## 3 Biomechanics of Articular Cartilage
### Lubrication Mechanisms in AC
In healthy diarthroidal joints, the AC surfaces allow rolling and sliding relative to each other. Effective lubrication of these surfaces is possible through a combination of hydrodynamic and boundary lubrication, the purpose of which is to reduce friction during movement. Typically, rolling generates less resistance than sliding, although both mechanisms affect energy dissipation within the joint [219]. Differences were observedat the knee joint in terms of AC surface sliding during the gait cycle. The tibial cartilage and meniscus surfaces exhibit limited sliding, typically measured in millimetres, while the femoral cartilage and meniscus surfaces experience greater sliding during the same cycle [218]. The disproportion in sliding speeds can have a significant impact on the optimal joint structure through differences in the cartilage composition of the shin and femur, and, consequently, on joint function [189]. In mixed lubrication mode, the initial coefficient of friction of AC may be around 0.002-0.02, while in the steady state, the equilibrium kinematic coefficient of friction increases to around 0.05-0.1, depending on the sliding speed, cartilage tissue deformation, or the load ratios attributed to sliding and rolling [227]. It has been empirically found that relative movement between contacting surfaces improves joint lubrication [228]. The viscosity of the SF is crucial not only for the rate at which the fluid is squeezed out of the fracture, but also for the rate at which it is drawn into the fracture via the relative motion between the fluid and the contact interfaces. SF is sucked into the gap at the \"leading edge\" of the sliding contact mainly due to the tendency of the liquid molecules and dissolved substances to attach to the surface and interact with each other as well as the effect of inertia on the flow [218]. Therefore, higher SF viscosity increases energy losses due to shear, while it promotes hydrodynamic lubrication due to more efficient fluid absorption [227]. The suction of SF into the contact interface at low speeds may explain why physiological SF is initially characterised by a viscosity about 40-50 times greater than water, because the increased viscosity favours its efficient entrainment under low-speed conditions [189]. As the suction speed increases, the viscosity may decrease, thus maintaining a similar level of fluid suction. The nonlinear rheological properties of SF support effective suction regardless of the speed. Appropriate surface roughness may support this process, but the mechanisms of this interaction are not yet well understood [219]. The interaction of friction and lubrication modes is captured on so-called Stribeck curves or Stribeck surfaces [219; 227]. In a mixed lubrication model, soft bearings made of a rough, porous material such as AC are characterised by complex interactions. When the roughness of two AC surfaces come into contact with each other, deformation occurs, resulting in changes in the geometry of the contact zone. Deformation can significantly increase the actual contact area due to prolonged loading. The increase in contact area improves AC surface matching and has the effect of lowering nominal contact stresses [219]. The increase in anatomical conformity within the articular surface leads to an increase in the AC contact area, which translates into tribological mechanisms occurring within the synovial joints [123]. Both the increased contact area and the higher degree of geometric conformity result in an increased mean drainage path, both along the articular interface and within the AC matrix [202]. In the context of the knee joint, increased participation of the meniscus promotes improved compliance between the femoral condyles and the articular surface of the tibia [229]. The meniscus plays an important role in load dissipation by reducing local contact stresses in the AC, increasing the effective contact area with the underlying bony matrix [230].