###### Abstract
Figure 1: (**a**) Schematic representation of the knee joint; (**b**) the different regions of the meniscus: posterior horn, central body, anterior horn and the three directions: vertical, radial and circumferential; (**c**) schematic representation of the confined compression test set up, (**d**) example of the variability of the porosity within meniscal samples from \(\mu\)CT scans; and (**e**) the results show that the central body of the meniscus exhibits higher permeability values. Understanding and appropriately modelling the fluid flow behaviour in the different portions of the meniscal tissue, considering a range of various loading conditions, is essential to gain insight into the biomechanical function of this tissue. To date, limited information is available on region-specific and anisotropic permeability in the meniscus, which is essential to understand the fluid flow evolution and its relationship with the internal architecture. Poromechanics experimental tests, such as confined compression tests--including both stress relaxation and creep--are currently used in order to characterize material parameters, such as the elastic modulus at equilibrium and permeability. In order to identify these two parameters, the main models used are based on the biphasic and poroelastic theories. Even though these theories developed from different roots, it can be shown that they are basically equivalent [12]. In this study, we focused our attention on the evaluation of the permeability in the meniscus and in particular on its variation in the different regions of the tissue and directions (See Figure 1b). Moreover, we showed that the biphasic model does not provide a good fit with the experimental curves; therefore, we proposed a poroelastic model in which the pore pressure diffusion equation is derived by adopting a modified version of [PERSON]'s law involving fractional derivatives [13]. It has been shown that, in a high porosity medium, there is a departure from [PERSON]'s law as the inertia (velocity-squared term), thermal dispersion, convective (development term) and boundary (no-slip condition) effects not included in the [PERSON]'s model may play a significant role [14]. Moreover, in the last few decades, experimental evidence of anomalous diffusive phenomena, i.e., not following a Darcian behaviour, has grown [15; 16; 17; 18; 19]. This is mainly due to the fact that the permeability, hence the rate of fluid flow, is not a constant quantity. Variations in permeability occur, for example, when the fluid flow impacts the geometry or the micro-structural features, such as the configuration of the pores. For example, experiments on water flow in building materials highlighted that the permeability changes during the flowing process as a result of the microstructural rearrangement of grains/pores. [PERSON] et al. [19] hinted that, during the compaction of sand, the permeability might decrease due to the fact that the fluid carries solid particles, which then close some of the pores. Essentially, the configuration of the medium, in particular the ratio between closed/open pores, changes during the process. Fluid might be trapped in the medium leading to a slower fluid flow rate. On the contrary, if during the fluid diffusion process, some of the pores open creating conductive microchannels, the permeability might increase. Therefore, fluid can be transported for a large distance in a reduced time resulting in a faster diffusion process. Modelling the anomalous fluid diffusion process is one of the key points when dealing with the poromechanics of biological tissues. Therefore, it is fundamental to develop a theory that can incorporate the change in microstructural features (for instance, the interaction between fluid particles and open pores/channels) during the transport process. Recently, the stochastic Continuous Time Random Walk (CTRW) framework was proposed for this purpose [20].