Understanding the Corneal Flap and How it is Lifted in LASIK

The Cornea is the vital tissue altered during the LASIK procedure. The Cornea is constructed of several layers, the most anterior of which is called the Epithelium. The Corneal Epithelium is an epidermal layer of cells which, when damaged, is quick to regenerate. For instance, mechanical trauma to the Cornea may heal completely in a 24 to 48 hour period. The top layer of Corneal epithelium is one of 5 layers which become increasingly compact as you go deeper through the Epithelial layer. Beneath the most compact, bottom layer of Epithelium is a very dense layer of collagen, Bowman's Layer. Beneath Bowman's Layer is the dense, central part of the Cornea, the Stroma. The Stroma consists of interdigitated sheets of collagen material that form the cornea lamellae, or collagen sheet bed. The collagen lamellae are glued together with hydrated cellular glue called glycos-aminoglycan material. The cornea is able to maintain optical transparency because the spaces between the interdigitated sheets of collagen are less than 1 wavelength of light apart. This allows light to travel unimpeded through the corneal substance. If the corneal endothelium (discussed below: responsible for maintaining the osmotic (water) balance within the cornea) malfunctions, water will enter the spaces between the corneal lamellae, separating them further, impeding the transmittance of light.

The density of the collagen sheets decreases as you go deeper through the stroma. The purpose of having a dense anterior stroma is to protect the eye from traumatic external forces. Water is a major component within the intra-stromal spaces where the glycos-aminoglycans are found, and they complex to form a hydrated glycos-aminoglycan ground substance which holds the collagen lamellar sheets together much like glue would hold layers of paper together. There are also Keratocytes (Corneal Cells) dispersed throughout the Corneal Matrix. These cells are active in the inflammatory and immunological response of the Cornea. Posterior to the stroma is another collagenous layer, descemets membrane which bounds the most posterior layer, the corneal endothelium. The endothelium acts as a pump, regulating the amount of water that enters the cornea from the anterior chamber just beneath the Corneal endothelium. If the endothelium is damaged, the influx of water will disrupt the smooth, equal layers of corneal stroma above and cause the cornea to become cloudy. There is a very delicate balance maintained between the posterior endothelial pump mechanism and the stroma in order to insure optical clarity of the cornea.

In standard Lasik Surgery, a device called a microkeratome is used to lift a cap of corneal epithelium and anterior stroma so the laser can reshape the lamellar corneal layers underneath. The microkeratome is analogous to a lathe. The microkeratome need only to cut through the epithelium. Once it has cut past the epithelium, the blade encounters the interdigitated sheets of collagen, and, runs through the layers of collagen, separating them, but for the most part not cutting through them. The microkeratome allows the surgeon to peel the upper layers of collagen away from the lower layers. This is why LASIK is called inter-lamellar surgery. A newer technology, IntraLase, uses the laser to cut the flap which has the advantage of forming a perfect flap every time. There is significant risk of cutting a flap with a standard microkeratome and intralase (femtosecond lasik) removes this risk from the procedure. After the flap is lifted, the part of the surgery that corrects the vision begins. The Excimer laser uses cool ultraviolet light to break molecular bonds of the collagen lamellae in the exposed corneal bed. The word excimer is derived from the terms "excited-dimer". The energy of the laser light imparted on a molecule (2 atoms or dimer) causes the two bonded atoms to separate and evaporate. In LASIK for nearsightedness (myopic lasik), the evaporation of the collagen lamellae is greater in the central part of the corneal bed and gradually lessens toward the flap-bed periphery, so when the flap in laid down, the overall corneal curve is flatter. In LASIK for farsightedness (hyperopic lasik), there is a need to make the central corneal curvature steeper. This is achieved by applying the laser in a ring fashion around the peripheral flap bed, leaving the central flap bed alone. This makes the peripheral ring area flatter than the central area of cornea, effectively steepening the curvature of the cornea. Astigmatism LASIK works by flattening corneal curvature in one meridian of the astigmatic axis of the cornea more than the other. (see website document on Astigmatism)