important. It is probably not sufficient to have a suitable head restraint if the rest of the seat is not designed to minimise neck injuries. The head restraint is an important factor in whiplash injury protection, but it is not the only factor that determines whether or not there will be an injury (Jakobsson et al., 1999). A study of U.K. data showed a statistically non-significant trend towards reduced neck-injury outcomes in instances of seat back damage (Morris et al. 1996). A study which included both single and multiple crashes showed that the whiplash injury frequency was approximately twice as high if the seat was undamaged compared to when the seat yielded (Parkin et al. 1995).

Tests done by Strother et al. (1987) to evaluate the seat back strength showed that the average seat back stiffness was 52 Nm/deg. In a later study, Molino (1998) showed that the average seat back strength had increased to 65 Nm/deg. The reason for this is to protect the occupant from cargo or passengers in the back of the vehicle and also to prevent total collapse of the seat, but there is some debate as to whether this causes an adverse effect.

Tests done by Håland et al. (1996) also emphasis the importance of the seat design. In their study, a Peugeot 205 and an Opel Corsa (which have the highest and the lowest risk of sustaining neck injuries in their weight class according to statistics from Folksam) were impacted from the rear. One of the front seats was exchanged between the two vehicles. It was found that the difference in the loading of the front seat occupant (represented by the Hybrid III dummy) in the two vehicles was mainly caused by the difference in the design of the seat and not by the difference in the design of the vehicle structure. The dummy movement was faster in the Opel Corsa.

In tests with a Hybrid III dummy Hofinger et al. (1999) concluded that cushion properties are also important. Both the kind of cushion and its shape and position have a big influence on seat characteristics. Basically the design of the seat should guarantee an early and nearly simultaneous support of the torso and head which requires a defined ‘diving’ into the seat back cushion to prevent or minimise a relative linear and angular displacement between head an torso.

A yielding seat is likely to be more forgiving if the occupant is out of position (ie. the head is not close to the head restraint, Benson et al. 1996). Furthermore, spine straightening tends to intensify and the axial compression force against the cervical spine is increased as the seat stiffness increases (Ono et al. 1998). The forward rebound phase can only be neglected in seat design with seat backs that deform plastically during low speed (Delta V=15 km/h) rear crashes with relatively low (6 g average) vehicle acceleration (Muser et al. 2000).

Both Delta-V and shape of the crash pulse most likely influence the injury risk in rear end crashes. A lower mean acceleration with no distinct peaks probably reduces the injury risk (Kullgren 1998, Eriksson et al. 1999). A further indication of this is that occupants of vehicles fitted with a towbar (that adds a stiff structure to the end of the vehicle) have a 22 % higher risk of sustaining long-term whiplash injuries if the vehicle is struck from behind (Krafft et al. 2000).

There are at least three different anti-whiplash seats on the market. There are two general principles that apply; one propels the head restraint forward to meet the head and thereby reduces the relative head torso movement; the second principle allows the occupant to move backward until both the head and the spine are supported from the seat closer in time. Thereafter if the forces are high, the seat reclines and thereby reduces the acceleration. The WHIPS seat (plastic deformation, Jakobsson et al. 1999) the SAHR seat (active headrestraint, Wiklund et al. 1998) and a seat made by Toyota (Sekizuka et al. 1998) that could reduce the relative motion between the vertebra are the three only anti-whiplash seats that thought to have been are installed in new vehicles. Several prototypes for anti whiplash systems exist.

DUMMIES Hybrid III is the most common dummy used in rear impact crash tests, but more sophisticated dummies are now available. It is probably not sufficient to only use a biofidelic neck on the Hybrid III, since the whole spine has to be considered in rear end crashes. The BioRID dummy has been developed especially to study the relative motions between head and torso (Svensson et al. 2000). Eriksson et al. (1999) has developed a MADYMO model of the whole BioRID.

Recent research has led to the development of a dummy called RID-II a that will be commercially available in the future (Wismans et al. 1999). It is possible to use a RID neck (Svensson et al. 1992) or a TRID neck (Thunnisen et al. 1996) on a Hybrid III 50th percentile dummy.

Conclusions

The following conclusions have been drawn in the literature review by Olsson et al. (2000):

• It is time to implement the knowledge about what causes whiplash injuries. During the recent years substantial knowledge has been gained and possible solutions to reduce the problem have been installed in some vehicles.
• During the 1990’s the risk of sustaining a whiplash injury has increased significantly. This can be to the fact that the seat force-deflection characteristics have changed (ie the average seat have become stiffer and/or stronger).
• Whiplash injuries occur in all impact directions, but the risk is highest in rear crashes. Around 50% of the whiplash injuries occur in rear crashes, 30% in frontal crashes and the rest in other types of accidents.
• Whiplash injuries usually occur within 24 hours of the impact. The most common symptoms are headache and neck pain followed by pain in the shoulder girdle. Thereafter comes paraesthesia and weakness in the arms.
• It is important to separate the analyses for short and long-term injuries since there may be different injury types with different injury mechanisms.
• Based on data provided by NSW Compulsory Third Party (CTP) insurers to the MAA’s Claims Register to the end of June 1998, whiplash is the single most frequently recorded crash injury. Whiplash was involved in 38.9% (49,344) of the total 126,923 CTP claims reported and accounted for 25% ($1466.3M) of the total incurred cost.
• Many people with whiplash injury also report pain elsewhere due to the crash.
• About 10% of all whiplash injuries become long-term injuries in rear crashes and about 5% in frontal crashes.
• In Sweden, during the last years around 60% of all long-term traffic injuries have been neck injuries.
• Frontal crash air bags in combination with pretensioners and load limiters are thought to be strong factors in reducing whiplash injuries in frontal crashes.
• In frontal crashes it is well known that wearing a seat belt increases the risk of whiplash injuries. But it is hard to find clear-cut evidence that wearing a seat belt in low severity rear end crashes increases the risk of neck injuries.
• In frontal crashes the deceleration of the vehicle directly after the occupant’s seat belt contact seems to explain the risk of sustaining long-term whiplash injuries.
• To reduce the injuries to the neck and the rest of the spine in rear crashes the vehicle seat and head restraint must be seen as one unit.
• A seatback with controlled deformation energy absorption offers better protection for out of position occupants in rear end crashes.
• Stiffer seatbacks tend to increase loadings on the cervical, thoracic and lumbar spine in rear end crashes.
• For lower severity rear crashes factors like the head restraint and seat back geometry and cushion properties are more important; at higher severity rear end crashes the seat force-deflection characteristics are more important.
• A head restraint should prevent extreme hyperextension of the neck and minimise the relative motion between the head and torso. Head restraints alone are not enough to prevent all whiplash injuries.
• A towbar adds structure to the end of the vehicle. Statistics from Sweden shows that a vehicle with a towbar has around 20% higher long-term whiplash injury risk if it is hit in a rear-end crash.
• There are at least three different anti-whiplash seats on the market. There are two general principles that apply; one propels the head restraint forward to meet the head and thereby reduces the relative head torso movement; the second principle allows the occupant to move backward until both the head and the spine are supported from the seat closer in time. Thereafter if the forces are high, the seat reclines and thereby reduces the acceleration.
• There is much evidence suggesting that the zygapophyseal joints are damaged during a whiplash event.
• Violent pressure gradients in the spinal canal during the whiplash movement can occur and probably cause nerve damage.
• In rear crashes a low NIC value most probably reduces the risk of muscle strain, upper neck lesion, zygapophyseal joint pinching as well as harmful pressure transients.
• A rear-impact dummy has been developed to measure the risk of whiplash injuries, in low-speed rear-end crashes. This dummy, the BioRID has been designed especially to study the relative motion of the head and torso. For tests representing crashes in which a vehicle is struck in the rear, BioRID can help researchers learn more about how seatbacks, head restraints, and other vehicle characteristics influence the likelihood of whiplash injury.
• To evaluate the whiplash injury risk in frontal crashes it seems possible to use both the NICprotraction value and the My flexion value and NIJ.

Recommendations:

• Further medical research is needed to establish the exact link between the whiplash movement and the whiplash injury, although much knowledge has been gained.
• There should be on-going collection of whiplash injury data from modern anti whiplash systems (eg WHIPS, SAHR).
• There is a need to establish adequate test methods to evaluate the whiplash injury risk for rear and frontal crashes.

Acknowledgements

We are grateful to Holden Ltd for their generous sponsorship of this research.

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