Depth and strain rate-dependent mechanical response of chondrocytes in reserve zone cartilage subjected to compressive loading.

The role of the growth plate reserve zone is not well understood. It has been proposed to serve as a source of stem cells and to produce morphogens that control the alignment of clones in preparation for the transition into the proliferative zone. We hypothesized that if such a role exists, there are likely to be mechanoregulatory stimuli in cellular response through the depth of the reserve zone. A poroelastic multiscale finite element model of bone/growth-plate/bone was developed for examining the reserve zone cell transient response when compressed to 5% of the cartilage thickness at strain rates of 0.18%/s, 5%/s, 50%/s, and 200%/s. Chondrocyte maximum principal strains, height-, width-, and membrane-strains were found to be highly dependent on reserve zone tissue depth and strain rate. Cell-level strains and fluid transmembrane outflow from the cell were influenced by the permeability of the calcified cartilage between subchondral bone plate and reserve zone and by the applied strain rate. Cell strain levels in the lower reserve zone were less sensitive to epiphyseal permeability than in the upper reserve zone. In contrast, the intracellular fluid pressures were relatively uniform with reserve zone tissue depth and less sensitive to epiphyseal permeability. Fluid shear stress, induced by fluid flow over the cell surface, provided mechanoregulatory signals potentially sufficient to stimulate reserve zone chondrocytes near the subchondral bone plate interface. These results suggest that the strain rate and tissue depth dependence of cell-level strains and cell surface fluid shear stress may provide mechanoregulatory cues in the reserve zone.