The consequences of dehydration-hydration on bone anisotropy and implications on the sublamellar organization of mineralized collagen fibrils.
In this study, the effect of dehydration-hydration based sublamellar dimensional change on bone anisotropy was used as a tool to understand sublamellar organization of mineralized collagen fibrils. Bone consists of hydroxyapatite, Type I Collagen, mucopolysaccharides and bone fluid, which associates with bone constituents and improves the mechanical properties of bone. Knowing that dehydration causes dimensional changes comparable to those observed in the mechanical testing of a bone sample, here, the dehydrated organic component of lamellar bone was modelled to contract towards the mineral, forming a contraction vector as the surface normal of the mineral plate. The amount of dehydration based contraction in rotated collagen fibrils was calculated for two models of sublamellar arrangements, namely A and B, where the mineral plate of the 0° (axial, [0 0 1]) sublamellar collagen fibril was oriented respectively along either (0 1 0) or (0 0 1) planes. Projections of sublamellar contraction vectors were denoted as u, v and w displacements at 10°-20°-30° angles and summed to give the lamellar total. Using the total displacements, anisotropy ratios of properties in directions parallel (W) versus perpendicular (U or V) to the osteonal axis were calculated. With dehydration, the osteonal lamellae in Model A (behaving as positive Poisson's ratio material) may display maximal planar expansion (at 1.4%) and peraxial contraction (-0.24%), which may even cause sample warping. The large variation in the wet and dry bone anisotropy ratios of the models demonstrates the effect of collagen orientation on bone mechanics.