Knee pain when bending can be caused by the following reasons: first, osteoarthritis of the knee, osteoarthritis of the knee can involve the soft tissues around the knee when serious, and the symptoms of pain in the tendons on the back side of the knee when bending, it is necessary to examine the X-ray and CT of the knee for early diagnosis and early treatment. Second, popliteal fossa disease, when there is a popliteal cyst or popliteal bursitis, can also have symptoms of tendon pain when the knee bends, then early examination, examination of the popliteal fossa ultrasound and MRI to diagnose. Third, patellar ligamentitis or quadriceps tendonitis, these two diseases are often caused by chronic sports injuries or strain, can have pain in the patellar ligament and quadriceps tendon when bending the knee, this time to check the body and examination, early MRI to diagnose and treatment.