There are several possible causes of pain in the calf and knee: first, osteoarthritis of the knee joint. When osteoarthritis of the knee joint severely involves the surrounding soft tissues, such as ligaments or synovial membranes or tendons, there can be pain in the knee and calf, which should be diagnosed and treated by examining x-rays and MRI of the knee joint. Second, thrombosis, if there is a thrombosis in the posterior intermuscular vein of the calf or the posterior popliteal vein of the knee, there can be pain in the knee and calf, often in the posterior muscles and tendons, and it is necessary to conduct relevant investigations and examine the ultrasound or angiography of the blood vessels of the lower limbs for early diagnosis and treatment. Third, tumor, when there is a tumor in the knee joint bone or surrounding soft tissue, there can be pain in the knee joint and calf, often accompanied by systemic symptoms, such as low-grade fever or wasting, then the MRI, CT, X-ray of the knee joint and calf should be examined for diagnosis and treatment.