Soreness and swelling of a left leg popliteal cyst can be caused by a popliteal cyst compressing blood vessels and nerves. A popliteal cyst in the left leg, which may be caused by trauma, infection, rheumatism, tuberculosis, etc., is a movable cystic soft-tissue mass in the back of the knee joint of the left leg that may cause pain and swelling in the back of the knee. Mostly because the popliteal cyst presses and irritates some of the blood vessels and nerves, triggering poor local blood circulation in the left leg, which leads to the soreness and swelling. Left leg popliteal cyst soreness may also have other causes, it is recommended to go to the regular hospital in a timely manner to improve the relevant examination, to clarify the cause of the disease, under the guidance of the doctor to carry out targeted treatment or treatment.