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

Abstract

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 NaCl 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 µm 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 7 MPa of nominal compressive stress over 11 and 13 min. The observed rupture rates for the dehydrated specimens loaded with 7 MPa over 60 and 90 s were 20% versus 40% and 20% versus 60% for healthy and degenerate tissues, respectively. At 8 MPa of nominal compressive stress over 60 and 90 s 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.

Effect of hydration on the frequency-dependent viscoelastic properties of articular cartilage

“…This is of relevance to the earliest stages of osteoarthritis where the cartilage takes in fluid. 9 Our finding is consistent with studies that have found hyper-hydrated cartilage to reduce the loading required for surface rupture 1,27 and that cartilage is stronger following fluid removal. 3 Although, such increased strength is combined with a reduction to the timeframe over which failure occurs 1 possibly by reducing the ability of cartilage to redistribute stress away from the loading site.…”

How the Structural Integrity of the Matrix Can Influence the Microstructural Response of Articular Cartilage to Compression

“…The ability of a healthy (intact) surface layer to transmit force over a wider loaded region has previously been proposed as a mechanism that AC uses to prevent rupture [34,35] and can also account for how the healthy (intact) surface layer is able to transfer force over a wider loaded area. The downward drawing of the tissues within the central pore region reflects the reaction of the tissue to these forces.…”

“…A flat-ended polished indenter (8 mm diameter) that contained a 450 μm diameter central pore/hole was selected because it provided a localized region where stresses could generate within the tissue during compression [34,35]. A creep compression rig was utilized for these tests and all tissues remained in distilled water during testing, consistent with a previous employed protocol [13].…”

“…The surfaceremoved tissues had a median of 30.5 AE 4.8% of their total tissue depth removed through the use of a microtome (Leica Microsystems Model SM2000R, Wetzlar, Germany). All samples were set in cylindrical stainless steel mounts and mounted identical to described previously [34,35].…”

“…A central pore within a flat-ended indenter would not only take into account the anisotropic nature of AC, but it would also produce a region that would allow cartilage to deform freely around the pore during compression [34,35]. Therefore, this study aims to investigate how AC deforms around a central pore within a flat-ended indenter.…”