Principal stiffness orientation and degree of anisotropy of human osteons based on nanoindentation in three distinct planes

Haversian systems or 'osteons' are cylindrical structures, formed by bone lamellae, that make up the major part of human cortical bone. Despite their discovery centuries ago in 1691 by Clopton Havers, their mechanical properties are still poorly understood. The objective of this study is a detailed identification of the anisotropic elastic properties of the secondary osteon in the lamella plane. Additionally, the principal material orientation with respect to the osteon is assessed. Therefore a new nanoindentation method was developed which allows the measurement of indentation data in three distinct planes on a single osteon. All investigated osteons appeared to be anisotropic with a preferred stiffness alignment along the axial direction with a small average helical winding around the osteon axis. The mean degree of anisotropy was 1.75±0.36 and the mean helix angle was 10.3°±0.8°. These findings oppose two well established views of compact bone microstructure: first, the generally clear axial stiffness orientation contradicts a regular 'twisted plywood' collagen fibril orientation pattern in lamellar bone that would lead to a more isotropic behavior. Second, the class of transverse osteons were not observed from the mechanical point of view.