## 4 Articular Synovial Parameters
### Composition and Properties of the Synovial Fluid  
Synovial fluid is a viscous fluid that is secreted by the inner cells of the synovial membrane and filtered from blood plasma. There is approximately 2.5 mL of SF in a healthy synovial joint [241]. The viscosity of the synovial fluid is a result of its high hyaluronic acid (HA) content. SF is extremely important as its function is to deliver nutrients to the AC, transport waste materials and provide lubrication with reduced friction, covering joint surfaces. Synovial fluids also include components such as proteoglycan 4 (PRG4), surfactant phospholipids (SAPL), collagenases, proteases and prostaglandandins [242]. The cellular components of SF also include synovial A and B cells, B and T lymphocytes, neutrophils and monocytes. Synovial fluid may also consist of macrophages, as well as mesenchymal stem cells, especially in pathological conditions. Lubricants are secreted by synovivocytes in the synovial membrane and chondrocytes in the AC. They are accumulated by the semi-permeable synovial lining in the synovial space. If the lubricating system is deficient, AC surface erosion can occur in arthritis conditions [243]. The normal pH value of healthy synovial fluid is in the range of approximately 7.31-7.64 [244; 245]. During inflammation, the pH value decreases, which affects the activity of metalloprotein, resulting in AC degradation. Due to its rheological properties, SF can be characterised as a non-Newtonian fluid with non-linear viscosity. Hyaluronate (HA) is the most abundant polymeric component in SF, in complex form with proteins, due to its flexible, highly hydrated polymer chain [236; 246]. Due to the viscoelastic properties, it is considered one of the key lubricating components in synovial joints [246; 247; 248]. The other proteins tested in the presence of HA showed a reduced friction coefficient [249]. In human SF, the concentration of HA is approximately 1-4 mg/mL, while higher values of this component are observed in people under 40 years of age [250; 251]. Osteoarthritis sufferers showed a decrease in HA, while the percentage of proteins increased, which correlates with a decrease in synovial fluid viscosity. This results in a reduction of its lubricating properties. To maintain adequate SF viscosity, HA is injected intra-articularly in OA patients [252]. PRG4, despite its low content in SF, constitutes a significant share in boundary lubrication. Its presence in different zones is crucial to the AC friction coefficient. The function of lubricin is to maintain appropriate lubricating properties at the cartilage-synovial fluid interface and to prevent cells from attaching to the joint surfaces [253; 254]. Its functions also include facilitating the transport and anchoring of phospholipids to the cartilage surface [255]. Another role of PRG4 is to provide adequate cartilage-protective properties to the AC, which can be enhanced through interaction with the HA by dissipating shear-induced energy [236; 246; 257; 258]. Lubricin, together with the other main components ofSF, is an effective biolubricant agent, whose concentration decreases in acute injury. An increase in this biolubricant was observed in patients undergoing arthrocentesis [259; 260; 261]. The most abundant protein in synovial fluid is albumin. It has been shown to improve boundary lubrication by adsorbing to the surface of the joint material, thereby protecting the joint surfaces from wear [249]. Albumin is pH-dependent, while no relationship was shown between albumin and lubricant viscosity at low angular velocity [236]. The function of albumin is to lubricate joint surfaces and also to interact with other components. Another important protein found in SF is globulin. It has an important role in AC boundary lubrication. Compared to the other components of the synovial fluid, globulin produces thicker layers in tribological tests at different concentrations [262].