There are several possible causes of knee flexion pain: First, meniscal injury, when the posterior horn of the meniscus is torn or damaged, there can be pain when the knee is flexed, early diagnosis and treatment should be carried out by performing a meniscal MRI. Second, osteoarthritis of the knee joint. When osteoarthritis of the knee joint involves the surrounding soft tissues or causes secondary damage to the meniscus, pain can occur when the knee joint is flexed, and X-ray and MRI of the knee joint should be performed for early diagnosis and treatment. Third, popliteal cysts, popliteal cysts of the knee joint can appear in the popliteal fossa of the knee joint pain, especially when the knee joint bending, squatting pain is more obvious, can be palpable cystic swelling, to perform ultrasound and nuclear magnetic examination of the knee joint for early diagnosis and treatment. Fourth, bursitis of the knee joint, severe bursitis of the popliteal fossa of the knee joint, can also appear knee pain, to conduct early examination for diagnosis.