4.a. Evaluation of Seat Performance Criteria for Rear-end Impact Testing: BioRID II and Insurance Data

Johan Davidsson (JD) presented a common study of Chalmers and Folksam on the evaluation of seat performance criteria based on insurance data (WCWID-01-03). The presentation was an update of GTR7-14-02. The study was performed within the EEVC work programme by EEVC WG12 (see report WCWID-01-09):

Approach for the study is to compare claim rates for different seats in the Folksam insurance data with BioRID II metrics from tests for the same seats.

The focus is on neck injury and very few thoracic or lumbar spine injuries/claims are contained in the Folksam database.

Two risk levels:
1. Symptoms for more than one month in case of initial symptoms
2. Permanent medical impairment in case of initial symptoms (with no financial incentive to claim this level of symptom)

The Chalmers/Folksam/EEVC study uses 150 cases per seat (IIHS used a minimum group size of 30 cases, which may explain the weak correlations that they reported).

In Sweden, the risk for a given vehicle has reduced over 10 years, indicating that the assessment of injury by the medical profession has become stricter. This has been compensated for in the study by normalising to the 2005 risk level.

The risk for different seats cf. the NIC was shown as an example. Philippe Petit (PP) asked if the differences were tested for significance, because the range is quite small. This was not the case. Norbert Praxl (NP) asked if confounding factors were controlled like e.g. delta-v?

JD replied that there was no information on delta-v, but ensured that the groups were not very light or very heavy vehicles. The Volvo and Saab had a step-change in seat design on a
vehicle platform of very similar mass and structure.

Koshiro Ono (KO) asked whether the patients’ medical records were re-reviewed by the study in order to confirm the diagnosis and the severity of WAD. This was not the case, but for the permanent injury rating there is a considerable effort that the patient has to go to in order to be classified, with little reward.

Overall, NIC showed the best correlation with risk, both at the symptoms > 1 month and the permanent medical impairment levels. OC-T1 x-axis relative displacement and L1 x-axis acceleration correlated with long-term injury risk. Neck extension and T1 x-axis acceleration may be candidates, but appear to be sensitive to inclusion or not of outliers.

Limitations of the study include:
• Changes to the dummy over time
• Changes since the seats in the study were tested
• Correlation coefficients were maximum 0.72.

PP noted that there is a very strong assumption that the pulse is the same over time, even for an identical vehicle. In France, it was observed that the average speed of fatal collisions
was increasing until automatic ticketing was introduced, when it dropped 5%. JD: 2009 pulses likely to be more severe than 1999, due to changes in the vehicle structures.

Agnes Kim (AK) noted that the study is based on Folksam data and that means Swedish drivers, who are more likely to sit in a ‘standard’ position than US drivers, who tend to have a
very poor, non-standard posture. BL: This is the data we need – the regulator can only assume that a driver is sitting in a good position with a correctly adjusted seat, and ensure that they are provided with a minimum level of safety in this case if they take this care. The seat-belt doesn’t protect you if you don’t wear it, but it is still required to be fitted. AK pointed out that in regulations to date we take due care to ensure that if an occupant is somewhat out of position they will still be protected, but the rear impact is somewhat different.

Annette Irwin (AI) asked whether the BioRID predicts that the risk is, as would be expected, lower in the heavier vehicles. JD replied that he can’t answer this question based on this data, but other studies have done this.

4.b. Status of JARI Research: Reviews on Injury Parameters and Injury Criteria for Minor Neck Injuries during Rear-end Impacts

JARI/Koshiro Ono (KO) presented a review on injury parameters and injury criteria for minor neck injuries during rear-end impacts (WCWID-01-05).

Objectives of this study: review the published papers and current knowledge for reducing minor neck injuries; review of injury evaluation parameters from human volunteer tests and accident reconstruction simulations.

A chain of evidence from different sources (PMHS tests, volunteer tests, animal tests etc.) was presented:

• Excessive deflection leads to soft tissue injury
• Facet joint is most common injury site
• Strain rate affects rupture strength of soft
• Stretch of facet capsule is related with pain

Symptoms from six volunteer tests were presented. It was noted that each subject received multiple tests, from 4 to 9.4 km/h delta-v. The strains in the facet joints were estimated from the high-speed x-ray data and safe and failure zones defined. The correlations of the strains and strain rates (principal and shear) with candidate injury criteria were presented. The injury criteria were calculated for each volunteer (not for the BioRID II). Head and neck loads were calculated using inverse dynamics and external forces from strain gauges on the head restraint supports.

Accident reconstructions (20 occupants in 15 cars) were then presented. The THUMS FE model was scaled based on the height and weight of the occupant (with the mass scaled by adjusting the density). The head restraint backset was not known, so a standard backset of 50 mm was used.

Injury classified as WAD 2+, so WAD 1 (pain only with no identifiable physical change) was classified as non-injury.

The correlation between the candidate criteria and the strain / strain rate in the simulations was presented. KM noted that the correlations are for a lower level of injury than we are
targeting with the regulation.

A comparison between the Japan and US approaches and suggested criteria was presented, followed by a comparison of facet joint strains for tensed and relaxed muscle conditions.

Upper facet joint strain was higher in the relaxed state; lower facet joint strains were higher in the tensed stated.

It seems not to be possible to evaluate neck injury risk from OC-T1 kinematics alone; NIC, and neck forces/moments are also required. This is because the head-neck motion is not achieved by a simple rotation of a straight line neck, but includes an s-shaped neck.

Concluding, various injury risk curves (IRC) were presented for human subjects, with 95% confidence intervals. Good correlations with WAD2+ were found for NIC, upper neck Fx & My, lower neck Fx & My.

For IV-NIC of 1.1, an 82.9% risk of WAD2+ was derived. As such, when the neck is flexed or extended 10% above the physiological range, an 82.9% risk of WAD2+ exists.

4.c. Status of NHTSA Research: Preliminary PMHS Injury Risk Curves and Potential Injury Criteria in Rear Impact

Yun-Seok Kang (YSK) presented the status of NHTSA research (WCWID-01-06).

The study is divided in two phases. In Phase 1 tests are performed using a laboratory seat and in Phase 2 one using a production car seats.

Aims of Phase 1 (laboratory seat) were:

• Evaluate the biofidelity of candidate dummies
• Investigate the mechanism of injury
• Relate injury to measured PMHS variables

The seat included load cells in the base, back and head restraint. Springs controlled the initial response, with dampers to control rebound. Tests were performed at 17 km/h, 8.5 g and 24 km/h, 10.5 g – two tests with each of seven PMHS (plus four low-speed tests with one additional PMHS).

A new technique for mounting instrumentation to each cervical body without disrupting the main musculature of the neck was developed. This gave the rotation and displacement of each vertebral body. It was found that intervertebral flexion dominated the response.

Disc ruptures, subluxations (representing WAD) and ligament lacerations were identified upon necropsy.

Correlated intervertebral kinematics (linear/angular acceleration, velocity and displacement) and injury; used this to suggest criteria and correlate with global kinematics in order to identify potential injury criteria on the dummy. Rotation about the y-axis had the best correlation, with angular velocity and facet joint slide reasonable. However, each intervertebral level may have a different threshold, so the data was normalised using the physiological range of motion.

JD commented that the data is being treated as though the measurements at each intervertebral level are independent, but they may not be – if you damage at one level it may release other levels. The rupture and laceration only occurred in the lab seat tests, due to over-riding the head restraint resulting in hyper-extension. In the car seat tests (to be presented later), hyper-extension did not occur and there were none of these more serious injuries – only subluxations.

IV-NIC rotation was the best injury predictor, and rotation measures were better correlated than displacements or strains
Potential ‘global’ PMHS injury criteria were NDCrot and NDCx (and NIC).

However, this test condition was designed for evaluating biofidelity, repeatability etc. – not to represent a real seat; rigid head restraint affects loads; upper and lower neck loads not accurate after head restraint contact due to ramping-up motion; and seat back rotation was more uniform that real seats. The lab seat induces flexion, which will left-censor the risk functions considerably – therefore the values from this study should not be used directly.

This, with other limitations, meant that tests were performed with production car seats.

There was some discussion regarding the lack of muscle tone in PMHS subjects. The muscle tone in vivo (once the head is upright) to maintain position is 1-5% of maximal exertion, and reaction to loading occurs after the period of interest, so no significant limitation to using PMHS.

Phase 2: Production car seats

• Paired BioRID II and PMHS tests
• Verify measures from experimental seats highly correlated to injury

Toyota Camry and Chevy Cruze chosen – one with good IIHS and Euro NCAP ratings, one with poor. Three pulses: FMVSS 202a, JNCAP, 24 km/h. Seven PMHS, with the combination of seat and pulse altering the dose. Aimed for 50 mm backset, which was achieved for five PMHS.

Global motion was rearward (extension), but at each cervical vertebral level, flexion again dominated. BioRID also showed flexion (lower magnitude at the higher, 24 km/h severity).

Again, injuries at each vertebral level were compared with intervertebral kinematic measurements. Rotation about the y-axis had the best correlation. NDCrot had the best correlation for these seats.

In conclusion:

• IV-NICrot was the best PMHS injury risk predictor.
• The most promising BioRID injury criteria were IV rotation and NDCrot.

KM noted that the IIHS and Euro NCAP assessments seem to be doing the right sort of thing, so could just add the metric(s) recommended from the GTR-7 work. JD noted that the metrics used in the consumer information tests are focussed on preventing hyper-extension, so an additional metric to prevent injurious flexion would be welcome.

KM noted that the subluxations cannot be observed on a live patient, and we are currently working with a neurosurgeon to determine whether the subluxations would be associated with WAD, rather than the more severe, conventionally AIS-coded injuries.

Presentation from KM on the alignment of the US and Japan studies:

Best injury predictor for a dummy:

• US – iV-NICrot
• Japan – iV-NICrot (well correlated with strain and strain rate)

Potential ‘global’ injury criteria

• IV-NICrot → NDCrot, NDC, NIC
• IV-NICrot → NIC, UNFx, UN My, LN Fx, LN My

Common ground

• NIC
• US: Investigated forces and moments in PMHS. May need to consider direct correlation with the forces and moments in the BioRID
• Japan: Investigated NDCrot and NDCx (well correlated to WAD2+)

Sensitivity analysis showed that the PMHS neck forces and moments were very sensitive to the position of the centre of the contact, which was not well characterised. Also, the BioRID skull cap force did not match the head restraint forces in 5 out of 7 tests.

Future work:

• Ensure the dummies represent the final regulatory tool
• Re-run 5 injury criteria sled tests using (both BioRIDs) with Camry and Cruze seats
• Conduct paired BioRID/Hybrid III sled tests
  • To get extension metrics to supplement the flexion metric
  • 5 seats (Cruze, Camry, Toyota Matrix, Ford F150, Honda Odyssey), all three
    pulses
  • Check BioRID metrics deliver at least the protection level of 202a with Hybrid III head angle metric – which is the minimum requirement for the US to be able to adopt BioRID, because the current safety level cannot be degraded.

4.d. Other Whiplash Injury Criteria work

Lex van Rooij (LvR) indicated that TNO performed a large study with validated numerical models of car seats, BioRID II and Active Human Models that could be beneficial to this group once compared against real-world injury data. Since the last step is currently being performed, no scientific findings from this project could be presented. As such, this ongoing work was not presented.

LvR did however show results from an analysis on Euro NCAP test results from 2010 and 2011. A total of 38 anonymous test results on NIC and upper neck Fx shear force and the correlation between the two was shown. The analysis showed no correlation between NIC and Fx, indicating that Fx is a criterion that is not captured with just using NIC. The presentation was shared with the group (WCWID-01-07).