The mechanical environment of the chondrocyte: a biphasic finite element model of cell-matrix interactions in articular cartilage

Mechanical compression of the cartilage extracellular matrix has a signific ant effect on the metabolic activity of the chondrocytes. However, the rela tionship between the stress-strain and fluid-flow fields at the macroscopic "tissue" level and those at the microscopic "cellular" level are not fully understood. Based on the existing experimental data on the deformation beh avior and biomechanical properties of articular cartilage and chondrocytes, a multi-scale biphasic finite element model was developed of the chondrocy te as a spheroidal inclusion embedded within the extracellular matrix of a cartilage explant. The mechanical environment at the cellular level was fou nd to be time-varying and inhomogeneous, and the large difference (similar to 3 orders of magnitude) in the elastic properties of the chondrocyte and those of the extracellular matrix results in stress concentrations at the c ell-matrix border and a nearly two-fold increase in strain and dilatation ( volume change) at the cellular level, as compared to the macroscopic level. The presence of a narrow "pericellular matrix" with different properties t han that of the chondrocyte or extracellular matrix significantly altered t he principal stress and strain magnitudes within the chondrocyte, suggestin g a functional biomechanical role for the pericellular matrix. These findin gs suggest that even under simple compressive loading conditions, chondrocy tes are subjected to a complex local mechanical environment consisting of t ension, compression, shear, and fluid pressure. Knowledge of the local stre ss and strain fields in the extracellular matrix is an important step in th e interpretation of studies of mechanical signal transduction in cartilage explant culture models. (C) 2000 Elsevier Science Ltd. All rights reserved.