The deformation behavior and mechanical properties of chondrocytes in articular cartilage

Introduction: Chondrocytes in articular cartilage utilize mechanical signal s to regulate their metabolic activity. A fundamental step in determining t he role of various biophysical factors in this process is to characterize t he local mechanical environment of the chondrocyte under physiological load ing. Methods: A combined experimental and theoretical approach was used to quant ify the in-situ mechanical environment of the chondrocyte. The mechanical p roperties of enzymatically-isolated chondrocytes and their pericellular mat rix (PCM) were determined using micropipette aspiration. The values were us ed in a finite element model of the chondron (the chondrocyte and its PCM) within articular cartilage to predict the stress-strain and fluid flow micr oenvironment of the cell. The theoretical predictions were validated using three-dimensional confocal microscopy of chondrocyte deformation in situ. Results: Chondrocytes were found to behave as a viscoelastic solid material with a Young's modulus of approximately 0.6 kPa. The elastic modulus of th e PCM was significantly higher than that of the chondrocyte, but several or ders of magnitude lower than that of the extracellular matrix. Theoretical modeling of cell-matrix interactions suggests the mechanical environment of the chondrocyte is highly non-uniform and is dependent on the viscoelastic properties of the PCM. Excellent agreement was observed between the theore tical predictions and the direct measurements of chondrocyte deformation, b ut only if the model incorporated the PCM. Conclusions: These findings imply that the PCM plays a functional biomechan ical role in articular cartilage, and alterations in PCM properties with ag ing or disease will significantly affect the biophysical environment of the chondrocyte.