Bone Density and Trochanteric Tissue Thickness Affect Fracture of the Female Pelvis in Side Impact Experiments

In spite of advances in occupant protection, pelvic fractures remain a source of morbidity and mortality in motor vehicle side impacts. Women are more frequently injured than men, leading automotive researchers to incorporate the SID-IIs (small female crash dummy) into side impact testing of passenger vehicles. Currently accepted fracture tolerances for the female pelvis are based on extrapolations from experimental results containing both men and women, and there is no unified theory that predicts pelvic fracture for a woman of a given body habitus under side impact conditions. The present report describes a two phase experimental study aimed at improving our understanding of pelvic fractures, with an emphasis on the roles of bone quality and soft tissues. The first phase, which explored the role of bone mineral density (BMD) on impact responses, involved drop tower tests on isolated bone-ligament specimens, both male and female. The second phase employed a linear impactor and intact specimens (females only) positioned on a sliding, inertially compensated seat, where the influence of trochanteric soft tissue thickness (T) along with BMD was studied. Positive correlations between the force to fracture the pelvic ring and BMD were observed in both sets of experiments. For the intact female specimens of Phase 2, the force to fracture correlated positively with BMD and T; while impulse correlated with T only. Maximum compression, viscous criterion, and energy to fracture, however, were independent of both BMD and T. As anticipated, the force to fracture an intact pelvis was greater than that of an isolated bone-ligament specimen, for a given BMD. The force tolerance at 25% probability of fracture for the intact female specimens (average age = 76) was 3.16 kN, which is substantially below previously reported estimates for the 5 percentile female.