THE ANISOTROPY OF OSTEONAL BONE AND ITS ULTRASTRUCTURAL IMPLICATIONS

The anisotropic elastic symmetry of osteonal bone reflects the ultrast ructural organization of collagen fibrils and mineral crystals within the osteons as well as the lamellar microstructure. Until recently, re ported values for bone's anisotropic elastic properties were limited i n their interpretation by poor precision and resolution of measurement techniques. Here, we report measurements of bone anisotropy using hig h precision acoustic microscopy. The elastic properties of canine femo ral bone specimens, taken from 23 femora, were measured at 10 degrees increments from the long axis of the bone. Half of the bone specimens subsequently were demineralized in EDTA solution, the other half were decollagenized in sodium hypochlorite solution, and the acoustic measu rements were repeated. We found the elastic symmetry of osteonal bone deviates significantly from orthotropic theory supporting the hypothes is that the lamellar microstructure forms a ''rotated plywood'' (Weine r and Traub, FASEB J 6:879-885; 1992). The principal orientation of bo ne mineral was along the long axis of the bone, while bone collagen ap peared to be aligned at a 30 degrees angle to the long axis. The misal ignment between the mineral and the collagen suggests that (1) a subst antial percentage of the mineral is extrafibrillar, and (2) the alignm ent of extrafibrillar mineral is governed by external influences, e.g. , mechanical stresses.