Thứ Sáu, 18 tháng 5, 2012

2012 PRK-LASEK


  • Journal of Refractive Surgery
  • January 2012 - Volume 28 · Issue 1: 65-71
  • DOI: 10.3928/1081597X-20111004-01

Abstract

PURPOSE: To evaluate visual outcomes following epi-LASIK compared to photorefractive keratectomy (PRK).
METHODS: Of a total 294 patients aged ≥21 years, 145 (290 eyes) underwent epi-LASIK and 149 (298 eyes) underwent PRK for low to moderate myopia or myopic astigmatism. Epi-LASIK was performed with the Amadeus II epikeratome (Abbott Medical Optics) and PRK with the Amoils rotary epithelial brush (Innovative Excimer Solutions). All ablations were performed using the same excimer laser system. Outcome measures included intraoperative complications, corneal reepithelialization, postoperative pain, uncorrected distance visual acuity (UDVA), manifest refraction spherical equivalent (MRSE), corrected distance visual acuity (CDVA), corneal haze, and quality of vision.
RESULTS: Mean preoperative MRSE was −2.97±1.19 diopters (D) for epi-LASIK versus −2.95±1.06 D for PRK. Complete reepithelialization was achieved by postoperative day 4 in 46.9% of epi-LASIK eyes versus 92.4% of PRK eyes, with superior UDVA at postoperative day 1 in the PRK group (P=.002). Using Fisher exact test, a significantly higher percentage of epi-LASIK eyes compared to PRK eyes achieved 20/15 or better at 1 month (25.8% vs 17.8%, P=.031), 3 months (62.3% vs 49.3%, P=.004), 6 months (77.1% vs 57.9%, P<.001), and 12 months (75.9% vs 61.9%, P=.002). A change in MRSE >0.50 D occurred in 8.4% of epi-LASIK eyes within the 3- and 12-month interval versus 17.7% of PRK eyes (P=.04). No differences were noted between the two procedures in CDVA or clinically significant haze.
CONCLUSIONS: Epi-LASIK showed superior refractive efficacy and stability but required more time for wound healing, resulting in inferior early visual outcomes and a tendency to overcorrect higher refractive errors compared to PRK. Both treatments were safe and comparable in terms of pain and haze formation.
From the Ophthalmology Service, Walter Reed Army Medical Center, Washington, DC (Sia, Coe, Ryan); the Department of Ophthalmology, University of Florida College of Medicine, Jacksonville, Florida (Edwards); and The Wilmer Eye Institute, Johns Hopkins University, Baltimore, Maryland (Bower).
Portions of this material were presented at the Association for Research in Vision and Ophthalmology Annual Meeting, April 27–May 1, 2008, and May 3–7, 2009, Ft Lauderdale, Florida.
The authors have no financial interest in the materials presented herein.
The opinions expressed in this manuscript are those solely of the authors and do not represent the views or official policies of the United States Army or Department of Defense.
AUTHOR CONTRIBUTIONS
Study concept and design (C.D.C., D.S.R., K.S.B.); data collection (C.D.C., D.S.R.); analysis and interpretation of data (R.K.S., C.D.C., J.D.E., D.S.R., K.S.B.); drafting of the manuscript (R.K.S., C.D.C., J.D.E., K.S.B.); critical revision of the manuscript (R.K.S., C.D.C., D.S.R., K.S.B.); statistical expertise (C.D.C.); administrative, technical, or material support (C.D.C.); supervision (C.D.C., D.S.R., K.S.B.)
Correspondence: Rose K. Sia, MD, Center for Refractive Surgery, Walter Reed Army Medical Center, 6900 Georgia Ave, Washington, DC. Tel: 202.782.8327; Fax: 202.782.4653
Received: January 04, 2011
Accepted: August 08, 2011
Posted Online: October 10, 2011
Although LASIK is the most common refractive surgery, surface ablation may be indicated in patients with thin corneas, dry eyes, anterior basement membrane dystrophy, and in patients at risk for trauma due to occupational or recreational considerations.1–3 However, reepithelialization typically takes 3 to 5 days, during which time patients frequently experience discomfort and fluctuating vision. In addition, photorefractive keratectomy (PRK) can result in corneal haze in up to 2% to 4% of patients.4,5
Alternative surface techniques have been developed to overcome postoperative PRK pain and haze. Epi-LASIK uses a microkeratome-like device to mechanically separate the epithelium from Bowman layer. The cell morphology and physiology of the resulting epithelial sheet is less affected compared to the chemical separation used in laser epithelial keratomileusis (LASEK)6,7 and remains intact for at least 24 hours postoperatively,7 acting as a mechanical barrier to tear cytokine mediators8,9 and controlling the corneal wound response. Preliminary studies have shown epi-LASIK to be a safe and effective treatment in the short term.10–13 This study evaluates the visual outcomes of epi-LASIK compared to PRK up to 12 months postoperatively.

Patients and Methods

The present report compares data from two separate non-randomized, prospective, clinical studies performed at Walter Reed Army Medical Center after approval by the institutional review board and conducted in accordance with the Declaration of Helsinki. After informed consent, consecutive enrollment and treatment were done between May 2005 and July 2006 for PRK and between November 2006 and December 2007 for epi-LASIK. Enrollment criteria and all study methods pre- and postoperatively were identical between the two study cohorts. All patients were aged ≥21 years with myopia or myopic astigmatism between −0.50 and −6.25 diopters (D) of manifest refraction spherical equivalent (MRSE), refractive cylinder up to 3.00 D, stable refraction for at least 12 months, and corrected distance visual acuity (CDVA) of 20/20 or better in both eyes.
All treatments were performed with the LADAR Vision 6000 (Alcon Laboratories Inc, Ft Worth, Texas). Treatments were conventional with a 6.5-mm optical zone; no wavefront-guided treatments were performed. Stromal ablation in PRK was performed after epithelial debridement with the Amoils rotary brush (Innovative Excimer Solutions, Toronto, Ontario). The stromal bed was irrigated with chilled balanced saline solution (BSS), moxifloxacin 0.5% ophthalmic solution and ketorolac 0.4% were administered, and a bandage contact lens applied. In epi-LASIK, a nasal-hinged epithelial flap was created using the Amadeus II epikeratome (Abbott Medical Optics, Santa Ana, California), using manufacturer-recommended epikeratome parameters. After the flap was reflected nasally with use of a microspatula, photoablation was performed on the underlying stroma. During flap creation, if a complication occurred in the first eye (right eye), a new epithelial separator was used for the second eye. Immediately following ablation, the stromal bed was irrigated with chilled BSS before repositioning the epithelial sheet, which was retained whenever possible. After the epithelial sheet was in position and adherent moxifloxacin 0.5% and ketorolac 0.4% were administered, a bandage contact lens was applied.
Postoperative medications for both groups included topical moxifloxacin 0.5% one drop four times daily for 1 week or until complete reepithelialization; fluorometholone 0.1% one drop four times daily for 4 weeks, followed by a 6-week taper; carboxymethylcellulose 0.5% one drop four to eight times daily for 2 weeks and then as needed; topical ketorolac 0.4% up to four times daily for the first 48 hours after surgery as needed; and oxycodone/acetaminophen 5 mg/325 mg orally as needed for postoperative pain. The bandage contact lens was removed on postoperative day 4 if there was no epithelial defect or replaced as needed until complete reepithelialization. Pain, corneal reepithelialization, uncorrected distance visual acuity (UDVA), and any complications such as corneal infiltrates were assessed on postoperative days 1, 4, and 7. Pain was assessed using a five-point scale10: 0 (none), 1 (minimal), 2 (mild), 3 (moderate), and 4 (severe/worst possible pain).
Uncorrected distance visual acuity, CDVA, MRSE, wavefront aberrometry, patient satisfaction questionnaire, 5% and 25% low contrast visual acuity, and slit-lamp microscopy were assessed at 1, 3, 6, and 12 months postoperatively. The questionnaire focused on visual difficulties and the general satisfaction of postoperative vision using a 10-point scale ranging from 1 (no symptoms) to 10 (severe, disabling symptoms).14 Low contrast visual acuity was performed using back illuminated logMAR charts with 5% and 25% contrast (Precision Vision Inc, La Salle, Illinois). Corneal haze was graded on a standard 5-point scale: 0 (completely clear), 1+ (trace), 2+ (mild), 3+ (moderate), and 4+ (severe).15
Aberrometry was performed using the LADARWave wavefront analyzer (Alcon Laboratories Inc) at 850 nm, and the wavefront error at 550 nm was estimated using the instrument’s proprietary chromatic aberration correction factor. Monochromatic aberrations were calculated using pupil diameters of 3 and 6 mm and a Zernike polynomial series up to and including 4th order was calculated for each measurement. For analysis, the absolute root-mean-square (RMS) wavefront error of each Zernike mode was determined and the absolute level of higher order aberrations calculated.
Visual outcomes of epi-LASIK and PRK were compared. Epi-LASIK eyes that had intraoperative complications resulting in flap amputation were excluded from comparative analysis. SPSS software version 16.0 (SPSS Inc, Chicago, Illinois) was used for statistical analysis. Fisher exact test was performed to compare visual outcomes, reepithelialization, and presence of clinically significant haze between epi-LASIK and PRK. The Mann-Whitney test was used to compare early postoperative UDVA, pain scores, and questionnaire data. Odds ratios (OR) were used to determine likelihood of flap complications. P<.05 was considered statistically significant for all testing.

Results

A total of 294 patients, mean age 34.2±7.8 years (range: 21 to 52 years), were enrolled; 145 were consecutively assigned to epi-LASIK and 149 to PRK. Age, gender, preoperative MRSE, UDVA, 25% low contrast visual acuity, central corneal thickness, and keratometry were comparable between groups (Table 1).

Preoperative Demographics and Clinical Characteristics of Eyes That Underwent Epi-LASIK and PRK for Low to Moderate Myopia
Table 1: Preoperative Demographics and Clinical Characteristics of Eyes That Underwent Epi-LASIK and PRK for Low to Moderate Myopia

Intraoperative Complications

A summary of intraoperative epi-LASIK complications is shown in Table 2. Overall, a flap complication occurred in 102 (35%) of 290 epi-LASIK eyes, 57 of which resulted in flap amputation. A relative increase in rate of complications was noted for age ≤34 years and the right eye, but only age was statistically significant (P=.02). Odds ratios are shown in Table 3. No intraoperative complications occurred in the PRK group.

Intraoperative Flap Complications That Occurred During Epi-LASIK
Table 2: Intraoperative Flap Complications That Occurred During Epi-LASIK

Preoperative Findings and Risk of Intraoperative Epithelial Flap Complications in Epi-LASIK
Table 3: Preoperative Findings and Risk of Intraoperative Epithelial Flap Complications in Epi-LASIK

Follow-Up Rate

Follow-up availability was comparable between the groups at 1 and 3 months. Significantly fewer epi-LASIK (82.8%) patients returned for 6-month follow-up compared with PRK patients (89.3%, P=.024), whereas significantly more epi-LASIK (86.9%) than PRK patients (75.8%, P=.001) were seen at 12 months. No significant differences were noted in age, gender, baseline MRSE, UDVA, CDVA, or 5% and 25% low contrast visual acuity between initial patients and those seen at each time point studied.

Pain, Epithelial Healing, and Early Visual Recovery

Few patients reported significant pain on postoperative days 1, 4, or 7. Of the 298 eyes treated with PRK and 233 eyes treated with epi-LASIK, reepithelialization was significantly faster in PRK eyes compared with epi-LASIK eyes by postoperative day 4 (92.4% vs 46.9%, P<.001) and postoperative day 7 (99.3% vs 86.3%, P<.001). Five epi-LASIK eyes required an additional visit on postoperative day 10. Epi-LASIK was associated with significantly worse UDVA at postoperative day 1 (P=.002); however, no other significant difference was noted in UDVA in the early postoperative period.

Visual Outcomes

Visual outcomes of eyes treated with epi-LASIK (n=233) and PRK (n=298) were compared.
Safety. Safety indices progressively improved over time for both groups (Table 4). No eye lost ≥2 lines of CDVA in either group at 12 months. Distribution of CDVA line changes in epi-LASIK and PRK groups 12 months postoperatively are shown in Figure A.

Safety and Efficacy Indices of Epi-LASIK and PRK
Table 4: Safety and Efficacy Indices of Epi-LASIK and PRK

Twelve-month outcomes of epi-LASIK and PRK. A) Change in corrected distance visual acuity. B) Uncorrected distance visual acuity. C) PRK spherical equivalent attempted vs achieved. D) Epi-LASIK spherical equivalent attempted vs achieved. E) Spherical equivalent refractive accuracy. F) Stability of spherical equivalent refraction.
Figure. Twelve-month outcomes of epi-LASIK and PRK. A) Change in corrected distance visual acuity. B) Uncorrected distance visual acuity. C) PRK spherical equivalent attempted vs achieved. D) Epi-LASIK spherical equivalent attempted vs achieved. E) Spherical equivalent refractive accuracy. F) Stability of spherical equivalent refraction.
Efficacy. Efficacy index of epi-LASIK was consistently higher compared to PRK. A statistically significant difference was noted in the number of eyes achieving UDVA of 20/20 or better at 3 months, favoring epi-LASIK (95.5% vs 86.9%, P=.001), but not at any other time point. A significantly higher percentage of epi-LASIK eyes compared to PRK eyes achieved 20/15 or better at 1 month (25.8% vs 17.8%, P=.031), 3 months (62.3% vs 49.3%, P=.004), 6 months (77.1% vs 57.9%, P<.001), and 12 months postoperatively (75.9% vs 61.9%, P=.002) (Fig B).
Predictability. The number of eyes within ±0.50 D of emmetropia was comparable between epi-LASIK and PRK at 1 month (69.5% vs 68.5%, P=.85), 3 months (86.3% vs 81.5%, P=.157), and 6 months (92.3% vs 89.2%, P=.295) but significantly lower for epi-LASIK than PRK at 12 months postoperatively (86.2% vs 92.5%, P=.04). Figures C and D illustrate scatterplots of 12-month attempted versus achieved MRSE. A strong correlation was noted between attempted and achieved MRSE in both groups, but with higher refractive errors, epi-LASIK tends to overcorrect whereas PRK tends to undercorrect. The distribution of final spherical equivalent refraction is shown in Figure E.
Stability. Between 3 and 12 months postoperatively, 17 (8.4%) epi-LASIK eyes compared to 40 (17.7%) PRK eyes had >0.50-D change in refractive spherical equivalent (P=.004). Mean MRSE at each time point is depicted in Figure F.

Quality of Vision

Subjective Visual Performance and Patient Satisfaction. Subjective optical quality was the same between the two procedures at all time points for vision fluctuations, double vision, glare, light sensitivity, halos, starbursts, patient satisfaction, postoperative vision quality, and the chance to have the procedure again. However, epi-LASIK patients reported more frequent artificial tear use at 3 (P<.001) and 6 (P<.001) months and slightly greater difficulty in their daily activities when compared to PRK patients (P=.012).
Low Contrast Visual Acuity. Both 5% and 25% low contrast visual acuity results were comparable between treatment groups at all time points except at 12 months postoperatively; 5% low contrast visual acuity was maintained or improved in 94% of epi-LASIK versus 78.6% of PRK eyes (P<.001) whereas 25% low contrast visual acuity was unchanged or better than preoperative in 87% of epi-LASIK versus 75.8% of PRK eyes (P=.024).
Higher Order Aberrations and Low Contrast Vision. Correlation analysis showed that for 3- and 6-mm artificial pupils, epi-LASIK demonstrated a significant but weak association between increasing higher order aberrations and diminished low contrast visual acuity (3-mm pupil: 25% low contrast visual acuity: r=0.16, P=.019; 6-mm pupil: 5% low contrast visual acuity: r=0.17, P=.015; 25% low contrast visual acuity: r=0.25, P<.001) at 12 months postoperatively. No significant correlation was noted for 3-mm pupils at 5% low contrast visual acuity.
Optical Quality. Mean absolute higher order aberrations increased postoperatively in PRK and epi-LASIK eyes. For a 3-mm artificial pupil, epi-LASIK eyes increased from 0.068±0.029 μm to 0.095±0.036 μm (P<.001) and PRK eyes increased from 0.075±0.10 μm to 0.089±0.043 μm (P=.03) postoperatively. For a 6-mm artificial pupil, epi-LASIK eyes increased from 0.66±0.28 μm to 0.99±0.39 μm (P<.001) and PRK eyes increased from 0.72±0.28 μm to 0.98±0.46 μm (P<.001) postoperatively. However, no significant difference was noted between epi-LASIK and PRK in postoperative optical quality for either a 3-mm pupil (P=.14) or 6-mm pupil (P=.61).

Corneal Haze

Clinically significant haze was observed in two epi-LASIK eyes at 1 month, one eye at 3 months, and one eye at 6 months postoperatively; each case progressively improved and resolved. No PRK eye developed clinically significant corneal haze at any time.

Visual Outcomes After Flap Complication

Two hundred ninety eyes planned to undergo epi-LASIK were subgrouped by whether a quality flap was made (n=188), the flap was removed by automatic amputation or surgeon amputation (n=57), or a complication occurred where the flap was retained (n=45). Measurements of safety, efficacy, predictability, and stability were found to be statistically comparable among the subgroups.
Stromal Incursion. Six (2.1%) of 290 epi-LASIK eyes had inadvertent stromal incursion of the epikeratome during epithelial separation. All treatments were completed during the same session as planned. Mitomycin C (MMC) was not used in any case. Postoperatively, complete reepithelialization was noted in 4 of 6 eyes at postoperative day 4, 1 eye at postoperative day 7, and 1 eye at postoperative day 10. One eye had clinically significant haze at 1 month postoperatively, which subsequently improved with topical steroids. Although midperipheral mild linear corneal scarring was observed in 2 eyes at 3 months postoperatively, no eye lost ≥2 lines of CDVA at any time point. No irregular astigmatism was detected in any of these patients. All eyes had UDVA 20/20 or better starting at 3 months postoperatively.

Discussion

Previous studies of epi-LASIK have shown promising results, with comparisons being made to off-flap versus on-flap epi-LASIK procedures as well as with LASEK procedures.13,16,17 Our study is different from previous studies as the comparison of epi-LASIK to PRK uses a rotating brush for epithelial removal in the PRK eyes rather than an epikeratome or blunt blade. Early postoperative results showed the two procedures to be equivalent when comparing pain and vision, except for postoperative day 1 when PRK eyes had significantly better UDVA. This is likely due to an edematous and hazy epithelial sheet present in the epi-LASIK eyes. Epi-LASIK eyes were also significantly slower to achieve reepithelialization than PRK eyes. This is consistent with results reported by Torres et al,18 with epi-LASIK requiring 4.75±1.44 days and PRK eyes requiring 3.95±1.39 days. In contrast, O’Doherty et al16 reported 72±24 hours for epithelial closure in epi-LASIK eyes, whereas PRK eyes took 96±24 hours. This discrepancy may be a result of different techniques of epithelial removal in the PRK eyes, as Torres et al18 used a beaver blade whereas O’Doherty et al16 used an epikeratome.
In the present study, epi-LASIK and PRK were found to be safe procedures but greater refractive stability and efficacy were seen in epi-LASIK than PRK. Although a strong correlation between attempted and achieved refraction was found in both treatments, epi-LASIK seemed to have a tendency to overcorrect higher refractive errors. This observation was similar to the initial results reported in LASEK,19,20 which were attributed to the PRK nomogram-based treatment and the slower but modulated wound-healing response probably due to the epithelial sheet acting as barrier between tear cytokines and the ablated stroma.20,21 In contrast, myopic regression and undercorrection tendencies in PRK are likely due to a more intense wound-healing response as more tissues are removed, possibly as a consequence of basement membrane disruption.7,21 Objective image quality in epi-LASIK seemed to be superior over PRK in terms of retaining or improving low contrast visual acuity but comparable to PRK in terms of absolute level of higher order aberrations. Epi-LASIK patients reported experiencing greater difficulty in performing daily activities but this may not be significantly different from PRK given that the mean score for epi-LASIK patients was 1.5 (±1.0) and PRK was 1.36 (±0.89).
Corneal clarity as measured by clinically significant haze was not detected in PRK eyes but was observed in some epi-LASIK eyes. Kalyvianaki et al17 reported similar results, with more on-flap epi-LASIK eyes developing haze at 1 and 3 months than off-flap epi-LASIK eyes. The low level of haze is likely a result of mild to moderate levels of myopia treated in the study.
In our study, 35% of epi-LASIK eyes had intraoperative complications, with 20% resulting in amputated flaps. This is similar to previous reports, including O’Doherty et al16 who reported 33% of eyes resulted in the failed creation of a flap and were converted to PRK. Our data suggest a higher rate of flap complication occurs in younger patients (≤34 years), which could be due to a more adherent epithelium in younger patients.22 Results also suggested complications were more likely to occur in the right eye than the left eye, but this observation is likely biased because the right eye was treated first in all cases. A new epithelial separator was used for the left eye every time a flap complication occurred in the right eye.
Although the study was not designed to compare onflap epi-LASIK versus off-flap techniques, our analysis showed retaining or removing the flap following an unsuccessful creation did not appear to affect the overall visual outcomes, which is consistent with the comparative study by Kalyvianaki et al.17
Stromal incursion occurred in 2.1% of eyes treated with epi-LASIK in our study, which is comparable to a previously reported 2.99% rate.12 Despite the small number and insignificant effect on visual outcomes found in our study, we cannot assume that this complication could not be potentially sight-threatening. Two eyes in our study developed midperipheral mild corneal scarring, which could have an adverse effect on their vision if not managed carefully. Widely used in refractive surgery to prevent postoperative haze and scarring, MMC21 was used by Katsanevaki et al12 following stromal incursion whereas no eyes in our study were treated with MMC prophylactically.
In conclusion, epi-LASIK required more time for reepithelialization, had inferior early visual acuity, a tendency to overcorrect higher refractive errors, and a significant number of amputated flaps necessitating conversion to PRK. However, when successful, epi-LASIK showed superior refractive efficacy and stability compared to PRK. Both treatments were comparable in terms of safety and haze formation. Despite being nonrandomized, the prospective design, large sample size, identical study criteria and methods, as well as good follow-up could provide considerable strength for the conclusion of this study. Because this study was limited to myopia up to −6.00 D, the results cannot be extrapolated to higher degrees of myopia or to hyperopic or mixed astigmatism.

References

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Preoperative Demographics and Clinical Characteristics of Eyes That Underwent Epi-LASIK and PRK for Low to Moderate Myopia
Epi-LASIK PRK PValue*
No. of patients (eyes) 145 (290) 149 (298) —
Female/male (%) 54/91 (62.8) 48/101 (67.8) .225
Age (y) 34.1 (21 to 49) 34.8 (21 to 52) .807
Sphere (D) −2.68±1.18 (−0.50 to −6.25) −2.65±1.04 (−0.25 to −5.75) .716
Cylinder (D) 0.56±0.51 (0.00 to 2.50) 0.59±0.56 (0.00 to 2.75) .588
MRSE (D) −2.97±1.19 (−1.00 to −6.25) −2.95±1.06 (−1.00 to −6.00) .819
UDVA (logMAR [Snellen]) 1.00 (20/200)±0.37 1.00 (20/200)±0.37 .678
CDVA (logMAR [Snellen]) −0.11 (20/16)±0.43 −0.10 (20/16)±0.48 <.001
5% LCVA (logMAR [Snellen]) 0.30 (20/40)±0.11 0.33 (20/43)±0.11 .001
25% LCVA (logMAR [Snellen]) 0.31 (20/41)±0.08 0.36 (20/46)±0.11 <.001
CCT (μm) 543±35 540±34 .258
Steep K (D) 44.39±1.58 44.36±1.46 .825
Flat K (D) 43.35±2.76 43.55±1.44 .257
Intraoperative Flap Complications That Occurred During Epi-LASIK
Complication No. of Eyes (%)
None 188 (64.8)
Automatic flap amputation 43 (14.8)
Buttonhole flap 17 (5.9)
Hinge tear 14 (4.8)
Surgeon amputation (hinge in treatment zone) 14 (4.8)
Incomplete flap/superficial resection 8 (2.8)
Stromal incursion 6 (2.1)
Total 290 (100.0)
Preoperative Findings and Risk of Intraoperative Epithelial Flap Complications in Epi-LASIK
Preop Finding Odds Ratio (95% CI) PValue*
Age ≤34 years 1.69 (1.04–2.76) .02
Right eye 1.63 (1.00–2.65) .07
Cylinder <1.00 D 1.26 (0.61–2.60) .59
CCT ≤500 μm 1.23 (0.58–2.58) .70
Flat K <42.00 D 1.21 (0.62–2.36) .61
Steep K >46.00 D 0.57 (0.26–1.26) .16
Safety and Efficacy Indices of Epi-LASIK and PRK
Follow-up (mo) Procedure Safety Index Efficacy Index
1 Epi-LASIK 0.61 −0.16
PRK 0.72 0.39
3 Epi-LASIK 1.09 0.60
PRK 1.11 0.36
6 Epi-LASIK 1.33 0.78
PRK 1.18 0.59
12 Epi-LASIK 1.33 0.85
PRK 1.29 0.67
AUTHORS
From the Ophthalmology Service, Walter Reed Army Medical Center, Washington, DC (Sia, Coe, Ryan); the Department of Ophthalmology, University of Florida College of Medicine, Jacksonville, Florida (Edwards); and The Wilmer Eye Institute, Johns Hopkins University, Baltimore, Maryland (Bower).
Portions of this material were presented at the Association for Research in Vision and Ophthalmology Annual Meeting, April 27–May 1, 2008, and May 3–7, 2009, Ft Lauderdale, Florida.
The authors have no financial interest in the materials presented herein.
The opinions expressed in this manuscript are those solely of the authors and do not represent the views or official policies of the United States Army or Department of Defense.
Correspondence: Rose K. Sia, MD, Center for Refractive Surgery, Walter Reed Army Medical Center, 6900 Georgia Ave, Washington, DC. Tel: 202.782.8327; Fax: 202.782.4653

Received: January 04, 2011
Accepted: August 08, 2011
Posted Online: October 10, 2011
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