|Year : 1990 | Volume
| Issue : 3 | Page : 107-113
Three year results of the Prospective Evaluation of Radial Keratotomy (PERK) study
George O Waring III, Michael J Lynn, William Culbertson, Peter R Laibson, Richard D Lindstrom, Marguerite B McDonald, William D Myers, Stephen A Obstbaum, J James Rowsey, David J Schanzlin
PERK Coordinating Centers. Emory University, Atlanta, GA, USA
George O Waring III
PERK Coordinating Centers. Emory University, Atlanta, GA
Source of Support: None, Conflict of Interest: None
The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center clinical trial of a standardized technique of radial keratotomy in 435 patients who had simple myopia with a preoperative refractive error between -2.00 and -8.00 diopters (D). We report results for one eye of each patient. The surgical technique consisted of eight incisions using a diamond micrometer knife with the blade length determined by intraoperative ultrasonic pachymetry and the diameter of the central clear zone determined by the preoperative refractive error. At three years after surgery, 58% of eyes had refractive error within one diopter of emmetropia; 26% were undercorrected, and 16% were overcorrected by more than one diopter. Uncorrected visual acuity was 20/40 or better in 76% of eyes. The operation was more effective in eyes with a preoperative refractive error between -2.00 and -4.37 diopters. Between one and three years after surgery, the refractive error changed by 1.00 diopter or more in 12% of eyes, indicating a lack of stability in some eyes.
|How to cite this article:|
Waring III GO, Lynn MJ, Culbertson W, Laibson PR, Lindstrom RD, McDonald MB, Myers WD, Obstbaum SA, Rowsey J J, Schanzlin DJ. Three year results of the Prospective Evaluation of Radial Keratotomy (PERK) study. Indian J Ophthalmol 1990;38:107-13
|How to cite this URL:|
Waring III GO, Lynn MJ, Culbertson W, Laibson PR, Lindstrom RD, McDonald MB, Myers WD, Obstbaum SA, Rowsey J J, Schanzlin DJ. Three year results of the Prospective Evaluation of Radial Keratotomy (PERK) study. Indian J Ophthalmol [serial online] 1990 [cited 2022 Jan 28];38:107-13. Available from: https://www.ijo.in/text.asp?1990/38/3/107/24525
In the 435 eyes, there were a small number of complications including six eyes that lost two or three lines of best corrected acuity, 15 eyes that experienced vascularization of the incisions, two eyes that developed delayed bacterial keratitis, and four eyes that had recurrent epithelial erosions.
The Prospective Evaluation of Radial Keratotomy (PERK) Study is a multicenter trial designed to determine the outcome of a single standardized technique of radial keratotomy. We report here the results of the PERK study at three years after surgery.
| Material and methods|| |
Details of the background, design surgical technique, and clinical examination methods and early results of the PERK study have been previously published. 
The PERK study is evaluating a single standardized technique of radial keratotomy by comparing measurements taken before and after surgery in one eye of each patient. The diameter of the uncut central clear zone, 4.0,3.5, or 3.0 mm, was determined according to the baseline spherical equivalent of the cycloplegic refractive error. Other aspects of the surgical protocol were the same for all patients. Patients selected for the study met strict entry criteria.
| Surgical technique|| |
A single standardized technique was specified for all eyes. A round marking trephine was centered on the visual axis mark. , The diamond blade of the micrometer knife was extended to a length equal to 100% of the thinnest of four intraoperative ultrasonic corneal thickness readings (speed of sound set at 1640 m/sec) taken at the 3, 6, 9, and 12 o'clock meridians just outside the trephine mark and verified with a calibrated, coin-shaped gauge block. The surgeon made eight freehand radial incisions that extended from the edge of the trephine mark to the limbal vascular arcade and that were spaced equidistantly around the cornea. After surgery, topical antibiotics, but no corticosteroids, were administered.
At each of the Clinical Centers an opththalmologist-examiner, who was not the surgeon, and a techniciancoordinator gathered all baseline and postoperative data as described previously.
Multiple regression analysis was done using the best subset selection method for determining important factors affecting the change in refractive error between baseline and three years after surgery. sub The predictability was characterized by the width of the 90% confidence interval of the change in refraction predicted from the regression equation for an individual patient.
| Results|| |
The study design specified a sample size of approximately 400 eyes in order to estimate the percentage of patients with visual acuity of 20/40 or better with a precision of about 5%. Surgery was done on the first eye of 435 patients between March 1982 and October 1983. Each of the nine clinical centers entered 43 to 50 patients into the study.
Of the 435 patients, 10 patients (2.3%) were lost to follow-up and eight additional patients (1.8%) missed a three year examination, leaving 417 patients (96%) examined approximately three years after surgery on the first operated eye. The three year follow-up interval ranged from three months before to six months after the three year anniversary after surgery; 90% of patients were examined within two months of the three-year anniversary and eight patients (2%) were examined four to six months after the three year anniversary.
We accounted for the 18 patients without a three year visit; their last follow-up visit after surgery was two weeks (2 patients), one year (8 patients), and two years (8 patients). Two of these 18 patients are no longer being sought, one who died and another who had further radial keratotomy surgery in both eyes by a physician not associated with the PERK study.
The refractive, keratometric, and visual acuity results are presented in this paper for the first operated eyes of 401 of the 417 patients examined at three years after surgery. We excluded 16 eyes (3.7%) because of surgery inconsistent with the protocol or because of events that altered the outcome of the surgery. These 16 eyes included: (1) eight eyes that received a clear zone inconsistent with the surgical protocol, seven receiving a clear zone 0.50 mm smaller than specified and one receiving a 0.50 mm larger clear zone; (2) two eyes that received a repeated radial keratotomy before six months after the initial surgery; (3) three eyes that were fitted with hard contact lenses; (4) one eye that underwent a cataract extraction before the three year follow-up visit; (5) one eye that was treated for glaucoma with timolol and had large changes in intraocular pressure and refraction;  and (6) one eye of a patient who received hyperbaric oxygen treatment to speed healing of a foot ulcer and who experienced a marked change in refraction.
In the presentation of complications, we have included all eyes that had surgery in the PERK study.
For the 57 eyes that had a repeated operation, we used refractive, keratometric, and visual acuity results from the follow-up visit preceding the reoperation six months (39 eyes), one year (15 eyes), and two years (3eyes) as described previously.  We reported the results in these eyes previously.  There were 16 eyes excluded from the presentation of the refractive and uncorrected visual acuity outcomes. To ensure that no relevant information remains unreported, we have presented this data elsewhere.
| Cycloplegic refraction|| |
Spherical Equivalent Refraction
The range of myopia before surgery was -2.00 and -8.00 D. The mean change in refraction between baseline and three years after surgery was a*decrease in myopia of 3.79 D (S.D = 1.58). The refractive change ranged from a 0.25 D increase in myopia to a 10.13 D decrease in myopia. In six eyes (1.5%) the change in refraction was less than 1.00 D.
[Figure - 1] shows the refractive error at three years compared to the baseline refractive error. At three years, 58% of eyes were within 1.00 D of emmetropia, -0.62 to -1.00, 13%; -0.12 to -0.50, 17%; piano, 4%; +0.12 to +0.50,13%, +0.62 to +1.00, 11 %. Twenty-six percent of eyes were undercorrected by more than 1.00 D and 16% of eyes were overcorrected by more than 1.00 D.
The refractive results in the three baseline refraction groups are given in [Figure - 2]. The mean change in refraction between baseline and three yers was:.lower group, 2.84 D; middle group, 3.66 D; higher group, 4.69 D. The range for the change in refraction was more than 5.50 D in each group.
The percent of eyes within 1.00 D of emmetropia at three years in the three baseline refraction groups was: lower, 77%; middle, 61 %; higher, 39%. The percent of overcorrections of more than 1.00 D was similar in the three groups; lower, 16% middle, 19%, higher, 13%. However, the percent of undercorrections of more than 1.00 D varied considerably in the three groups: lower, 7%; middle, 19%; higher, 48% [Figure - 2].
| Change in astigmatism|| |
The change in astigmatism ranged from a decrease of 1.25 D to an increase of 2.75 D. In 41 eyes (10%), astigmatism increased by 1.00 D or more and in four eyes (1 %) the increase was 2.00 D or more. For seven eyes (2%) the astigmatism decreased by 1.00 D or more. For the 305 eyes with astigmatism at baseline and three years, the axis of the cylinder changed 0 to 20 degrees in 56%, 21 to 40 degrees in 14%, 41 to 60 degrees in 7%, and 61 to 90 degrees in 23%.
| Uncorrected visual acuity|| |
At baseline, 69% of all eyes had an uncorrected visual acuity of 20/200 or worse, with 48% unable to read the top line of the chart (20/200); 30% of all eyes saw 20/50 to 20/160 and one eye saw 20/40; no eye saw better than 20/40. At three years, 51.% of all eyes saw 20/20 or better and 76% of all eyes saw 20/40 or better; 2% of all eyes saw 20/200 or worse.
[Figure - 3] displays the results in the three baseline refraction groups. At three years, 20/40 or better uncorrected visual acuity was achieved by 90% of eyes in the lower group, 81 % of eyes in the middle group, and 60% of eyes in the higher group. An uncorrected visual acuity of 20/200 or worse occurred in none of the lower group, 1 % of the middle group, and 6% of the higher group.
| Central keratometric measure|| |
The average reduction in corneal power in the three baseline refraction groups was: lower, 2.81 D; middle, 3.16 D; higher, 3.70 D.
| Stability of refractive error|| |
We evaluated the stability of the refractive error by computing the change between one and three years after surgery for 339 eyes that had not received a ireoperation. Fifty-six -percent of the eyes changed by less than 0.50 D. Changes of more than 0.50 D were more frequently a decrease in minus power (34%) than an increase in minus power (10%). A change of 1.00 D or more occurred in 12% of eyes; 9% of eyes had a decrease in minus power and 3% an increase in minus power.
| Predictability of outcome|| |
A detailed evaluation of the predictability of radial keratotomy at one year after surgery in the PERK study has been published.  At three years, regression analysis of the three groups combined showed that, similar to the one year results, the diameter of the clear zone and patient age were the most important factors affecting the outcome. Together these two factors explained 33% of the variability in refractive change. The average depth of the incision scar explained an additional 5% of the variability. No other factors significantly improved the fit of the model. As indicated by the regression coefficients, a 0.5 mm reduction in the diameter of the clear zone was related to a 0.85 D increase in the average change in refraction between baseline and three years. Also, an increase in 10 years of age was related to a 0.70 D increase in the average change in refraction.
We quantified the predictability using 90% confidence interval for the change in refraction predicted by the regression equation for an individual eye. The confidence interval gives a range for the change in refraction that would include 90% of eyes with similar values for clear zone, age, and depth of incision. The width of the interval indicates the precision with which the outcome can be predicted. The widths of the confidence intervals in the three baseline refraction groups were lower: 2.80 D; middle, 3.80 D; higher, 5.18 D. For all three groups combined the width was 4.12 D. The confidence intervals at three years are 0.40 to 1.00 D wider than at one year.
| Complications|| |
In reporting complications, we have included all 435 eyes that had surgery in the PERK study.
| Decrease in best corrected visual acuity|| |
Since a loss of best corrected vision is a serious complication, we have included all 435 eyes, regardless of their postoperative course, using data from follow-up visits before the three year visit for the 18 patients who missed that visit. We have interpreted the gain or loss of one Snellen line of vision as a possible normal variation and therefore considered a change of 2 lines or more as noteworthy.
At baseline, all eyes had a visual acuity of 20/20 or better with the best manifest refraction. At three years, nine eyes saw worse than 20/20 (eight saw 20/25 and one saw 20/40). In six of these eight eyes the vision decreased by one line.
Five eyes (1 %) lost 2 lines and one eye lost 3 lines of best corrected visual acuity [Figure - 4]. The eye losing three lines to 20/40 developed glaucoma after surgery; other details of this case are described in a later section. Interestingly, 16 eyes (4%) gained 2 lines in best corrected acuity after the surgery. In general there appeared to be little change in best corrected acuity three years after radial keratotomy.
| Overcorrection and uncorrection|| |
[Figure - 1] demonstrates that the percentage of overcorrections was similar (approximately 16%) in all three refraction groups, but the percentage of undercorrections was six times as great in the 3.0 mm group (48%) as in the 4.0 mm group (8%). Details are presented in the section on refractive outcome.
| Irregular astigamatism|| |
On the keratographs of the majority of eyes, the central two or three circles were regular because they overlaid the central clear zone and the peripheral six or seven circles exhibited a slight irregularity, because they overlaid the corneal scars. We think that this irregularity is caused by slight elevation of each corneal scar with a relative slight depression both between and in the flattened center of the cornea. This topography is the anatomic basis for the creation of the stellate iron line. 
One patient complained of bothersome monocular diplopia that persisted five years after surgery because of a diffusely gray hypertropic scar in the 7:30 axis. The keratograph demonstrated focal pleating and irregularity over the scar. The patient insisted on treatment for his diplopia, so five and one-half years after his initial surgery, he had revision of the scar under topical anesthesia. Surface irregularity and monocular diplopia were less severe, but persistent.
| Effect of intraocular pressure on refraction|| |
Busing and colleagues  reported a PERK patient whose intraocular pressure increased from 12 to 26 mm Hg two years after surgery, with flattening of the central cornea and a change in refraction from +2.00 D to +5.75 D. Topical timolol decreased the intraocular pressure to 10 mm Hg, with steepening of the cornea and lessening of the overcorrection to +1.75 D. After subsequent cessation of timolol, the intraocular pressure rose to 20 mm Hg and the refractive error again increased to +5.00 D.
| Ptosis|| |
Linberg and colleagues  documented the appearance of ptosis in approximately 105 of eyes after radial keratotomy at one PERK center. The cause of the ptosis was not clearly determined, but compression of the levator palpebrae aponeurosis by a heavy eyelid speculum was suspected. Carroll and colleagues  reported a similar case at another PERK center.
| Discussion|| |
The Prospective Evaluation of Radial Keratotomy (PERK) study conforms to the principles of modern multicenter clinical trials.  Follow-up was excellent, 96% of the 435 patients having a three-year examination at a PERK center, 90% of these within two months of the three-year anniversary date of their surgery.
To study the effect of the radial keratotomy operation itself, we excluded those eyes in which events occurred that would alter or obscure the effect of the surgery, reporting the uncorrected visual acuity and refractive results in eyes that conformed strictly to the surgical and follow-up protocol. To ensure that all untoward events in the entire PERK population are reported, we include reports of complications in all 435 patients.
| Change in refraction and visual acuity measurements|| |
Our data confirm those of all other studies published since Fyodorov's initial paper in English in 1979: [18 ] radial keratotomy is most effective for lower amounts of myopia, generally 5 D or less [Figure - 1][Figure - 2][Figure - 3]. Thus, patients in the lower myopia group, with a baseline refraction of -2.00 D to -3.12 D had a refraction at three years of ±0.50 D in 46°%o and ± 1.00 D in 76% of eyes, with uncorrected visual acuity of 20/20 or better in 74% and 20/40 or better in 91 % of eyes, while those in the higher myopia group with a baseline refraction of -4.50 D to -8.00 D had a residual refractive error of ± 0.50 D in 20% and ±1.00 D in 39% of eyes and uncorrected visual acuity of +20/20 or better in 33% and 20/40 or better in 60% of eyes.
| CORRELATION AMONG CHANGES IN REFRACTION, VISUAL ACUITY, AND KERATOMETRIC READINGS|| |
Numerous studies of radial keratotomy have documented the lack of a direct correlation between the changes in central keratometric measurement, cycloplegic refraction, and visual acuity.  Generally, there is a greater change in refraction than in corneal, curvature, a phenomenon probably explained by the induction of greater asphericity in the cornea and by the fact that the keratometer measures the radius of curvature of the cornea only between two 50 micron spots approximately 3 mm apart, not the curvature of the entire cornea. Similarly, patients with a residual myopic refractive error generally have a better visual acuity than individuals with a similar refractive error who have not had radial keratotomy, a phenomenon partially explained by the increased spherical aberration of the paracentral cornea, which produces a larger blur circle on the retina. sub On the other hand, many patients who see 20/20 or better without correction are overcorrected by more than one diopter (17% in PERK), emphasizing
that a good visual outcome does not always mean a good refractive result.
| Stability of refraction|| |
The unsutured radial keratotomy wounds in the avascular cornea heal slowly, requiring at least four to five years to completely eject the epithelial plug  and to -remodel the stroma adjacent to the incision scar  (personal observations). Thus, at three years, we expected these wounds to produce some instability of refraction.
We have reported elsewhere persistent diurnal fluctuation of vision three years after surgery.  Fifty-two patients were examined before 10:00 a.m and after 5:30 p.m. on the same day. Between the morning and evening examinations, 31 % of the eyes had an increase in minus power of 0.50 D to 1.50 D, and 19% had a decrease in uncorrected visual acuity of 2 to 5 Snellen lines.
Wyzinski  demonstrated that the largest daily changes in refraction occur soon after awakening.
Between one and three years after the surgery, 12% of the eyes changed by 1.00 D to 2.50 D, 9% showing a continued decrease in minus power, that is, a continued effect of the surgery sometimes called "progressive hyperopia".  This is a much lower percentage than reported by Deitzand Sanders, sub 1.00 D or more in 31 of eyes between one and three to four years after surgery.
Other authors have presented stability results only in terms of the average change in the minus power of the refraction: Sawelson and Marks,  0.17 between 18 months and 3 years, and Arrowsmith and Marks,  0.23 D between one and two years. The average change in the PERK study was a decrease in minus power of 0.23 D. Deitz and Sanders sub reported an average decrease in minus power that was greater than in these other studies, 0.53 D between one and four years.
All evidence indicates that there is not a loss of the effect of radial keratotomy during the first years after surgery; whether there is a clinically meaningful, systematic progression of effect in individual eyes needs further study.
| Predictability of outcome|| |
We have previously quantified the predictability of radial keratotomy by calculating the 90% confidence interval of the change in refraction one year after surgery, an interval that spanned approximately 3.50 D. sub The confidence interval was wider at three years, 4.0 D, because changes in refractive errors between one and three years after surgery caused an increase in the variability of the refractive outcome at three years compared to one year.
To improve the predictability of radial keratotomy for an individual patient, we must control the inherent surgical and biological variability of the surgery. Potential methods include using preoperative factors in the choice of surgical technique, , refinement of existing surgical techniques, , and pharmacologic manipulation of corneal wound healing with drugs such as corticosteroids, human epidermal growth factor, fibronectin and beta aminopropionitrile.
| Complications|| |
Decrease in Best Corrected Visual Acuity
The most important indicator of the safety of radial keratotomy is the change in best corrected visual acuity before and after surgery. A loss of best corrected visual acuity, for whatever reason, is a serious complication of. this elective surgery.
Six eyes (1.4%) lost 2 to 3 line of best corrected acuity at three years [Figure - 4], only one seeing worse than 20/25 (acuity was 20/40), indicating that vision - threatening complications occurred rarely.
| Refractive complications|| |
The most common complications in the PERK study were overcorrection (16°'0) and undercorrection (26%) of more than 1.00 D [Figure - 1][Figure - 2]. Unexpectedly overcorrections occurred with similar frequency in the three clear zone groups. Overcorrection is an undesirable phenomenon that will hasten the onset of symptomatic presbyopia. The incidence of overcorrections m' ht be reduced by using four instead of eight incisions or a larger optical zone. Most of the undercorrections occurred in the eyes with 4.50 D to 8.00 D of myopia, a problem that may be reduced through improved surgical planning and technique. Because we cannot identify before surgery the patients who will underrespond or overrespond, we do not know which individuals will require modification of the surgical technique.
The increase of 0.50 D or more in astigmatism in 34% of eyes presumably resulted from asymmetrical incisions and wound healing. We have also demonstrated that repeated operations further increase the amount of astigmatism. 
| Irregular astigmatism|| |
The corneal surface has not returned to normal smoothness at three years, as indicated by persistence of slightly irregular keratograph mires over the minimally elevated scars and the persistence of the stellate iron line  in the minute troughs between the scars and in the flattened center of the cornea. We did not identify symptoms associated with this mild irregular astigmatism, but moderate amounts of irregular astigmatism can cause symptomatic diplopia as demonstrated by the patient who required revision of one radial scar.
| Technique of surgery|| |
We designed the surgical technique for the PERK study in 1980 to be as safe and effective as possible and to be reproducible by a reasonably trained ophthalmologist.
A synthesis of information from the PERK study and other reports allows some speculation for modifying the PERK technique to improve outcomes. Further clinical trials will be required to test the effectiveness of these suggestions.
Four incisions instead of eight can be used in patients with lower amounts of myopia to reduce the amount of corneal scarring and the incidence of overcorrections, , although small clear zones and repeated operations are needed more often.
Older patients have a greater decrease in myopia from the surgery, ,, and the approximate 0.70 D increase
change for each 10 year increase in age should be considered in surgical planning.
Deeper incisions have a greater effect.A singlepass, centrifugal incision with the PERK knife did not consistently achieve a depth of 90%. -2 A number of techniques are available to increase the achieved depth of the incision and these need comparative testing.
In determining the center of the clear zone, Uozato and Guyton sub showed that the center of the clear zone should be in the center of the patient's entrance pupil, to decrease postoperative glare.
-The thickness of the cornea varies in different regions, the superior axis being the thickness followed by the nasal, inferior, and temporal axis.  If the sequence of incisions is made to cut the thicker portions of the cornea first, there will be less chance of perforation early in the operation.
The surgical technique in the PERK study was standardized, but in clinical practice there is great variation in individual surgeons' instruments, surgical plans, and manual techniques, so that each surgeon must study his own results and modify his technique accordingly.
| References|| |
Waring GO, Moffitt SD, Gelender H, et al: Rationale for and design of the National Eye Institute prospective evaluation of radial keratotomy (PERK) study. Ophthal 90:40-58. 1983.
Waring GO, Lynn MJ, Gelender H, et al: Results of the prospective evaluation of radial keratotomy (PERK) study one year after surgery. Ophthalmol 92:177-198, 1985.
Lynn MJ, Waring GO, Sperduto RD, et al: Factors affecting outcome and predictability of radial keratotomy in the PERK study. Arch Ophthalmol 105:42-51, 1987.
Schanzlin DJ, Santos VR, Waring GO, et al: Diurnal change in refraction. corneal curvature, visual acuity, and intraocular pressure after radial keratotomy in the PERK study. Ophthalmol 93:167-175, 1986.
Mandelbaum S, Waring Go, Forster RK, et al: Late development of ulcerative keratitis in radial keratotomy scars. Arch Ophthalmol 104:11561160, 1986.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]