ANISOTROPHY AND NONLINEARITY OF CARTILAGE MECHANICS

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Articular cartilage is the bearing material of diarthrodial joints. Its primary mechanical function is to transmit large loads across the articular surfaces of joints with minimal friction and wear; under normal conditions, cartilage can maintain this function for seven to eight decades. From an engineering perspective, the mechanical behavior of cartilage is considered to be remarkable, unmatched by any traditional engineering bearing material. However, despite several decades of sophisticated biomechanical studies of cartilage, an accurate understanding of cartilage mechanics remains elusive due to its remarkable versatility and complexity. Studies have demonstrated that the mechanical response of articular cartilage may vary as a function of duration and rate of loading or deformation, i.e., cartilage exhibits viscoelasticity. Furthermore, it has been shown that the tensile stiffness of cartilage differs when testing the tissue parallel and perpendicular to the split line directions, i.e., it exhibits anisotropy. It has also been established that the stiffness of cartilage in compression may be one to two orders of magnitude smaller than in tension, i.e., it exhibits tension-compression nonlinearity. Various studies have also confirmed that these measured properties may vary from the superficial to the deep zone of cartilage, i.e., the tissue exhibits depth-dependent inhomogeneity. To date, no single constitutive model of articular cartilage has been able to describe its mechanical response under the various testing conditions described in the literature. The hypotheses of this proposal are that (1) cartilage is orthotropic, requiring more material constants than have been measured to date to describe its mechanical response; (2) the tension-compression nonlinearity of cartilage requires that some, but not all of these constants have different values in tension and compression; and (3) that a theoretical framework encompassing cartilage orthotropy and tension-compression nonlinearity, using mixture theory, can provide agreement between theory and experiment for all testing configurations; this agreement improves when incorporating tissue inhomogeneity in the analysis. Therefore, the specific aims of this proposal are to test human patellar cartilage samples in tension, compression, shear and permeation, along the three mutually perpendicular directions which are hypothesized to characterize the planes of material symmetry, such as to determine experimentally a complete set of elastic and permeability constants of cartilage; to determine whether these constants indeed describe an orthotropic material; to experimentally assess the depth-dependent inhomogeneity of cartilage; and to compare transient and equilibrium experimental responses to corresponding predictions from a newly proposed biphasic, octantwise orthotropic, conewise linear elasticity model with depth-dependent inhomogenous properties. To achieve these aims, it is proposed to use the most current techniques for measurement of tissue mechanical properties.