Hi shakira112, in the hands of someone comfortable with ptosis surgery, the external (levator advancement) and internal (muller muscle resection) ptosis repairs can have equivalent outcomes. The decision for which surgery is performed depends on multiple factors including need for other procedures, anatomical reasons and other issues that can be illucidated with careful examination. Each procedure has its advantages and disadvantages. With an external repair, precise tightening of the tendon for the eyelid raising muscle is required but if skin or fat removal is desired, a second incision isn't required. In addition, to establish symmetry patient cooperation is needed for external surgery and the patient must be awake (not feeling anything) for the repair. Internal repairs can be performed under general anesthesia without cooperation but revision following the repair is difficult and if skin/fat removal is desired, a second incision is required. Regardless, in the hands of a competent eyelid specialist, the outcomes are quite similar. I would recommend someone trained in ophthalmic plastic surgery, you can find a local surgeon on the American Society of Ophthalmic Plastic Surgeons (ASOPRS) website.
Thank you for sharing your question. Most Oculoplastic Surgeons are comfortable performing both procedures. The technique depends on the needs of the patient. In general, the operation performed behind the eyelid lifts the eyelid less than the one done through the skin. I hope this helps.