Interstitial fluid pressurization during confined compression cyclical loading of articular cartilage

The objective of this study was to experimentally verify the well-accepted but untested hypothesis that cartilage interstitial fluid pressurizes vario usly under the action of an applied cyclical stress in confined compression over a range of loading frequencies, contributing significantly to the car tilage dynamic stiffness. Eighteen bovine cartilage cylindrical samples wer e tested under load control using a porous indenter in a confined compressi on chamber fitted with a microchip pressure transducer at its bottom. Over a static stress of 130 kPa. a cyclical stress of amplitude 33 kPa was appli ed with the indenter at frequencies ranging from 0.0001 to 0.1 Hz. The cart ilage interstitial fluid pressure and deformation were measured simultaneou sly as a function of time. The displacement response at the lowest tested f requency was curvefitted in the time domain to determine the linear biphasi c material properties, H-A=0.70 +/- 0.10 MPa and k(o) = 2.4 x 10(-16) +/- 0 .64 x 10(-16) m(4)/N s. These properties were employed in the biphasic theo ry to predict the interstitial fluid pressure response and compare it to ex periment, resulting in nonlinear coefficients of determination ranging from r(2)=0.89 +/- 0.15 to 0.96 +/- 0.03 depending on frequency. It was found f or the samples of this study that above a characteristic frequency of 0.000 44 Hz, the magnitude and phase of fluid pressurization marched the applied stress, reducing the tissue strain at the impermeable bottom surface to ne arly zero. The findings of this study verify the hypothesis that cartilage dynamic stiffness derives primarily from flow-dependent viscoelasticity as predicted by the linear biphasic theory; they demonstrate experimentally th e significance of interstitial fluid pressurization as the fundamental mech anism of cartilage load support over a wide range of frequencies. (C) 2000 Biomedical Engineering Society. [S0090-6964(00)00202-2].