The deformation behavior and viscoelastic properties of chondrocytes in articular cartilage

Chondrocytes in articular cartilage utilize mechanical signals in conjuncti on with other environmental factors to regulate their metabolic activity. H owever, the sequence of biomechanical and biochemical events involved in th e process of mechanical signal transduction has not been fully deciphered. A fundamental step in determining the role of various factors in regulating chondrocyte activity is to characterize accurately the biophysical environ ment within the tissue under physiological conditions of mechanical loading . Microscopic imaging studies have revealed that chondrocytes as well as th eir nuclei undergo shape and volume changes in a coordinated manner with de formation of the tissue matrix. Through micromechanical experiments, it has been shown that the chondrocyte behaves as a viscoelastic solid material w ith a mechanical stiffness that is several orders of magnitude lower than t hat of the cartilage extracellular matrix. These properties seem to be due to the structure of the chondrocyte cytoskeleton, and in part, the viscoela stic properties of the cell nucleus. The mechanical properties of the peric ellular matrix that immediately surrounds the chondrocyte significantly dif fer from those of the chondrocyte and the extracellular matrix, suggesting that the pericellular matrix plays an important role in defining the mechan ical environment of the chondrocyte. These experimentally measured values f or cbondrocyte and cartilage mechanical properties have been used in combin ation with theoretical constitutive modeling of the chondrocyte within arti cular cartilage to predict the non-uniform and time-varying stress-strain a nd fluid flow environment of the cell. The ultimate goal of these studies h as been to elucidate the sequence of biomechanical and biochemical events t hrough which mechanical stress influences chondrocyte activity in both heal th and in disease.