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.

A biomechanical investigation of the dynamic shear failure of the osteochondral region of immature, adolescent, and mature bovine cartilage bone laminates was performed. The osteochondral junction was loaded in pure shear under impact conditions through the cartilage layer only. The results indicate the adolescent tissue fails at a nominal shear stress of 2.0 MPa, whereas the immature and the mature tissues fail at 3.8 MPa and 2.6 MPa, respectively. The adolescent tissue had a significant reduction in the fracture toughness of its osteochondral junction compared with that of the immature or mature tissues. The fracture toughness, describing the energy required to initiate and propagate a crack to failure, was 3.6 kN/m, 2.3 kN/m, and 10.2 kN/m for the immature, adolescent, and mature bovine tissues, respectively. This significant reduction associated with the adolescent osteochondral junction is explained in terms of the structural changes occurring within this important anchoring region during maturation. These findings question the wisdom of subjecting the adolescent joint to high levels and rates of loading.

A method of assessing foot movement suitable for use in clinical practice is presented. The method assesses the component of movement in the horizontal plane which is produced by rotating the calcaneum about the axis of the subtalar joint.