It is very common to see patients with knee effusion as the main complaint in clinical work, and knee effusion is only a clinical symptom. It is essential to differentiate between different diseases causing knee effusion in order to guide patients to scientific subspecialty consultation and treatment. In general, synovial fluid is present in all synovial joints, and normal synovial fluid is colorless or pale yellow against a white background. The amount of synovial fluid exudation is a preliminary indication of the severity of localized inflammation or infection of the joint. The greater the amount of exudate, the more fluid is aspirated by arthrocentesis, and the more severe the possible joint lesion. The following is a brief comparison of common disease manifestations and the general appearance of joint fluid color. 1. Osteoarthropathy of the knee joint (proliferative arthritis). The onset of the disease is mostly in middle-aged and elderly people, with women over 50 years old being the most common. There is a history of knee pain for many years, often with a sudden increase in knee pain, swelling, limited flexion, and difficulty squatting. The amount of fluid drawn by puncture is large, usually ranging from 30 to 70 ml, and the synovial membrane is yellowish or yellow. It is non-inflammatory synovial fluid, often combined with the presence of N-fossa cysts. 2. Acute tears of the meniscus and cruciate ligament of the knee. The onset is mostly in adolescents who play a lot of sports and have a history of acute knee injury. The knee joint swells rapidly after the injury, accompanied by severe pain. The puncture fluid is red and bloody, and the amount of bleeding correlates with the degree of tissue damage. 3. Intra-articular fracture of the knee. After acute injury, the knee joint is painful and rapidly swollen, and the puncture fluid is fresh and bloody with fatty microdrip. 4, chronic synovitis after injury. It is common in adolescents, with a history of acute sprains, and then recurrent episodes of knee effusion, sometimes light and sometimes heavy, with increased knee effusion after excessive exercise, which can be reduced after rest, and can be accompanied by mild pain and weakness and other symptoms, and the puncture fluid is mostly light yellow fluid. The course of the disease is long and the treatment is quite difficult. 5. Bacterial infectious arthritis. It is mainly in children aged 7 to 11 years old, with rapid onset, fever as the main manifestation, accompanied by inflammatory symptoms such as redness, swelling and heat pain in the knee joint, and purulent fluid in the puncture fluid. In severe infections, the presence of a large number of leukocytes in the synovial fluid may turn the fluid white. 6. Tuberculous knee osteoarthritis. The knee joint is painful with swelling, and the local skin temperature may be mildly increased, but the congestion and redness are not obvious, and may be accompanied by high or low fever. The puncture fluid is white and cheese-like. 7. Rheumatoid arthritis of the knee. The swelling of the knee joint is symmetrical and can also develop in one knee, mostly accompanied by swelling or deformity of the small joints of the hands and feet. 8. Gouty arthritis of the knee. This disease occurs mostly in middle-aged men, obese people, mostly after drinking alcohol and high-fat diet acute onset, the knee joint can be manifested as severe pain, accompanied by redness and swelling and other manifestations, puncture synovial fluid is characteristic white, emulsion-like, interspersed with white debris. 9. Knee hyperpigmented villous nodular synovitis. This disease is most common in middle-aged men. The knee joint pain is not heavy, but the swelling gradually increases, especially the swelling of the suprapatellar capsule is the most obvious, soft hyperplastic synovial tissue can be palpated, the joint fluid is not much, the puncture fluid is serous or orange or dark brown. These are common clinical knee disorders, and sometimes knee swelling and pain are often the first clinical manifestations. Along with a thorough physical examination and blood chemistry and imaging, arthrocentesis fluid microscopy, glucose and latex determination, protein quantification, immunological tests and pathogen culture are necessary and should be included as routine tests. Only with a clear diagnosis can treatment be targeted.