Interfacial bonding between mineral platelets in bone and its effect on mechanical properties of bone

Bone is a composite material consisting principally of apatite mineral, collagen fibrils, non-collagenous proteins, and other organic species. Recent electron microscopy studies have shown that the mineral in bone occurs as stacks of thin polycrystalline sheets ("mineral lamellae," MLs) which surround and lie between the collagen fibrils. We focus on the effect of the interface between these mineral lamellae on the mechanical properties of bone. Previous studies on bone treated with sodium hypochlorite (NaClO) to remove all organic material showed a greatly weakened mineral framework. Here, we treated femoral cortical bone with ethylenediamine (EDA), which only removes collagen, to study the effect of its removal on bone properties. We tested the degree of completion of the treatment by Raman spectroscopy and thermogravimetric analysis. When only collagen is removed, a continuous mineral structure remains and is less weakened than by NaClO treatment. Transmission electron microscopy study of finely ground particles of the EDA treated bone shows that stacks of MLs remain joined, whereas in NaClO treated bone, only isolated crystals are present. Thus, we infer that the MLs in bone are held together in stacks by an organic glue, which is destroyed by NaClO, but which survives the EDA treatment. We show that this glue may contribute to the stiffness, strength, and energy absorption of bone. Further studies are needed to discover the chemical nature of this glue. This study provides a starting point for such investigations.