MICROSTRUCTURE-MICROHARDNESS RELATIONS IN PARALLEL-FIBERED AND LAMELLAR BONE

Understanding the mechanical function of bone material in relation to its structure is a fascinating but very complicated problem to resolve . Part of the complexity arises from the hierarchical structural organ ization of bone. Microhardness measurements, initially on relatively s imply structured parallel-fibered bone, show a marked anisotropy in th ree orthogonal directions, This may, in part, be due to the highly ani sotropic structure of the basic building block of hone, the mineralize d collagen fibril, Microhardness measurements made face-on to the laye rs of crystals and collagen triple helical molecules, show much lower values than those made edge-on to these layers, Microhardness measurem ents of the much more complex ''rotated-plywood'' structure of lamella r bone, reveal the well-known general tendency toward anisotropy in re lation to the long axis of the bone. A detailed examination of microha rdness-microstructure relations of lamellar bone, however, shows that only in certain orientations can microhardness values be related direc tly to a specific attribute of the lamellar structure. Clearly, the gr adual tilting and rotating of the mineralized collagen fibrils that fo rm this structure produce a material that tends toward having isotropi c microhardness properties, even though its basic building block is hi ghly anisotropic, This may be an important structural attribute that a llows lamellar bone to withstand a variety of mechanical challenges.