A bulge in the leg fossa behind the knee is considered to be possibly due to popliteal cyst, lipoma, infection and other factors. 1. Popliteal cyst: due to knee joint lesions such as knee osteoarthritis, meniscus injury, bursitis, etc., resulting in knee joint fluid entering the popliteal fossa through the weak area of the joint capsule at the back of the joint, and it is difficult for the fluid entering the popliteal fossa to flow back into the joint cavity, which gradually increases and forms cystic swellings. 2. Lipoma: common in obese people, due to inflammation stimulation of connective tissue degeneration, abnormal metabolism of adipose tissue and other reasons leading to abnormal accumulation of fat. If the abnormal accumulation is in the popliteal fossa, it may protrude from the skin surface to form a bulge. 3. Infection: local infection can cause pus in the soft tissues behind the knee, the pus collects under the skin, forming a bulge, accompanied by local skin redness, swelling, elevated skin temperature and pain. When there is a bulge in the leg socket behind the knee, it is recommended to actively seek medical attention and treatment under the guidance of a physician.