Nanoindentation discriminates the elastic properties of individual human bone lamellae under dry and physiological conditions

Mechanical properties of single lamellae of human compact and trabecular bone tissue were measured with a combined atomic force microscopy (AFM) and nanoindentation technique. This combination allows for both characterization of bone surface topography and indentation of the bone extracellular matrix (ECM) with depths of between 100 and 600 nm. Four bone structural units (BSUs) were tested with 400 indents under dry conditions, and four BSUs with 160 indents were tested in a liquid cell under physiological conditions. A correspondence was established between the optical appearance of bone lamellae and the topography of the polished bone surface. The indentation modulus and hardness of bone ECM were investigated as a function of lamella type and indentation depth under wet and dry conditions. For low depth indents, thick lamellae showed a higher indentation modulus than thin lamellae. With increasing indentation depth, thick lamellae exhibited a significant decrease in indentation modulus and hardness, whereas, for thin lamellae, the effect of indentation depth was much less significant. These trends were similar for dry and physiological conditions and support compositional and/or ultrastructural differences between thick and thin lamellae.