Pilar 4: Protección social

Older occupants are especially vulnerable to thoracic injury. In NASS CDS data, Hanna and Hershman found that the proportion of crash-involved occupants with AIS 2+ thoracic injury increased five-fold from 1.5 percent of the occupants age 25 to 44 to 6.5 percent of the occupants 75 or older. By contrast, head-injury rates increased only from 1.9 percent to 2.6 percent.

Abdomen, arm, and leg injury rates doubled and spinal-injury rates tripled. The risk of thoracic injuries appears to be intrinsic to aging, as it occurs for frontal as well as side impacts, in cars as well as LTVs, at all seat positions, for belted as well as unbelted occupants. Ribcage and sternum fractures are especially prevalent.⁹ Wang and Rupp reported that bone density in the ribcage decreases strongly with age.¹⁰

According to Kent and Patrie, “Chest deflection injury threshold is strongly dependent on the age of the subject. This is true regardless of whether injury onset or severe injury is considered. A 30-year-old has a 50 [percent] risk of sustaining one rib fracture at a chest deflection level of 35 [percent]. This threshold drops to 13 [percent] deflection for a 70-year-old. A 30-year-old has a 50 [percent] risk of sustaining more than six rib fractures at a deflection level of 43 [percent], while a 70-year-old can tolerate only 33 [percent] deflection before reaching this threshold.

These findings are…presumably due to multiple characteristics of aging. First, the failure strain of both cortical and trabecular bone decreases with age. Second, geometric changes associated with aging may predispose ribs to fracturing for older subjects under conditions where they might deflect non-injuriously in a younger subject. These geometric changes include a decrease in the proportion of the rib cross-section that is cortical bone and a general decrease in rib slope.

Finally, material changes such as calcification of the costal cartilage and decreasing bone mineral density also are likely contributors to the decreased chest deflection tolerance.”¹¹ The Center for Injury Biomechanics and Wake Forest University is currently quantifying the variation of rib cortical thickness, bone density, and ribcage geometry by age and gender, based on data collected through CT scans.¹²

Zhou, Rouhana, and Melvin statistically analyzed numerous existing sets of test results with cadavers of varying ages to estimate the effect of aging on thoracic-injury tolerance. The

9 Hanna, R., & Hershman, L. (2009). Evaluation of Thoracic Injuries Among Older Motor Vehicle Occupants.

(Report No. DOT HS 811 101). Washington, DC: National Highway Traffic Safety Administration.

10 Wang, S., & Rupp, J. (2006). Alterations in Injury Patterns and Body Composition With Aging. (PowerPoint presentation. Ann Arbor, MI: University of Michigan Transportation Research Institute

http://www.nhtsa.gov/DOT/NHTSA/NVS/CIREN/2006%20Presentations/MI_0306b.pdf

11 Kent, R., & Patrie, J. (2005). Chest Deflection Tolerance to Blunt Anterior Loading Is Sensitive to Age but Not Load Distribution. Forensic Science International, 149, pp. 121-128.

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underlying hypothesis is that bones change with age: Their modulus of elasticity and bending strength decrease after age 30 due to increased porosity and demineralization. The rate of

deterioration speeds up after age 40 and then even more after age 60. Muscles and arterial tissues also decrease in strength with age. However, the extent of deterioration in strength or tolerance is not uniform, but varies with the type of tissue and the manner of loading. Here are their estimates of the percent loss of strength or injury tolerance from age 25 to 75.¹³

Percent Reduction from Age 25 to 75

72 Belt force needed to produce AIS 3 injuries 55 Compact bone fracture toughness

50 Vertebral bone ultimate tensile strength (UTS) 40 Abdominal muscle wall UTS

35 Cardiac muscle UTS 29 Arterial tissue UTS

27 Side-impact velocity needed to produce AIS 3 injuries 21 Blunt-impact force needed to produce AIS 3 injuries

In other words, the effect of aging is more severe in belt loading than in blunt impact force. They believe that belt loading is a more static, less dynamic load than blunt impact, and thus has a less linear response. Also, belt force is concentrated on bone, rather than soft tissue. Bone

deteriorates more rapidly with age than soft tissue. (This does not imply that belt use is harmful for older occupants, merely that belts can be relatively less effective for older than for young occupants.)

In FARS and CDS data, Austin and Faigin identified that side impacts account for an increasing share of the fatalities as occupants get older. For any given delta V, injury risk is significantly higher, at all AIS levels, for older occupants and females. Furthermore, the higher the delta V, the more risk increases with age.¹⁴

Ridella, Rupp, and Poland developed logistic regression analyses for 2000-2010 NASS-CDS data on MY 2000-2010 vehicles to estimate an occupant’s odds of AIS ≥ 3 injury as a function of that occupant’s age, gender, belt use, BMI, height, and seat position; the vehicle type (car or LTV); the impact type; and the delta v (or other measure of severity such as number of quarter turns in a rollover).¹⁵ The database has one record per occupant, unlike the double-pair

comparison analyses of Evans that consider pairs of occupants exposed to the same crash; on the other hand, the ability to control for delta v or other measures of severity in CDS data is useful for controlling for differences in the distribution of crash severities for occupants of different

13 Zhou, Q., Rouhana, S. W., and Melvin, J. W. (1996). “Age Effects on Thoracic Injury Tolerance,” 40^th Stapp Car Crash Conference Proceedings, Paper No. 962421. (Publication No. P-305). Warrendale, PA: Society of

Automotive Engineers.

14 Austin, R. A., & Faigin, B. M. (2003). Effect of Vehicle and Crash Factors on Older Occupants. Journal of Safety Research, 34, pp. 441-452.

15 Ridella, S. A., Rupp, J. D., & Poland, K. (2012). Age-Related Differences in AIS 3+ Crash Injury Risk, Types, Causation and Mechanisms. International IRCOBI Conference on the Biomechanics of Impact.

ages. The backwards-stepwise regression method starts by considering all of the variables and then dropping those that are non-significant.

Table 1-1 shows the estimated increase in the odds of AIS ≥ 3 injury for aging 10 years. Aging significantly increases risk to almost every region of the body in all types of crashes. However, the largest age effect was observed for the thorax, in frontal crashes, for female occupants.

Separate odds ratios were calculated for men and women for body region and crash mode combinations for which an age*gender interaction was significant.

Table 1-1: Adjusted Odds Ratios (and 95% CIs) for AIS ≥ 3 Injury to Different Body Regions Associated With a Decade Increase in Age¹⁶

A supplementary analysis of injury causation in 1,289 CIREN cases found thoracic and spinal injuries increase with age and lower extremity and head injuries, as a percent of the total injuries experienced in each age group, decrease as age increases. There were no significant gender differences in these trends. For frontal crashes, thoracic injury type changed from soft-tissue injuries such as lung contusions to bony-tissue injuries such as rib and sternum fractures as age increased. The contribution of the air bag and steering wheel decreases and the seat belt begins to play a larger role in injury causation for the oldest age groups. Comorbid factors such as

osteoporosis, osteopenia and obesity were more common for the older occupants. Crash severity was consistently lower for the older age groups and outcomes were worse in the older occupants even for similar crash severity and injury.

NHTSA’s review of 122 front-seat occupant fatalities in CDS frontal impacts – belted, protected by air bags, and in MY 2000+ cars or LTVs – identified 16 cases where the occupant’s age was a primary factor (i.e., the crash would otherwise not likely have been fatal) and 15 where it was a secondary factor. Here are two individual examples of older occupants who did not survive. Both

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occupants were quite fragile; the second, in addition, may have been exceptionally frail due to a pre-existing medical condition:

“As an example of a case in which the occupant’s age was deemed critical, see Case No. 2004-50-147, in which the 80-year-old driver sustained a number of thoracic injuries leading to her demise. CDS did not report any pre-existing medical conditions. The crash was not very severe with a coded [delta v] of 22 mph…, and it did not result in any intrusions into the occupant’s space. The team did not see signs of contact with the steering wheel and concluded that her numerous thoracic injuries were caused by loading from the shoulder belt. She suffered a number of fractured ribs and a heart laceration; the thoracic cage typically shows a greater tendency to sustain fracture with elevated age. Given that the chest injuries were responsible for her demise, and that this crash was not severe, the team felt that her age was a critical element leading to her death.

Case No. 2004-50-147 Case No. 2002-75-53

“Case No. 2002-75-53 is an example of a fatality that was likely due to [the occupant’s fragility and/or frailty due to old age and] a pre-existing medical condition. The minor frontal crash, with a [delta v] of 14 mph, of the Toyota 4Runner led to the death of the right-front passenger, who suffered from advanced lung cancer that had metastasized to the liver.

“Her injuries included a heart laceration, a subdural hematoma and a cervical spine fracture.

Although an older occupant (71 years old) who was of smaller stature (4’11”), she was in the right front passenger seat, making it unlikely that she was sitting close to the [instrument panel]

(as may be the case for a driver of her stature). In fact, the seat was noted to be in the mid-to-rear track position, so [thoracic] interaction with a deploying bag was unlikely and there would be little reason for her type of injuries to occur in a typical adult [unless head interaction with the air bag was also a factor]. Due to her advanced cancer, the team concluded that her body’s condition was weakened, making her exceptionally fragile and more susceptible to injury in this minor crash [and perhaps also frail, increasing the likelihood of a fatal outcome for these injuries]. The

two other occupants in this vehicle were either not injured or only suffered minor contusions – suggesting that the pulse of this crash should not be injurious to a normal belted occupant.”¹⁷ Bose, Segui-Gomez, and Crandall used logistic regression to analyze injury risk of belted drivers in 1998-2008 CDS data as a function of the driver’s gender, controlling for a number of other variables. “Results from the multivariate regression analysis indicated that the odds of a belt-restrained female driver sustaining an MAIS 3+ and MAIS 2+ injury were 47 percent (95%

CI=27%, 70%) and 71 percent (95% CI=44%, 102%) higher, respectively, than those of a belt-restrained male driver when we controlled for the effects of age, mass, BMI category, crash [delta v], vehicle body type, number of events, and crash direction…For chest and spine AIS 2+

injuries, the odds of an effectively belted female driver to sustain the injury was 38 percent (95%

CI=1%, 89%) and 67 percent (95% CI=34%, 109%) higher, respectively, than those of a belted male driver in comparable crash conditions.

“To account for the correlation between sex and anthropometric size, the regression

methodology used in the study specifically controlled for the effects of BMI and overall mass as measures of size. Tolerance to traumatic injury may also be predicted as a function of sex-specific properties. Specifically, female occupants are at a higher risk for sustaining whiplash injuries because of differences in neck anthropometry, strength, and musculature, and the relative positioning of the head restraint. Similarly, a higher risk of lower extremity injuries has been reported for female drivers as a result of their relatively short stature, preferred seating posture, and a combination of these factors yielding lower safety protection from the standard restraint devices.” They did not find a statistically significant difference between belted females and males in the CDS data for fatality risk and AIS 2+ head injuries.¹⁸

NHTSA’s review of 122 belted, frontal fatalities of late-model vehicles equipped with air bags in CDS cases did not attribute any fatalities to anatomical or physiological vulnerability specifically linked to gender – e.g., a female’s neck injury that a male with a thicker neck might have

avoided. But it rather often cited anatomical features that are more common in females than males. Short stature was a secondary factor in eight cases, six of them females. Four occupants had been displaced out of position, two of them lightweight females. Eleven occupants were obese and 5’6” or shorter, a combination that taxes existing restraint systems (because these people usually sit close to the steering wheel): ten of them were females. However, eight of these 11 cases also involved exacerbating circumstances such as exceedingly high delta v,

corner/oblique impact, or underride. In two of the three remaining cases (nos. 2004-79-49 and 2007-12-180) it appeared that the driver’s short stature and sitting close to the wheel contributed to poor driver-air bag interaction; in case no. 2006-41-64, the obese driver bottomed out the air bag (whose deployment was perhaps delayed because of the centered impact with a narrow object).¹⁹

17 Bean, J. D., Kahane, C. J., Mynatt, M., Rudd, R. W., Rush, C. J., & Wiacek, C. (2009, September). Fatalities in Frontal Crashes Despite Seat Belts and Air Bags. (Report No. DOT HS 811 202, pp. 40-41). Washington, DC:

National Highway Traffic Safety Administration Available at www-nrd.nhtsa.dot.gov/pubs/811102.pdf

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