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 variously under the action of an applied cyclical stress in confined compression over a range of loading frequencies, contributing significantly to the cartilage dynamic stiffness. Eighteen bovine cartilage cylindrical samples were tested under load control using a porous indenter in a confined compression 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 applied with the indenter at frequencies ranging from 0.0001 to 0.1 Hz. The cartilage interstitial fluid pressure and deformation were measured simultaneously as a function of time. The displacement response at the lowest tested frequency was curvefitted in the time domain to determine the linear biphasic material properties, H(A) = 0.70+/-0.10 MPa and k0=2.4x10(-16)+/-0.64x10(-16) m4/N s. These properties were employed in the biphasic theory to predict the interstitial fluid pressure response and compare it to experiment, resulting in nonlinear coefficients of determination ranging from r2 = 0.89+/-0.15 to 0.96+/-0.03 depending on frequency. It was found for the samples of this study that above a characteristic frequency of 0.00044 Hz, the magnitude and phase of fluid pressurization matched the applied stress, reducing the tissue strain at the impermeable bottom surface to nearly 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 the significance of interstitial fluid pressurization as the fundamental mechanism of cartilage load support over a wide range of frequencies.