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Eyelid layers
The eyelid is built in layers. Through an upper lid incision the surgeon passes skin, orbicularis muscle, the orbital septum and the fat behind it to reach the levator aponeurosis, which lifts the lid and attaches to the tarsus.
Outside to inside
Eyelid skin loses elasticity and thins with age. Redundant lid skin and muscle is called dermatochalasis and is usually noticed in the fourth decade. The upper crease sits about 8 to 9 mm above the lash line in women and 7 to 8 mm in men. At least 20 mm of vertical lid skin is kept so the eye can close.[1][2]
The orbicularis muscle lies under the skin. In a randomized left-right study of 22 patients, lids with wide muscle removal had more sluggish closure, lagophthalmos and dry eye than skin-only lids, with the same appearance. A meta-analysis of 12 trials found 7.98 times the odds of lagophthalmos with muscle removal.[1][2]
The upper lid has two fat compartments, medial and central, and the lower lid has three, medial, central and lateral. Dye injected into one compartment during blepharoplasty stained only that compartment. The inferior oblique muscle separates the medial and central lower pads and is at risk when fat is removed.[1][2]
The levator aponeurosis and Muller muscle sit behind the fat and attach to the tarsus, the firm plate of the lid. The crease forms where levator fibers insert into the skin at the upper border of the tarsus. Age-related separation of the levator is the most common cause of upper lid ptosis.[1][2]