If you can’t straighten your leg and your knee is bent, it may be caused by osteoarthritis, knee injury, joint effusion, popliteal cyst and so on. 1. Osteoarthritis: Degenerative changes in the knee joint such as osteophytes and wear and tear of the articular cartilage can lead to osteoarthritis, which can cause pain and limitation of activities in the knee joint, leading to the phenomenon that the knee joint can not be straightened. 2. Knee injuries: such as knee fractures, ligament injuries or meniscus injuries, can cause knee pain and swelling, resulting in knee dysfunction, and the leg can not straighten the knee is bent symptoms. 3. Joint effusion: often associated with knee trauma, gouty arthritis, rheumatoid arthritis, rheumatoid arthritis and other diseases, can cause joint pain and swelling, and then lead to the joint can not be straightened. 4. Popliteal cyst: a benign soft tissue mass at the back of the knee, when the cyst is large, it also compresses the tissues around the knee, resulting in stiffness or inability to straighten the knee. The symptoms of the leg is not straight knee is bent, it is recommended that patients go to the hospital as soon as possible, clear the cause of the targeted treatment, so as not to delay the condition.