The deformation behavior and mechanical properties of articular chondrocytes are believed to play an important role in their response to mechanical loading of the extracellular matrix. This study utilized the micropipette aspiration test to measure the viscoelastic properties of chondrocytes isolated from macroscopically normal or end-stage osteoarthritic cartilage. A three-parameter standard linear solid was used to model the viscoelastic behavior of the cells. Significant differences were found between the mechanical properties of chondrocytes isolated from normal and osteoarthritic cartilage. Specifically, osteoarthritic chondrocytes exhibited a significantly higher equilibrium modulus (0.33 +/- 0.23 compared with 0.24 +/- 0.11 kPa), instantaneous modulus (0.63 +/- 0.51 compared with 0.41 +/- 0.17 kPa), and apparent viscosity (5.8 +/- 6.5 compared with 3.0 +/- 1.8 kPa-s) compared with chondrocytes isolated from macroscopically normal, nonosteoarthritic cartilage. The elastic moduli and relaxation time constant determined experimentally in this study were used to estimate the apparent biphasic properties of the chondrocyte on the basis of the equation for the gel relaxation time of a biphasic material. The differences in viscoelastic properties may reflect alterations in the structure and composition of the chondrocyte cytoskeleton that have previously been associated with osteoarthritic cartilage. Coupled with earlier theoretical models of cell-matrix interactions in articular cartilage, the increased elastic and viscous properties suggest that the mechanical environment of the chondrocyte may be altered in osteoarthritic cartilage.

Biomechanical factors are believed to play a n important role in regulating the metabolic activity of chondrocytes in articular cartilage. Previous studies suggest that cytoskeletal proteins such as actin, vimentin, and tubulin influence cellular mechanical properties, and may therefore influence the mechanical interactions between the chondrocyte and the surrounding tissue matrix. In this study, we investigated the role of specific cytoskeletal components on the mechanical properties of individual chondrocytes isolated from normal or osteoarthritic hip articular cartilage. Chondrocytes were exposed to a range of concentrations of chemical agents that disrupt the primary cytoskeletal elements (cytochalasin D for F-actin microfilaments, acrylamide for vimentin intermediate filaments, and colchicine for microtubules). Chondrocyte mechanical properties were determined using the micropipette aspiration technique coupled with a viscoelastic solid model of the cell. Chondrocyte stiffness (elastic modulus) was significantly increased with osteoarthritis. With increasing cytochalasin D treatment, chondrocyte stiffness decreased by up to 90%) and apparent viscosity decreased by up to SOYO. The effect of cytochalasin D was greater on normal chondrocytes than those isolated from osteoarthritic cartilage. Treatment with acrylamide also decredsed the moduli and viscosity, but only at the highest concentration tested. No consistent changes in cell mechanical properties were observed with colchicine treatment. These findings suggest that microfilaments and possibly intermediate filaments provide the viscoelastic properties of the chondrocyte, and changes in the structure and properties of these cytoskeletal elements may reflect changes in the chondrocyte with osteoarthritis.