Mechanical characterization of articular cartilage degraded combinedly with MMP-1 and MMP-9.

Matrix metalloproteinases (MMPs) degrade the extracellular matrix (ECM) subsequently damaging cartilage and altering biomechanical properties. Collectively, MMPs cleave every ECM macromolecule. However, MMPs present complex substrate interactions and digest differing ECM components making it difficult to understand the individual role each MMP plays in cartilage degradation. To understand the combined impact MMPs have on cartilage biomechanical properties, MMPs from two subfamilies: collagenase and gelatinase were investigated. Three ratios of MMP-1 (c) and MMP-9 (g), c1:g1, c1:g0 and c0:g1 were considered. Cartilage plugs (n = 30) were collected from the femoral condyles of 3 bovine stifle joints. In groups of 10, samples were treated with MMP-1, MMP-9, or a combination. Samples were subjected to indentation loading up to 20% bulk strain and were assessed mechanically and histologically to determine the degradative impact. Young's modulus and peak load were compared between the control and degraded explants. In comparison to samples degraded by MMP-1 or MMP-9 individually, cartilage degraded with both enzymes resulted in a 9-15% greater reduction in stiffness and peak load. Individually, MMP-1 and MMP-9 have a minimum effect on cartilage micromechanical properties, but synergistically the two enzymes digest ECM components and a much greater degradative effect is observed.