Excimer Laser Photorefractive Keratectomy for the Correction of Hyperopia Using an Erodible Mask and Axicon System

Excimer Laser Photorefractive Keratectomy for the Correction of Hyperopia Using an Erodible Mask and Axicon System David P. S. O'Brart, MD, FRCS, FRCOphth/ Chris G. Stephenson, FRCS, FRCOphth, 1 Katherine OUver, MCOptom, BSc/ John Marshall, PhD 1 Purpose: The purpose of the study is to evaluate photorefractive keratectomy for the correction of hyperopia using the erodible mask and Axicon system. Methods: Forty-three patients (43 eyes) with a mean refraction (spherical equivalent) of +4.54 diopter (D) (range, + 1.75 to + 7.50 D) were treated using a Summit Technology "Apex Plus" excimer laser. This system uses an erodible mask to create a 6.50-mm diameter hyperopic correction over the axial cornea. An Axicon then is used to fashion a 1.50-mm "blend zone" around the correction. On the basis of preoperative refractions, patients were assigned to 3 groups: 2 groups of 14 patients underwent either "+2.00 D" or "+3.00 D" corrections and 15 patients had "+4.00 D" corrections. Results: All patients had a reduction in their hyperopia with an overcorrection, especially in the first month after surgery and some stability in the refractive change at 3 to 6 months. The mean manifest refraction (n = 43) at 6 months was -0.17 D (range, +4.50 D to -3.125 D). Patient satisfaction was high. At 6 months, all eyes had an improvement in unaided near visual acuity. Unaided distance acuity was improved in 37 eyes (86%). A ring of haze 6.5 mm in diameter appeared in all eyes 1 month after surgery. Night halo measurements at 6 months showed no differences from preoperative levels. Flicker contrast sensitivity and forward light scatter (glare) measurements showed no differences after surgery. Conclusions: In this short-term study, photorefractive keratectomy for hyperopia using the erodible mask and Axicon system appeared to be a promising procedure. Visual performance, in terms of flicker contrast sensitivity, forward light scatter, and night halos, was not compromised. There was an overcorrection based on the manufacturer's algorithms. Manipulation of the treatment algorithms should improve future predictability. Ophthalmology 1997; 104:1959-1970

Originally received: February 25, 1997. Revision accepted: August 6, 1997. 1 Department of Ophthalmology, United Medical and Dental Schools, St. Thomas' Hospital, London, England.

2

Department of Optometry and Vision Sciences, Glasgow Caledonian University, Glasgow, United Kingdom.

Supported by the Iris Fund for Prevention of Blindness in relation to both the purchase and maintenance of the laser and provision of a Research Fellowship (COS). Dr. Stephenson holds a research fellowship sponsored by the Iris Fund for Prevention of Blindness. Professor J. Marshall is a consultant for Summit Technology. Reprint requests to David P. S. O'Brart, MD, FRCS, FRCOphth, Department of Ophthalmology, St. Thomas' Hospital, Lambeth Palace Road, London SEl 7EH, United Kingdom.

The surgical correction of hyperopia represents a great challenge. In recent decades, numerous techniques, such as thermokeratoplasty, hexagonal keratotomy, keratophakia, and keratomileusis, have been developed. However, such procedures have met with only limited success, with reports of poor predictability, stability, and sight-threatening complications. I-JO More recently, excimer laser photorefractive keratectomy (PRK) has been the subject of intensive research. The use of short-wavelength, highenergy ultraviolet radiation to reshape the corneal surface has heralded a new era in the treatment of refractive errors.11-18 Clinical trials have shown PRK to be safe and effective for the correction of low-to-moderate degrees of myopia and astigmatism. 11 - 18 There have been few published studies investigating its efficacy for hyperopia.

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In contrast to PRK for myopia, in which the central cornea is flattened, hyperopic PRK requires reshaping the peripheral cornea to steepen the central curvature. This generally requires larger ablation diameters than for myopic corrections. There are a number of attendant problems in the manufacture of excimer lasers with large beam diameters, because it is difficult to maintain a homogeneous distribution of energy across a large beam crosssection. This can be improved by optical processing, but as every optical surface reduces the emitted energy, manufacturers need to keep the optical train to a minimum. The use of more powerful lasers could reduce this problem, but the attendant cost and size would be great. Because of such limitations, prototype ophthalmic excimer lasers typically were restricted in the size of their ablation diameters to 6.00 mm or less. As such, they were unable to steepen the corneal surface over a wide enough area to create a stable and effective hyperopic correction. An alternative to increasing the beam diameter is to use scanning systems to move the beam. Dausch et al 19 published the results of hyperopic PRK on 23 eyes using a MEL 60 (Aesculap-Meditec) laser. This used a 7.00 mm X 1.00 mm scanning beam with a rotating spiral mask to create an ablation diameter of 7.00 mm, although the actual diameter of the hyperopic correction zone was only 4.00 mm. At 12 months, they reported 80% of eyes with corrections between + 2.00 and + 7.50 diopters (D) were within :±: 1.00 D of that intended. In a ''higher'' correction group ( + 11.00 D to 16.00 D), only 37% was within :±: 1.00 D. A second study from Anschutz 20 using the same laser system reported poorer predictability. In their study, 66% with corrections between +2.00 D and +5.75 D and 38% with corrections between +6.00 D and + 10.00 D were within :±: 1.00 D of that intended at 12 months. Further follow-up also has suggested that the correction may not be stable, with some regression after the first year. 20 In both studies, problems with decentration in combination with the small 4.00-mm correction zone resulted in a number of eyes losing best-corrected visual acuity. 19 ·20 The use of larger correction zones for hyperopic PRK should limit complications resulting from inaccurate centration. It also might improve the predictability and stability of the refractive change. This certainly has been noted in myopic PRK. 21 - 23 The Summit Technology "Apex Plus" laser (Summit Technology, Boston, MA) has a unique delivery system that combines an erodible mask with an Axicon system to produce an overall ablation diameter of 9.50 mm, with a hyperopic correction zone of 6.50 mm. The erodible mask is a small, polymethylmethacrylate button that is placed within the laser in the path of the beam and acts as a template for the ablation. 24 It is used to create a hyperopic correction over the axial cornea with an actual diameter of 6.50 mm. An Axicon then is used to refract the circular cross-section excimer beam into a large diameter anulus and ablate a 1.50-mm anular "blend zone" around the central correction. To assess the efficacy of the erodible mask and Axicon system with its ability to produce large diameter, smoothsurfaced hyperopic ablations, we conducted a prospective

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study. A total of 43 patients (43 eyes) were treated. Follow-up was 6 months in all cases.

Materials and Methods Patients After Ethical Committee approval, a series of 43 patients (43 eyes) underwent PRK for the correction of hyperopia. The mean age was 53.2 years, with a range of 21 to 79 years. There were 23 female patients and 20 male patients. The mean preoperative refraction (spherical equivalent) was +4.54 D (range, + 1.75 to + 7.50 D). Patient Assessment From our database of prospective ''volunteer'' patients, those with hyperopia were sent written information in which the research nature of the project was emphasized. Those who wished to participate in the study were interviewed and counseled to select patients with realistic expectations who fully understood the experimental nature of the procedure. All subjects were older than 18 years of age. Patients with pre-existing ocular pathology, previous anterior segment surgery, diabetes, or connective tissue disorders were excluded. Before surgery, a detailed ocular examination was performed. This included refraction, autorefraction, keratometry, biomicroscopy, tonometry, and mydriatic funduscopy. All cylinders were refracted in negative cylinders. During subjective refraction, reliance was placed on fogging techniques, in particular the + 1.00 D blur test, to ensure that an accurate endpoint was reached consistently. This test assumes that when a further, unnecessary + 1.00 Dis added, the patient will be "fogged" to approximately 20/60. If, with this extra + 1.00 D, Snellen acuity is significantly better than 20/60, then too much minus has been prescribed and the spherical component can be revised accordingly. In addition to the examinations described above, a series of computerized measurements was undertaken to introduce a degree of objectivity into patient assessment. Measurements of corneal haze were made with a charge coupled device (CCD) camera system, the calibration and data analysis of which have been described previously. 25 This measured gray-scale disturbance caused by the combined signal of light reflected and scattered back from the cornea or back-scattered light alone. A CCD-camera mounted on a Haag-Striet slit lamp was used to capture an image of the cornea on a frame grabber. The image was digitized and analyzed using in-house software. To discriminate between reflected and scattered light, linear polarizing filters within the CCD-camera and slit-lamp light source were used. Flicker contrast sensitivity and forward light scatter (glare) were assessed using a previously described computerized technique. 26 In summary, this was a two-part test in which flicker visual contrast sensitivity was first measured with central test stimulus generated on a highresolution monitor. This stimulus flickered at 7.5 Hz be-

O'Brart et al · PRK for Hyperopia Correction Using Erodible Mask and Axicon System tween the background level and a match level. The patient adjusted the contrast between the match luminance and background to minimize flicker by means of buttons on a computer keyboard. The test then was repeated with a bright anular light source flickering in counterphase and surrounding the central test stimulus. This "stray-light" provided an additional luminance for the forward scatter of light. Night halo was assessed using a computerized system, which has been described previously.27. 28 The test was performed under scotopic conditions, with the patient at 1 m from a high-resolution monitor. The subject fixated the light source, a bright white circle 40 minutes of arc in diameter, located in the center of an otherwise dark screen. A small, white spot acted as a cursor and was moved by means of a computer mouse either centripetally or centrifugally, until it coincided with the perceived edge of the halo. The movement of this cursor was restricted to 12 radii at 30° intervals, passing through the center of the halo source. When the subject considered that the cursor was coincident with the halo edge, its location was recorded by pressing the mouse button. The cursor then moved automatically to the next meridian. Once the position of the halo edge was recorded at all 12 meridia, the area within these points was calculated in square millimeters. Corneal topography was assessed using a computerized photokeratoscope, computed anatomy, TMS-1. Attention was paid to the mean corneal power (calculated from the average simulated K values) and keratometric cylinder within the central 3.00-mm zone, as well as the surface asymmetry and surface regularity indices. The latter are specially developed corneal statistical indices that provide quantitative indicators to monitor changes in corneal topography. 29 ' 30 The Laser A Summit Technology Apex Plus laser with an emission wavelength of 193 nm, a pulse repetition rate of 10 Hz, and a fixed radiant exposure of 180 mJ/cm2 at the cornea was used. This system uses an erodible mask with an Axicon system to create a hyperopic correction with an overall diameter of 9.50 mm. The Erodible Mask This is a small, precisely shaped polymethylmethacrylate button mounted in the center of a 1.5-mm thick, 10-mm diameter quartz substrate. The quartz substrate, unlike the polymethylmethacrylate mask, is transparent to 193-nm radiation. Its role is to support the mask during photoablation. The thinnest sections of the mask are less than 111m thick and the thickest sections between 30 and 100 11m. Its diameter is 8.00 mm. Before PRK, the mask on its quartz substrate is placed in a cassette that is inserted into a port in the laser, so that the mask lies in the path of the laser beam. Throughout the mask procedure, the beam diameter remains fixed at 6.50 mm. Each laser pulse removes approximately 0.22 11m of the mask surface by photoabla-

a

~

PMMA button

Quartz substra~

b

c

Figure 1. A, the erodible mask is a polymethylmethacrylate button that is placed on a quartz substrate in the path of the excimer beam. B, with each laser pulse, a little of the mask surface is ablated. With further pulses, the thinner regions of the mask are perforated and the laser energy reaches the eye. C, by completely ablating (eroding) the mask in this way, its surface profile can be transferred to the cornea.

tion (Fig 1A). As PRK proceeds and further pulses are delivered, the thinner regions of the mask are perforated and laser energy can pass through the quartz substrate and reach the surface of the eye (Fig 1B). When the mask material has been removed totally, the shape of the corneal ablation approximates the original shape of the mask (Fig 1C). This system can transfer almost any shape to the corneal surface. Unlike traditional approaches to PRK, there is no need to use complex computer-controlled irises, apertures, and slits to control the shape of the ablation. The mask has been shown to produce smoother-surfaced ablations than conventional iris diaphragm technology.Z4 For hyperopic PRK, the masks have a convex surface shape (Fig 1), the exact curvature of which depends on the degree of hyperopia to be corrected and is based on treatment algorithms developed by Munnerlyn et al. 31 In this pilot study, the manufacturer added an additional correction or "compensation" factor of approximately 50% to the Munnerlyn-based algorithm to offset wound healing. The Axicon An Axicon is a prismatic optical lens system. It is constructed from quartz and placed within a cassette. The

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a

Volume 104, Number 11, November 1997

Circular beam

Axicon Annular beam

b

"blend" zone

Figure 2. A, an Axicon, an optical lens system that converts the circular excimer beam into an anulus. B, it is used to create a "blend zone" around the hyperopic correction.

cassette is inserted into the cassette port of the laser and supports the Axicon in the path of the beam. It is used to refract the circular 6.50-mm beam cross-section into a 1.50-mm wide anulus with an inner diameter of approximately 6.50 mm and an outer diameter of 9.50 mm (Fig 2). At its inner diameter, the radiant exposure at the cornea is approximately 180 to 200 mJ/cm2 and falls linearly to 0 mJ/cm 2 at its outer diameter. The anular beam, with its energy gradient, is used to fashion a blend zone around the hyperopic correction (Fig 2). The Procedure The aim of the study was not to fully correct each patient's refractive error, but to assess the predictability of this new system. On the basis of their preoperative refraction, patients were assigned to one of 3 groups: 2 groups of 14 patients underwent either "+2.00 D" or "+3.00 D" corrections and 15 patients had "+4.00 D" corrections. The mean attempted correction, based on the manufacturer's algorithms, was + 3.02 D, and an undercorrection was attempted in most patients. All treatments were performed by a single surgeon (DO'B). Before treatment, a series of tests designed to confirm beam homogenicity was carried out. The eye to be treated was first aligned beneath the laser aperture and the other eye taped shut to facilitate fixation. The patient practiced fixating a target light within the laser aperture for the predicted duration of the procedure. Sterile precau-

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tions were adopted. Topical amethocaine 1% and pilocarpine 4% were instilled. The noise and bum-like smell were shown by exposing the corneal epithelium to a short series of pulses. The laser was programmed for the hyperopic correction and the erodible mask fixed in the mask cassette. The cassette was placed in the cassette port. A lid speculum was inserted, and an area of central corneal epithelium approximately 10.00 mm to 11.00 mm in diameter was removed with a #64 Beaver blade. Care was taken not to remove epithelium from over the limbus. After epithelial debridement, Bowman layer carefully was wiped clean of debris and the mask correction performed. Once completed, the mask cassette was removed and replaced with the Axicon cassette. A further series of laser pulses was delivered to fashion the blend zone around the hyperopic correction (177 for +2.00 D corrections, 275 for +3.00 D corrections, and 372 for +4.00 D corrections). The central hyperopic correction zone was 6.50 mm. The blend zone was approximately 1.50 mm. The overall ablation diameter was 9.50 mm. The maximum depth of ablation occurred at 6.50 mm and was approximately 42 J.Lm for the +2.00 D, 63 J.Lm for the +3.00 D, and 84 J.Lm for the +4.00 D corrections. Between the mask and Axicon treatments, the surface of the cornea was moistened with a drop of 0.9% saline and then dried with absorbent-coated swabs (Ophtha-sticks). This was performed in an attempt to maintain corneal hydration. The time taken from beginning epithelial debridement to completion of the procedure was less than 4 minutes in all cases. Accurate alignment of the eye during the procedure was crucial and was facilitated by two helium neon lasers, one at each side of the laser aperture. During laser exposure, the intersect of these beams was maintained by the surgeon on the corneal apex over the entrance pupil center. This was facilitated by observing the locations of the beams on the iris and maintaining them at the ''3 and 9 o'clock" positions. Postoperative Treatment and Assessment Topical chloramphenicol 1% ointment was applied immediately after the procedure. Oral analgesics were prescribed, and then the eye was padded overnight. Chloramphenicol eyedrops were administered four times a day from the time the pad was removed for 10 days. On the basis of two published studies showing that topical corticosteroids appear to make no permanent contribution to the refractive outcome or visual performance for lowto-moderate myopic PRK corrections, 32•33 we did not prescribe them after surgery. Postoperative examinations were carried out at 1, 2, and 4 weeks and 3 and 6 months. At each visit, a full refraction, autorefraction, and slit-lamp examination were performed. Objective measurements of haze and backscattered light were made with our CCD-camera system. 25 Flicker contrast sensitivity, forward light scatter, night halo, 26 - 28 and corneal topography also were measured. The CCD-camera system has been shown to be useful for the measurement of haze over the axial cornea. 25 Because of problems with the inferior tear meniscus and upper lid, it is only of limited value in measuring haze in

O'Brart et al · PRK for Hyperopia Correction Using Erodible Mask and Axicon System 6

3

+3.00D

+2.00D

(

4

n= l 4

2

I

n= l4

I F------------------ -- -------------- -or-~~----------~~--------------~-4

or-~~------=t=============~

-I

-1

-2

~

-2

-3

-3 0

4

12

16

20

24

16

12

0

28

20

28

24

Weeks

Weeks Figure 3. Mean changes in refraction with time for"+ 2.00 D" corrections (n = 14 ). Dotted line = the intended correction based on the manufacturer's algorithms (includes an additional correction to offset wound healing). Solid line = emmetropia. Error bars = :!: 1 standard deviation.

Figure 4. Mean changes in refraction with time for"+ 3.00 D" corrections (n = 14 ). Dotted line =the intended correction based on the manufacturer's algorithms (includes an additional correction to offset wound healing). Solid line = emmetropia. Error bars = :!: 1 standard deviation.

the corneal periphery. To assess disturbances in peripheral corneal transparency, a subjective grading of stromal haze was made at each visit. This was based on the following criteria: 0 = no haze; 0.5 = trace/just perceptible; 1 = easily seen with slit lamp; 2 = moderate haze; 3 = pronounced haze, iris details visible; and 4 = scarring, iris details obscured.

D of the intended correction. Because an undercorrection had been attempted in most patients, the mean manifest refraction at 6 months was -0.17 D (range, +4.50 D to -3.125 D) and patient satisfaction was high. Forty-one percent oftreated eyes were within ± 1.00 D of emmetropia at 6 months. Astigmatic Change and Vector Analysis

Vector Analysis To investigate any surgically induced mean vectorial astigmatic change in the manifest refraction, vector analysis was performed in all eyes according to the system described by Waring. 34 Statistical Methods Wilcoxon rank-sum tests were used to compare measurements of haze, flicker contrast sensitivity, forward light scatter, and halo. Results with P < 0.05 were considered statistically significant.

The mean preoperative manifest refractive cylindrical correction was -0.84 D : :': : standard deviation 0.68 D at a mean axis of 85.7° (n = 43). At 6 months after surgery, the mean manifest refractive cylindrical correction was -1.24 D : :': : standard deviation 1.07 D at a mean axis of 83.SO (n = 43). In 36 eyes (84%), the change in manifest refractive cylinder was less than 1.00 D. In only one patient was the change greater than 2.00 D. Vector analysis of all 43 treated eyes showed a surgically induced mean vectorial change in the manifest refraction at 6 months after surgery of 0.8 D (range, 0 D to 2.79 D). There were no significant differences among

Results Refractive Outcome

6

After surgery, all patients had a reduction in their hyperopia. The mean changes in refraction with time for the three treatment groups are shown in Figures 3, 4, and 5 and Table 1. In all groups, there was an overcorrection, especially during the first few weeks after surgery. This diminished slightly over the next few months. At 3 to 6 months, there appeared to be some stability in the refractive change, with only minimal changes in the refractive correction between these postoperative visits (Table 1). At 6 months, the mean reduction in manifest refraction for all 43 eyes was -4.70 D (range, -0.75 D to -9.125 D). This represented an overcorrection 1.56 times more than expected based on the manufacturer's algorithms. With this algorithm, only 32% of eyes were within : :': : 1.00

4

+4.00D n= l 5

0

4

12

16

20

24

28

Weeks Figure 5. Mean changes in refraction with time for "+4.00 D" corrections (n = 15). Dotted line= the intended correction based on the manufacturer's algorithms (includes an additional correction to offset wouNI healing). Solid line = emmetropia. Error bars = :!: 1 standard deviation.

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Table 1. Mean Preoperative Refraction and Mean Change in Refraction after Hyperopic Photorefractive Keratectomy

Treatment Groups

Mean Preoperative Refraction [D (SD)]

+2.00 D*

+2.93

(0.63)

+3 .00 D*

+4.21 ( 1.24) +6.37 (0.9)

+4.00 D* D

= diopter;

SD

Mean Change in Refraction [D (SD)]

= standard

1 Week

2 Weeks

4 Weeks

-4.32 (0.94) -6.12 (1.04)

-4.8 (1.2) -5.99 (0.66)

-8.76

( 1. 79)

3 Months

6 Months

-4.6

-3.77

-3.46

-5.43

-4.6

-4.3

(0.9)

(0.84)

(1.09) -6.83

(0.98) -8.23 (1.49)

-8.65

(1.55)

(0.86)

( 1.52) -6.27 (2.19)

( 1.53)

deviation.

*These corrections are based on the manufacturer's algorithms, which contain an additional correction of approximately 50% to the Munnerlyn-based algorithm."

the three treatment groups. In 31 eyes (72% ), the vectorial change was less than 1.00 D. In only one eye (2%) was the change greater than 2.00 D. Corneal topography showed no decentration of the ablation zone in this eye or significant irregular astigmatism. Analysis of the simulated keratometry values in this eye showed a postoperative change at 6 months of only 1.2 D.

surgery. Twelve eyes (28%) lost lines of Snellen BSCVA, 2 (5 %) of which lost 2 lines. No eyes lost more than two lines. Thirty-one eyes (72%) either had an improvement or showed no change in BSCV A. Ten eyes (23%) gained a line of BSCV A and one patient gained two lines after treatment. Corneal Haze

Unaided Visual Acuity Figures 6 and 7 illustrate the changes in unaided near and distance acuity for all 43 eyes at 6 months after surgery. Unaided near acuity improved in all 43 eyes (100%). Thirty-two patients (74%) could read N8 or better unaided with their treated eyes compared with only 1 patient (2%) before surgery. Distance acuity improved in 37 eyes (86%). In six eyes, it remained unchanged. Twenty-seven patients (63%) could see 20/40 or better unaided with their treated eyes compared with only 2 patients (5%) before surgery. Three treated eyes were amblyopic and did not have a visual potential of 20/40 before surgery. Best Spectacle,corrected Visual Acuity Figure 8 shows the changes in best spectacle-corrected visual acuity (BSCV A) for all 43 eyes at 6 months after

In 9 (21%) of the 43 treated eyes, slit-lamp examination results showed the presence of very faint subepithelial haze (grade 0.5-1) over the axial cornea at the 3- and 6month postoperative assessments. In the remaining 34 eyes (79%), the central corneas remained completely clear on slit-lamp examination throughout the follow-up period. Figure 9 shows gray-scale measurements of reflected and scattered light and scattered light alone over the axial cornea with time for all 43 treated eyes. 25 There was a small but statistically significant increase in both measurements compared with preoperative levels in all three treatment groups at all stages after surgery (P > 0.05). There were no significant differences between values at different points during the follow-up period and no differences among the three treatment groups.

18 16 14 12

No. of patients

No. of patients

10 8 6 4 2 0

N48 N36 N24 N18 N14 N10 N8

N6

N5 N4.5

ILl Preop II 6 month I Figure 6. Change in unaided near visual acuity at the 6-month postoperative visi t compared with preoperative level (n = 43 ).

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0.1 0.17 0.25 0.33 0.5 0.67 or less EJ Preop 11 6 months

I

1.2

I

Figure 7. Change in unaided distance visual acuity at the 6-month postoperative visit compared with preoperative level (n = 43 ).

O'Brart et al · PRK for Hyperopia Correction Using Erodible Mask and Axicon System

20 18 16 14 12 No. of 10 patients 8 6 4 2 0 Figure 8. Change in best-corrected visual acuity at the 6-month postoperative visit compared with preoperative level (n = 43 ).

In all treated eyes, a faint ring of subepithelial haze, 6.5 mm in diameter, was apparent by the forth week after surgery. At 3 months, the intensity of this haze ring had increased. At 6 months, its appearance remained largely unaltered from that seen at 3 months (Fig 10). In most eyes, the intensity of the haze ring was not uniform. Typically, haze was more prominent in the inferior cornea and was less apparent in the superior cornea. We were unable to obtain accurate measurements of peripheral corneal haze with our CCD-camera system, because of problems with the inferior tear meniscus and upper lid and lack of homogenicity of the haze ring. At 6 months, the intensity of the haze ring, using the grading system described above, did appear to be more apparent with +3.00 D and +4.00 D corrections compared with +2.00 D corrections (Table 2). No patients undergoing +2.00 D corrections had peripheral haze grading greater than +1 compared with 7 patients (50%) with +3.00 D and 11 patients (73%) with +4.00 D corrections. In two patients (5% ), the peripheral ring of haze was particularly prominent (grade 4), appearing at 6 months as a subepithelial plaque (Fig llA). This appearance was associated in both eyes with significant regression with a loss of more than 70% of the refractive correction. These were the only two eyes in our cohort with such marked regression. Careful slit-lamp examination results showed in-filling of the peripheral ablation by the plaque. A loss of the central steepening effect was shown by corneal topography (Fig liB). In both patients, the central cornea remained clear, and BSCV A was unaltered at the 6-month assessment.

Table 2. Haze Gradings (Percent of Total) in the Corneal Periphery for All Three Treatment Groups Treatment Groups

+2.00 D

+3.00 D

0

0 0

None Trace of haze +/Minimal haze + Moderate haze + + Marked haze + + + Scarring + + + +

7 (50) 7 (50)

0

0 0

7 (50) 6 (43) 0

1 (7)

+4.00 D 1 3 7 3 1

0

(7) (20) (46) (20) (7)

were no differences among the three treatment groups. No patients reported night vision disturbances after surgery. Corneal Topography In all eyes, there was an increase in central corneal power that correlated closely with the change in manifest refraction (Table 3 and Fig 12). There was some surface irregularity during the first month after surgery. Thereafter, the surface regularity and asymmetry indices 29 •30 showed an improvement in all groups with a return to normal levels in the +2.00 D group at 6 months (Table 3). These indices still were slightly high at 3 to 6 months in the +3.00 D and +4.00 D groups. These findings are consistent with those seen after PRK for myopia. 23 In three eyes, there was slight decentration of the ablation zone relative to the entrance pupil center. In all patients, the decentration was inferior and was less than 0.6 mm. Epithelial Healing In all patients, the epithelial defect created by surgery was healed at 1 week after surgery. In two patients, there were epithelial irregularities during the first month, manifested as punctate epithelial erosions and small areas of heaped epithelium. These patients were the oldest of our cohort and were both in their sevent-

70

Grey Scale Units

Forward Light Scatter Flicker contrast sensitivity and glare (forward scatter) measurements 26 showed no significant differences between preoperative and postoperative measurements. Night Halo Measurements Night halo measurements 27 ·28 at 3 to 6 months showed no significant differences from preoperative levels. There

Figure 9. Mean gray-scale measurements of reflected and scattered light and scattered light alone over the axial cornea with time for all 43 treated eyes.

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Figure 10. Anterior segment photograph of a 58-year-old woman who underwent a +"3.00 D" correction 6 months previously. There is a 6.5-mm ring of anterior stromal haze. The axial cornea is clear.

Figure 11. Left, anterior segment photograph of a 36-year-old woman who underwent a "+ 3.00 D" correction 6 months previously. The peripheral haze is particularly prominent (grade 4 ), appearing as a subepithelial plaque. The central cornea remained clear, and best-corrected acuity was unaltered. Right, corneal topographic map of the same eye, which shows regression of the refractive correction associated .

.9? .. 7!i

..

\

60 I

·0011

285

Figure 12. Corneal topographic map of a 59-year-old man who underwent a "+3.00 D" correction. Left, preoperative appearance and, Right, 6 months after PRK.

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O'Brart et al · PRK for Hyperopia Correction Using Erodible Mask and Axicon System Table 3. Mean Values for Surface Regularity Indices (SRI), Surface Asymmetry Indices (SAl), Cylinder, and Central Power with Time for the Three Treatment Groups Obtained from Analysis of Computerized Videokeratoscopy* Treatment Groups

Indices

Preoperative

2 Weeks

4 Weeks

13 Weeks

26 Weeks

+2.00 Dt

SRI SAl Cylinder (D) Central power (D) SRI SAl Cylinder (D) Central power (D) SRI SAl Cylinder (D) Central power (D)

0.6 0.28 0.78 43.76 0.73 0.43 1.05 43.6 0.47 0.3 1.19 43.7

2.8 1.1 1.7 47.53 3.48 1.55 2.4 48.54 4.07 2.25 3.86 49.94

0.9 0.87 0.95 47.72 2.1 1.5 2.36 48.78 2.9 1.85 2.59 51.01

0.67 0.6 0.84 47.01 1.26 0.98 1.74 48.3 2.39 1.1 2.4 50.57

0.92 0.6 0.79 46.6 1.24 0.74 1.6 47.9 1.8 1.24 2.12 50.21

+3.00 Dt

+4.00 Dt

*Normal corneas generally have SAis less than 0.5 and SRis less than 1.00. 29 •30

t These corrections are based on the manufacturer's algorithms, which contain an additional correction of approximately 50% to the Munnerlyn-based algorithm. 31

ies. In both patients, the epithelium on slit-lamp examination appeared to be smooth and regular at 6 months, with no punctate staining. However, corneal topography in these patients did show some slight central irregularities, and both patients had lost two lines of Snellen corrected acuity at 6 months. (These patients have now been observed for a year and have regained at least one line of BSCV A, with further improvement of topographic irregularities.) One patient experienced a recurrent erosion problem 2 months after surgery. This settled after 1 month with the patient using lubricant ointments. He has had no further episodes for 6 months. Complications A list of complications for all three treatment groups is given in Table 4. At 6 months, ten eyes lost one line of BSCVA, and two eyes lost two lines. One eye had a problem with a recurrent corneal erosion and two had irregular epithelial healing. In two eyes, there was a pe-

ripheral plaque of subepithelial haze associated with regression of the refractive correction. These problems appeared to be more frequent in the +3.00 D and +4.00 D treatment groups.

Discussion Study Design For PRK to become a clinically acceptable procedure for the correction of hyperopia, the induced correction must be both predictable and stable, with no significant deficit in visual performance after surgery. This current study was designed to collect data in a systematic fashion with as many variables as possible kept constant. Patients were allotted to three treatment groups within which all individuals received the same standardized PRK treatment and postoperative regimen. Refractive Outcome and Stability

Table 4. Number of Eyes with Complications after Hyperopic Photorefractive Keratectomy at 6 Months for All Three Treatment Groups (%)

0 0 0 0

1 (7) 1 (7) 0 1 (7)

1 1 1 1

0

0

+2.00 D

Irregular epithelial healing Reduced BCVA > 2 lines Recurrent corneal erosion Regression of correction >50% Astigmatic change > 2.00 D (vector analysis) BCVA = best-corrected visual acuity.

+3.00 D +4.00 D (%)

Treatment Groups

(7) (7) (7) (7)

1 (7)

From Figures 3, 4, and 5 and Table 1, it can be seen that there was an overcorrection of hyperopia, especially in the first month after PRK. Although this diminished over the next few months, the mean-achieved correction at 6 months still was 1.56 times greater than intended. This could be explained by the compensation factor added to the correction algorithm by the manufacturer to offset wound healing. This factor approximated to an additional 50% correction to the Munnerlyn-based algorithm?' Thus, a correction with a +2.00 D mask would in fact approximate to a +3.00 D correction. Similarly, a treatment with a +3.00 D mask would approximate to a +4.50 D correction and that with a +4.00 D mask to a +6.00 D correction. When this is taken into consideration, the mean-achieved corrections at 6 months agree closely with

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the algorithm in all three groups and are within 0.5 D of that expected (Table 1). Even with the compensation factor taken into consideration, there still was an overcorrection during the first weeks after PRK. This is similar to the overcorrection seen in the early postoperative period after myopic PRK."- 16·22·23 In order to explain such changes, it is important to remember that these events occur during the first few weeks after surgery. The corneal epithelium heals by replacement, not repair, and re-epithelialization is rapid. Thus, changes occurring within the immediate postoperative period probably are related to epithelial healing. It may be postulated that similar mechanisms are effecting such changes in both myopic and hyperopic PRK and that regression toward the expected correction in the first weeks is likely to be the result of epithelial healing and the buildup of epithelial cell layers. Although a 6-month follow-up period is not long enough for a final analysis of refractive stability, it does allow certain inferences to be made. Figures 3, 4, and 5 and Table 1 show that in the first 3 months after PRK, there was some regression of the hyperopic correction, with an average reduction of 21%. Between 3 and 6 months, there appeared to be little change, with a mean reduction of less than 8%. This pattern appears to be similar to that reported after PRK for low-to-moderate myopic corrections, in which most patients retain the correction they had at 3 months."- 16'22'23 Previous studies, albeit with smaller correction zones, have suggested that with hyperopic PRK, refractive stability may take several months with some regression even after the first year in higher order corrections. 2°Clearly, a period of longer follow-up is required to determine long-term refractive stability, and our cohort will continue follow-up for several years. Studies of myopic PRK have shown that its accuracy varies inversely with the degree of attempted correction."-16'22·23 In this study, similar trends were noted. Taking the manufacturer's "compensation" factor into consideration, 71% of eyes treated with +2.00 D masks were within :::':::1.00 D of the expected (+3.00 D) correction at 6 months. With + 3.00 D masks, 57% of eyes were within : :': : 1.00 D of the expected ( +4.50 D) correction, whereas only 27% of eyes treated with +4.00 D masks were within : :': : 1.00 D of the expected ( +6.00 D) correction. The diminished accuracy of PRK associated with higher degrees of myopic correction has been associated with both a trend toward undercorrection and a reduction in predictability, which is reflected in an increase in the standard deviations of higher diopter treatment groups."- 16·22 ·23 From Figures 3, 4, and 5 and Table 1, it can be seen that the inaccuracy of PRK associated with the correction of higher degrees of hyperopia did not appear to be associated with an undercorrection, but a dramatic increase in the standard deviation of the higher diopter treatment groups. Compared with previous studies of myopic PRK with similar orders of correction, 22 ·23 the standard deviations of our hyperopic PRK groups seem rather high and reflect a general trend toward poorer predictability even with low-order corrections. It must be remembered, however, that these are preliminary studies with prototype

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masks and algorithms. We have reported previously a significant improvement in the predictability of myopic PRK associated with an increase in the diameter of the ablation from 5.00 mm to 6.00 mm. 22·23 By optimizing the diameter and shape of the ablation in hyperopic PRK, it may be possible to improve the predictability of the procedure in the future. Astigmatic Change and Vector Analysis Previous studies of spherical myopic PRK have reported the induction of astigmatism in eyes that previously did not have astigmatism and an increase in those with preexisting astigmatism. 34 In most patients, the change is small and has not been deemed clinically significant. In this current study, there were similar findings. Vector analysis showed a mean change of 0.8 D at 6 months, with an alteration of less than 1.00 Din 72% of patients. In only one patient was the vector change greater than 2.00 D. This patient had the highest preoperative refractive cylinder (-2.50 D), and at 6 months after surgery, this patient had a vector change of 2.79 D. The increase in astigmatism in this patient and others is difficult to explain. It can be hypothesized that the induction of astigmatism may result from decentration of the ablation zone, misalignment of the laser optics, variations in laser fluences, and irregular epithelial and stromal wound healing. Corneal topography in this eye, however, showed no decentration of the ablation zone or significant irregular astigmatism, corneal haze was minimal, and analysis of simulated keratometry showed a postoperative keratometric change at 6 months of only 1.2 D. Uncorrected Visual Acuity The mean age of our cohort was 53.2 years, with only 6 of the 43 patients younger than 40 years of age. The majority of our patients were presbyopic as well as hyperopic and without refractive correction had no clear point of focus either for distance or near. After surgery, unaided near visual acuity improved in all eyes, with 32 patients (74%) able to read N8 or better unaided at 6 months. Distance acuity improved in 37 patients (86% ), with 27 patients (63%) able to see 20/40 or better unaided (3 eyes [7%] were amblyopic and did not have a visual potential of 20/40 before surgery). Patient satisfaction was therefore high. Even in cases in which eyes were overcorrected and became myopic, patients had a clear point of focus without spectacle correction for near and generally were pleased with the result. At this time, more than 90% of patients have elected to have their second eyes treated. Disturbances in Corneal Transparency Because PRK is undertaken on healthy eyes, any change in the postoperative transparency of the cornea, especially the axial cornea, is of concern. Although gray-scale measurements showed a small but statistically significant disturbance in axial corneal transparency after surgery, the values obtained were far less than we have reported in previous studies of myopic PRK, using the same CCD-

O'Brart et al · PRK for Hyperopia Correction Using Erodible Mask and Axicon System camera system calibrated in an identical fashion. 22 •23 •32•33 In myopic PRK, subepithelial haze develops over the axial cornea by the forth postoperative week, with maximal disturbances of transparency at 3 to 6 months. In our study, measurements were no greater at 3 and 6 months than at 1 and 2 weeks, when the only disturbances are very faint epithelial irregularities associated with healing. Slit-lamp examination results showed axial corneal haze in nine eyes (21 %), and in all patients, it was minimal. The changes in gray-scale measurement in this study probably reflect the sensitivity of our measuring system rather than any visually significant disturbance of central corneal transparency. In the corneal periphery, where the ablation depth was maximal, the situation was different. In all eyes, a ring of subepithelial haze, 6.5 mm in diameter, was apparent by the fourth week after surgery. At 3 months, the intensity of this haze ring had increased (Fig 10). This pattern is similar to the development of haze over the axial cornea seen after myopic PRK. 11 - 16' 22' 23 •32' 33 A number of preclinical studies have indicated that with increasing depths of stromal ablation, disturbances in corneal transparency are greater. 35 - 37 Although the accumulating clinical database has highlighted the need to consider other parameters such as ablation diameter and wound profile, the general trend of increasing haze with increasing ablation depth was seen in this study. Haze predominated in the corneal periphery at the site of maximal tissue ablation and occurred with greater intensity with + 3.00 D and +4.00 D corrections compared with + 2.00 D corrections (Table 2). It was observed that the intensity of the haze ring was not uniform and typically was more prominent in the inferior cornea. The reason for this finding is unclear. It may be that the upper lid offers a protective effect, which might be related to corneal hydration. Certainly, the haze ring was more evident in the interpalpebral region, where tear-film anomalies and evaporation might perhaps result in epithelial defects and alterations in corneal hydration. The observation of an increase in interpalpebral haze requires further investigation because it may lead to the development of strategies to reduce haze and alter wound healing after PRK. In a study of myopic PRK with a 5.00-mm ablation diameter, Durrie et al38 retrospectively assigned the patients into three classes of wound healing responses on the basis of their refractive outcome and the development of corneal haze. One group was described as "aggressive" healers. In these patients, there was marked scarring associated with significant regression of the refractive correction. The occurrence of severe haze with marked regression has been documented by other investigators. 11 - 16•32'33 In two (5%) of our patients, the ring of haze was very pronounced (grade 4), appearing as a subepithelial plaque (Fig 11A). In both these eyes, there was significant regression of the refractive correction. This is an important observation, because it suggests that the mechanisms associated with regression may be similar in both myopic and hyperopic PRK. It can be hypothesized that the subepithelial deposition of collagen and glycosarninoglycans resulting from the complex interactions of epithelial and stromal wound healing produces in an in-filling of the ablation and loss of effect (Fig 11B).

Epithelial Healing The overall ablation diameter in this study was 9.50 mm. Before ablation, an area of epithelium approximately 11.00 mm in diameter was removed. This is a much larger area of epithelial removal than that required for myopic PRK. In addition, hyperopic eyes tend to be smaller than myopic eyes with smaller corneal diameters. In such eyes, virtually all the epithelium, except for that over the limbus, has to be removed before PRK. The creation of such large epithelial defects has led to concerns about problems with epithelial healing. In this study, the epithelial defect was closed in all eyes by the first postoperative week. There were two patients with epithelial irregularities during the first few months, manifested as punctate epithelial erosions and small areas of heaped epithelium. These patients were the oldest of our cohort and were both in their seventies. In both patients, slit-lamp examination results showed that the epithelium appeared to be normal at 6 months, although corneal topography did show some slight irregularities. At 12 months, topography was normal. Both patients have had their second eyes treated and have not experienced any epithelial problems with these eyes. However, caution should perhaps be taken in very elderly patients, and they should be warned that it may take several months for their eyes to settle and vision to clear.

Complications In hyperopic PRK, the main site of ablation is in the corneal periphery, and the axial corneal is largely left untouched. Indeed, examination results of the cornea immediately after hyperopic ablation showed a small central area, 0.5 to 1.00 mm in diameter, where no ablation had occurred and Bowman membrane was intact. Although disturbances in transparency do occur, they mainly develop in the periphery and do not impair vision. In this study, even with severe haze (grade 4 ), BSCVA was not compromised. After surgery, visual performance, in terms of flicker contrast sensitivity, glare (forward light scatter), and night halo measurements, was not impaired. Although a number of eyes did lose lines of BSCVA during the follow-up period, these findings are similar to those seen after myopic PRK. With further follow-up, patients should regain lost lines as corneal remodeling continues. 11-16,22,23,32,33 In conclusion, in this short-term study, PRK for hyperopia using the erodible mask and Axicon system appeared to be a promising procedure. Although a peripheral ring of haze developed in all eyes, transparency over the axial cornea largely was undisturbed. Visual performance, in terms of flicker contrast sensitivity, forward light scatter, and night halos, was not compromised. There was an overcorrection based on the manufacturer's algorithms. Manipulation of the treatment algorithms should improve future predictability. Acknowledgment. The authors thank Mrs. Ann Patmore for technical support.

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