Many studies have demonstrated that articular cartilage is anisotropic in tension, based on tensile tests of tissue strips harvested parallel to the articular surface, along and perpendicular to the local split line direction (e.g., Akizuki et al., 1986; Kempson et al., 1968; Schmidt et al., 1990; Woo et al., 1979). The observed differences in the tensile modulus suggest that the material symmetry of cartilage in tension is no higher than orthotropy, since two orthogonal planes of symmetry (with unit normals parallel and perpendicular to the split line direction) automatically define a third plane of symmetry mutually perpendicular to the other two. However, the properties of articular cartilage differ significantly in tension and compression (Cohen et al., 1998; Soulhat et al., 1998) and it remains to be established whether cartilage is anisotropic in compression as well. Only one previous preliminary study has investigated the compressive modulus of cartilage along two mutually perpendicular directions (Jurvelin et al., 1996), reporting significant differences.

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