Articular cartilage surface failure: an investigation of the rupture rate and morphology in relation to tissue health and hydration

This study investigates the rupture rate and morphology of articular cartilage by altering the bathing environments of healthy and degenerate bovine cartilage. Soaking tissues in either distilled water or 1.5 M NaCI saline was performed in order to render the tissues into a swollen or dehydrated state, respectively. Creep compression was applied using an 8 mm flat-ended polished indenter that contained a central pore of 450 microm in diameter, providing a consistent region for rupture to occur across all 105 tested specimens. Rupture rates were determined by varying the nominal compressive stress and the loading time. Similar rupture rates were observed with the swollen healthy and degenerate specimens, loaded with either 6 or 7MPa of nominal compressive stress over 11 and 13 min. The observed rupture rates for the dehydrated specimens loaded with 7 MPa over 60 and 90s were 20% versus 40% and 20% versus 60% for healthy and degenerate tissues, respectively. At 8 MPa of nominal compressive stress over 60 and 90s the observed rupture rates were 20% versus 60% and 40% versus 80% for healthy and degenerate tissues, respectively; with all dehydrated degenerate tissues exhibiting a greater tendency to rupture (Barnard's exact test, p <0.05). Rupture morphologies were only different in the swollen degenerate tissues (p <0.05). The mechanisms by which dehydration and swelling induce initial surface rupture of mildly degenerate articular cartilage differ. Dehydration increases the likelihood that the surface will rupture, however, swelling alters the observed rupture morphology.