Knee effusion is medically called synovitis. Synovitis is a specific manifestation in synovial tissue caused by a variety of diseases such as trauma, osteophytes, tuberculosis, joint degeneration, rheumatic diseases, pigmented villous nodular synovitis, surgery, etc. These diseases can cause mechanical, biological, and chemical stimulation of the synovial membrane, resulting in congestion and edema of synovial tissue, increased vascular permeability, excessive secretion of synovial fluid, and decreased absorption, leading to clinical symptoms such as joint swelling, pain, and restricted movement. Knee effusion in the elderly is more commonly induced by osteophytes, meniscal injuries, and mechanical biological and chemical irritation produced by free bodies, secondary to synovial membrane of the knee, edema, exudation, and effusion. Chinese medicine believes that synovitis is divided into two aspects: trauma and chronic strain injury. The knee joint suffers from fracture, dislocation, ligament rupture, cartilage injury and other lesions, all of which can cause simultaneous damage to the synovial membrane, making it very easy for stasis and fluid to accumulate after injury. Humidity and heat can cause heat, swelling, burning, muscle contracture, and joint flexion and extension disorders in the knee joint, resulting in acute synovitis. If the injury is mild or long-term chronic strain, combined with wind, cold and dampness, the knee can gradually become swollen and dysfunctional, resulting in chronic synovitis.