In vivo strains in the femur of the nine-banded armadillo (Dasypus novemcinctus)

Abstract

Understanding of the interplay between bone loading patterns and bone material properties in mammals has been based primarily on evidence from upright eutherians, which show limb bones that are loaded predominantly in anteroposterior (AP) bending with minimal torsion. However, loading patterns from the femora of marsupial opossums using crouched limb posture, show appreciable torsion while the bone experiences mediolateral (ML) bending. These data indicated greater locomotor loading diversity than was previously recognized, and suggested the possibility that ancestral loading patterns found in sprawling reptiles might have been retained among basal mammals. To further test this hypothesis, in vivo locomotor strains were recorded from the femur of the nine-banded armadillo (Dasypus novemcinctus). Orientations of principal strains and magnitudes of shear strains indicate that their femora are exposed to a limited amount of torsion, while loading is dominated by ML bending that places the medial aspect of the femur in compression and the lateral aspect in tension. This orientation of bending is similar to that found in opossums, but planar strain analyses indicate much more of the armadillo femur experiences tension during bending, potentially due to the actions of large muscles attached to the robust third trochanter (T3). Comparisons of peak locomotor loads to evaluations of femoral mechanical properties lead to estimates of limb bone safety factors between 2.3--5.0 in bending, similar to other eutherians, but lower than opossums and most sprawling taxa. Thus, femoral loading patterns in armadillos show a mixture of similarities to both opossums (ML bending) and eutherians (limited torsion and low safety factors), along with unique features (high axial tension) that likely relate to their distinctive hindlimb anatomy.