Structural and material changes in the aging thorax and their role in crash protection for older occupants

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.