How LASIK Corrects Imperfections in the Cornea

Many patients do not realize that the eye is an imperfect optical system. As light rays from distant objects pass through the individual optical components of the eye, they are subject to being distorted by the imperfections found in the cornea and the crystalline lens.

The Cornea. The cornea is the "window" of the eye. It is the transparent front part of the eye that covers the iris, pupil and anterior chamber, providing most of an eye’s optical power. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus.

The cornea is made up of five layers of strong clear tissue. The first layer (epithelium) is made up of highly regenerative cells that allow for quick healing (24-48 hours) of superficial injuries. The deeper four layers add rigidity and provide a barrier against infection. Those four layers are the bowman’s layer, the stroma, descemet’s membrane and endothelial.

Crystalline Lens. This normally transparent structure is responsible for one-third of the refraction of light that enters the eye. Located just behind the pupil, it can change in shape to allow focus to change from distant to near objects. This focus change is called accommodation. As a person matures, the lens hardens which makes accommodation more difficult.

Retina. This membrane lines the inside wall of the eye. It contains photoreceptors (rods and cones) that transmit light into sight by changing light energy into electrical impulses. These messages are transferred from the retina to the brain and reinterpreted as images.

All laser vision correction surgeries work by reshaping the cornea, or clear front part of the eye, so that light traveling through it is properly focused onto the retina located in the back of the eye.

The word laser is the abbreviation for Light Amplification by Stimulated Emission of Radiation. The Excimer Laser's precise, ultraviolet beam reshapes the front surface of the cornea giving the cornea a new optical profile so that light entering the cornea is bent to focus properly on the retina. The more tissue that is removed from the cornea the greater the amount of refractive error that is corrected. Following treatment, the nearsightedness, farsightedness and astigmatism can be reduced and in most cases eliminated.

In the LASIK procedure, a liquid anesthetic is dropped into the patient's eye, numbing it for surgery. The surgeon then props the eyelids open and marks the cornea with water-soluble ink to guide in the later repositioning of the flap. A suction ring is placed on the eye to secure the eye and maintain pressure within the eye while the cornea is drawn outward. Simultaneously, a microkeratome (a small, automated scalpel) is placed in the track of the suction ring. The blade of the microkeratome then moves across the cornea, creating a flap of corneal tissue some 30-40% deep into the total corneal thickness. This layer is not cut away completely, but remains attached at one side and is then opened like a door on a hinge to reveal the stromal bed beneath.

The surgeon folds the flap back out of the way, then removes some corneal tissue underneath using an Excimer Laser. The Excimer Laser uses a cool ultraviolet light beam to precisely remove ("ablate") very tiny bits of tissue from the cornea to reshape it. When the cornea is reshaped in the right way, it works better to focus light into the eye and onto the retina, providing clearer vision than before. The flap is then laid back in place, covering the area where the corneal tissue was removed.

The computer on the laser is programmed by the surgeon to precisely control the amount of tissue to be removed. The area involved in the corneal sculpting in less than 8mm in diameter and the depth sculpted is much often less than the thickness of a human hair.

To correct myopia, the laser trims the cornea's center, making it flatter. For hyperopia, a doughnut shaped ring of tissue is removed. The laser is programmed to ablate the necessary amount with a modified version of the patient's glasses or contact lens prescription. The corneal flap is then repositioned to its original position on the stromal bed.

One way that eye care professionals deal with presbyopia is by producing monovision. Normally, both your eyes work together equally when you look at an object, to produce what's called binocular vision. However, you probably have a dominant eye that your brain tends to favor for "sighting" (most right-handed people are right-eye dominant, for example). Contact lens fitters often take advantage of this "one-eye dominance" to produce monovision (think of it as the opposite of binocular vision) with the contacts: they fit one eye for distance vision (typically the dominant eye) and one for near vision.

Therefore, in monovision, one eye does more work (sighting) than the other. If one of your eyes is set for distance vision and the other is set for near vision, the distance eye will do most of the work when looking at objects in the distance, and the near vision eye will do most of the work when looking at objects close by.

Some LASIK surgeons will produce monovision in their presbyopic patients by purposely leaving the non-dominant eye slightly nearsighted so that these patients can see up close without glasses (out of one eye). Many are wary of the technique because not everyone can become accustomed to the absence of binocular vision. It's better to try monovision with contact lenses or trial lenses in the doctor's office first to be sure you can adapt.