PRK and LASIK tutorial
- The 4 basic refractive errors and how they blur vision
- The Excimer Laser: PRK vs. LASIK
- Wavefront guided laser treatments for better vision
- Potential risks
- Testimonials
The 4 basic refractive errors:
Refractive errors are simply the eye's errors of focusing, which cause blurry vision. As the video above illustrates, the two structures in our eye that are responsible for focusing light are the cornea (the clear window in front of the eye where a contact lens would rest) and the lens (located just behind the iris, where cataracts form). If the cornea and lens focus well, images are clear on the back of the eye on the macula. The macula is the center of the retina, acting like the film in a camera, taking a picture to send back to the brain. When the cornea and lens aren't focusing light correctly, there will be blurry vision from one or more refractive errors.
Mypopia: Also called nearsightedness, myopia occurs when light is focused in front of the retina. Nearsighted people noticed that near objects are in better focus than distance objects. Usually there is a near point where objects are in perfect focus, corresponding to how nearsighted the eye is. Nearsightedness usually occurs because the cornea is too steep, or the eye is too long. The goal of corneal refractive surgery is to flatten the cornea (in the case of LASIK and PRK) thereby refocusing light rays further back onto the macula.
Hyperopia: Also called farsightedness, hyperopia occurs when light is focused behind the retina. People who are farsighted usually notice that objects in the distance are in better focus than those at near, although even the distance objects may be quite blurry. Hyperopia usually occurs because the cornea is too flat or the eye too short. The goal of refractive surgery is to steepen the cornea (in the case of LASIK or PRK) thereby refocusing light forward onto the macula. An eye with a large accomodative amplitude (see presbyopia below) can mask hyperopia for many years. Hyperopic eyes may show presbyopia earlier than average.
Astigmatism: Astigmatism occurs when the eye has more than one curvature, like a football. If the eye has no astigmatism, it is shaped more like a basketball. In astigmatic eyes, light is focused at different distances, resulting in blurry vision. The goal of refractive surgery is to remold the cornea into a perfect sphere, thereby refocusing all light onto one spot on the macula.
Presbyopia: Presbyopia is a condition of the lens of the eye. An eye with presbyopia cannot focus from distance to near. Nearly everyone is born with a large focusing ability (accomodative amplitude) that allows a large depth of focus from distance to very near. As we age, the amplitude decreases, perhaps because the lens of the eye becomes stiff or the intraocular ciliary muscles responsible for changing the lens' shape become less effective. Presbyopia usually becomes apparent in our 40's and worsens over time. Corneal surgery with the excimer laser cannot treat presbyopia. The FDA approved treatments for presbyopia are multifocal intraocular lens implants (such as the ReZoom, ReStore, Tecnis, or Crystalens), which are implanted after a lens or cataract has been removed, and Conductive Keratoplasty (which steepens the cornea with radio waves). Read more about these lens implants under the cataract link in the menubar above
The Excimer Laser:
The Excimer Laser is used when having LASIK or PRK to reshape the cornea. The laser essentially imprints the glasses correction onto the cornea to improve uncorrected vision. It has been in use since 1987. The excimer laser is FDA approved to treat myopia, hyperopia, and astigmatism, but not presbyopia. The Excimer laser is a very precise instrument -- it has the ability to remove corneal tissue at 0.25 microns (0.00004 of an inch) per pulse. The human cornea is about 550 microns thick. Most refractive errors will require removal of about 50-100 microns of cornea.
LASIK stands for Laser In-situ Keratomileusis. During LASIK, a thin flap of the cornea is created. The excimer laser then reshapes the exposed corneal bed under the flap according to your refractive error. The flap is then laid back down. Quick healing, little discomfort, and a rapid return of useful vision are the advantages of LASIK over PRK.
PRK stands for Photorefractive Keratectomy. PRK has been performed since 1987. During PRK, the excimer laser reshapes the front of the cornea according to your refractive error. There is no cutting of a flap like in LASIK. PRK is therefore somewhat safer than LASIK, but recovery is slower and more uncomfortable. PRK may be a good option for those with corneas too thin to have LASIK. PRK may be preferred for very active individuals who may be at risk of scratching their cornea (after LASIK, a surface trauma could disclocate the flap, a potentially serious injury). Outcomes of both LASIK and PRK are similar at one to three months after treatment.
Wavefront guided laser treatment and "higher order" refractive aberrations:
The cornea has small hills and valleys that are just microns tall (one-thousandths of a millimeter), which in essence are tiny refractive errors called higher order aberrations. They contribute to blurry vision and glare, in addition to the primary refractive errors discussed above (myopia, hyperopia, astigmatism). The technology is now available to measure and treat these small refractive errors. The treatment is called "wavefront" or "custom" PRK / LASIK. It is "custom" because a person's wavefront is thought to be as unique as their fingerprint. The potential advantages of custum over standard laser treatments are less glare at night, increased contrast sensitivity (vision in low light), and better acuity.
Risks of excimer laser treatment:
All surgical procedures entail some risk. Some risks to consider are: under or overcorrection which may require a second treatment, nightime glare or blurring, infection, scarring, double vision or ghosting, and the need for glasses to read. Serious complications that would lead to permanent vision loss are very rare, but possible, after laser treatment. Some complications may require additional treatments or surgeries. The laser treatment cannot be reversed. As the years go by, small adjustments to the refraction will normally occur as they would have if you didn't have surgery. Rare inherited conditions of the cornea, such as keratoconus, that are not apparent prior to treatment may appear afterwards.
Your doctor will discuss these and other potential risks in more detail during your preoperative evaluation. You will receive a detailed consent form to read and sign. Our staff and doctors are available to discuss any concerns you have at your visit.