NANOMETER-SCALE POLYCRYSTALLINE APATITE PLATES ARE THE RE-BARS OF BONE (Invited Presentation)

Bone is a composite material composed of collagen fibers mineralized with apatite. The nanometer-sized crystals of apatite must be visualized using transmission electron microscopy (TEM). Earlier TEM studies of bone using sections cut by an ultramicrotome, found that the apatite was mainly inside the collagen fibrils, in periodically spaced 40 nm-wide zones. Using ion milling we showed however that most of the mineral is in the form of polycrystalline plates up to 100s of nm across, and 5 nm thick which surround or lie between the fibrils; they are brittle and were always shattered by the ultramicrotome. The plates are mosaics of single 5 nm thick apatite crystals which are flattened on the 10-10 plane, and are either anhedral in other directions, or weakly elongated along the c axis. Stacks of 4 to >20 plates (“mineral lamellae”) are separated by <1nm gaps apparently containing a tough organic glue: stacks break across the stack rather than delaminating.

Although described in the bone mineral literature as hydroxyapatite, <20% of the c-axis channel sites in the mineral are occupied by OH. Chemically, it is a CO₃-HPO₄ apatite. All XRD reflections are consistent with the hexagonal P6m/3 space group although the flattened shape of the crystals implies violation of this symmetry.

Finite element modeling of the mechanical properties of bone show that a model including such features generates greater stiffness and strength than are obtained using conventional models in which most of the mineral, in the form of isolated crystals, is situated inside collagen fibrils. The mineral lamellae act as stiffening elements that are preferentially aligned parallel to the long axis of long bones, essentially acting as re-bars to a material otherwise composed of soft collagen.