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 ;
(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 . The data obtained from these tests were used with a transversely isotropic biphasic model to estimate material parameters.