Common causes of a bulge in the leg socket behind the knee include popliteal cysts, lipomas, and tumors. 1. Popliteal cyst: most cases of bulge in the leg socket at the back of the knee are a cystic solid mass, i.e. popliteal cyst. In the early stage of popliteal cysts, there are no obvious symptoms, and there is no obvious effect on the mobility of the knee joint. When the mass interferes with life or affects the appearance, it can be surgically removed. 2. Lipoma: caused by a large accumulation of adipose tissue, it is usually a solid swelling with a solid sensation when touching the swelling. It is asymptomatic and can be left untreated. 3. Tumor: first of all, it needs to be examined to determine its benign or malignant nature. If it is benign, surgical resection can be performed. If malignancy is suspected, puncture examination is needed to determine the nature before deciding the next treatment plan. The bulge in the popliteal fossa behind the knee joint may be caused by other factors, such as hemangioma, etc. It is recommended that the patient consult a doctor in time to clarify the cause of the disease and then carry out targeted treatment.