Quasi-cylindrical, nanometer-scale columns of mineral platelets contribute to stiffness of bones

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Abstract
Bone is constructed of mineral platelets surrounding and lying between collagen fibrils. TEM images of ion-beam milled sections of long bones of humans and other terrestrial vertebrates cut normal to the axes of collagen fibrils display ~5nm thick polycrystalline mineral platelets (MPs) arranged either as stacks of MPs wrapping closely around collagen fibrils, or as stacks of curved MPs lying between the fibrils. Previous finite-element modelling showed that circular cylinders composed of MPs would contribute to the torsional and bending stiffness of bone. Here we used TEM to study sections of cortical bone of long bones cut normal to collagen fibrils. We found closed loops up to 570 nm in diameter on their long axis. They consist of stacks of2-5 MPs, and are mostly elliptical in form; while some are almost circular. They enclose random arrangements of cylindrical collagen fibrils closely surrounded by thicker stacks of MPs. The loops are cross sections of quasi-cylindrical columns extending for unknown distances above and below the plane of the section. The columns make up about 1/3 of the volume of cortical bone. They would contribute to the stiffness of the bone with respect to torsion and bending, and presumably evolved for that reason.

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