24 Sep PRK Astigmatism and LASIK Astigmatism
What is Astigmatism?
Astigmatism means that your eye is shaped more like an egg than a perfectly round ball. This makes object out of focus
Can PRK Correct Astigmatism and can LASIK Correct Astigmatism?
Many patients are surprised to find out that PRK and LASIK can correct astigmatism and the modern tratment profiles do a very good and accurate job of correcting regular astigmatism.
How Does LASIK and PRK Correct Astigmatism?
Both PRK and LASIK use an eximer laser to reshape your cornea and make it more round. This treats the astigmatism to restore good focus. Different laser systems accomplish this rounding of your cornea in different ways. Some flatten the steep part of the cornea, others steepen the flap part of the cornea, and some systems, like the Allegretto, do both.
History of PRK and LASIK Astigmatism
The FDA trial for PRK was started in 1985 and it was finally FDA approved in 1996. This first approval was only to correct myopia. Further FDA trials were conducted and PRK astigmatism correction was FDA approved in 1997. Since that time, we have had the ability to treat both your myopia and your astigmatism in the same PRK laser treatment.
Even though we have been correcting astigmatism with laser eye surgery for almost 20 years there are still many patients with with the false assumptions that we can’t help them. LASIK was first performed in the U.S. in 1997. The astigmatism correction patterns where already available by then, so LASIK Astigmatism correction has been available since 1997.
Advances in PRK Astigmatism and LASIK Astigmatism
There have been a number of important advances in the quality of the lasers and the laser patterns used for PRK astigmatism and LASIK astigmatism correction in the last twenty years. These advances have improved the safety of the procedure and the quality of vision. Both PRK and LASIK use the same excimer lasers to correct your vision. As such, when a laser improves astigmatism correction for PRK, it also improves it for LASIK.
How is Night Vision after Astigmatism Correction?
Your cornea is 12 millimeters (mm) in diameter, but only the more central part matters for vision because only the light that can pass through the pupil has any effect. Light that hits the iris out side the pupil is blocked by the iris and never makes it to the back of the eye. PRK and LASIK only need to treat the central 6 mm of your cornea to give good focus for daytime vision.
The original laser eye surgery treatments zone was generally 6 mm treatment for myopia but the size of the astigmatism treatment zone was smaller, sometimes as small as 3.5 mm depending on the amount of astigmatism treated. This pattern is called the “traditional” pattern on the VISX laser system. It does a good job for most patients.
There were two problems with this early treatment zone pattern. First, many patients have pupils that were larger than 6 mm at night. Some patients could see the abrupt transition where the laser pattern stopped. This caused nighttime glare and haloes. In the late 1990’s, laser engineers improved the “traditional” laser pattern by adding a “blend zone” feature so that the edge of the laser pattern was gently blended into the rest of the cornea. This improved night vision for many patients. The quality of the blend zones has improved over the years.
The second problem with the “traditional” laser pattern was the small size of the astigmatism treatment pattern. Further improvements in the Visx system brought the “Custom” treatment patterns which increased the diameter of the astigmatism treatment a bit further out. This trend was pushed even further by the Allegretto EX 500 system by Alcon where the treatment zone for astigmatism is a full six mm on all patients.
Smoother Treatment Patterns
Another significant advance in laser astigmatism management came with much smoother treatment profiles. A smoother cornea after your laser vision correction is good for optical quality. The “traditional pattern” used two plates that opened in progressive stages to treat astigmatism. This delivery method was effective, but not ideal as microscopic ridges were created at each succesive stage as the plates opened.
When the VISX “custom” treatments became available, a variable spot size was used to smooth and customize the pattern. The most modern patterns like the Alegretto EX 500 use small laser spots delivered at 500 times per second. The treated cornea is further smoothed using this method. This improves the quality of vision, decreases the chances of corneal haze, and increases the speed of epithelial healing. All of these make both PRK astigmatism and LASIK astigmatism treatments better and safer.
Eye Tracking Systems
The precise placement of these spots is controlled by eye trackers that monitor the position of your eye. FDA approval for treatment with an eye tracker was granted in the year 2,000. This tracker checked your eye position 80 times per second. The more modern trackers now check eye position 1,000 times per second and adjust the direction of the laser spots to the correct location.
What is Mitomycin C
Mitomycin C is a drug that inhibits the action of fibroblasts that make scar tissue. The military has shown that applying mitomcyin for a few seconds after the laser correction procedure decreases the chances of having any corneal haze complications after PRK astigmatism repair. This has resulted in an increase in safety for the PRK astigmatism procedure. It is essential that you not be pregnant at the time of the procedure if mitomycin is used. Corneal haze is less common after LASIK astigmatism correction, so mitomycin is not needed.
Advice for your PRK/LASIK Astigmatism
Request a modern treatment pattern
The Allegretto EX 500 only has advanced treatment patterns with large zones for all astigmatism treatments. It is my personal favorite at this time. We also have the VISX laser. It has two patterns available, custom and traditional. If your surgeon only has the VISX, insist on the custom pattern. It has larger astigmatism patterns and a smoother bed than the “traditional” pattern that uses the old opening diaphragm/plates with smaller astigmatism zones. The custom treatment charges a higher royalty fee per use to the surgeon but it is worth the extra money.
I am also very impressed with the treatment patterns on Schwind system, which is not currently available in the U.S. I do not personally recommend the Nidek system, though some surgeons are pleased with it.
Choose and Experienced PRK Astigmatism Surgeon
Some surgeons perform LASIK almost exclusively and only rarely perform PRK. Ask your surgeon what percentage of his/her practice is PRK if you desire PRK astigmatism correction. Experience matters with eye surgery.
Mitomycin for PRK?
If you desire PRK, ask your surgeon if mitomycin will be used to decrease the chances of corneal haze formation.
If I can answer any other questions about LASIK astigmatism correction or PRK astigmatism, please schedule a time to meet with me or post a comment.