The Biomechanical Function of the Collagen Fibril Ultrastructure of Articular Cartilage

An inhomogeneous, layered, anisotropic, continuum model of the in-situ articular cartilage-on-bone structure is developed. The model, based on the collagen fibril ultra-structure of the tissue, incorporates an anisotropic (transversely isotropic) layer modeling the superficial tangential zone. The stresses, strains and displacements generated in this model by a static, axisymmetric, parabolically distributed shear-free surface load are determined, using integral transforms and numerical methods. The predicted deformation under the modeled normal physiological loading condition is examined along with deformation changes resulting from abnormal changes in loading geometry and material properties. The results of this analysis of the modeled, layered, inhomogeneous structure, when extrapolated to normal in-vivo conditions, imply that the articular surface is not exposed to continuum tensile stresses, but may be predisposed to possible long term damage due to fatigue by tensile strains. Abnormal decreases in the loaded area of the joint, perhaps due to loss of congruency or degeneration in the joint, cause changes in the magnitude and in the character of the deformational quantities that are detrimental to the long-term survival of the tissue.