|Advanced Surface Ablation (Photorefractive Keratectomy, LASEK, PRK)
WHAT IS PRK?
LASIK is the most popular form of refractive eye surgery in the world today to correct moderate degrees of nearsightedness, farsightedness, and astigmatism. However a certain number of patients, primarily because their corneas are abnormally thin or because of slight defects in the cornea such as very mild forms of keratoconus, are not good candidates for LASIK. LASIK performed in this situation can actually weaken the cornea and because of the normal pressure inside the eye cause the weakened cornea to bulge forward thus recreating the nearsightedness and astigmatism the patient required correction for in the first place.
BE OF HEART, THERE IS CORRECTION AVAILABLE IN MANY CIRCUMSTANCES. This is called advanced surface ablation or photorefractive keratectomy. Photorefractive Keratectomy (PRK) was actually performed for years prior to LASIK in all patients when Excimer lasers were first used to etch the cornea. Side by side comparisons show the results with PRK to be as equally successful as LASIK but the cornea is usually not weakened by this procedure.
Photorefractive Keratectomy (PRK) fell out of favor because it required approximately four days to a week for recovery as compared to LASIK which recovered much more quickly with less discomfort.
Because of its slightly longer recovery time and slightly more discomfort, and the possibility of slight haze formation it fell out of favor in relation to LASIK.
More modern lasers which are gentler on the cornea can produce very, very good PRK results today. Part of the reason the results are better and higher degrees of nearsightedness, farsightedness, and astigmatism can be treated with PRK than before is the use of a chemical used topically at the time of surgery to reduce or eliminate the possibility of haze.
This medication is called Mitomycin-C and when used in proper concentrations on a small sponge over the center of the cornea immediately after laser treatment haze is almost nonexistent following surgery. Thus, (PRK) is possible for people who are not good candidates for LASIK. PRK is extremely safe and highly successful. Indeed the results parallel those of LASIK nicely and some people who are adverse to the idea of having a flap produced either by laser or by microkeratome blade would prefer to have advance surface ablation, or PRK, in the first place.
Before surgery to determine whether PRK (advanced surface ablation) or LASIK is the best procedure for you, numerous measurements will be taken. It is important to know the contours of the cornea in great detail as obtained by the Orbscan machine. Regular topography (Contour measurement) which looks in more detail at the front curve of the cornea is also necessary. The size of the pupil in dim light is measured. A Wavescan picture of the eye which measures not only the amount of nearsightedness and farsightedness and astigmatism present on the cornea but all the minor defects within eye that we all have is also taken. This can be fingerprinted onto the cornea thus producing a customized or personalized treatment that is specific to each person often allowing resultant vision that is better than 20/20. Tear production must be adequate and measured carefully as this affects healing. All these measurements are similar to those taken for LASIK and will determine which is best for you.
If advanced surface ablation (PRK) is chosen as the best choice then the operative procedure is similar to that performed for LASIK except that a flap is not produced. A sponge with a light concentration of alcohol is placed over the cornea for thirty seconds to loosen the skin on the corneal surface. The skin (epithelium) is then gently brushed away over a six to seven millimeter area of the central cornea exposing the underlying stroma (the underlying tough connective tissue layer). The Visx Star IV Excimer laser is then used to etch through the stroma into the cornea the specific treatment necessary to correct the nearsightedness, farsightedness, and/or astigmatism plus the higher order aberrations within the eye as mentioned previously to hopefully produce an exquisite result.
Following this a small sponge of Mitomycin-C is placed over the area treated and left in place for 15 seconds to 2 minutes depending on the degree of correction attempted.
Because a large abraded area of six to seven millimeters on the cornea is created for the PRK procedure it is more uncomfortable than the LASIK in healing. However certain measures are taken to ensure that the patient will indeed be comfortable after surgery. First, a bandage contact lens is applied. This is simply a soft contact lens with no power in it which hugs the cornea and is left in place for three to four days. No manipulation on the part of the patient is necessary. With the contact lens in place the lid can blink over it rather than over the raw surface. The nerve endings exposed on the raw surface are thus not irritated and the skin can grow more rapidly underneath the contact lens.
On average approximately three days are necessary for the skin surface to completely cover the defect. At this point the bandage lens can be removed and the patient will begin to have good vision with comfort. During this three to four day period of time the patient will be more light sensitive and spending time in a slightly darkened environment is appropriate. Ice packs are always used throughout the first two to three days which markedly decrease the discomfort. Eyedrops in the form of antibiotics and cortisone and drops that have non-steroidal anti-inflammatory properties similar to aspirin are used to also diminish the discomfort. Between the bandage contact lens, the drops, and the ice pack, people are generally comfortable without requiring any oral (by mouth) pain medication. However, as a precautionary measure pain medication will be provided at the end of the procedure to help people sleep at night.
After surgery using modern surface ablation (PRK) the recovery is markedly different from LASIK. Within four hours the LASIK patient derives good vision and moderate to mild degrees of discomfort disappear. However with PRK because of the abrasion created for the treatment the vision is always more blurry and discomfort is more severe at first. In a nutshell, at the end of a week the patient who has undergone advanced surface ablation (PRK) will see equally well and be as pain free as was the LASIK patient at the end of the first day.
Because of this, and this alone, LASIK is our preferred procedure provided all of the measurements suggest it can be done safely. Do not despair! Advanced surface ablation (PRK) carries the same results!! When one considers the options of four to seven days of discomfort with PRK over a lifetime compared to four hours to one day of discomfort with LASIK, to most people it does not feel like a severe sacrifice.
OTHER FORMS OF SURFACE TREATMENT.
Advanced surface ablation (PRK) can be performed by replacing the skin (epithelium) which is discarded. It can be saved before the Excimer laser treatment is carried out and replaced after the cornea is reshaped. This goes under the general title of LASEK. In other words in this procedure an attempt is made to save the thin layer of epithelial cells - skin surface that is removed with the alcohol and brushed away during routine advanced surface ablation (PRK). This is accomplished by simply peeling back the epithelium and saving it. It is then placed back over the raw surface in the position it was located previously. This is an attempt to decrease discomfort. A modified keratome can also be used creating a pass across the eye and bunching up the epithelium so it can be spread out again over the raw surface afterwards. These procedures have not become popular in most circumstances because the skin that is replaced is not viable (it does not live). It simply serves as a covering until the new skin grows under it over the surface of the cornea. The same drop regiman, the same amount of pain medication, the same use of ice packs, and the same use of a bandage contact lens needs to be applied as with PRK. My feeling is that since the new skin has to grow back over the treated area under the old skin it is best to remove old epithelium totally allowing a smooth base for the new skin to grow.
In my experience the level of discomfort is no different between LASEK and (PRK). Therefore advanced surface ablation whether it is performed saving the old skin (epithelium) or not results in same recovery time with approximately the same discomfort level.
- Advanced surface ablation (PRK) gives exactly the same results as LASIK but with less risk in people who have thin corneas and/or slightly abnormal corneas. The main disadvantage is a slightly longer recovery time.
- The patient is equally safe having either procedure done if the proper indications are followed.
- Advanced surface ablation (PRK) is much safer than before because of the use of Mitomycin-C to prevent fibrosis and haze following the procedure which was a problem in the early days.