Crash investigation studies show that reduction of thoracic injuries, particularly from restraint loading, is important for elderly occupants. Reducing these injuries is pred- icated on an understanding of the various thoracic changes that occur with aging. In this study, it was hypothesized that in addition to the well established material changes like density reduction, there are structural changes - rib angle increase and cage aspect ratio increase - that also affect thoracic response (stiffness and injury) to restraint loadings.
The hypothesis was first evaluated with a computed tomography (CT) study of 13 post-mortem human subjects (PMHS) aged 54 to 85 years. It was attempted to identify aging-related material and structural changes measured through CT, and correlate them with thoracic stiffness and rib fractures outcomes available from pre- vious tests. However, due to the limited number of subjects and small age range, no definite conclusions could be drawn. The study, however, proved useful by revealing the rib cage density distribution: maximum laterally in a rib and usually increasing with rib level.
As the PMHS CT study could not lead to definite conclusions about the hy- pothesis, another CT study, of 161 living humans aged 18 to 89 years, was performed. Significant aging related changes in thoracic material and structural properties were found: linear regression analysis showed bone density reduction at the rate of -8.6% per decade (p < 0.001), rib angle (measured in lateral view as the angle between the vertical and the rib in upright rib cage position) at +1.6% per decade (p = 0.012), and cage aspect ratio at +3% per decade (p = 0.066).
The findings of the living human CT study were incorporated into a simple ana- lytical model approximating the rib cage with a hollow elastic ring. The ring showed an increase in compression-stiffness with aspect ratio increase and a decrease with density loss or cortical thinning. When all these effects were combined to represent aging, the stiffness of the ‘old’ and the ‘young’ ring were found to be similar. The model further showed density loss as more important in increasing rib fractures than cortical thinning.
Finally, H-Torso (a commercial finite element model) was modified to repre- sent ‘elderly’ thorax and subjected to four restraint loading conditions - hub, diago- nal, double diagonal, and distributed. Comparisons of ‘elderly’ model response with the baseline ‘young’ model response validated the findings of the analytical model: material and structural changes played approximately equal roles in affecting the force-deflection response and all the factors, material and structural, contributed to increasing the number of rib fractures with aging under restraint loading. Density loss was again found to be the dominant factor contributing to rib fractures with aging.
It is concluded, therefore, that structural factors like rib angle and chest aspect ratio change with age and offset the effect of material changes on thoracic stiffness while contribute to contribute to rib fractures. Thus, combining the effect of both the structural and material changes, the thoracic stiffness does not change much, while the tolerable level of chest deflection is, in agreement with previous studies, reduced substantially with aging. The primary contribution of this thesis is the apportionment of various factors affecting response of an aging human thorax.