## 3 Biomechanics of Articular Cartilage
### Mechanical Properties of Articular Cartilage
[178] developed a theoretical basis for a biphase contact mechanics model for the indentation test, which provided a starting point for analytical solutions to assess the flow and deformation of the solid skeleton during local AC loading. This allowed for a more accurate fit of the models to the experimental results. In the work of [PERSON] et al. [173], numerical and experimental studies were carried out to validate the biphase indentation model, which enabled the precise determination of the Young's modulus and the hydraulic permeability coefficient based on the time response of the AC to the indenter load. [PERSON] et al. [51] determined the usefulness of this method in the analysis of spatial inhomogeneities and anisotropies of cartilage. The indentation test enables the assessment of mechanical properties in small, localised areas, which is crucial when examining different morphological zones of tissue. [PERSON] et al. [172] used this method in combination with optical analysis to determine the Poisson's ratio. Observation of the lateral displacement around the indenter enabled determination of the elastic behaviour of bovine AC. Later, [PERSON] et al. [180] used the indentation test to spatially map the elastic properties of the AC in the dog's knee. The obtained topographic data showed significant regional differences in AC stiffness, confirming the high utility of this method in local and comparative studies. In his work, [PERSON] [45] demonstrated that the indentation test is one of the methods that provides information on the surface stiffness of tissues with minimal invasiveness. As an advantage, it showed the possibility of performing in situ tests, as well as measuring dynamic and time-dependent behaviour. [PERSON] et al. [179] used this measurement method to study the influence of pore geometry on the local stiffness and dynamic response of porous materials that stimulate AC. In order to investigate the mechanical properties of AC under tensile deformation conditions, one of the basic test methods is the tensile constant strain rate. The specimen is subjected to tensile forces at a predetermined rate, and the stresses and their corresponding strains are measured, allowing parameters such as the tensile modulus, maximum stress, elastic limit and time-dependent response of the tissue to be determined. Using this method, [PERSON] et al. [154] showed that there are changes in tensile properties with age. Tissue from older individuals was less susceptible to stretching, which may be due to collagen reorganisation and changes in collagen density. The team of [PERSON] et al. [174] used this method at a constant strain rate to assess the mechanical properties of bovine AC. By analysing the relationship between animal age and the tensile properties of the tissue, they found that stiffness also increased with age and correlated with an increase in collagen II content, with a decrease in proteoglycan levels. In their subsequent work, [PERSON] et al. [181] assessed the development of the mechanical properties of AC cultured in vitro. Their study showed a relationship between the increase in biomechanical parameter values during stretching and the structure and organisation of collagen fibres. Thus, the tensile constant strain rate can be used to monitor tissue maturation and functionality in tissue engineering studies. Another measurement method mentioned is tensile stress relaxation. It was included in the work of [PERSON] et al. [182] to assess the effect of the osmotic environment on the mechanical properties of tissue under axial tension. AC samples taken from the medial condyle of the femur were subjected to axial tension at a constant strain, after which the decreasing stress over time was recorded. The results confirmed that the initial stress level, as well as the rate and extent of stress relaxation, are significantly influenced by the osmotic environment. The dynamic shear test is a method that is used to assess the mechanical properties of tissues by applying cyclic shear deformations.