The most common cause of a bulge in the leg behind the knee is a popliteal cyst, which is caused by a buildup of fluid in the joint cavity or tendon sheath due to inflammation or other factors, and does not require special treatment for mild symptoms, but needs to be treated surgically for more severe symptoms.
The fossa behind the knee is called the popliteal fossa, and a bulge here indicates the presence of popliteal cysts, which may be caused by fluid accumulation in the joint cavity or tendon sheath, and is commonly seen in sports injuries, osteoarthritis, rheumatoid arthritis, and intra-articular infections, etc. When the symptoms are mild, it does not need to be treated, and can be observed, but when the symptoms are more severe, it needs to be completely treated with surgery.
If there is a bulge in the fossa of the leg behind the knee, it may also be lipoma, inflammatory edema, etc. If it is not eliminated, it is necessary to go to the hospital in time, and ask the professional physician to make a judgment and take relevant measures to intervene.