Leg joint pain when bending consider osteoarthritis, synovitis, meniscus injury, rheumatoid arthritis and so on. 1. Osteoarthritis: joint hyperplasia and degeneration, articular cartilage wear and tear, narrowing of the joint space caused by knee pain, serious deformation of the joints, squatting activities are limited. 2. Synovitis: Hyperplasia and hypertrophy of synovial membrane in the joints, aseptic inflammation and secretion of fluid, swelling of joints and restriction of joint activities. 3. Meniscus injury: meniscus injury caused by joint sprain, flexion and extension joint pain, popping, do knee MRI film can be a clear diagnosis. 4. Rheumatoid arthritis: symmetrical swelling of joints, pain, morning stiffness, limited joint movement. Laboratory tests related to rheumatism can make a clear diagnosis. There are many reasons for leg joint pain when bending, it is best to go to the hospital to check and clarify the cause of the disease for treatment.