How do lasers used in LASIK or PRK treat Refractive Error?

To improve vision the focusing problems of the eye need to be corrected. Laser technology is used to reshape the cornea. By making the cornea more flat or more curved, the light rays are bent differently and focusing can be improved. The Excimer (excited-dimer) laser uses light energy to break chemical bonds in the cornea. The ultraviolet light energy (wavelength of 193 nm) comes from an Argon-Fluoride laser, which produces very short pulses of energy (10 nanoseconds). These pulses of light energy actually dissolve or ablate the collagen fibers. By using a computer to guide the location of each laser application into a complex pattern of overlapping spots, the shape of the cornea is sculpted to a new and improved shape, resulting in better focus and clearer vision.

What is PRK? Photorefractive keratectomy

With PRK the cornea is sculpted with an Excimer laser. The thin skin that covers the cornea must first be removed. The laser is then applied. Over the next 3-4 days, the skin (epithelium) grows back over the cornea. PRK is a simpler procedure than LASIK. PRK has a longer visual recovery than LASIK because the vision will not be fully improved until the epithelium has completely healed and become smooth again. There is also more discomfort with PRK as the nerves in the cornea are exposed when the thin skin is removed. Because PRK is a simpler procedure there is less potential for some complications that can happen with a LASIK flap (see below). Because nerves are not cut, there is less chance for dry eye problems after the procedure.

What is LASIK?

LASIK also uses an Excimer laser to sculpt the cornea. However, before treating the eye with the laser a few layers of the cornea are removed as a thin flap. The laser is then applied to the cornea below the flap and the flap is placed back into position. By cutting the flap and applying the laser to the internal collagen fibers of the cornea there is less pain and a quicker visual recovery. The presence of a flap may lead to complications in rare cases such as an eye injury, or an excessively thin cornea leading to ectasia.

What are the risks associated with LASIK and PRK?

LASIK and PRK are very safe procedures with a track record of success over 15 years. Treatments and technology improve each year, bringing the opportunity to achieve better vision with minimal risk. However, each person responds differently to surgeries, and any surgical procedure can have risks involved. Some of the main risks include…

Night Vision Problems

Following LASIK surgery some patients will be aware of increased glare or halos at night. This tends to occur in patients with night vision symptoms before the surgery, in patients with a larger correction, and patients with larger pupils. The glare is related to light rays coming into the eye from the edges of the cornea where the normal cornea meets with the ablated cornea.

Dry Eye

Because the nerves are damaged with the flap creation and corneal ablation it is common to have some dry eye symptoms for up to 3 months after the procedure. If you have a dry eye problem before surgery, LASIK may not be the best option and PRK or ICL should be considered. You will likely need to use artificial tears during the day until your corneal nerves recover.


Because the protective surface of the eye is removed with either PRK or LASIK there is a chance that bacteria get into the cornea and cause an infection. Antibiotic eye drops are used to prevent an infection, but even with this protection, rare infections can develop. The risk of an infection is less than 1 in 1,000.

Corneal Ectasia

LASIK and PRK make the cornea thinner, which in some people can weaken the structure of the cornea. If the cornea becomes weak it can change shape, leading to worsening vision. Pre-operative testing and good decision-making can usually help prevent this complication.

Flap Complications (LASIK)

Using a flap allows for faster vision recovery with LASIK, but the process of creating a flap does lead to some other risks. With a simple trauma to the eye, the flap can dislodge, requiring a procedure to replace it. Small cells from the epithelium (thin skin that covers the eye) can grow underneath the flap requiring a procedure to lift the flap. If there are problems when the flap is cut you may have to postpone your LASIK procedure for another day.

Stromal Haze (PRK)

The cornea is amazing clear tissue. An injury to the cornea can lead to scar tissue formation, which causes the cornea to become cloudy in the area of the scar. PRK is a very mild trauma to the eye and therefore some people develop a slight haze to the cornea. In most cases, this does not have a significant impact on the quality of the vision, but if haze is extensive it may make your vision less clear. Severe scarring would be unusual. In the case of severe scarring, a corneal transplant surgery may be necessary.

Over-correction or Under-correction

Your eye is a living organ. Very precise measurements need to be made and then appropriate treatments applied to achieve the best possible vision. In some cases, it is difficult to get accurate measurements and different people can respond differently to the treatments. Because of this, some treatments will lead to over-correction or under-correction, meaning that the vision does not turn out to be 20/20 after the procedure. If this occurs an enhancement procedure can be completed. Usually, the eye is allowed to heal for at least 3 months before an enhancement is initiated. You may need to wear glasses or contact lenses for a few months during this time.

How do I know if I am a good candidate for LASIK/PRK?

Not everyone is an ideal candidate for refractive surgery. We get terrific results because we are careful to select patients who are good candidates for each procedure. For LASIK and PRK there are several items necessary to achieve good results. They include…

  • A generally healthy eye and cornea
  • No uncontrolled dry eye symptoms
  • Adequate corneal thickness – measured at your pre-op with a safe and simple test (pachymetry)
  • Eyelid and facial structures that allow the laser to get near the eye
  • Dissatisfaction with glasses and/or contact lenses
  • A desire to have improved vision with a willingness to accept some risk to achieve this goal

All of these items are assessed at your refractive surgery screening visit. After a thorough review, Dr. DeBry and/or Dr. Mu will help you to understand your options and make recommendations for the procedure that will help you achieve your best vision.

What is All-Laser LASIK?

As described previously, LASIK treatments require a thin flap to be made on the cornea. In the past, this flap was created with a very sharp blade called a microkeratome. In a small percentage of people, the microkeratome-created unique problems as the flap were cut with imperfections or centering issues. To lower the risk of irregular flaps a laser was developed for flap creation. This Femtosecond laser cuts tissue with an accuracy of 0.01 mm and makes perfectly circular flaps with a very consistent thickness. The use of all-laser LASIK has made LASIK a safer procedure. It has also added a little more cost to the procedure because now 2 lasers are used instead of one. An interesting side note, the femtosecond laser fires a laser pulse 0.000000000000001 seconds in duration. This rapid burst of energy causes a microscopic explosion in the tissue, breaking bonds.

What happens on the day of the surgery?

Having eye surgery is a bit like a trip to the dentist. It is a little uncomfortable, takes about 15 minutes to have it done, and has great benefits. It can be scary as some people don’t like things coming up close to their eyes. To help relax you we will provide a prescription for relaxing medication, a sleeping pill commonly used to relax anxious nerves. If you are small in stature and not an anxious person taking ½ of a pill is usually fine. If you have a lot of anxiety or are a bigger person you can take a whole pill or in some cases even more.

During the procedure, you will lie down on a comfortable bed with a firm headrest. Get comfortable, as it is very important to hold still once the procedure starts. The eyelids are cleansed and sticky tape is put around the eyelashes to keep them out. A small wire is used to gently hold open the eyelids. During the treatment, there are several steps…

  • The Femtosecond laser will cut the flap. A suction ring is placed on the eye during this step. It takes only 15 seconds but is a bit uncomfortable due to the pressure from the suction ring. Your vision will fade to black during this time.
  • Next, the flap will be lifted and the corneal tissue prepared for the excimer laser. During this step, your vision will be blurry.
  • The excimer laser treatment is next. This takes between 15 and 45 seconds depending on the degree of correction being treated. There will be a red or green light during this time and it is important that you focus on the colored light to keep the laser treatment centered on the cornea.
  • After the laser, the flap will be irrigated. You will feel some cold water around the eye. Then the flap will be placed back into position and allowed to seal for a few minutes. The wire holding the lids will be removed and the treatment repeated on the other eye.

After the procedure, we encourage you to go home and take a nap. You will be sleepy from the relaxing medication and it is helpful to keep your eyes closed for a few hours to start the healing process. The vision will be a little blurry for the first 24 hours but will improve even more over the next several days as the cornea starts to heal.

PRK treatments are even easier as there is no Femtosecond flap created. You will simply lie under the laser table for a few minutes as the lids are prepared, and the corneal epithelium is removed. The excimer laser application is done right on the surface of the cornea and takes 15-45 seconds. A bandage contact lens is placed on the eye after the procedure to help with comfort and healing of the epithelium.

Choosing Bilateral Surgery

You have the choice to do PRK/LASIK in both eyes on the same day, or separate the two surgeries into two different days. The safest option is to do the eyes on two different days. This allows Dr. DeBry the opportunity to assess the outcome of the first surgery before the second procedure. Based on the first procedure, small adjustments in laser power can be made prior to the second eye being done. However, many people have scheduling constraints with work or family responsibilities that make it difficult to take extra days off to do the surgeries on different days. If you choose to have the surgeries done on the same day we will have you sign an extra consent form acknowledging this choice. The risk of a severe bilateral problem such as an infection is very rare. Plan on your vision being blurry for the first day or two while the eye is healing from the procedure. We recommend you don’t plan any important meetings or travel for at least a few days after the procedure to allow your vision time to improve.

Are there any restrictions after the procedure?

The most important thing to avoid after the procedure is rubbing your eyes. This can displace the flap if you had LASIK or the bandage contact lens if you had PRK. Otherwise, there are very few restrictions. You can bathe and shower like normal, just avoid getting soap and water in your eyes. It would be a good idea to avoid visually demanding tasks for a few days after the procedure. You should stay out of the swimming pool or hot tub for a few weeks after the procedure.

What if I need an enhancement?

LASIK and PRK change the shape of the cornea, but there can be regression over time. Also, some people don’t achieve perfect results after the first treatment and a second treatment needs to be applied. Some people require an enhancement procedure years after LASIK/PRK. Our policy is that enhancements are included with the original surgery fee for the first year. If an enhancement is needed years later there will be a new fee for those services.

What is custom LASIK (wavefront-guided LASIK)?

Normal LASIK uses the refraction numbers based on your glasses to create a treatment plan. Studies have shown that vision can be improved and some negative visual symptoms decreased if the laser is programmed to treat higher order aberrations along with the refraction numbers. To achieve this, additional measurements are made on the eyes to measure the individual “fingerprint” of your eye. These measurements are then put into the LASIK excimer computer to provide a custom treatment designed just for you. This may give better results depending on your corneal structure and refraction.


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