Pain when bending the knee is due to some pathological changes in the joint area, such as arthritis, synovitis, joint effusion, which are more common diseases that can cause pain when bending the knee. The human body is like a machine, and the knee is a machine part that can’t bend, so there must be some pathology around it. The most common is joint effusion, more fluid will swell, when bending inside the tension is big, pressure can not go down, bending pain, sometimes can not bend. Tendon, joint capsule, bone, cartilage, and meniscus changes can also lead to limited joint movement. The knee joint is like a very precise instrument, each part fits very well, when there is pain when bending the knee joint, you must seek medical treatment in time, let the doctor find the cause of the disease, symptomatic treatment, and early treatment, because delayed to the back of the joints to further degeneration, aging occurs, then there is an irreversible situation, the prognosis is not too good, so you need to seek medical treatment as soon as possible, so the doctor to diagnose and treat.