Eyelid skin is the thinnest in the body with relatively sparse subcutaneous fat. This allows free movement of the lid in closure and blinking. The upper eyelid skin is thinner than that of the lower lid. The skin itself has many fine hairs as well as sebaceous and sweat glands. Healing occurs quickly in this area and scarring is usually minute.
The lid crease of the upper eyelid is formed by the insertion of the levator aponeurosis fibers into the skin and the orbicularis oculi muscle. It is approximately 8–12 mm superior to the lash line and lies just at the level of the upper edge of the tarsal plate. Medially and laterally, the crease is closer to the lid margin and has an arc shape across the lid. The Asian eye usually lacks this crease due to the lower insertion of the levator aponeurosis on the tarsus.
The lid fold describes the tissue above the lid crease and may extend throughout the length of the upper lid or it may be more localized. Excess tissue may develop in the aging face and sag over the lid crease, sometimes obscuring vision. A combination of excess skin, hypertrophied orbicularis oculi muscle, and herniated fat can be responsible for this process.
The orbicularis oculi muscle provides the main mimetic function to the eyelid. It receives its innervation from the temporal and zygomatic branches of the facial nerve. The muscle is elliptical and divided into three bands (the pretarsal, preseptal, and preorbital), which attach to the bony orbit at the medial and lateral canthal tendons. The muscle can become hypertrophied over time and result in a full appearance of the eyelids.
Orbital fat cushions the globe and its associated structures, and its anterior limit is the orbital septum. In the upper eyelid, the fat separates the levator aponeurosis posteriorly and the orbital septum anteriorly. Here it is divided into two fat compartments: central and medial. In the lower lid, there are three fat compartments: lateral, central, and medial (Figure 74–1).
The orbital fat is divided into the upper medial and central compartments, and the lower medial, central, and lateral compartments.
The levator muscle acts to elevate the upper eyelid and has its origin in the periorbita posteriorly. The muscle runs above the superior rectus and fans out anteriorly to become the levator aponeurosis. Insertion occurs at the level of the tarsus, as previously described, forming the lid crease (Figure 74–2). Its innervation is by cranial nerve III (the oculomotor nerve).
Coronal view of the orbit and its structures.