Bending the leg and knee pain may be a sign of meniscus injury, knee joint effusion or rheumatoid arthritis. 1. Meniscus injury: meniscus is a kind of cartilage tissue in the knee joint, when there is a tear or other injury, it may affect the bending of the leg, and there will be knee pain when bending the leg. 2. Knee joint effusion: when there is effusion in the knee joint, the effusion will squeeze the joint cavity when bending the leg, which may cause localized dull pain and the range of bending the leg will be reduced. 3. Rheumatoid arthritis: After rheumatoid arthritis occurs, inflammatory changes will occur in the joint tissues, increased exudation, when bending the leg will also have knee pain symptoms. The symptoms of knee pain when bending the leg, is likely to be the manifestation of some diseases, need to consult a doctor in a timely manner to determine the cause of the disease, for standardized treatment.