The biomechanical role of the collagen fibril ultrastructure of articular cartilage

Abstract

The biomechanical role of the collagen fibril ultrastructure of articular cartilage has been examined in two problems. A continuum model of the articular cartilage on bone structure was developed to examine the effect of the fibrous structure on the stresses, strains and displacements occurring in articular cartilage in physiological loadings. Also, a discrete fibril model of the fibrous structure of soft connective tissues of which articular cartilage is one, was developed to examine the macroscopic behavior of these tissues in terms of the microscopic actions of the fibrous network.

The biological aspects of these problems were not considered.

A discrete fibril model of soft connective tissues was developed wherein fibrils) whose properties were random variables) take up deformed states under the influence of the continuum strain field. The model was simplified such that linear behavior of the individual fibrils was assumed) and only one fibril property) the curl factor C was a random variable. Using the mean value and the standard deviation of the curl factor distribution as parameters, stress-strain expressions of the model were fitted to experimental tensile test data for tendon and articular cartilage available in the literature. The best fit parameters for the tendon tests were associated with observed structural features of the tissue. The model provides an alternative viewpoint for the study of the fibrous component of articular cartilage.

The modeled behavior of articular cartilage was found to differ from that found by other investigators studying homogeneous isotropic models of articular cartilage. The inhomogeneous cartilage on bone structure prevented the creation of tensile stresses on the articular surface. However, tensile strains were induced by the inhomogeneous structure at the articular surface under the load, the most common area of xiv osteoarthritic degeneration. It is suggested that strain rather than stress is the cause of the mechanical insult involved in the etiology of osteoarthritis. While decreased stiffness of the middle and deep zones of articular cartilage and increased stiffness of the subchondral bone had detrimental effects on the deformation of the tissue, the most drastic effects were caused by changes in the area over which the load was distributed. The unique fibrous structure of the superficial tangential zone decreased the magnitudes of the stresses and strains to which the articular surface was exposed. The sole mechanical function of articular cartilage is to provide an efficient bearing surface for the joint.

Linear elasticity theory and Hankel transform methods were used to solve the axisymmetric boundary value problem of a normal surface traction on the surface of the layered model. The transformed solutions were inverted numerically using a computer.

The continuum model of articular cartilage, based on scanning electron microscope observations of the fibrous ultrastructure, was a layered medium. The material symmetries of the layers were chosen to reflect the fibrous ultrastructure of each region. Material properties required by the model, which incorporated a transversely isotropic superficial layer, were taken from experimental data available in the literature.