The leg fossa is also known as the popliteal fossa, and the common causes of lumps in clinical practice include two main categories: 1, common benign masses on the body surface, such as sebaceous cysts or subcutaneous lipomas. The most common clinical cause of a mass in the popliteal fossa is a popliteal cyst caused by excessive mucus secretion from the knee joint cavity due to excessive knee joint activity. Generally, after reducing knee activities, the cyst can shrink or even disappear. If they do not shrink or disappear for a long time, or even cause localized pain and discomfort, surgery is usually required, but there is a possibility of recurrence of popliteal cysts after surgical treatment.