Viscoelastic isotonic (pH7.3) solution for injection into joint cavities 1% sodium hyaluronate obtained by fermentation. It is used to treat symptoms of osteoarthritis. It can be applied in the knee, or in any other joint that is classified as "synovial". Synovial fluid, which is viscoelastic as a result of hyaluronic acid, is found in all joints, where it provides normal, pain-free movements as a result of its lubrication and shock absorption properties. It is also responsible for feeding the joint. In the case of degenerative joint diseases such as osteoarthritis, the viscoelasticity of the synovial fluid is significantly reduced, and thus its functions as a lubricant and shock absorber. This increases the mechanical load on the joint and the destruction of the cartilage resulting in pain and limited movement of the affected joint. Supplementing the synovial fluid by intra-articular injection with highly purified hyaluronic acid can moderate the viscoelastic properties of the synovial fluid. This also improves its lubrication properties and reduces the mechanical overload of the joint. As a rule, this result reduces pain and improves the mobility of the joint, and the effect lasts several months after applying a cycle of five intra-articular injections.