What is refractive error?
The goal of laser vision correction is to reduce refractive error and, by doing so, reduce or eliminate a patient’s need for glasses or contact lenses. Refractive error is an imbalance between the light-focusing power of the eye and the length of the eye, which leads to blurred images on the retina. Refractive error is expressed as the glasses’ strength in diopters (D) required to bring a distance image into focus on the retina.
There are several types of refractive error that can be addressed with laser vision correction:
- Myopia (nearsightedness): Myopia is a condition in which the cornea focuses light too strongly relative to the length of the eye. In myopia, a distant image is focused in front of the retina rather than on it, which makes it harder to see than a near object. Laser vision correction can be used to selectively remove tissue from the center of the cornea, flatten its curvature, and move the focal point backwards onto the retina to allow better distance vision without glasses.
- Hyperopia(farsightedness): In hyperopia, the cornea’s focusing power is too weak relative to the length of the eye. In this condition, images are blurred because the light rays reach the retina before they ever join together to a focused image. Although hyperopic patients are commonly referred to as “farsighted” because their distance vision is usually better than their near vision, these patients do not necessarily see well at distance, especially as they age and lose the lens’s near-focusing power (see presbyopia, below). Contrary to laser vision correction for myopia, laser treatments for hyperopia remove tissue from around the central cornea to steepen the center and increase its optical power. This moves the focal point forward onto the retina.
- Astigmatism: Astigmatism can occur at the same time as myopia or hyperopia, and is usually targeted for correction in the same treatment. Astigmatism describes blur that results from asymmetry (unevenness) in the focusing power of the cornea or lens. A useful way to picture astigmatism is to compare an American football and a basketball. The curvature of a basketball, a perfect sphere, is the same no matter what perspective it is viewed from. A football, however, has different curvatures depending on what angle it is viewed from. Similarly, the cornea often has some degree of shape difference (astigmatism) that affects image focus, and spectacles (glasses) with “cylinder” correction or toric contact lenses are often used to correct this. In laser vision correction, the laser treatment profile is specially programmed to include a correction for astigmatism. Patient with very high astigmatism or irregular patterns of astigmatism require special consideration; their physicians may talk to them about alternatives to laser vision correction.
- Presbyopia: Presbyopia is different from the first three refractive problems above because it eventually occurs in everyone, gradually progresses into the person’s late 60s, and usually doesn’t affect baseline distance vision (except for some people with hyperopia). As we enter our 40s, the lens, an auto-focusing structure that is suspended behind the cornea, begins to lose its ability to relax (accommodate) into a steeper configuration for reading and other near work. People with no distance vision problems end up needing reading glasses because of this process. While laser vision correction does not reverse this process, your specialist can discuss your options for reducing dependence on reading glasses.
What are photorefractive keratectomy (PRK) and LASIK?
Both of these laser vision correction techniques use an excimer laser to rapidly and precisely remove a defined pattern of tissue from the cornea to correct the refractive errors discussed above. These systems use sophisticated eye tracking technology and high laser repetition rates that have improved outcomes over earlier systems.
In PRK, the surgeon removes the corneal surface cells known as epithelium to expose the layer underneath that will be reshaped using the excimer laser (the stroma).
In LASIK, an instrument called a microkeratome or a femtosecond laser produces a thin flap on the surface of the cornea that remains attached to the surface by a hinge. The flap is folded back and the excimer laser correction is applied beneath the flap; the flap is then returned to its original position. The flap holds without stitches. LASIK accounts for about 85 to 90% of laser vision correction procedures.