Nonuniform swelling-induced residual strains in articular cartilage

Swelling effects in cartilage originate from an interstitial osmotic pressu re generated by the presence of negatively charged proteoglycans in the tis sue. This swelling pressure gives rise to a non-zero residual strain in the cartilage solid matrix in the absence of externally applied loads. Previou s studies have quantified swelling effects in cartilage as volumetric or di mensional change of excised samples in varying osmotically active solutions . This study presents a new optical technique for measuring two-dimensional swelling-induced residual strain fields in planar samples of articular car tilage attached to the bone (i.e., in situ). Osmotic loading was applied to canine cartilage-bone samples by equilibration in external baths of varyin g NaCl concentration. Non-zero swelling induced strains were measured in ph ysiological saline, giving evidence of the existence of residual strains in articular cartilage. Only one component of planar strain (i.e., in thickne ss direction) was found to be non-zero. This strain was found to be highly non-uniform in the thickness direction, with evidence of compressive strain in the deep zone of cartilage and tensile strain in the middle and surface zones. The obtained results can be used to characterize the material prope rties of the articular cartilage solid matrix, with estimated values of 26 MPa for the tensile modulus for middle zone cartilage. The method provides the basis to obtain material properties of the cartilage solid matrix from a simple, free-swelling test and may be useful for quantifying changes in c artilage properties with injury, degeneration and repair. (C) 1999 Elsevier Science Ltd. All rights reserved.