RK Vs PRK
Do Both Modalities Have a Place in the Future
of Refractive Surgery?
James J. Salz, MD and Spencer P. Thornton, MD
RK has its place, but PRK is method of choice
I would like to compare photorefractive keratectomy (PRK) and radial keratotomy (RK) from the following standpoints: efficacy, stability, safety, side effects, economics and future developments.
EFFICACY: Jeffrey Hong, MD, analyzed my personal PRK and RK results over the past three years. Both the low and moderately myopic patients obtained comparable results from the two procedures, but my RK patients required additional surgery in approximately 20% of the eyes, compared to only one of the eyes in the PRK group. In comparing the efficacy of the two procedures, it is important to remember that the amount of correction that can be obtained with RK is age-dependent, while PRK is not. For example, I recently performed PRK with astigmatic correction (elliptical program) on the more myopic right eye of a 27-year-old patient with anisometropia. Her refractive error was: OD -9.50 -2.50 X165 = 20/40; OS -2.50 = 20/20.
Six months later, her uncorrected vision in the operated eye is 20/50 with a refraction of +0.75 -0.50 X165. RK would have been an inappropriate procedure for this patient. There are also times, however, when incisional surgery may be the more appropriate procedure.
STABILITY: A significant percentage of RK patients experience a continued flattening of the cornea, the so-called progressive hyperopic shift. Although long-term follow-up of PRK patients is not yet available, preliminary reports indicate that the procedure appears to stabilize between six and 12 months for the low myopes treated with the VISX laser. In general, the eyes are usually mildly hyperopic for the first few months and slowly regress toward emmetropia, stabilizing between 18 to 24 months.
SAFETY: The only vision-threatening complications I encountered were an early postoperative keratitis in an RK patient, which cleared without sequelae and a grade 2 corneal haze in a PRK eye, which gradually improved to grade 1 over a two-year period.
Published studies on RK and PRK support the safety of both procedures. The only vision-threatening complication encountered in the Prospective Evaluation of Radial Keratotomy (PERK) study was delayed bacterial keratitis in three eyes. The national Summit and VISX PRK Phase III Food and Drug Administration trials reported no vision-threatening complications.
The amount of correction that can be obtained with RK is age dependent, while PRK is not.
Another measure of the safety of a refractive surgical procedure is an analysis of the percentage of eves that lost two or more lines of best-corrected visual acuity. Although 31% of the eyes in the PERK study at five years lost two or more lines of BCVA, only one eye had a BCVA worse than 20/40 and that was secondary to a cataract presumably unrelated to the surgery.
In the two-year analysis of the VISX Phase III trial, only four cases (0.7%) out of 586 lost two or more lines of BCVA and none were worse than 20/30. The Summit results revealed a similar low incidence of loss of BCVA of only 2% of 585 eyes at one year.
The most common side effects reported in the PERK RK study were fluctuation in vision and glare. I believe PRK has an advantage because sensitivity to bright light is less common, diurnal fluctuation of vision is rare and difficulty with night vision is likely to be even less of a problem with the larger ablation diameters currently in use.
ECONOMIC FACTORS: From either the surgeon or patient perspective RK has, and will continue to have, a significant economic advantage over PRK.
FUTURE DEVELOPMENTS: Although there have been significant improvements in RK instrumentation, technique and surgical planning over the past 15 years, it is unlikely that there will be a major breakthrough that will dramatically improve the procedure. PRK, on the other hand, is still in the early phase of its development. Although the results of the U.S. FDA clinical trials are already quite respectable compared to advanced RK techniques, we should remember that these reported results were obtained with lasers and techniques that are already five years old. By simply enlarging the ablation zone from 5 mm to 6 mm - the current practice in the ongoing U.S. studies and abroad - significantly better results have already been reported and presented at recent meetings.
Although the excimer laser has been used primarily for the correction of myopia and myopic astigmatism, other applications are on the horizon. Hyperopic corrections have successfully been performed in Germany for the past two years. Incisional surgery for hyperopia (hexagonal keratotomy) has been associated with an unacceptable incidence of complications.
Both RK and PRK are currently in wide use throughout the world for the correction of myopia and myopic astigmatism with great success and a high percentage of satisfied patients. As PRK becomes readily available in the U.S., it will probably be the method of choice for the majority of myopic patients.
Incisional surgery will survive because it will be more appropriate for certain refractive errors, it may be used to "touch up" residual refractive errors following PRK and it may be selected by some patients and surgeons for economic reasons, as it is likely to be significantly less expensive than corneal laser surgery.
James J. Salz, MD, is a pioneer of refractive surgical techniques and
is in private practice in Los Angeles; with offices at Cedars Sinai Medical Tower, Ste.
39OW 8635 West Third St., Los Angeles, CA 90048. Toll Free (888) LVC-EYES
RK Offers Immediate Visual Improvement
With all the publicity that photorefractive keratectomy (PRK) and automated lamellar keratoplasty (ALK) have been getting, you might think that radial keratotomy was history. At least it seems that this is what the promoters want you to think. But before we look at the actual advantages and disadvantages of each approach it would only be fair to recognize that many of the proponents of PRK and ALK have a vested interest in those modalities. Some are paid consultants to the equipment manufacturers. With rare exceptions, this is not and never has been the case with RK. The proponents of PRK would have you believe that, with just a push of the button, you can get accurate, precise, predictable and permanent changes in the refraction of the cornea without the "problems" of RK. They claim that, for errors of over 3D, PRK is more dependable and more accurate.
I would like to compare PRK and RK in terms of patient comfort, range, irregular astigmatism, reoperation rates, precision and visual results.
PATIENT COMFORT: Compared with PRK patients, RK patients recover a lot faster. They see well in one day, and with very little discomfort. Most PRK patients do eventually end up with good acuity, but not without an ordeal of pain, overcorrection, haze and prolonged postop medication.
RANGE: No one knows exactly what the range of error is for PRK, but most proponents claim that there doesn't seem to be any limit. We still don't know the long-term results of smaller diameter ablation in higher errors and going deeper into the central cornea with resulting central corneal weakness. RK has been shown to be relatively precise and predictable up to about 5 D or 6 D in the hands of skilled surgeons, with greater variability and progressive hyperopic changes increasing with higher errors.
IRREGULAR ASTIGMATISM: The early hope that PRK would result in fewer cases of induced irregular astigmatism has not become reality. With PRK, the epithelium must be debrided before ablation. It can take a long time to do accurately and can result in corneal dehydration, which affects the accuracy of the ablation. In RK, dehydration is minimal and is directly related to the time involved in the procedure itself. Reports indicate that irregular astigmatism is just as frequent with PRK as with RK.
REOPERATION RATES: Although there are few dependable reports in the literature,
PRK reoperation rates within one year appear to be in the range of 10% to 20%. The
retreatments appear to have a fairly good success rate, but not as good as first
treatments. If all retreatments are included in the overall results, then about 98% of
patients see 20/40 or better at one year. With the Russian RK approach used by many U.S.
surgeons, the reoperation rate is in the range of 40% to 60%. With the American system,
the reoperation rate is in the 10% to 20% range, about the same as that claimed for PRK.
The overall results are also comparable, with about 98% of RK patients seeing 20/40 or
better uncorrected at one year.
PRECISION: The excimer laser is said to be an incredibly precise instrument, capable of removing tissue to within 0.25 m of the determined amount. Unfortunately, in PRK, that potential precision does not always translate into a precise refractive result. Both the stroma and the epithelium can heal in unexpected ways, and the keratocytes responsible for the formation of new collagen can cause a persistent haze in the anterior stroma, resulting in loss of best corrected vision. RK, on the other hand, does not alter the clarity of the central cornea.
VISUAL RESULTS: Loss of two or more lines of best-corrected vision at one year has been reported in less than 1% of PRK patients, but even this low number far exceeds that reported with RK. Even loss of one line of best-corrected vision is rare with RK and the reason may lie in the fact that RK does not affect the clarity of the visual pathway, whereas PRK frequently produces central islands of unablated or underablated cornea with resulting haze and visual distortion. These central islands seem to resolve in a year or two but produce some concern in the meantime. The more myopic the patient, the longer the time required for visual recovery. With PRK, it can take up to a year before full visual rehabilitation is achieved.
The flap-and-zap, or laser in situ keratomileusis (LASIK) procedure has been promoted as an alternative to PRK with the premise that it reduces the role of wound healing and results in a clearer, more comfortable eye sooner. We do not yet know if it will live up to its promises, but the reported complications are anything but minor. If the microkeratome blade is not perfectly clean, perfectly aligned, perfectly adjusted, perfectly set and perfectly operated, the potential for disaster is always there. Slight misalignments may produce tremendous amounts of irregular astigmatism. Slight operation problems may cause permanent scars in the visual pathway. Long-term follow-up studies are not available to show the extent or seriousness of complications, but surgeons have reported everything from lost corneal caps (even with flaps) to cataract formation to iris prolapse due to perforations by the keratome. It is a demanding procedure and requires a high degree of skill.
At the Thornton Eye Center in Nashville, two of my colleagues, Jim Hays and Dale Pilkinton, and I offer all three procedures, and in looking to the future we feel certain that RK will continue to have a significant place in our surgical approach to myopia and astigmatism. RK demands a great deal of skill on the part of the surgeon, but it offers immediate visual improvement, a high degree of predictability, patient comfort and few complications.
Spencer P Thornton, MD, is a pioneer of refractive surgical techniques and is in private practice in Nashville, Tenn.; with offices at 2010 Church St., Nashville, TN 37203.
Ocular Surgery News Vol. 14, No. 2 January 15, 1996
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