1, Hyaluronic acid (HA) physicochemical properties HA is a unique linear mucopolysaccharide, cross-linked by N-acetylphthalide glucosamine and D-glucuronic acid, widely distributed in the extracellular matrix of human tissues (such as vitreous humor, synovial fluid, cartilage, etc.), with strong hydrophilicity and high viscoelasticity. HA has endogenous HA and exogenous HA. There are two types of HA: endogenous, i.e. HA secreted by human body, with molecular weight of about 4~10×106 Da and concentration of about 3.5g/L in synovial fluid; exogenous HA, i.e. HA supplemented by foreign sources, including low molecular weight HA (LMW HA, 0.5~1.5×106 Da) and high molecular weight HA (HMW HA, 6~7×106 Da). When the endogenous HA production and metabolism is abnormal, resulting in tissue and organ dysfunction, the exogenous HA can be supplemented. 2.The mechanism of action of HA in the treatment of bone and joint diseases HA has been widely used in the treatment of bone and joint diseases, especially in osteoarthritis, and its mechanism of action is mainly reflected in the following three aspects: ① Covering, lubrication and buffering stress: HA in synovial fluid has a high degree of In addition, hyaluronic acid has a barrier effect on bacteria, toxins, immune complexes, etc., and protects cartilage from enzymes, chemicals and toxins. For example, Yasuda et al. found that the increase of inflammatory cytokines and matrix degeneration products in the joints of osteoarthritis patients can induce the production of catabolic enzymes (such as collagenase and degenerative enzymes), leading to degeneration of articular cartilage; while HA can inhibit the degradation of cartilage by catabolic enzymes, thus achieving the effect of cartilage protection. ②Reducing inflammatory response: HA binding to HA receptors CD44 and CD168 can reduce the number of inflammatory cells and the synthesis and release of interleukins, prostaglandin E2 (PGE-2) and matrix metalloproteinases, thus achieving anti-inflammatory effects. ③Promote endogenous macromolecule synthesis: By promoting endogenous macromolecule HA synthesis to improve the properties of pathological joint fluid, it achieves sustained symptom relief and delays the progression of osteoarthritis; and promotes cartilage regeneration by promoting the synthesis of proteoglycans and collagen. Because of the short half-life of HA, it is speculated that the sustained improvement of symptoms after exogenous HA injection may be related to its promotion of endogenous HA synthesis. 3.Safety HA has good biocompatibility, can be completely metabolized in vivo, non-toxic, sterile, non-chemotactic, good safety. The most common adverse reactions are mild pain and swelling in the injection area and joints, occasionally accompanied by headache, fever and drug rash, which are mostly tolerated by patients and do not require special treatment. Complications of vascular and nerve damage have not been reported in the literature, and allergic reactions and septic joint infections are rare. However, patients with allergy to poultry and eggs should use HA with caution and discontinue the drug immediately if allergy is detected, and take appropriate anti-allergic treatment. HA is safe and effective in the treatment of bone and joint diseases, and can relieve symptoms and improve joint function. The application of HA has greatly reduced the dosage of NSAIDs, lowered medical costs and reduced drug side effects.