Deformation and rupture of the articular surface under dynamic and static compression.

Cartilage-on-bone samples were dynamically and statically compressed at various stress levels to determine the deformation and rupture behaviour of the articular surface (AS). Instantaneous deformations were captured photographically by using a transparent indenter in combination with a ultra high speed flash. Principal strains (PS) were evaluated using large deformation theory. The tensile strains induced indirectly in the AS were a function of the rate at which the direct compressive force was applied. At the same compressive stress the tensile strains induced statically were approximately twice those induced dynamically. Rupture of the AS occurred in about 60% of those specimens tested statically at 15 MPa and followed approximately the split-line direction. By contrast, no rupture was observed dynamically even at stresses as high as 28 MPa. In terms of joint function the research demonstrates that the AS is considerably more resistant to rupture under dynamic than under static loading. The biomechanical parameter governing rupture appears to be the level of indirectly induced surface strain rather than the directly applied compressive stress. The very different mechanisms controlling the compressive deformation of articular cartilage (AC) at high vs low rates of loading clearly influence the levels of in-plane strain induced in the AS.