The extracellular matrix, interstitial fluid and ions as a mechanical signal transducer in articular cartilage

Objective: (1) Provide an overview of the biomechanical factors that are re quired to analyze and interpret biological data from explant experiments; ( 2) Present a description of some of the mechano-electrochemical events whic h occur in cartilage explants during loading. Design: A thorough and provocative discussion on the effects of loading on articular cartilage will be presented. Five simplest loading cases are cons idered: hydrostatic pressure, osmotic pressure, permeation (pressure loadin g), confined compression and unconfined compression. Details of how such su rface loadings are converted or transduced by the extracellular matrix (ECM ) to pressure, fluid, solute and ion flows, deformation and electrical fiel ds are discussed. Results: Similarities and differences in these quantities for the five type s of loading are specifically noted. For example, it is noted that there is no practical mechanical loading condition that can be achieved in the labo ratory to produce effects that are equal to the effects of osmotic pressure loading within the ECM. Some counter-intuitive effects from these loadings are also described. Further, the significance of flow-induced compression of the ECM is emphasized, since this frictional drag effect is likely to be one of the major effects of fluid flow through the porous-permeable ECM. S treaming potentials arising from the flow of ions past the fixed charges of the ECM are discussed in relation to the flow-induced compaction effect as well. Conclusion: Understanding the differences among these explant loading cases is important; it will help to provide greater insights to the mechano-elec trochemical events which mediate metabolic responses of chondrocytes in exp lant loading experiments.