It is possible that the following diseases may trigger the symptoms of not being able to straighten the knee of the leg: 1. The most common disease that can cause this symptom is synovitis of the knee joint. Synovitis can be triggered by acute, sudden trauma that leads to joint damage. Or long-term joint strain, stimulating the synovial membrane, which thickens and produces inflammation, and the symptoms of the knee dare not straighten. In some patients with severe synovitis, the synovial membrane may adhere to the surrounding soft tissues, resulting in restricted joint movement and even loss of joint function. Patients with synovitis, in addition to not daring to straighten their legs, are also afraid to fully bend their legs and do a full squat. Synovitis can cause symptoms of joint swelling; 2. Patients with fat pad inflammation of the knee joint, who are also afraid to fully straighten their legs, are mostly caused by joint damage. Because when straightening the knee, the fat pad will be squeezed and painful symptoms will occur, so patients with fat pad inflammation are afraid to straighten their legs, but when the knee is bent or when squatting, there will be no significant pain.