There are several possible causes for knee pain when bending: First, it is meniscal injury. When there is injury or tear in the posterior horn of the meniscus, knee pain can occur when the knee is bent, that is, when the knee is flexed, and early examination of MRI is required for early diagnosis and treatment. Second, popliteal cysts, when the knee popliteal cysts, can also appear when bending knee pain, you can check the popliteal fossa cystic swelling, to check the popliteal fossa ultrasound, MRI, to early diagnosis. Third, osteoarthritis, severe osteoarthritis can involve the soft tissue around the knee, resulting in pain when bending the knee, early examination of the knee X-ray, CT to diagnose. Fourth, patellofemoral osteoarthritis, patellofemoral osteoarthritis in severe cases, can appear when the knee bends in front of the knee pain, to early examination of the knee X-ray, early nutrition of cartilage, anti-inflammatory and pain relief treatment.