Left leg bend inside the pain is also known as the left popliteal fossa pain, there may be the following reasons: First, popliteal cysts, popliteal cysts can appear in the left leg bend, that is, the popliteal fossa pain, then there can be obvious swelling, can be touched with cystic swelling, this time the popliteal fossa ultrasound and MRI to carry on the early diagnosis and treatment; Second, the bursitis, when the popliteal fossa bursa chronic aseptic Second, bursitis, when the bursa of the popliteal fossa has chronic aseptic inflammation, pain in the left popliteal fossa can occur, and then we should carry out early diagnosis and treatment with physical examination and nuclear magnetic examination; third, meniscus injury, when the posterior horn of the meniscus is injured, pain in the left popliteal fossa can occur in the knee flexion or squatting, and then we should carry out the magnetic examination of the meniscus to carry out diagnosis and treatment; fourth, osteoarthritis of the knee, when the knee has osteoarthritis involving soft tissues, and then we should carry out the magnetic examination of the meniscus for early diagnosis and treatment. Fourth, osteoarthritis of the knee joint, when osteoarthritis of the knee joint involves the soft tissues, the pain in the left leg can appear, then we should carry out the X-ray film of the knee joint and the nuclear magnetic examination to carry out the early diagnosis and treatment.