## 3 Biomechanics of Articular Cartilage  
Articular cartilage can be described as a thin layer of specialised connective tissue that is characterised by specific viscoelastic properties. The main function of AC is to provide a smooth and lubricated surface for low-friction articulation. It also facilitates the transfer of loads to the underlying subchondoral bone [2]. A special feature of articular cartilage is its ability to withstand high cyclic loading with little or no signs of damage or degenerative changes [2; 135; 136; 92]. The AC consists of two basic phases: a solid phase that is porous and permeable, consisting of collagen fibres, proteoglycans, glycoproteins and chondrocytes, and a liquid composite phase consisting of interstitial fluid containing water and ions such as sodium, potassium, calcium and chloride [3; 137; 138]. The solid phase accounts for about 15-32% of the total cartilage mass, with the proportion reduced to 10% in diseased cartilage immediately prior to its destruction [139]. The liquid phase accounts for about 68-85% of the total mass. The resistance to compression of articular cartilage is provided by the relationship between aggregates of proteoglycans and interstitial fluid, and more specifically, by the negative forces of electrostatic repulsion [14; 120; 140]. The immediate increase in interstitial fluid pressure is associated with the initial and rapid application of contact forces to the joint during loading. The effect of such a local increase in pressure is that interstitial fluid flows out of the ECM, generating high frictional resistance on the matrix [120; 138; 141; 142; 143]. Interstitial fluid flows back into the tissue when the compressive load ceases. Due to the low permeability of the AC, interstitial fluid is not quickly squeezed out of the matrix [141; 144]. The cartilage under the contact surface is limited by the opposing bones and surrounding cartilage, which has the effect of reducing mechanical deformation. Articular cartilage is viscoelastic, and its behaviour is dependent on the time and exposure to constant load or deformation [2; 145]. The viscoelasticity of cartilage is associated with a flow-dependent and a flow-independent mechanism [8; 146]. The flow-dependent mechanism is related to the interstitial fluid, as well as the frictional resistance that is associated with this flow [128; 137; 138; 147]. The displacement of cartilage is a function of time, which is related to the inability of the fluid to immediately escape from the matrix [45]. The increase in interstitial fluid pressure is influenced by the application of joint contact forces during joint loading. In the case of deformity, fluid flow occurs through the layers of cartilage, as well as the surface of the joint, resulting in fluid flowing out of the ECM. Fluid can also flow through layers of tissue if a pressure differential is applied to a section of cartilage. Initially, the displacement is quite rapid and corresponds to a larger or smaller flow of fluid from the cartilage [4; 5; 148]. The fluid flow slows down as the rate of displacement decreases, and its value approaches a constant value [45]. Displacement is constant at equilibrium, and the fluid flow is stopped. The flow-independent component of viscoelasticity is due to the intrinsic viscoelastic behaviour of the collagen-proteoglycan matrix [149; 150]. The fluid pressure is an important component of the total load support, which has the effect of reducing the stresses acting on the solid matrix. In the articular cartilage, aggregated molecules swell against the collagen skeleton, which is its mechanical response. The compression resistance of articular cartilage is provided by the association between proteoglycan aggregates and interstitial fluid, due to the negative forces of electrostatic repulsion [2; 4; 49]. In an aqueous environment, the aggregated proteoglycan molecule spreads under the influence of mutual repulsion of negative charges, occupying a large volume [45].