“…Indeed, both the strain energy density,Ŵ sκ , and the hydraulic conductivity, k, are isotropic [see (10a) and (29a)], and all the parameters appearing in their constitutive expressions, including the referential volumetric fraction of the solid phase, φ sR , are set equal to constants. If, on the one hand, the model could be acceptable for studying the structural evolution of tumour tissues, which are often assumed to be elastically and hydraulically isotropic [1,42,84], it fails to be accurate for tissues, such as articular cartilage, in which the presence of reinforcing collagen fibres induces anisotropy [35,78,79,94], and the constitutive laws are strongly dependent on material points. In these cases, whereas the balance laws (44a) and (44b) only need to account for the contribution of the fibres to the strain energy density and hydraulic conductivity, the plastic flow rule (44c) should be reformulated.…”