Video microscopy to quantitate the inhomogeneous equilibrium strain within articular cartilage during confined compression

The objectives of this study were to develop a method to quantitate the displacement and strain fields within articular cartilage during equilibrium confined compression, and to use the method to determine the variation of the equilibrium confined compression modulus with depth from the articular surface in bovine cartilage. The method made use of fluorescently labeled chondrocyte nuclei as intrinsic fiducial markers. Articular cartilage was harvested from the patellofemoral groove of adult bovines and trimmed to rectangular blocks 5 mm long, 0.76 mm wide, and 500 microns deep with the articular surface intact. Test specimens were stained with the DNA binding dye Hoechst 33258, placed in a custom confined compression chamber, and viewed with an epifluorescence microscope equipped for video image acquisition. Image processing was used to localize fluorescing chondrocyte nuclei in uncompressed and compressed (approximately 17%) specimens, allowing determination of the intra-tissue displacement profile. Strain was determined as the slope of linear regression fits of the displacement data in four sequential 125-microns-thick layers. Equilibrium strains varied 6.1-fold from the articular surface through 500 microns of cartilage depth, with the greatest compressive strain in the superficial 125-microns layer and the least compressive strain in the two deepest 125-microns layers. Thus, the four successive 125-microns layers have moduli that are 0.44 (superficial), 1.07, 2.39, and 2.67 (deep) times the apparent modulus for a 500-microns thick cartilage sample assumed to be homogeneous.