Transplantation of chondrocytes has shown promise for augmenting the repair of articular cartilage defects. An in vitro model system was developed to examine the role of static compression on chondrocytes transplanted to articular cartilage explants. In addition, as a major step toward understanding how dynamic loading may influence chondrocyte-mediated repair, the fields, forces and flows that result from the loading of full-thickness cartilage that has inhomogeneous properties was theoretically and experimentally assessed. These studies used continuum electromechanics, biomechanical testing, biochemical assays, and immunohistochemistry techniques to gain an understanding of the biomechanical-metabolic relationships in cartilage.
Transplantation of bovine chondrocytes onto articular cartilage disks at densities up to 650,000 cells/cm² was achieved. The transplanted cells expressed the normal chondrocyte phenotype as indicated by synthesis of aggrecan that was able to aggregate with hyaluronate and link protein, and expression of Type II collagen.
The in vitro studies also examined proteoglycan biosynthesis by bovine chondrocytes after transplantation onto and compression between articular cartilage disks. After short seeding times (1 h), chondrocyte retention was susceptible to loading. Compression of up to 0.6 MPa inhibited proteoglycan biosynthesis.
These studies suggest that mechanical forces regulate the metabolic response of transplanted chondrocytes. Static compression due to the tightness of the “press fit” at sites of chondrocyte transplantation may affect subsequent integrative repair.
A new method of assessing streaming potentials in articular cartilage was developed using the confined compression creep test. The electrokinetic material property, ke, was determined using pulses of compressive stress and the relationship AV=keΔσ. Enzymatic treatment of normal cartilage by chondroitinase ABC or trypsin demonstrated that ke is dependent upon tissue composition. The material property, ke, was also estimated at different depths from the articular surface and the static offset stress using pulse and oscillatory confined compression experiments.
Analysis of cartilage mechanics and electromechanics using homogeneous and layered (inhomogeneous) models indicated significant differences in the streaming potential and stiffness behavior of full-thickness cartilage.
The departures of the potential and stiffness behavior, in a frequency- and offset-compression-dependent manner, of articular cartilage from that predicted for a homogeneous sample may be useful characteristic indicators of normal tissue.
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