Swelling with effusion after arthroscopic surgery may be related to surgical trauma, intra-articular infection, synovitis, etc. It can be treated with proper rest, NSAIDs, and surgery. 1. Surgical trauma: Arthroscopic surgery is an invasive surgery, and trauma may occur after surgery resulting in a small amount of fluid in the joint cavity. In this case, the small amount of fluid in the joint cavity can be slowly absorbed by resting and reducing the weight of the joint. 2. Intra-articular infection: After the surgery, a small amount of blood will continue to seep from the trauma site, which, if not properly cared for, can lead to an infection resulting in a fluid buildup in the joint cavity. When it is determined that the cause of the effusion is infection, targeted antibiotics should be selected under the guidance of the doctor to control the inflammation, such as levofloxacin and so on. 3. Synovitis: The trauma caused by arthroscopic surgery may lead to rupture and congestion of the synovium, causing synovitis, which may lead to a large amount of fluid in the joint cavity. Inflammatory synovium, exudate, and fluid can be removed surgically after a large amount of fluid has accumulated, and a drain should be placed to help control bleeding after the removal procedure. When swelling and fluid buildup occur after arthroscopic surgery, it is important to go to the hospital and receive professional treatment from your doctor.