A cyst in the crook of the leg, also called a popliteal cyst, is a cyst that forms mainly in the synovial position on the posterior side of the knee joint and is filled with a jelly-like substance, usually due to either congenital or acquired appearance. The congenital form is present from birth and may grow in size until it compresses the nerves and blood vessels and affects the surrounding tissues, which should be treated aggressively. In other cases, such as local trauma or strain, the cyst may appear in the synovial membrane. If there is a more pronounced compression of the surrounding tissues, there are usually two treatments: the first, puncture with an empty needle to aspirate the jelly-like substance inside, and then bury absorbable thread, betamethasone, and lidocaine injections through the empty needle, which can be effective in getting the walls of the cysts to append by stimulating the walls of the cysts. The second, if the former treatment is not good, usually involves surgical excision.