The Cornea is the part of the eye that is altered during the LASIK procedure. The Cornea is an optically clear tissue composed of several layers, the most anterior of which is the Corneal Epithelium. The Corneal Epithelium is an epidermal layer of cells which, when damaged, regenerates quickly, often within 24 hours or sooner. Mechanical trauma to the Corneal Epithelium 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 the layers deepen from exterior toward the interior of the eye. Bowman’s layer is a very dense layer of collagen just below the Epithelium. 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 corneal lamellae, or collagen sheet bed (Stroma). The collagen lamellae are glued together with hydrated cellular glue called Glycosaminoglycan (GAG). The Cornea is able to maintain optical transparency because the spaces between the interdigitated sheets of collage are less than 1 wavelength of light apart. This allows light to travel unimpeded through the Corneal substrate.
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 GAG’s are found and they complex to form hydrated GAG ground substance which holds the collagen lamellar sheets together much like glue would hold layers of paper together. There are also cells called Keratocytes dispersed throughout the GAG-Collagen matrix which 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 most inferior layer of the Cornea, the Endothelium, acts as a “pump”, regulating the amount of water that can enter the Corneal lamellae. If the Corneal Endothelium malfunctions, water enters the spaces between the Corneal lamellae, separating them further, impeding the transmittance of light (see Fuch’s Dystrophy; see Endothelial Cell Dropout). There is a very delicate balance maintained between the Endothelium and the Stroma which has the regulatory function of ensuring optical clarity of the Cornea.
The Corneal lamellae are lightly bound together. When separated and then replaced, the layers are able to bind together again. The first surgical step in LASIK involves running a surgical blade tangentially across the cornea in order to separate the upper layers from the lower layers and create a “flap”. The device the blade is housed in is called a Microkeratome and is analogous to the body of a lathe. The microkeratome makes a cut through the epithelium and for the most part just separates a layer of stroma as the blade runs across the cornea from within. Similar to peeling the skin off a grape, the microkeratome makes the separation necessary for the surgeon to “peel” a superficial layer of cornea which becomes known as the flap. The flap occurs by separating the layers, or lamellae of the cornea thus LASIK is sometimes. A laser called a Femtosecond laser is often employed to create the flap. The Femtosecond laser has several advantages over the bladed microkeratome, the major advantage being that the risks of having a blade touch the eye is eliminated and the second advantage being that the flap depth can be consistently controlled for.
When the flap is separated, it is gently lifted and folded aside, exposing the lower layers of Corneal lamellar stroma, the most superficial of which will be altered by the energy imparted to it by the Excimer laser during the refractive/corrective part of the procedure. The Excimer laser uses cool ultraviolet light to break the molecular bonds of the collagen lamellae in the exposed corneal bed. The word excimer is derived from the terms “excited” and “dimer”. The energy of the laser light is absorbed by a molecule (dimer), causing it 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 in the periphery. When the flap is 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.