Most of the joints should have a layer of joint capsule on the outside, mainly with the surrounding ligaments, muscles and tendons to play a role in stabilizing the joint, in addition to closing the joint into a cavity with synovial fluid, which can promote joint movement. The joint capsule itself is divided into two layers, one is the fibrous membrane, the outer layer is the fibrous membrane, which often contains nerves and blood vessels, so there is painful stimulation and pressure sensation. Its inner layer is a synovial epithelium, which can secrete synovial fluid as well as absorb it, and often the secretion and absorption can reach a dynamic balance. Therefore, a small amount of synovial fluid is always maintained in the joint cavity to lubricate the joint. Synovial fluid is actually a nutrient metabolic carrier for articular cartilage and meniscus. This is why the joint capsule is so important in joint movement. If a patient is immobilized due to some disease, such as a fracture, or if the joint is immobilized for a long time due to a disease that requires prolonged bed rest, a contracture of the joint capsule can occur and limited joint mobility occurs at this time. If the joint is traumatized, the tissues inside the joint will bleed due to trauma, and the inflammatory factors caused by the injury will stimulate the synovial epithelium of the joint capsule to secrete synovial fluid, resulting in fluid accumulation in the joint cavity. If there is more fluid, there will be swelling and increased pressure, and the patient will experience discomfort, which will require joint cavity puncture for decompression. Of course, for some patients with osteoarthritis, because of cartilage wear and tear, bone growth, some bone flab irritation, and some inflammatory factors release, it will lead to increased secretion of synovial fluid in the synovial membrane of the joint capsule, which is often called synovitis of the joint capsule. Therefore the joint capsule is very important for our physiological functions and is a structure that is prone to inflammatory reactions to various diseases.