Monthly PI Article March 2019
OUT OF POSITION OCCUPANTS HEAD AND NECK INJURIES
By Matthew J. DeGaetano, DC and John Doe, DC
Certified in Whiplash and Brain Traumatology
Certified Colossus Consultant
Based on the frontal airbag OOP injury data, the Technical Working Group (TWG) believes neck injuries will be the most critical out of position (OOP) injury risk from side airbags. Experience with frontal airbags indicates that rupture of the connective tissues between the head and neck (occipital condyles – atlas region) is a primary cause of the fatalities observed among OOP children and adults. Accordingly, the TWG considered a number of neck injury indicators that can be measured at the upper neck load cells of the dummy necks.
One approach is to impose limits on the peak force and moment values that are measured by
the upper neck load transducer located at the dummy’s head/neck interface, occipital condyles.
Limit values for these measurements were proposed by AAMA (1998) for OOP assessment of frontal airbags.
A second approach is to place limits on an index. The Neck Injury Risk (Nij) combines the effects of the forces and moments as was proposed by Prasad and Daniel (1984) for tension and extension moment. In its rulemaking activities regarding the assessment of OOP injury risk from frontal airbags, NHTSA proposed using the Nij concept and extended the analysis to include the combinations of tension-flexion, compression-flexion, and compression-extension. In its
comments on the NHTSA proposal, the Alliance (1999) developed injury risk curves for the
combined effect of tension-extension moment based on its re-analysis of the animal
injury/dummy response correlation data of Mertz et al. (1982) and Prasad and Daniel (1984).
The Alliance recommended setting the limit for tension-extension moment at 2 percent risk of
AIS ≥3 neck injury. The 5 percent risk line was not chosen as the limit line because 5 animals
with AIS ≥3 neck injury were below the 5 percent limit line. There were no animals with AIS ≥3
neck injury below the proposed 2 percent limit line.
In addition, the Alliance was concerned that the Nij concept allowed high axial forces when the
bending moments were low. Because the most sensitive indicator of animal neck injury was
peak neck tension (Mertz et al., 1997), the Alliance proposed to limit peak tension and peak compression. The limits for peak neck tension were set at 3 percent risk of AIS ≥3 neck injury.
The limits for peak neck compression were set at the currently used Injury Assessment
Reference Values (IARV). These peak force limits are the same as those proposed by AAMA
(1998). NHTSA agreed with the Alliance proposal and incorporated these limits into FMVSS
208 for regulating the OOP performance of frontal airbags.
The TWG reviewed the two approaches and chose to use the combined index, Nij=1 and the
peak force limits, that were proposed by the Alliance and are now the upper neck limit
requirement of FMVSS 208 for OOP regulation of frontal airbags. The intercept values of the
Nij limit lines and the peak tension and peak compression limit values are given in Table 2.
Some manufacturers and suppliers have expressed concern that extension and flexion bending
moments measured on a dummy in a given test may not always reflect neck injury risk to the
corresponding human. As noted above, injury risk assessment on anthropomorphic dummies is
a reasonable surrogate for assessing injury risks to humans and it is widely used in the
automotive engineering community. It is, nevertheless, an imperfect science that occasionally
yields invalid results for a variety of reasons. For this reason, the TWG agreed in general that
whenever a test result is obtained in excess of a reference value, and if there is reason to
question the validity of this result, the manufacturer (or other testing entity) should use
additional analyses to assess the validity of the result. If, however, the result is validated, then
appropriate countermeasures should be pursued.
Therefore, if a manufacturer or other testing entity obtains an extension or flexion bending moment in excess of the reference values and has reason to question the validity of that result, the response should be the same as the general guidance adopted by the TWG; namely, to use additional analyses to assess the validity of the result and, if the result is validated, to take appropriate actions