The structure and mechanical properties of bone lamellae at the nano-level have recently been intensively investigated. However, while the mineralized collagen fibrils are undoubtedly the basic elementary units of bone, the exact organization of those fibrils in osteonal lamellae is still under debate. Determining the fibril organization in osteonal lamellae and establishing the correlation between the structure of the lamellae, the presence of water and the local mechanical behavior, are important tasks from both biological and material science points of view.

To investigate the local mechanical properties, variations in Young’s modulus of individual lamellae around a single osteon have been measured by nanoindentation in three orthogonal directions. The main findings include the following:​ (i) The difference between the two directions perpendicular to the osteon axis (OA) is statistically insignificant in all osteonal lamellae;​ (ii) If the ratio of Young’s moduli in directions parallel and perpendicular to the OA, respectively, is defined as the anisotropy ratio (AnR), most osteonal lamellae exhibit AnR values slightly larger than 1 under dry conditions, suggesting slight, though significant, mechanical anisotropy;​ (iii) The AnR unexpectedly decreases under wet conditions, at length scales both higher and lower than the individual lamellae, which may be related to a change in fibril orientation upon rehydration;​ (iv) In general, the individual osteonal lamellae under dry and wet conditions appear to exhibit a significantly lower AnR compared to the whole bone or its cortical part.

To investigate the fine structure of osteonal lamellae, the fibril organization was studied using a serial surface view method, exposing parallel slices every 8-10nm with a dual beam microscope (FIB-SEM). It was found that the structure consists of both unidirectional sub-layers of fibrils at several orientations, and randomly oriented sub-layers always present between each pair of unidirectional sub-layers. This finding is at variance with the expected classical “rotated plywood” structure. The average fibril orientation in a lamella, as measured by FIB-SEM, is in good agreement with the observed mechanical AnR.

Finally, the potential link between Young’s modulus and the structure of individual lamellae was investigated, accounting for the fact that the structure consists of an assembly of stacked cylinders. Successive parallel slices were cut through the osteon in directions parallel and perpendicular to the OA. Due to the curvature of the structure, the resulting specimens exposed portions of lamellae with varying fibril orientation. The ensuing mechanical properties of a significant number of osteonal lamellae appeared to be correlated to the curvature.