The goals of this research were to (i) develop a tissue model system for studying the microstructure of matrix produced by chondrocytes, (ii) characterize the biochemical and mechanical properties of the chondrocyte culture tissue, (iii) evaluate the response of the chondrocyte culture tissue to various stimulants (retinoic acid, interleukin-ip, and xyloside), (iv) investigate the roles of proteoglycan and collagen in the tearing and tensile properties of a chondrocyte culture tissue, and (v) develop a finite element model of the chondrocyte culture tissue microstructure to study its tensile pre-failure properties Tearing properties of cartilage are important since tensile strength testing alone cannot predict when failure in cartilage will occur. Failure testing is valuable because it is known that cracks exist and propagate from the cartilage surface in osteoarthritic joints

Proteoglycan and collagen are two microstructural molecules in cartilage extracellular matrix that may determine the tear and tensile properties of cartilage-like tissues. The roles of these molecules were explored by experimentation using a cultured cartilage tissue, and by development of a theoretical finite element model which related the cartilage tissue microstructure to its macroscopic properties.

The cartilage culture tissue tested was characterized, and was digested free of specific matrix molecules in an attempt to identify the roles of each molecule This cartilage culture tissue was similar to immature normal cartilage in that the collagen fibril diameters were in the low range for normal cartilage, while the water content was in the high range for normal cartilage, and the material tensile stiffness was in the low range.

It was found that collagen was important for providing the material stiffness of the cultured tissue, and that both collagen and proteoglycan were important for providing the tear toughness of the tissue. It was also found that as the collagen density or collagen material stiffness increased, the material stiffness of the cultured tissue increased, and as the proteoglycan or collagen densities increased, the tear toughness of the tissue increased.

A three-dimensional finite element microstructural model of cartilage was developed, and consisted of linear elastic collagen fibrils embedded in a linear viscoelastic proteoglycan solid matrix. Fluid flow in the cartilage matrix was not included in this model since the tissue experimentally tested in this research was loaded slowly, which would not permit interstitial fluid flow to influence the stiffness measurements. Therefore a viscoelastic time dependent behavior was an appropriate model for the cartilage The results of this model were comparable to the experimental results, as well as to past continuum models of cartilage. Both collagen and proteoglycan material stiffnesses were found to be important in determining the material stiffness of the cartilage. As the material stiffness of the proteoglycan decreased, the failure strain of the cartilage increased, while the ultimate strength remained unaffected. This model may be applicable to the poorly understood joint disease, osteoarthritis, where both proteoglycan loss and collagen degradation occur.