It has been shown that the stress relaxation response of articular cartilage in unconfined compression is not adequately described by an isotropic biphasic model [1,2]. Two possible explanations have been advanced to account for this observed disagreement between theory and experiment:

(i) friction at the indenter/tissue interface that the model assumes to be negligible [3];

(ii) cartilage behaves as a transversely isotropic material [4,5].

In the present study, stress-relaxation experiments were performed in both confined and unconfined compression on immature bovine knee cartilage. High aspect ratio (height/diameter) specimens were used to minimize the effect of friction at the indenter/tissue interface [3]. The data obtained from these tests were used with a transversely isotropic biphasic model to estimate material parameters.

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