Orthotropic index for bone.

An orthotropic index (OI) is proposed to indicate the existence of a preferred material direction in each of the symmetry planes of an orthotropic material such as bone. Currently, this function is performed by the anisotropy ratio (AR) of any two Young's moduli or compressive (A(c)) and shear (A(s)) anisotropy factors comprised of complicated functions of the elastic constants. The OI incorporates the four independent engineering constants (the shear modulus and Poisson's ratio in addition to the two Young's moduli) in each symmetry plane into a single index. The OI thus improves upon the AR by reflecting orthotropy in a more holistic sense and upon the AR, A(c) and A(s) by taking on a unique value (zero) only when the material is in fact isotropic.