**2. Transversely isotropic or orthotropic material properties**

In bone, like wood and many other biological structures, there is a "grain" or preferred direction associated with the structure. The mechanical behavior of bone and other directional composites is dependent upon the direction of the applied load. Bone material is assumed as anisotropic, and as many as twenty-one independent elastic constants are required to completely characterize their mechanical behavior. Most materials have planes of symmetry that reduce the number of material constants [1, 2]. For example, materials having properties that differ in each of two mutually perpendicular directions are termed orthotropic. Nine elastic constants are required to fully characterize their mechanical behavior. To determine the nine independent elastic coefficients of an orthotropic material the following mechanical tests are required. Compressive tests in each of three mutually perpendicular material directions; three lateral deformation tests to obtain Poisson ratios. Standards from the ASTM C469, D1621 [3, 4] have been adapted for mechanical testing procedures on biological tissue [5]. Bone is assumed to be highly anisotropic. This anisotropy in different issues may vary. In the long bone appetite needles, collagen fibers, lamellae, blood vassal, etc. show a clear tendency to be oriented along the length of the bone. In general most of the loads in bone are likely to be acting along its length.
