The knee socket is medically known as the popliteal fossa, and common causes of popliteal cysts, sebaceous cysts, and multiple lipomas are popliteal cysts. 1. Popliteal cyst: due to knee osteoarthritis, there is an increase in synovial fluid in the joint and high pressure in the joint, which results in swelling of the bursa at the back of the knee, leading to the formation of popliteal cysts. 2. Sebaceous cysts: Sebaceous cysts are formed when sebum accumulates in the sebaceous glands in the popliteal fossa. 3. Multiple lipomas: subcutaneous cysts formed by adipose tissue can lead to the appearance of a bag in the popliteal fossa. There may be other reasons for the swelling in the popliteal fossa, so we suggest that you go to the hospital in time, improve the examination to clarify the cause of the disease, and carry out standardized treatment under the guidance of the doctor.