Mechanical forces are thought to modulate the metabolic activity and promote the structural adaptation of a variety of connective tissues including cartilage. However, "abnormal" forces may predispose cartilage to degeneration, as in osteoarthritis, and modeling errors, as in ball and socket ankle joint. During dynamic loading of cartilage, a number of physical phenomena occur naturally and are inexorably coupled; compression of cartilage results in deformation of cells and extracellular matrix, hydrostatic pressurization, fluid flow, streaming potentials, and physicochemical changes associated with increased matrix charge density. The objectives of this thesis were (1) to characterize the effects of static and dynamic compression on the synthesis, assembly, and degradation of extracellular matrix constituents, and (2) to relate metabolic responses to possible biophysical regulatory mechanisms.
Chambers were designed to allow uniaxial radially unconfined compression and mechanical testing of cartilage disks while maintaining an organ culture environment. Dynamic stiffness measurements of 3-mm diameter disks identified a characteristic frequency (0.001 Hz) that separated low- and high-frequency regimes in which different flow and deformation phenomena predominated. At 20.01 Hz, oscillatory strains of only ~1-5% caused hydrostatic pressurization within disks and stimulated biosynthesis (SH-proline and SS-sulfate incorporation) by ~20-40%. At f<0.001 Hz, oscillatory compressions of <5% caused fluid exudation from cartilage disks with little pressurization, but did not affect synthesis. In contrast, static compression depressed synthesis in a dose-dependent manner.
Proteoglycans endow cartilage with a large swelling pressure due to their high fixed charge density. Most cartilage proteoglycans are aggregated on a central hyaluronate backbone to form a macromolecular complex; the size of the aggregate effectively immobilizes proteoglycans within the collagenous meshwork. Newly synthesized proteoglycans undergo an extracellular conversion process whereby their binding affinity for hyaluronate increases. Static compression slowed the c process, as did incubation in acidic media (without compression), both in a dose dependent manner. Oscillatory compression [f=0.001-0.1 Hz) did not affect the conversion. The similarly delayed kinetics of affinity conversion with compression and acidic media suggest that this delay may be due in both cases to a physicochemical mechanism of decreased intratissue pH. Such a regulatory mechanism may have a physiologic role in the development or remodeling of the cartilage matrix, even in unloaded tissue, favoring deposition of newly synthesized proteoglycans in the low charge (proteoglycan-poor) regions.
The turnover of proteoglycans is increased in experimental osteoarthritis, and may be influenced by physical or biological mechanisms. Slow cyclic compression of cartilage disks, radiolabeled with ³⁵S-sulfate and ³H-proline, led to increases in the release of radiolabeled macromolecules as well as an enrichment of the culture media with collagenous [hydroxyproline-containing) ³H-residues and non-aggregating $5S-proteoglycans. The patterns of release were consistent with several physical phenomena (convection, diffusion, and disruption of the collagen meshwork) as well as biological effects [decreased deposition of proteoglycan).
These studies provide a framework for identifying both physical and biological mechanisms by which static and dynamic compression can modulate the accumulation, maintenance, or degradation of a mechanically functional extracellular matrix in cartilage. The culture and compression methodology potentially allows in vitro evaluation of clinical strategies of physical therapies (e.g., continuous passive motion) to stimulate cartilage remodeling.
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