Estimation of in situ elastic properties of biphasic cartilage based on a transversely isotropic hypo-elastic model

Articular cartilage is known to behave nonlinearly for large deformations. Mechanical properties derived from smell strain experiments yield excessive ly large deformations in finite element models used in the study of severe blunt impact to joints. In this manuscript, a method is presented to determ ine the nonlinear elastic properties of biphasic cartilage based on a trans versely isotropic hypo-elastic model. The elastic properties were estimated by fitting two force-displacement curves (in rapid loading and at equilibr ium) obtained from large deformation indentation relaxation tests on cartil age using a nonporous spherical indentor. the solid skeleton of the cartila ge was modeled as transversely isotropic hypo-elastic material and a commer ical finite element program was employed to solve the problem of a layer in dented by a rigid sphere. Components of the hypo-elasticity tensor were mad e dependent on deformation according to the variations defined by a transve rsely isotropic hyperelastic formulation given earlier by others. Material incompressibility was assumed during the initial stage of rapid loading. Th e analysis was utilized for the determination of in situ properties of rabb it retropatellar cartilage at large deformations. the model was able to fit the material response to rapid loading and equilibrium indentation test da ta to approximately 50 percent strain. This material model suggested even h igher percentage of stress supported by the fluid phase of cartilage than g iven earlier by smaller deformation theories of biphasic cartilage.