Abstract
Articular cartilage is one of several biological tissues in which swelling effects are important in tissue mechanics and function, and may serve as an indicator of degenerative joint disease. This work presents a new approach to quantify swelling effects in articular cartilage, as well as to determine the material properties of cartilage from a simple free-swelling test. Samples of nondegenerate and degenerate human patellar cartilage were subjected to osmotic loading by equilibrating the tissue in solutions of varying osmolarity. The resulting swelling-induced strains were measured using a noncontacting optical method. A theoretical formulation of articular cartilage in a free-swelling configuration was developed based on an inhomogeneous, triphasic mechano-chemical model. Optimization of the model predictions to the experimental data was performed to determine two parameters descriptive of material stiffness at the surface and deeper cartilage layers, and a third parameter descriptive of thickness of the cartilage surface layer. These parameters were used to determine the thickness-averaged uniaxial modulus of cartilage, H(A). The obtained values for H(A) were similar to those for the tensile modulus of human cartilage reported in the literature. Degeneration resulted in an increase in thickness of the region of "apparent cartilage softening," and a decrease in the value for uniaxial modulus at this layer. These findings provide important evidence that collagen matrix disruption starts at the articular surface and progresses into the deeper layers with continued degeneration. These results suggest that the method provides a means to quantify the severity and depth of degenerative changes in articular cartilage. This method may also be used to determine material properties of cartilage in small joints in which conventional testing methods are difficult to apply.