Ptosis is also known as the droopy eyelid disease, in which the upper eyelid is abnormally lower than the usual. The disease can affect one eye or both of them. You can be born with it (congenital ptosis), or you may observe it develop as you age.
The basic surgery is determined by whether the eyelid raising LPS muscle (levator palpebrae superioris muscle) is strong or very weak. If the muscle is relatively strong, the surgery generally consists of operating on the muscle that elevates your eyelid, & making it stronger and thereby able to work better, which is elevating your eyelid higher (LPS eyelid surgery).
In other cases, where the eyelid raising muscle is very weak, the eyelid muscle itself may not be able to elevate the eyelid & in such cases, the forehead muscle needs to be connected to the eyelid muscle, to raise it.