The synovial capsule of the knee is covered by synovial membrane, which accounts for 70% of the body’s synovial volume and is the largest synovial cavity in the human body. The knee bursa starts from the edge of the articular cartilage of the femoral condyles, reflexes at the upper edge of the patella at 4 horizontal fingers, and ends downward at the lower edge of the tibial plateau, forming a closed capsule. The surface synovial cells secrete yellowish viscous synovial fluid, which nourishes the non-vascular articular cartilage, lubricates the joint surface, reduces friction, and dissipates heat when the joint is active. The synovium is rich in blood vessels and is prone to injury and bleeding, resulting in traumatic synovitis. It is divided into acute traumatic synovitis and chronic traumatic synovitis. (a) Acute traumatic synovitis: Swelling and pain in the joint rapidly after injury, swelling usually occurs immediately after the injury or within 1 to 2 hours afterwards, and the symptoms may gradually worsen, resulting in restricted movement due to protective spasm of the muscles around the knee joint. The skin temperature around the knee joint is elevated, with localized redness and generalized hypothermia. On palpation there is tension in the skin or swelling, localized pressure pain, and a positive floating patella test. (2) Chronic traumatic synovitis: Usually caused by two conditions: 1) acute traumatic treatment is incomplete and becomes chronic; 2) the knee joint is accumulated by repeated minor trauma. The pathological changes are mainly synovial congestion, swelling, hypertrophy or mechanized adhesions. The synovial fluid of the joint cavity is dark yellow sticky flocculent material. The main clinical manifestations are recurrent swelling, soreness, and restricted movement of the joint, with no severe pain and no redness or fever in the knee joint. Physical examination reveals swollen, full joints, positive floating patella test, and friction sounds and light pressure pain on palpation of the knee joint if there is hypertrophic synovium. Auxiliary examinations: In the acute phase, the joint puncture fluid is mostly pink or dark red, and the fluid is not coagulated. In the chronic phase, the fluid is yellow, unclear, and negative for bacterial culture. x-ray examination, traumatic knee synovitis bone is mostly abnormal. The amount of joint effusion can be seen as joint capsule swelling and joint gap widening. MRI examination can reveal significant knee joint effusion and can detect whether there is meniscal, cartilage and cruciate ligament injury. Diagnosis: Acute traumatic synovitis should be differentiated from intra-articular fractures, collateral ligament injuries, cruciate ligament injuries, meniscal tears, and other injuries. According to statistics, knee swelling that occurs within 24 hours after knee trauma has a 70% or higher probability of cruciate ligament injury, excluding fracture factors. Chronic traumatic synovitis should be distinguished from osteoarthritis, pigmented villous nodular synovitis, synovial chondromas, chondromalacia, synovial tuberculosis, hemophilic arthritis, rheumatoid arthritis, etc. Treatment: In acute traumatic synovitis, if the joint effusion is obvious, the blood should be extracted in time to avoid secondary hematoma or mechanized adhesions. Improperly treated disease can become chronic, with secondary hypertrophy of synovial membrane and destruction of articular cartilage. Other treatment includes icing, compression bandaging, elevation of the affected limb, and fixation of the knee in the straight position with a cast or knee brace for 2 weeks, with isometric contraction of the quadriceps muscle feasible at this stage to place muscle atrophy. For chronic traumatic synovitis, activity should be limited when symptoms are obvious, and then gradually increase the activity after symptoms are reduced. When fluid accumulation is obvious, joint cavity aspiration is feasible to avoid the formation of adhesions in the joint, leading to dysfunction. In addition, physical therapy and ultrasound can be applied. If local steroid injections are used, they should be given once a week, no more than three times. If conservative treatment is not effective, arthroscopic subsurface membrane debridement can be considered.