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05S-03 Stapp Car Crash Journal, Vol. 49 (November 2005), pp. Copyright © 2005 The Stapp Association Structural and Material Changes in the Aging Thorax and Their Role in Crash Protection for Older Occupants Richard Kent, Sang-Hyun Lee, Kurosh Darvish Center for Applied Biomechanics, University of Virginia Stewart Wang, Craig S. Poster, Aaron W. Lange, Chris Brede, David Lange University of Michigan Fumio Matsuoka Toyota Motor Corporation __________________________________ ABSTRACT – The human body undergoes a variety of changes as it ages through adulthood. These include both morphological (structural) changes (e.g., increased thoracic kyphosis) and material changes (e.g., osteoporosis). The purpose of this study is to evaluate structural changes that occur in the aging bony thorax and to assess the importance of these changes relative to the well- established material changes. The study involved two primary components. First, full-thorax computed tomography (CT) scans of 161 patients, age 18 to 89 years, were analyzed to quantify the angle of the ribs in the sagittal plane. A significant association between the angle of the ribs and age was identified, with the ribs becoming more perpendicular to the spine as age increased (0.08 degrees/year, p=0.012). Next, a finite element model of the thorax was used to evaluate the importance of this rib angle change relative to other factors associated with aging. A three-factor, two-level factorial design was used to assess the relative importance of rib cage morphology (“young” and “old” rib angle), thickness of the cortical shell (thick = “young” and thin = “old”), and the bone material properties (“young” and “old”) on the force-deflection response and injury tolerance of the thorax. The simulations showed that the structural and material changes played approximately equal roles in modulating the force- deflection response of the thorax. Changing the rib angle to be more perpendicular to the spine increased the effective thoracic stiffness, while the “old” material properties and the thin cortical shell decreased the effective stiffness. The offsetting effects of these traits resulted in similar effective thoracic stiffness for the “elderly” and baseline thoracic models, which is consistent with cadaver data available in the literature. All three effects tended to decrease chest deflection tolerance for rib fractures, though the material changes dominated (a four- to six-fold increase in elements eliminated using a maximum strain criterion). The primary conclusion, therefore, is that an older person’s thorax, relative to a younger, does not necessarily deform more in response to an applied force. The tolerable sternal deflection level is, however, much less. KEYWORDS – Aging, Thoracic Response, Restraints, Finite Element Model, Anatomy, Thoracic Injury __________________________________ INTRODUCTION the total population by 2010. Recognizing this changing demographic, the U.S. National Highway Importance of an Aging Population Traffic Safety Administration (NHTSA) (1993) Life expectancy in the U.S. has doubled since the identified eight Problem Identification Projects – beginning of the 20th century (Oskvig 1999) and by deficient research areas that will need consideration 2030 25% of the population will be age 65 or older in a proposed national traffic safety plan for older (OECD 2001). Due to longer life expectancy and drivers. Two of the areas identified as needing decreasing birth rates, the population growth rate of additional research are 1) knowledge about crash risk older Americans is expected to be 3.5 times that of for specific medical/functional conditions (which are strongly correlated with age) and 2) analysis of vehicle crashworthiness for older occupants. Address correspondence to Richard Kent, PhD, 1011 Linden Avenue, Charlottesville, VA 22902. Electronic mail: Aging is not just a U.S. domestic public health issue. [email protected] China will have 285 million people over the age of 1 2 Kent et al. / Stapp Car Crash Journal 49 (November 2005) 60 by 2025. The international Organization for Marcus et al. (1983) using laterally struck cadavers. Economic Co-operation and Development (OECD) Schmidt et al. (1975) found a similar age trend for concurred with NHTSA’s findings and identified a cadavers restrained by 3-point seat belts in frontal “pressing need” (OECD 2001) for research to sled tests. At 40 km/h, the average number of rib improve older people’s ability to survive crashes. In fractures increased from one fracture at age 20 to fact, they recommended that governments mandate approximately 10 at age 60. At 50 km/h, the 20-year- improved vehicle safety for older occupants. old average was 2 rib fractures, while the 60-year-old average was approximately 15 fractures. Similar Older drivers are unique in both the circumstances albeit less pronounced trends were also shown with and the outcomes of their collisions. For example, 2-point belt loading. Bending tests of rib segments NHTSA (Cerelli 1998) found that older drivers had from these cadavers indicated a substantial decrease higher belt usage than younger (Figure 1), and Morris in failure load with aging, but the study does not et al. (2002, 2003) identified several characteristics of indicate how the material properties and other factors older-driver crashes, including the disproportionate (e.g., the cross-sectional size and shape of the tested importance of chest injuries for older drivers. bone segment) contributed to this finding. Foret- Thoracic injury from restraint loading was shown to Bruno et al. (1978) studied injury severity as a be particularly significant for older drivers. Kent et function of shoulder belt force using cadavers in al. (2005a) corroborated these findings and showed staged collisions and living humans in field crashes. that 47% of drivers over 64 years of age who died in In both populations, the number of rib fractures for a a frontal crash sustained a fatal chest injury. In given belt load increased with aging. contrast, this number was only 22% for drivers age 16-33. There was a clear shift from fatal head The age-related decrease in thoracic injury tolerance injuries in the younger population to fatal chest has been observed in lateral impacts, as well. In the injuries in the older (Figure 2). Analysis of side development of the Thoracic Trauma Index (TTI), impacts revealed the same over-representation of Eppinger et al. (1984) showed that age was a chest injuries in the older population. Furthermore, significant modifier of the TTI threshold. To account many of these fatal thoracic injuries are not for this sensitivity, they included an age term with the particularly severe. A recent study conducted using kernel TTI function in an injury prediction algorithm. data from the Crash Injury Research and Engineering Kallieris et al. (1992) confirmed this sensitivity in a Network (CIREN) found that rib fractures were the series of lateral cadaver sled tests and included age as most serious injury sustained by 40% of patients over a covariate in a logistic regression model of injury 60 who died of chest injuries from automobile risk, as did Cavanaugh et al. (1993) in their collisions (Wang 2001). As the population continues development of the average spine acceleration (ASA) to age, it is likely that rib fracture frequency and the injury criterion for lateral thoracic loading. resulting morbidity and mortality will increase, and that chest injuries from restraint loading will become More recent studies have compiled larger datasets in a more significant scenario. In order to optimize attempts to quantify the age effects more precisely. restraint performance for an aging population, it is Zhou et al. (1996) developed piece-wise linear necessary to understand and to be able to model the regression models of rib fracture risk using data from force-deflection and injury tolerance changes 107 cadaver tests compiled by Eppinger (1976), 195 associated with the aging thorax. living humans compiled by Foret-Bruno et al. (1978), and 66 cadaver tests performed by Kroell et al. (1971, Decreased Thoracic Injury Tolerance 1974). They found that the tolerance to belt force decreased substantially with age. The oldest subjects That thoracic injury tolerance decreases with aging is (age 66 to 85) retained only about 20% of the well established in the literature. Both cadaver youngest (age 16-35) group’s belt force tolerance. studies and field data have shown that older subjects The sternal deflection tolerance to blunt hub loading generally sustain more rib fractures and other did not decrease as markedly with age: the oldest thoracic injuries for a similar impact severity. Using group retained about 80% of the youngest group’s the cadaver blunt hub impact data collected by Kroell deflection tolerance. It is important to note that the et al. (1971, 1974), Neathery (1974) found both chest difference in loading condition (belt versus blunt deflection and age to be significant predictors of hub) is not the only factor contributing to this injury outcome as defined by the Abbreviated Injury difference. The loading rates were greater on average Scale (AIS). At a given sternal deflection magnitude, with the hub loading, and different criteria (force for the AIS level (dictated primarily by rib fractures) was the belt loading and deflection for the hub loading) found to increase by 0.031 for each year of age, were considered. In 2004, Kent and Patrie compiled which is similar to the 0.025 factor identified by Kent et al. / Stapp Car Crash Journal 49 (November 2005) 3 chest deflection data from 93 cadaver tests involving 1.0 belt, hub, distributed, and combined belt-and-bag 0.9 loading to assess the role of age using a consistent 0.8 injury criterion. They found age to be a significant 0.7 Age = 70 covariate in a logistic regression model of rib fracture 0.6 Age = 30 risk. The 50% risk of rib fracture onset occurred at 0.5 13% chest deflection for age 70 and at 35% P(Injury) 0.4 deflection for age 30 (Figure 3).

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