Articular cartilage is a connective tissue covering the surfaces of diarthrodial joints. It contributes to weight-bearing, energy dissipation and load distribution in the joint. Osteoarthritis is a degenerative joint disease associated with breakdown of cartilage. Studies of cartilage mechanics and changes in cartilage properties with osteoarthritis contribute to the understanding of the degeneration mechanism and may be useful for the diagnosis and treatment of the disease.

This study presented a new method to determine cartilage material properties and describe changes in these properties with osteoarthritis. A new optical technique was developed to measure the two-dimensional distribution of swelling-induced strains in cartilage equilibrated in physiological and hypotonic saline solutions, relative to the hypertonic reference configuration. A new theoretical model for a free-swelling problem of cartilage was proposed based on the principles of a triphasic mechano-chemical theory. Cartilage was modeled as an isotropic linearly elastic material with both homogeneous and inhomogeneous behaviors considered. The effects of the entropic interactions on the total swelling pressure in cartilage were also considered. This model was used to determine the uniaxial modulus of healthy canine cartilage, as well as healthy and degenerated human cartilage, from the experimentally measured swelling-induced strains.

The distribution patterns of swelling-induced strains in the thickness direction differed significantly between healthy canine or human cartilage and degenerated human cartilage, with a dramatic increase in the strain magnitude at the surface of the degenerated samples. The strain distribution for degenerated cartilage could only be described by the inhomogeneous model. The values for the uniaxial modulus of canine and human cartilage were found to reflect the tensile, and not the compressive, behavior. A region of a reduced tensile modulus existed near the surface of both normal and degenerated cartilage. Degeneration was associated with a decrease in the uniaxial modulus at the surface, and with an increase in the depth of the layer with a reduced modulus. The entropic effects were found to make a significant contribution to the total swelling pressure in cartilage in the physiological and hypotonic configurations.

This method has the potential to quantify cartilage material properties without the need to contact the sample, and can be applied to study mechanical properties of cartilage in small animal models of osteoarthritis.