Some people have pockets behind the knee, both large and small, which are generally unlikely to be tumors and may be cysts. Some small children will develop cysts, which are congenital. In older people, cysts are often the result of a lesion in the joint, usually a meniscal lesion. After meniscal lesions or inflammation after cartilage lesions, the intra-articular pressure increases and a hole appears on the posterior side of the joint cavity, in the folded part of the joint capsule, and the surface of the gastrocnemius muscle keeps protruding backward with joint movement, often with one-way valve-like tissue that can only flow from the joint into the cyst, thus causing the cyst to grow larger and larger. If the cyst does not produce serious symptoms, it can usually be left untreated. If it is accompanied by intra-articular pain, fluid accumulation, and recurrent swelling, the cyst can be shaved off incidentally during intra-articular arthroscopic management, and if an additional 1-2 holes are made under the arthroscope the cyst can be completely removed.