## 3 Biomechanics of Articular Cartilage
### Lubrication Mechanisms in AC

Table 3: _Cont._The actual surfaces of solids are not perfectly smooth; at the microscopic level, various types of irregularities known as surface roughness can be observed. When two such surfaces come into contact, there is an interaction between them that accounts for much of the observed initial static friction coefficient. Experimental studies have shown that, for many materials, the equilibrium static friction coefficient is around 0.3. This value is typical for materials such as dry wood surfaces, steel, as well as AC in a phosphate buffer medium (PSB) without the presence of surface glycoproteins in the form of PRG4 [218]. The value of the static friction coefficient may be higher compared to its equilibrium value due to mechanical interlocking of micro-peaks or adhesive interactions. Local contact stresses can affect the interlocking mechanism, which in turn alters the friction coefficient under cyclic loading and unloading conditions. Increased loads result in both increased frictional forces and greater material wear [191]. As the load increases, the rate of wear also increases. When the value of the initial friction coefficient is higher than the equilibrium value at low slip speed, \"stick-slip\" friction is possible. This mechanism involves a cyclical transition between the \"stick\" and \"slick\" adhesion phases, resulting in an increased rate of surface wear. The equilibrium value of the friction coefficient can be reduced when the spaces between the micro-peaks are filled by the lubricant, reducing the bespoke surface contact. When a lubricant is not subject to rapid extrusion or abrasion, it is referred to as a boundary lubricant. The glycoprotein PGR4, synthesised by AC surface zone chondrocytes, plays a key role in biological systems. PRG4 effectively reduces the equilibrium friction coefficient of AC, lowering its value from 0.3 to 0.1 in the presence of physiological SF [219; 214]. PRG4 shows strong binding to denatured, amorphous and fibrillar collagen, which influences its sustained adsorption on the AC surface. When PRG4 is present on both contacting surfaces, it generates significant repulsive forces, which increase with the increasing PRG4 concentration [220]. At the same time, a significant reduction in the coefficient of friction is observed at high concentrations of PRG4, reaching values three or four times lower than in the absence of this component [220]. Other macromolecules such as HA, phospholipids and aggregates also play an important role in the mechanism of boundary lubrication. Previous studies have shown that the presence of these components can have a significant impact on the friction coefficient value, and their removal can lead to disruption of the lubricating function as well as an increase in frictional resistance [191; 221; 222; 223; 224]. Polymer networks can form structures resembling \"polymer brushes\" to which hydration layers adhere, according to hydration lubrication theory [218]. The purpose of these structures is to counteract the extrusion of water from the microscopic contact joints while maintaining the fluid shear resting characteristics [203; 225]. Such a mechanism can be considered in an osmotic context--as the surface gel is compressed, the local osmotic pressure increases, which causes water to be retained within the gel structure and maintains its hydrated character [226]. A characteristic feature of such a hydrated gel is low shear resistance, making it an effective lubricant under boundary friction conditions. When the key components of the surface gel, such as HA, aggrecan and collagen, are removed, the lubricating properties deteriorate and the contact surfaces wear faster [224]. Therefore, boundary lubrication occurs if surface asperities are in contact and transmit normal forces. The working distance at which boundary lubrication occurs depends on the asperity morphology and the extent of the repulsive forces generated by the PRG4 lubricant components, which at high concentrations can generate repulsion at a distance of up to 0.2 \(\upmu\)m [218].