PRK Changes anterior corneal surface by affecting the center of the cornea.
RK Changes anterior corneal surface by affecting periphery of the cornea.
PRK Photoablation of the central corneal surface. Involves a very small amount of central cornea with no incisions. PRK reshapes or sculpts the cornea by a shallow, rapid photoablation of microscopic amounts of tissue (often < 15% of total corneal depth).
RK Multiple deep peripheral corneal incisions. Involves peripheral cornea with multiple wound incisions (approx. 90% of total corneal depth).
PRK At this time, for the excimer correction of myopia, specific variables such as intraocular pressure, thickness of the cornea, keratometry readings (flat or steep), scleral rigidity, etc. do not indicate that they would directly affect the surgical outcome. There is some speculation that age may affect the corrective changes achieved following excimer laser PRK.
RK Many variables affect the outcome of RK surgery, such as age, intraocular pressure, thickness of the cornea, flat keratometry readings, scleral rigidity, etc. All these variables must be considered in addition to the degree of myopia present when performing RK surgery.
PRK Central corneal epithelium removed mechanically or with laser prior to photoablation; normally replaces itself in 3 days.
RK Peripheral epithelium incised or cut with surgical blade.
PRK Immediate postop comfort level: 90% have no significant postop pain. One can have severe, moderate, or little pain following PRK. With appropriate medications, the immediate postop pain can be markedly reduced. With the new postop medication regimen, postop pain is generally less severe than that following RK.
RK Immediate postop comfort level: One can have severe, moderate, or little pain following RK. Generally more painful than Excimer Laser PRK. 20-30% of RK patients have significant postop pain.
PRK A therapeutic soft bandage lens will be placed over the immediately postop eye until cornea re-epithelializes, normally in 3 days. It is recommended that the operated eye be shielded at night for 1 week postop with a plastic shield for protection during sleep.
RK Therapeutic bandage lens is not generally applied postop. The eye is not usually patched postop during waking hours, but it is recommended that the operated eye be shielded at night for 1 week postop with a plastic shield for protection during sleep.
PRK Potential for much more standardized results with computer-controlled process. For most corrections of -1.00 to -7.00 diopters, PRK can achieve maximum desired correction with only 1 application.
RK Surgeon-dependent procedure (less standardized results). For approximately 10% to 30% of those patients who undergo RK to correct myopia of -1.00 to -7.00 diopters, in order to achieve maximum desired correction it is necessary for the patient to undergo enhancement by a second and often a third RK procedure on that same eye.
PRK No significant structural weakening of cornea.
RK Structural corneal weakening common.
PRK Can be some regression of effect during the first year, usually stable at 3 to 6 months.
RK Can be regression of effect for months after RK surgery with unstable vision, especially with high number of incisions and redeepened incisions.
PRK Vision fluctuation during the day: So far, vision fluctuation during the day has not been a problem following excimer laser PRK.
RK Vision fluctuation during the day: Fluctuation of vision following RK can be a problem and is seen following RK in a limited number of patients. Because of the instability of the cornea, especially following maximum number of incisions and redeepening, the cornea weakens to such a degree that vision fluctuation may become a permanent situation in some cases.
PRK Microperforation eliminated
RK Microperforation possible since cuts are made to 90% of corneal depth.
PRK Less chance of damage to corneal endothelial cells.
RK Greater chance of endothelial cell damage.
PRK Less chance of glare problems at night from car lights. Glare problems with excimer or RK are not usually incapacitating at night and tend to be diminished to an insignificant level after 6 months to 1 year. Myopic people experience glare naturally prior to surgery and even more so with contact lenses. Glare following excimer is rarely worse than with contact lenses and usually less than that experienced with contact lenses. With the larger ablation zone, night glare is generally less than with RK.
RK Greater chance of glare problems at night from car lights. There is a greater chance of glare problems with starburst effect at night from car lights. However, this is usually not incapacitating, but potentially could be. This also depends on the optical zone size and the number of incisions. A patient with a large optical zone size and a few incisions may not have much glare or starburst, whereas a patient with a very small optical zone size and a large number of incisions with redeepening may experience this problem.
PRK Normally no scarring and no significant damage to adjacent tissue. By far and away, the majority of PRKs done show no scarring or opacification of the cornea after 1 year. Generally, there is expected to be a small degree of corneal haze present for a few months following the surgery, but this disappears. It is very unusual for this normal amount of corneal haze to cause any interference in visual acuity. Through the years, the minimal haze formation on the cornea has not proved to be clinically significant. There may be a transient temporary clinically significant haze and regression of the myopic effect that may develop in 2-3% of patients following PRK that is normally controlled and reversed by short-term potent corticosteroid drop treatment. It usually clears rapidly with reversal of the regression effect. If this occurs, it more commonly occurs in the higher degrees of myopia (i.e. -5.00, -6.00, or greater). In those rare cases where the central corneal haze becomes clinically significant and is not reversed by medical means (short-term corticosteroid drops), haze normally disappears or becomes clinically insignificant after 1-2 years. Out of those few patients who have had clinically significant corneal haze after 1 year (less than 1% in the -1.00 to -7.00 group), some have chosen to wait 6 months to 1 year longer and the haze has become insignificant. However, there have been some people who did not want to wait and wanted the haze to be removed by the excimer laser. This has been done in a few cases worldwide with clearing of the regression and haze in over 90% of cases following repeat excimer laser surgery that is directed toward removing this corneal haze. For the most part (over 99%), the corneal haze has been clinically insignificant thus far, especially in the -1.00 to -7.00 myopic group. However, the incidence of clinically significant haze increases in correction of higher degrees of myopia (-7.00 and beyond).
RK Scarring of corneal incisions present. Normally there is a mild or moderate amount of scarring that occurs with the RK incisions. Remember that these incisions are peripheral; but can contribute to some glare phenomenon, especially at night, which is normally inconsequential.
PRK Best corrected vision: Most of the time, best corrected vision is attained by 3 months, but it can take as little as 1 week or as long as 6 months or longer to attain best corrected vision.
RK Best corrected vision: It may take as little as 1 day or as long as 3 to 6 months to attain best corrected vision with radial keratotomy.
PRK Less chance for contact lens intolerance following PRK. It should be easier to fit contact lenses following excimer surgery vs RK surgery. However, one may or may not be able to wear contact lenses successfully after Excimer laser surgery. (The majority of patients would be able to wear contacts successfully if they wore contacts successfully prior to PRK.) One should wait 3 to 6 months before attempting contact lens wear.
RK Greater chance for contact lens intolerance following RK. If contact lenses are necessary to improve vision after RK, one should wait 3 to 6 months before fitting contacts. One may or may not be able to wear contact lenses successfully after RK surgery. (The majority of patients would be able to wear contacts successfully if they wore contacts successfully prior to RK.) However, because the deep RK incisions (90% deep) do not completely heal for 3 to 7 years or never (even though the surface cells heal over after a few days), it is felt that contact lens wear after RK versus after Excimer Laser PRK is more risky and it would be easier to develop a wound infection due to the chronic irritation of the contact lens over the radial incisions.
PRK Long term microbial keratitis and corneal stability: should not be a problem with PRK. One of the reasons why this is not a problem with PRK is that the corneal surface area of ablation generally is healed in 6 months to 1 year with complete stability in most cases. Since the ablation is only superficial, this area is generally considered healed 6 months to 1 year following excimer laser PRK.
RK Long term microbial keratitis (corneal infection). Occasional long-term incisional abscesses, particularly in patients who are eye-rubbers and/or have to go back to wearing their contact lenses. This can be a sight-threatening problem, especially if an incisional abscess occurs months to years following the initial RK surgery in association with contact lens wear.
PRK Generally slower visual recovery (sometimes only by a few days).
RK Generally quicker visual recovery.
PRK Phenomenon of progressive hyperopia thus far has not been seen with PRK.
RK Progressive hyperopia: A unique variable thus far seen with RK is referred to as progressive hyperopia, or consecutive hyperopia, or hyperopic "creep," which can be seen in a minority of cases. The eye continues to correct even years after the initial surgery has been done, and therefore the eye becomes progressively more and more farsighted. In the early RK years, there has been an incidence of 20-30% of patients who have exhibited this phenomenon. However, with modern-day surgery the incidence is less, about 5-20%. If this phenomenon is going to be seen, it is normally seen in high myopes who have had a maximum number of RK incisions with redeepening of the incisions. This apparently destabilizes the cornea to such a degree that allows progressive hyperopia. But, some cases of 4 and 8 cut RK have been reported. Therefore, in a certain percentage of RK patients, the eye never stabilizes after surgery.
PRK Shorter track record.
USA: 1988
RK Longer track record.
USA: 1978
Russia: 1974 - first human cases
(1972 research began)
PRK Number of cases done:
USA: thousands
Worldwide: hundreds of thousands
RK Number of cases done:
USA: over 1,000,000
Worldwide: several million
PRK Excimer Laser PRK may be used to correct undercorrected RK.
RK RK may be used to correct undercorrected Excimer Laser PRK.
PRK Excimer Laser PRK plus RK may correct higher degrees of myopia than either procedure alone.
RK RK plus Excimer Laser PRK may correct higher degrees of myopia than either procedure alone.
PRK Excimer Laser PRK more efficiently corrects higher degrees of myopia (-5.00 and greater) than does RK for patients who are in the 20's to early 30's.
RK RK is not as efficient in correcting higher degrees of myopia (-5.00 and greater) as is Excimer Laser PRK for patients in the 20's to early 30's.
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