Surgical results of photorefractive keratectomy with different operative modes

Surgical results of photorefractive keratectomy with different operative modes Wei-Li Chen, MD, Fung-Rong Hu, MD, I-Jong Wang, MD, Huai-Wen Chang, MS ABSTRACT Purpose: To compare the predictability, efficacy, and safety of photorefractive keratectomy (PRK) using different operative modes. Setting: National Taiwan University Hospital, Taipei, Taiwan. Methods: One hundred fifty-three eyes of 80 patients who had PRK for myopia with a follow-up of at least 6 months were studied. All patients were sequentially assigned to 1 of the following surgical modes: mode 1: PRK with the Summit OmniMed excimer laser; mode 2: PRK with the Summit Apex Plus laser; mode 3: PRK with the Summit Apex Plus laser with anti-central-island pretreatment. Results: Six months after treatment, a homogeneous topographic pattern was seen in 76% of mode 1 eyes, 70% of mode 2 eyes, and 88% of mode 3 eyes. In the low myopia group (ⱕ⫺6.0 diopters [D]), the mean residual refractive error was ⫺0.79 D ⫾ 0.59 (SD) in mode 1, ⫺0.94 ⫾ 1.02 D in mode 2, and ⫺0.31 ⫾ 0.42 D in mode 3. In the high myopia group (⬎⫺6.0 D), it was ⫺1.93 ⫾ 1.51 D, ⫺1.54 ⫾ 0.88 D, and ⫺0.70 ⫾ 0.81 D, respectively. Uncorrected visual acuity of 20/25 or better was achieved in 81% of mode 1 eyes, 56% of mode 2 eyes, and 89% of mode 3 eyes in the low myopia group, and in 48%, 28%, and 72%, respectively, in the high myopia group. Conclusions: Photorefractive keratectomy appears to be a predictable and effective procedure. The best results were achieved with the Summit Apex Plus laser with anti-central-island pretreatment, followed by the Summit OmniMed laser. The Summit Apex Plus laser without anti-central-island pretreatment produced less satisfactory results. J Cataract Refract Surg 2000; 26:879 – 886 © 2000 ASCRS and ESCRS

E

xcimer laser photorefractive keratectomy (PRK) is known to be an effective treatment for myopia, with the aims of creating a smooth anterior corneal surface, correcting refractive errors, and thus obtaining a clear and undistorted visual outcome. Attaining these results depends on the homogeneity and preprogrammed dis-

Accepted for publication January 28, 2000. Reprint requests to Fung-Rong Hu, MD, Department of Ophthalmology, National Taiwan University Hospital, 7, Chung-Shan South Road, Taipei, Taiwan. E-mail: [email protected]. © 2000 ASCRS and ESCRS Published by Elsevier Science Inc.

tribution of the laser energy and on environmental surface factors during the procedure.1–5 However, irregular topographic change is frequently seen postoperatively.6 – 8 Although it tends to resolve with time, its possible association with decreased visual function, monocular diplopia, and the need for retreatment makes the resolution of this problem important.6,9 –11 Several well-designed studies report the post-PRK steep central island incidence, mechanisms, and visual outcomes, but few controlled trials have been performed to determine the influence of different laser beam pro0886-3350/00/$–see front matter PII S0886-3350(00)00371-0

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

files, which might play a role in central island formation. In this study, we compared the results of PRK with 2 Summit excimer lasers, the OmniMed and Apex Plus. They have the same parameters except the OmniMed has a laser beam profile with Gaussian distribution and the Apex Plus has a top-hat distribution. Comparing the postoperative results of these lasers might help us understand the effects of different laser beam distribution patterns on the clinical outcomes of PRK. In addition to comparing the topographic pattern, predictability, efficacy, and complications of PRK, we evaluated the clinical effects of an anti-central-island pretreatment program, which is designed to combat the central island problem after PRK, especially in lasers with a top-hat distribution pattern.

Patients and Methods Study Design and Patient Selection Patients were eligible for the study if they were 20 years of age or older and had given informed consent; had a best corrected visual acuity (BCVA) of 20/30 or better in both eyes and a refractive astigmatism of less than 1.0 diopter (D) with mild to high myopia (⫺1.0 to ⫺12.0 D); and were available for follow-up examination for at least 6 months. The exclusion criteria included severe dry eye syndrome, clinical or topographic evidence of keratoconus, previous ocular surgery, and systemic diseases or therapy that might influence corneal wound healing. All patients were sequentially assigned to receive 1 of 3 surgical modes: mode 1: March 1994 to December 1994, PRK with the OmniMed laser; mode 2: September 1996 to January 1997, PRK with the Apex Plus laser; mode 3: January 1997 to January 1998, PRK with the Apex Plus laser and anti-central-island pretreatment. The OmniMed had a laser beam profile with Gaussian distribution and a fluence of 180 mJ/cm2 at a repetition rate of 10 Hz. The pulse duration was 14 nanoseconds, resulting in a stromal tissue ablation rate of approximately 0.25 ␮m/pulse. The ablation zone diameter in mode 1 was 6.0 mm. The Apex Plus had the same parameters as the OmniMed except that the laser beam profile had a top-hat distribution. The ablation diameter was 6.0 mm and 6.5 mm in modes 2 and 3, respectively. The anti-central-island pretreatment in mode 3 is shown in Table 1. 880

Table 1. Anti-central-island pretreatment settings in mode 3 eyes. Pretreatment Attempted Correction (D)

Ablation Zone (mm)

Pretreatment Dose (D)

ⱕ⫺3.0

2.0

⫺0.5

⬎⫺3.0, ⱕ ⫺8.0

2.5

⫺1.0

⬎⫺8.0

3.0

⫺2.0

Photorefractive Keratectomy Photorefractive keratectomy was performed by 3 surgeons and was standardized in all patients. The pupil was constricted preoperatively with 1 drop of pilocarpine 1% given twice within 5 minutes. The eye was anesthetized with topical proparacaine 0.5% solution. Two training sessions were performed to familiarize patients with the procedure and to ensure proper fixation. The patients were instructed to fix their gaze on the target inside the laser equipment. The first training session involved application of methylcellulose 1% to the cornea before ablation to block the incoming laser beam. The second session was performed on dry epithelium using 25 pulses of the laser at its maximum aperture. After the training sessions, a 6.5 mm optical zone marker centered on the entrance pupil was pressed onto the cornea to mark the treatment zone. The epithelium within the optical zone was removed completely using a #64 Beaver blade, and then excimer laser ablation was performed. The PRK procedure was performed as described in modes 1 and 2. In mode 3, anti-central-island pretreatment was performed immediately after epithelial debridement, followed by routine photoablation. A bandage contact lens was applied immediately after surgery. Artificial tears and gentamicin 0.3% solution were applied 4 times daily until the cornea re-epithelialized. After complete re-epithelialization, the contact lens was removed and fluorometholone 0.1% was applied 4 times daily for 1 month. The fluorometholone dosage was tapered over 3 months by 1 drop per day each month and then discontinued. Patient Examinations All patients were scheduled to receive follow-up examinations 1, 3, and 7 days and 1, 3, and 6 months postoperatively. Before surgery and 1, 3, and 6 months postoperatively, the following parameters were evalu-

J CATARACT REFRACT SURG—VOL 26, JUNE 2000

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

Table 2. Preoperative data of the patients receiving PRK. Low Myopia Group (<ⴚ6.0 D)

High Myopia Group (>ⴚ6.0 D)

Treatment Mode

Number of Eyes

Age (Mean Years ⴞ SD)

Refractive Error (Mean D ⴞ SD)

Number of Eyes

Age (Mean Years ⴞ SD)

Refractive Error (Mean D ⴞ SD)

1

24

30.4 ⫾ 7.2

⫺4.72 ⫾ 1.15

25

30.0 ⫾ 6.2

⫺7.25 ⫾ 0.66

2

20

35.7 ⫾ 8.0

⫺4.75 ⫾ 1.07

42

34.1 ⫾ 5.9

⫺7.47 ⫾ 1.04

3

43

32.9 ⫾ 3.7

⫺4.85 ⫾ 0.91

46

34.0 ⫾ 7.9

⫺8.05 ⫾ 1.23

ated: uncorrected visual acuity (UCVA) and BCVA, cycloplegic and manifest refractions, slitlamp biomicroscopy to assess corneal clarity, tonometry, and computerized corneal topography (Topographic Modeling System, TMS-I, Computed Anatomy Inc.). Data Analysis and Statistical Methods In each treatment mode, the patients were divided into low and high myopia groups according to their preoperative myopia. The low myopia group comprised patients with a preoperative refractive error equal to or less than ⫺6.0 D; the high myopia group comprised patients with a preoperative refractive error of more than ⫺6.0 D. Patterns of ablation were determined by viewing the subtraction map, which was derived by subtracting the preoperative from the postoperative power at each point on the topographic power maps.12 Predictability was evaluated by the mean postoperative manifest refractive spherical equivalent (SE) and the percentage of eyes within ⫾1.0 D of emmetropia. The percentages of eyes with UCVA better than 20/40 and 20/25 were also analyzed to represent the treatment’s efficacy. Subepithelial corneal haze was graded clinically on a scale of 0 to 4⫹: 0 ⫽ completely clear; 0.5⫹ ⫽ trace of haze seen only with indirect illumination; 1⫹ ⫽ mini-

mal haze seen with difficulty in direct illumination; 2 ⫹ ⫽ mild haze seen easily; 3⫹ ⫽ moderate dense opacity partially obscuring iris detail; and 4⫹ ⫽ dense opacity completely obscuring details of intraocular structures. Data were calculated as means and standard deviations or frequencies. Statistical analyses of the results were performed by the Student t test for postoperative SE and haze, and by the chi-square test for percentage of homogeneous topographic pattern, percentage of UCVA greater than 20/40 or 20/25, and percentage of eyes within ⫾1.0 D of emmetropia. A P value of less than 0.05 was considered significant.

Results Initially, there were 200 eyes in the study. Two hundred eyes were observed at 1 month, 185 eyes at 3 months, and 153 eyes at 6 months. The data from the 153 eyes (80 patients) examined at 6 months were analyzed. There were 46 eyes (25 patients) in mode 1, 56 eyes (28 patients) in mode 2, and 51 eyes (27 patients) in mode 3. Table 2 shows the preoperative data of the 200 eyes. There were no significant differences in age, sex, preoperative UCVA, or manifest refractive SE among the patients in the 3 modes. Table 3 shows the percentage of homogeneous topographic pattern after PRK. There was a trend toward

Table 3. Percentage of homogeneous topographic pattern after PRK. Percentage (Number of Eyes)

P Value*

Postoperative Months

Mode 1

Mode 2

Mode 3

Mode 1 vs Mode 2

Mode 2 vs Mode 3

Mode 1 vs Mode 3

1

37 (49)

35 (62)

62 (89)

NS

⬍.001

⬍.001

3

64 (47)

56 (60)

78 (78)

NS

⬍.001

.029

6

76 (46)

70 (56)

88 (51)

NS

.002

.027

NS ⫽ not significant *Chi-square test for comparison of frequencies between 2 modes

J CATARACT REFRACT SURG—VOL 26, JUNE 2000

881

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

progressive homogenization in the 3 modes. At all times, the mode 3 eyes had the highest percentage of homogeneous topographic pattern, followed by the mode 1 eyes. The mode 2 eyes had the highest percentage of irregular topographic pattern. The differences among the 3 modes were significant. Predictability, defined as the percentage of eyes within ⫾1.0 D of emmetropia, and mean postoperative manifest refractive SE are summarized in Tables 4 and 5. Myopic regression occurred in all 3 modes, but the refractive changes appeared to stabilize gradually 3 months after surgery. At 6 months, there were significant differences among the 3 modes in the postoperative manifest refractive SE and in the percentage of eyes within ⫾1.0 D of emmetropia.

The percentages of eyes with UCVA equal to or better than 20/40 or 20/25 are summarized in Tables 4 and 5. Regardless of the level of myopia, the percentage was highest in mode 3 eyes and lowest in mode 2 eyes. Anterior stromal haze appeared in the ablation zone after PRK in all 3 modes in both low and high myopia groups (Tables 4 and 5). Haze tended to decrease over time. At 6 months, the low and high myopia groups in all 3 modes showed trace to 1⫹ haze, but no significant differences were found.

Discussion The purpose of this study was to determine whether different laser machines and laser setting modes influ-

Table 4. Postoperative results of PRK in the low myopia group. P Value Variables Within ⫾1.0 D of emmetropia (%)

UCVA ⬎20/40 (%)

UCVA ⬎20/25 (%)

Refractive error (D) (Mean ⫾ SD)

Haze (Mean ⫾ SD)

Number of eyes

Postoperative Month

Mode 3

Mode 1 vs Mode 2

Mode 2 vs Mode 3

Mode 1

Mode 2

Mode 1 vs Mode 3

1

95

70

90

⬍.001*

⬍.001*

NS*

3 6

88

70

92

.002*

⬍.001*

NS*

86

75

95

.05*

⬍.001*

.031*

1

88

65

95

⬍.001*

⬍.001*

NS*

3

100

72

97

⬍.001*

⬍.001*

NS*

6

100

73

100

⬍.001*

⬍.001*

NS*

1

54

35

65

NS*

.025*

NS*

3

77

35

79

.006*

⬍.001*

NS*

6

81

56

89

NS*

.025*

NS*

1

⫺0.33 ⫾ 0.46

⫺0.66 ⫾ 0.92

⫺0.10 ⫾ 0.52

NS†

.003†

NS†

3

⫺0.72 ⫾ 0.61

⫺0.88 ⫾ 0.90

⫺0.35 ⫾ 0.41

†

.003

†

.007†

†

.006†

.002†

6

⫺0.79 ⫾ 0.59

⫺0.94 ⫾ 1.02

⫺0.31 ⫾ 0.42

1

0.94 ⫾ 0.51

1.01 ⫾ 0.60

1.19 ⫾ 0.67

NS†

NS†

NS†

3

0.87 ⫾ 0.79

1.03 ⫾ 0.64

1.05 ⫾ 0.65

NS†

NS†

NS†

6

0.73 ⫾ 0.75

0.83 ⫾ 0.70

0.82 ⫾ 0.64

†

†

NS†

1

24

20

43

3

22

20

37

6

21

16

19

UCVA ⫽ uncorrected visual acuity; NS ⫽ not significant *P value tested by chi-square test † P value tested by Student t test

882

NS

J CATARACT REFRACT SURG—VOL 26, JUNE 2000

NS

NS

NS

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

Table 5. Postoperative results of PRK in the high myopia group. P Value Variables Within ⫾1.0 D of emmetropia (%)

UCVA ⬎20/40 (%)

UCVA ⬎20/25 (%)

Refractive error (D) (Mean ⫾ SD‡)

Haze (Mean ⫾ SD)

Number of eyes

Postoperative Month

Mode 1

Mode 2

Mode 3

Mode 1 vs Mode 2

Mode 2 vs Mode 3

Mode 1 vs Mode 3

1

48

51

70

NS*

⬍.001*

⬍.001*

3

60

50

71

NS*

.002*

NS*

6

52

30

60

.002*

⬍.001*

NS*

1

79

52

85

⬍.001*

⬍.001*

NS*

3

86

76

88

NS*

6

92

67

94

⬍.001*

.027*

NS*

⬍.001*

NS*

1

32

29

54

NS*

.013*

NS*

3

36

25

59

NS*

.002*

NS*

6

48

28

72

NS*

⬍.001*

NS*

1

⫺1.03 ⫾ 1.71

⫺0.97 ⫾ 0.99

⫺0.40 ⫾ 0.99

NS†

.008†

NS†

3

⫺1.41 ⫾ 1.10

⫺1.20 ⫾ 0.78

⫺0.66 ⫾ 0.67

NS†

⬍.001†

6

⫺1.93 ⫾ 1.51

⫺1.54 ⫾ 0.88

⫺0.70 ⫾ 0.81

†

1

0.92 ⫾ 0.45

0.97 ⫾ 0.66

1.01 ⫾ 0.66

NS†

NS†

NS†

3

0.88 ⫾ 0.64

0.97 ⫾ 0.53

0.87 ⫾ 0.65

NS

†

†

NS

NS†

6

0.43 ⫾ 0.42

0.70 ⫾ 0.65

0.64 ⫾ 0.59

NS†

NS†

NS†

1

25

42

46

3

25

40

41

6

25

40

32

NS

⬍.001

†

⬍.001† .012†

UCVA ⫽ uncorrected visual acuity; NS ⫽ not significant *P value tested by chi-square test † P value tested by Student t test

ence PRK results in patients with simple myopia. This was achieved by comparing objective measures of topographic patterns, predictability, efficacy, and haze formation after treatment. The best results were achieved by the Apex Plus with anti-central-island pretreatment, followed by the OmniMed. Less satisfactory results were achieved by the Apex Plus without anti-central-island pretreatment. The goal of PRK is to create a corneal surface that improves visual acuity and minimizes optical aberration. We found that the ablation surface was not always as smooth and homogeneous as expected. Identifying postoperative topographic irregularity is important because it is associated with complications such as undercorrection, loss of BCVA, and increased glare and halos.6,8,9,13,14 The formation of steep central islands after

PRK was first observed and reported predominantly by investigators using the VISX Twenty-Twenty excimer laser.7 Since then, numerous investigators have observed similar topographic changes with different lasers. Several factors influence the topographic pattern after PRK: (1) an inhomogeneous excimer laser beam profile; (2) damage to the optical system of the laser that may attenuate the incoming beam, leading to underablated areas; (3) vortex plume formation that may interfere with the passage of successive pulses of laser light; (4) the impact of the laser pulse in the cornea producing a shock wave that could drive water centrally, decreasing central ablation and leading to the central island pattern; and (5) both epithelial and stromal wound healing and remodeling.10,15–18 The central island height in standard PRK is strongly correlated with

J CATARACT REFRACT SURG—VOL 26, JUNE 2000

883

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

ablation zone size and intended correction. An inverse relationship between pulse frequency and central island height in the PTK mode has also been reported.19 Some investigators have attempted to eliminate central island formation. Blowing nitrogen gas over the corneal surface appears to dry the surface, removing water or plume byproducts or both and, thus, results in the absence of central island formation. However, this practice was discontinued because of corneal dryness, surface irregularity, and increased haze formation. After nitrogen gas blowing was stopped during the procedure, steep central island formation was noted.20,21 Blowing an aerosol does not dry the surface but might produce a more uniform level of hydration, thereby reducing central island formation more than removing fluid from the corneal surface does.22 There are many laser designs and machines on the market. In at least 3 types of wide-field excimer lasers, including the Schwind Keratom with standard software, the Summit OmniMed, and the VISX Twenty-Twenty B with standard software, steep central islands have been clearly detected in clinical cases or experiments.12,23 Initially, steep central islands were seen most often with the VISX laser system, and in only a few cases in which the Summit laser was used. One possible explanation is that the VISX laser tends to have a flat, top-hat beam profile, and the Summit laser system tends to have a slightly more Gaussian beam profile that may counteract the interference of ablation by a vortex plume or by centrally expressed corneal surface fluid. Another observation is the larger ablation zone diameters of the VISX laser seem to be associated with a greater incidence of steep central island formation. In this study, we compared the PRK results with 2 Summit lasers, the OmniMed and the Apex Plus. The OmniMed came on the market in 1994. It is designed with Gaussian beam distribution, with greater energy density centrally. For astigmatism correction, this kind of uneven distribution pattern may have some limitations. The Apex Plus came on the market later than the OmniMed. Its laser parameters are nearly the same as those of the OmniMed except that it has a top-hat beam distribution pattern. The top-hat profile has homogeneous energy emission from the center to the periphery and thus makes astigmatism correction with an erodible mask possible. The addition of a multizone 884

treatment mode also broadens the Apex Plus’s applicability. This is why we decided to change from the OmniMed to the Apex Plus after the latter came on the market. Because Gaussian beam distribution of laser emission may ablate more tissue and partially remove the undercorrected portion of the central cornea, possibly mitigating central island formation, we assume that central islands occur less frequently than with a tophat distribution pattern. This may explain why the mode 2 eyes had a higher incidence of irregular topographic change during the follow-up than the mode 1 eyes. To reduce the risk of corneal irregularity with the Apex Plus, we set up an additional ablating algorithm focused on the central cornea. Based on the attempted myopic correction before ordinary PRK, 3 settings were designed on the theory that the degree of undercorrection was proportional to the attempted correction. The incidence of postoperative irregular topography decreased with no significant hyperopia shift in mode 3. This is compatible with the report by Maguen and Machat,24 which attempted to eliminate steep central island formation using several strategies. They found that the best was to pretreat patients with an extra 1.0 ␮m/D in the central 3.0 mm zone. VISX has implemented a central island factor software program designed to compensate for the tendency of central island formation. Early results using this software, which simultaneously adds 1.5 ␮m/D of tissue removal within the central 2.5 mm optical zone, shows marked improvement in the reduction of steep central island formation, with only minimal hyperopia developing in a few cases.14 McGhee and Bryce25 also report a reduced prevalence of topographic islands in cases of PRK treated with VISX 4.01 software with a profile adjustment anti-island software modification. Although there were significant differences among the 3 laser modes, an inhomogeneous topographic pattern changed to a smoother topographic pattern over time in both low and high myopia groups in all 3 modes, which is compatible with results of previous reports.8,13–14 Mode 3 had the best predictability and efficacy results in both low and high myopia groups, followed by modes 1 and 2. This is compatible with topographic changes and implies that central island formation may

J CATARACT REFRACT SURG—VOL 26, JUNE 2000

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

play a role in postoperative undercorrection. Clinically significant undercorrections may also be caused by surgical system error (incorrect data entered into the laser’s computer or incorrect algorithm) or postoperative regression caused by epithelial hyperplasia and corneal wound healing. This may explain why myopia regression progressed despite gradual homogenization of topographic patterns over time. Partial loss of corneal clarity is common after PRK, and the severity of corneal haze is influenced by the amount of attempted correction, steep wound contours, and excess intraoperative stromal hydration.26 –29 The severity of haze after excimer laser keratectomy is also time dependent.4,30,31 We found that the severity of haze was mild in all groups. There was no significant difference among the 3 modes in the high or low myopia groups. This study was limited by different laser ablation zone sizes in each mode. It is well known that in addition to beam distribution profile and attempted correction, factors such as optical zone decentration, ablation zone size, and a multizone approach influence the refractive results after PRK.32–34 In our study, the optical zone was 6.0 mm in modes 1 and 2 and 6.5 mm in mode 3. The different surgical results in mode 3 may have been partially influenced by this factor. In conclusion, this study showed that excimer laser PRK is an effective procedure for the treatment of myopia. Different laser machines with different beam profiles may influence postoperative results. Excimer lasers with top-hat beam distribution may result in more central island formation than those with Gaussian beam distribution. With the addition of anti-central-island pretreatment software, the incidence of irregular topographic change can be reduced. In this study, the Summit Apex Plus laser with anti-central-island pretreatment profile provided the best results, followed by the Summit OmniMed; the Summit Apex Plus laser without anti-central-island pretreatment had less satisfactory results. Future studies are needed to determine the optimal dosages of anti-central-island pretreatment for each excimer laser.

3.

4.

5.

6.

7.

8.

9.

10.

11. 12.

13.

14.

15.

16.

17.

References 1. Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol 1983; 96:710 –715 2. Munnerlyn CR, Koons SJ, Marshall J. Photorefractive

18.

keratectomy: a technique for laser refractive surgery. J Cataract Refract Surg 1988; 14:46 –52 Seiler T, Wollensak J. Myopic photorefractive keratectomy with the excimer laser; one-year follow-up. Ophthalmology 1991; 98:1156 –1163 Piebenga LW, Matta CS, Deitz MR, et al. Excimer photorefractive keratectomy for myopia. Ophthalmology 1993; 100:1335–1345 Maguen E, Salz JJ, Nesburn AB, et al. Results of excimer laser photorefractive keratectomy for the correction of myopia. Ophthalmology 1994; 101:1548 –1556; discussion by MJ Mannis, 1556 –1557 Lin DTC, Sutton HF, Berman M. Corneal topography following excimer laser photorefractive keratectomy for myopia. J Cataract Refract Surg 1993; 19:149 –154 Parker PJ, Klyce SD, Ryan BL, et al. Central topographic islands following photorefractive keratectomy. ARVO abstract 509. Invest Ophthalmol Vis Sci 1993; 34(suppl): 803 Hu FR, Tan CY, Chang SW, Chang HW. Analysis of corneal topography after excimer laser photorefractive keratectomy. J Formos Med Assoc 1998; 97:159 –164 Levin S, Carson CA, Garrett SK, Taylor HR. Prevalence of central islands after excimer laser refractive surgery. J Cataract Refract Surg 1995; 21:21–26 Genth U, Schmidt-Petersen H, Wollensak J, Seiler T. Quantitative comparison of central islands during photoablation of the cornea performed with different laser types. ARVO abstract 2645-B490. Invest Ophthalmol Vis Sci 1996; 37:S574 Krueger RR. Steep central islands: have we finally figured them out? (editorial) J Refract Surg 1997; 13:215–218 Lin DTC. Corneal topographic analysis after excimer photorefractive keratectomy. Ophthalmology 1994; 101:1432–1439 Hersh PS, Schwartz-Goldstein BH. Corneal topography of Phase III excimer laser photorefractive keratectomy; characterization and clinical effects. Ophthalmology 1995; 102:963–978 Krueger RR, Saedy NF, McDonnell PJ. Clinical analysis of steep central islands after excimer laser photorefractive keratectomy. Arch Ophthalmol 1996; 114:377–381 Puliafito CA, Stern D, Krueger RR, Mandel ER. Highspeed photography of excimer laser ablation of the cornea. Arch Ophthalmol 1987; 105:1255–1259 Krueger RR, Krasinski JS, Radzewicz C, et al. Photography of shock waves during excimer laser ablation of the cornea; effect of helium gas on propagation velocity. Cornea 1993; 12:330 –334 Manstein D, Forster W, Schrenberg M, et al. Damage mechanisms for 193nm excimer-laser corneal ablation. ARVO abstract 3486 –5. Invest Ophthalmol Vis Sci 1994; 35:2012 Noack J, To¨nnies R, Hohla K, et al. Influence of ablation

J CATARACT REFRACT SURG—VOL 26, JUNE 2000

885

PRK RESULTS WITH DIFFERENT OPERATIVE MODES

19.

20.

21.

22.

23.

24.

25.

26.

886

plume dynamics on the formation of central islands in excimer laser photorefractive keratectomy. Ophthalmology 1997; 104:823– 830 Fo¨rster W, Clemens S, Bru¨ning S, et al. Steep central islands after myopic photorefractive keratectomy. J Cataract Refract Surg 1998; 24:899 –904 Campos M, Cuevas K, Garbus J, et al. Corneal wound healing after excimer laser ablation; effects of nitrogen gas blower. Ophthalmology 1992; 99:893– 897 Maguen E, Nesburn AB, Papaioannou T, et al. Effect of nitrogen flow on recovery of vision after excimer laser photorefractive keratectomy without nitrogen flow. J Refract Corneal Surg 1994; 10:321–326 Krueger RR, Campos M, Wang XW, et al. Corneal surface morphology following excimer laser ablation with humidified gases. Arch Ophthalmol 1993; 111:1131– 1137 Shimmick JK, Telfair WB, Munnerlyn CR, et al. Corneal ablation profilometry and steep central islands. J Refract Surg 1997; 13:235-245 Maguen E, Machat JJ. Complications of photorefractive keratectomy, primarily with the VISX excimer laser. In: Salz JJ, ed, Corneal Laser Surgery. St Louis, MO, CV Mosby Co, 1995; 143–158 McGhee CNJ, Bryce IG. Etiology and avoidance of steep central islands (letter). J Cataract Refract Surg 1999; 25: 302–304 Caubet E. Course of subepithelial corneal haze over 18 months after photorefractive keratectomy for myopia. Refract Corneal Surg 1993; 9(suppl):S65–S70; errata, 236

27. Ehlers N, Hjortdal JØ. Excimer laser refractive keratectomy for high myopia; 6-month follow-up of patients treated bilaterally. Acta Ophthalmol 1992; 70:578 –586 28. Kim JH, Hahn TW, Lee YC, et al. Photorefractive keratectomy in 202 myopic eyes: one year results. Refract Corneal Surg 1993; 9(suppl):S11–S16 29. Seiler T, Derse M, Pham T. Repeated excimer laser treatment after photorefractive keratectomy. Arch Ophthalmol 1992; 110:1230 –1233; errata, 1708 30. Gartry DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive keratectomy; 18-month follow-up. Ophthalmology 1992; 99:1209 –1219 31. Yu JH, Hu FR, Chang SW, Wang IJ. Excimer laser photorefractive keratectomy for myopia. J Formos Med Assoc 1996; 95:225–230 32. Heitzmann J, Binder PS, Kassar BS, Nordan LT. The correction of high myopia using the excimer laser. Arch Ophthalmol 1993; 111:1627–1634 33. O’Brart DPS, Gartry DS, Lohmann CP, et al. Excimer laser photorefractive keratectomy for myopia: comparison of 4.00- and 5.00-millimeter ablation zones. J Refract Corneal Surg 1994; 10:87–94 34. Rosa N, Gennamo G, Pasquariello A, et al. Refractive outcome and corneal topographic studies after photorefractive keratectomy with different-sized ablation zones. Ophthalmology 1996; 103:1130 –1138 From the Department of Ophthalmology, National Taiwan University Hospital, Taipei,Taiwan. None of the authors has a financial interest in any product mentioned.

J CATARACT REFRACT SURG—VOL 26, JUNE 2000