Mostafa A. El-Husseiny, Olfat A. Hassanin, Iman M. Eissa and Mohamed Yasser Farag

COMPARISON OF LASER IN SITU KERATOMILEUSIS (LASIK) AND PHOTOREFRACTIVE KERATECTOMY (PRK) IN CORRECTING LOW TO MODERATE MYOPIA WITH OR WITHOUT ASTIGMATISM Mostafa A. El-Husseiny, Olfat A. Hassanin, Iman M. Eissa and Mohamed Yasser Farag Cornea and Refractive Department, Research Institute of Ophthalmology (RIO), Giza, Egypt. Department of Ophthalmology, Faculty of Medicine, Cairo University, Egypt. Volume-4 Issue-4 May-2014 Page 1

Abstract: Purpose: To compare refractive and visual outcomes in patients treated with Photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) in cases of mild to moderate myopia with or without astigmatism in a prospective randomized clinical trial. Setting: Research Institute of Ophthalmology (RIO) and RCC refractive center, Giza, Egypt. Methods: Refractive and visual acuity results in a cohort of 80 eyes of 40 consecutive patients were randomized to receive either PRK or LASIK by the first two surgeons. All patients had mild to moderate myopia (spherical equivalent [SE] ranging from -2.5 to -6.5 diopters [D]) with or without astigmatism. Patients were evaluated at 1 day, 1 week, and at 1, 3, 6, and 12 months post operatively. Results: At 1 month, uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), induction of higher-order aberrations (HOAs), and subjective symptoms of visual clarity, vision fluctuation, ghosting, and overall self-assessment of vision were worse in the PRK group. By 3 months, most of these differences had resolved except for trefoil aberrations which remained worse in the PRK group until at 12 months post operatively. At 12 months follow up, the mean spherical equivalent was reduced to ± 0.5 D in 97% of cases in the LASIK group and 96% of cases in the PRK group and reduced to ± 1D in 100% of cases in the LASIK group and 99 % of cases in the PRK group. Refractive stability was achieved in the LASIK group after one month and in the PRK group at 3 months. There were no intraoperative or postoperative flap complications in the LASIK group. There was no incidence of corneal haze in the PRK group. Conclusion: Both PRK and LASIK seem to be equally effective refractive surgical options to treat mild to moderate myopia, however, LASIK offers faster symptom recovery, an initially lower incidence of high order aberrations and faster refractive stability. No statistically significant differences were found in visual and/or refractive outcome after 12 months follow up with both procedures. Keywords: Simple myopia, PRK, LASIK, refractive stability, moderate myopia. INTRODUCTION Two modalities of excimer-laser-based photorefractive surgery are now enjoying widespread use: photorefractive keratectomy (PRK) and laser in situ keratomeilusis (LASIK). Each can be performed as a spherical or astigmatic treatment. Although excellent data exist on the efficacy and safety of each modality, there is a paucity of literature comparing the 2 procedures performed by one surgeon in the treatment of myopia and myopic astigmatism. As the choice of a surgical procedure is decided by the patient with the advice of the surgeon, it is imperative to understand the similarities and differences in outcomes between these procedures to provide informed counseling (Knorz et al., 1996; Fiander and Tayfour, 1995; Stephenson et al. 1998; Hersh, 1997; Yu et al. 1996; Goes, 1996; Hamberg-Nystro m et al., 1996 and Waring et al., 1995). Several prospective multi surgeon studies compare spherical zone LASIK and PRK. Hersh et al. (1998) reported generally similar refractive and visual results with comparable safety between these 2 methods in patients with 6.0 to 14.0 diopters (D) of myopia. El-Maghraby et Volume-4 Issue-4 May-2014 Page 2

al. (1999) and El Danasoury and coauthors (1999) also report similar final visual acuity and refractive outcomes with LASIK and PRK. These 3 studies support several general conclusions. Laser in situ keratomileusis offers more rapid visual recovery than PRK, although patients having either procedure generally demonstrate similar ultimate refractive and visual outcomes. Both procedures are comparably predictable, although in another study LASIK tended to produce a greater myopic result than PRK and PRK showed a tendency toward a less stable refractive result (Steinert and Hersh 1998). Based on the results of a study done by Russell et al. (2012), several conclusions were drawn for patient counseling which recommended that with low myopia, both LASIK and PRK yield a comparable visual and refractive outcome, although the recovery time for BCVA would be faster in flap-based procedures. There does not appear to be an increased risk of complication causing loss of BCVA with a flap-based procedure. Patients should be advised that there may be a small increase in astigmatism when having spherical PRK. With higher myopia (higher than 6.0 D), both visual and refractive outcomes appear to be superior in patients having a flap-based procedure. Astigmatic corrections offer no increased risk of poor visual outcome and are highly effective at reducing cylinder in most cases. In another prospective study done by Van De Pol et al. (2007) evaluation of 20 military Black Hawk pilots pre-surgically and at 1week, 1 month, and 6 months post-surgery was conducted to assess both PRK and LASIK visual and flight performance outcomes on the return of aviators to duty. Of 20 pilots, 19 returned to flight status at 1 month after surgery; 1PRK subject was delayed due to corneal haze and subjective visual symptoms. Improvements were seen under simulator night and night vision goggle flight after LASIK; no significant changes in flight performance were measured in the aircraft. Results indicated a significantly faster recovery of all visual performance outcomes 1 week after LASIK versus PRK, with no difference between both procedure outcomes at 1 and 6 months. Low contrast acuity and contrast sensitivity only weakly correlated to flight performance in the early post-operative period. Overall flight performance assessed in this study after PRK and LASIK was stable or improved from baseline, indicating a resilience of performance despite measured decrements in visual performance, especially in PRK. The study showed that contrast tests are more sensitive to the effects of refractive surgical intervention and may prove to be a better indicator of visual recovery for return to flight status. The present study, comparison between PRK and LASIK was done as regards refractive outcome and stability, overall patient satisfaction, presence and/or persistence of high order aberrations, best corrected visual acuity (BCVA) and the presence or absence of complications between two groups of patients (20 patients each) at 1,3,6 and 12 months postoperatively. PATIENTS AND METHODS Eighty eyes of 40 patients with myopia with or without astigmatism were randomly divided into two equal groups which received either PRK or LASIK. The first group (40 eyes of 20 patients) had undergone LASIK and the second group has undergone PRK with the Allegretto Wave Light Eye-Q 1010 (400 Hertz) with active eye tracker (400 Hertz) excimer laser (Alcon surgical, Novartis, USA), Inclusion criteria included a stable refraction with a change of less than Volume-4 Issue-4 May-2014 Page 3

0.25 diopters (D) of sphere or cylinder in the last year, discontinuation of soft contact lens wear at least 14 days prior to preoperative evaluation, best-corrected visual acuity of 20/20 or better, age 21or more, and the willingness to participate in follow-up examinations for at least 12 months following refractive surgery. Patients were excluded for the use of rigid gas permeable contact lenses, severe dry eye or chronic blepharitis, corneal pathology (recurrent corneal erosion, keratoconus, irregular corneal mires on central keratometry), corneal pachymetry of 500 microns or less or that LASIK procedure would result in less than 300 microns of remaining posterior corneal thickness below the flap postoperatively, baseline standard manifest refraction with a difference of 0.75 D or more in sphere power or 0.50 D in cylinder power as compared to the baseline standard cycloplegic refraction, history of herpes zoster or herpes simplex keratitis, corneal warpage, and certain systemic diseases or conditions (connective tissue disease, diabetes, pregnancy, lactation, immune-compromised state and severe atopy). After being informed about the nature of the study before their participation, all patients gave an informed consent in accordance with the guidelines in the Declaration of Helsinki. Patients who met the preceding criteria underwent a comprehensive preoperative evaluation, including history and examination with slit-lamp biomicroscopy, Goldmann applanation tonometry, detailed dilated fundus examination, manifest and cycloplegic refraction, and computerized corneal topography and pachymetry (Pentacam rotating Scheimpflug camera Oculus, Wetzlar, Germany). Patients also had their best-corrected visual acuity (BCVA) measured. Patients completed a questionnaire detailing subjective symptoms, which included a quantitative grading on a scale of 0 (no symptoms) to 10 (severe symptoms) for each of the following symptoms: glare under night and day conditions, haze, halos, clarity under night and day conditions, dry eye symptoms, gritty or scratchy sensation, vision fluctuation, and ghosting. Patients were also asked to grade their overall vision. The questionnaire used in this study has been employed and validated in previous refractive surgery eye studies (Golas & Manche, 2011 and Chan & Manche 2011). The questionnaire was administered preoperatively and postoperatively at 1, 3, 6, and 12 months. The LASIK and PRK surgeries were performed in a bilateral simultaneous fashion. Therefore, any learning curve would be negated because of the simultaneous fashion of the surgery and completion of the questionnaire during the same session. Patient questionnaire 1. People have difference experiences with their vision. Some people have problems with glare or light sensitivity. Please indicate whether you now that is, within the last two weeks have problems with glare or light sensitivity in the following situations. On a scale of 0 to 10 where 0 stands for no glare and 10 stands for disabling glare, how much trouble do you have with glare? Glare No glare Disabling glare At night 0.1.2.3.4.5.6. 7.8..9.10 Volume-4 Issue-4 May-2014 Page 4

2. People have difference experiences with their vision. Some people have problems with hazy or foggy vision. Please indicate whether you now that is, within the last two weeks have problems with hazy or foggy vision. On a scale of 0 to 10 where 0 stands for no haze and 10 stands for disabling haze, how much trouble do you have with haze? Haze No haze Disabling haze 0.1.2.3.4.5.6. 7.8..9.10 3. People have difference experiences with their vision. Some people have problems with halos, rings or star-bursts around objects or lights. Please indicate whether you now that is, within the last two weeks have problems with, halos, rings or starbursts. On a scale of 0 to 10 where 0 stands for no halos and 10 stands for disabling halos, how much trouble do you have with halos? Halos No halos 0.1.2.3.4.5.6. Disabling halos 7.8..9.10 4. People have difference experiences with their vision. Some people have problems with sharpness or clarity. Please indicate whether you now that is, within the last two weeks have problems with sharpness or clarity. On a scale of 0 to 10 where 0 stands for no problems and 10 stands for disabling problems, how much trouble do you have with sharpness or clarity? Clarity No problem Disabling problem At night 0.1.2.3.4.5.6. 7.8..9.10 During the day 0.1.2.3.4.5.6. 7.8..9.10 The next set of questions asks you to describe your vision as you go about your daily activities, both when you are at work and when you are not at work. Please enter your scores in the boxes provided for each eye. 5. My vision is excellent Strongly agree Strongly disagree 0.1.2.3.4.5.6. 7.8..9.10 6. Do you have problems with dry eyes? No problem Disabling problem 7. If you do have problems with dry eyes, how severe is this problem? 0.1.2.3.4.5.6. No problem 0.1.2.3.4.5.6. 7.8..9.10 Disabling problem 7.8..9.10 8. Do you have problems with gritty, scratchy None of the time All of the time Volume-4 Issue-4 May-2014 Page 5

or sandy feelings in your eyes? 0.1.2.3.4.5.6. 7.8..9.10 9. Do you have problems with your vision fluctuating over the day? 10. Do you have difficulty because of double vision or ghost images? None of the time 0.1.2.3.4.5.6. No difficulty 0.1.2.3.4.5.6. All of the time 7.8..9.10 Extreme difficulty 7.8..9.10 Questionnaire completed. Surgical procedure: All surgeries were performed at Research Institute of Ophthalmology (RIO) and Rowad correction center (RCC), Giza, Egypt. Topical proparacaine hydrochloride 0.5% (Benox; Epico, 10 th of Ramadan city, Egypt) and moxifloxacin hydrochloride ophthalmic solution 0.5% (Vigamox; Alcon, Fort Worth, Texas) were administered immediately before the procedure in all eyes. Both procedures started by proper sterilization with Povidone-Iodine 10% (Betadine) followed by application of speculum. For PRK, the epithelium was removed mechanically using epithelial scrubber to create a central 8.0-mm zone centered over the pupil. Proper drying of the bed was performed. Photoablation was achieved using the Allegretto Wave Light Eye-Q 1010 with active eye tracker (400 Hertz) excimer laser system. This is followed by proper wash by balanced salt solution No mitomycin C was used in any of the cases. For eyes undergoing PRK, a bandage contact lens (Acuvue Oasys; Johnson & Johnson Vision Care, USA) was placed for 4 to 5 days until complete epithelial regeneration. Postoperative medications included topical moxifloxacin hydrochloride 0.5% (Vigamox; Alcon, Fort Worth, Texas), nepafenac ophthalmic suspension 0.1% (Nevanac; Alcon, Fort Worth, Texas) and Sodium Hyaluronate 2mg (HyFresh; Jamjoom, Jeddah, Saudi Arabia) 4 times a day until the epithelium had healed then Nevanac eye drops are replaced by fluorometholone ophthalmic solution 0.1 % (FML; Allergan, Irvine, California ) four times a day for 2 weeks and then two times a day for another 2 weeks only and HyFresh eye drops continue for 3 months.. For LASIK, flaps were created using the Moria 2. A 9.2-mm diameter microkeratome (Antony, France), a superior hinge with 100 microns programmed flap depth setting was used in all cases. The standard LASIK technique was done. Postoperative medications included topical moxifloxacin hydrochloride 0.5% (Vigamox; Alcon, Fort Worth, Texas) four times daily for 7 days and prednisolone acetate 1.0 % (Econopred; Alcon, Fort Worth, Texas) four times daily for 7 days and Sodium Hyaluronate 2mg (HyFresh; Jamjoom, Jeddah, Saudi Arabia) 4 times a day for 3 months. Patients were prospectively evaluated at 1 day, 1 week, 1 month, 3 months, 6 months and 12 months. Primary outcome measures included UCVA, refractive stability, predictability, Pentacam testing, subjective questionnaire, loss of best spectacle-corrected visual acuity (BSCVA), and adverse event profile. Statistical analysis Volume-4 Issue-4 May-2014 Page 6

The data was coded and entered using the statistical package SPSS version 15 (SPSS Inc, Chicago). The data was summarized using descriptive statistics: mean, standard deviation, median, minimal and maximum values for quantitative variables and number and percentage for qualitative values. Statistical differences between groups were tested using independent sample t test for independent group comparisons while paired sample t test was used for paired data comparisons. Analysis of variance (ANOVA) was used to compare measurements before and after refractive surgery. A 1-way Kolmogorov-Smirnov test was used to examine the normality of the data. For all statistics, a P value of < 0.05 was considered statistically significant. RESULTS At 1 month postoperatively, the refractive outcome (predictability) in the PRK group showed that 85 % of eyes were within ±0.50 D and 97 % were within ±1.0 D (SE). At 3 months, 92% of eyes were within ±0.50 D and 95 % were within ±1.0 D. At six months 96% of eyes were within ±0.50 D and 98 % were within ±1.0 D, and at 1 year 96% of eyes were within ±0.50 D and 99 % were within ±1.0 D (SE). For the LASIK group the refractive outcome (predictability) at 1 month showed that 97 % of eyes were within ±0.50 D and 100 % were within ±1.0 D (SE). At 3 months, 96% of eyes were within ±0.50 D and 100 % were within ±1.0 D. At six months 97% of eyes were within ±0.50 D and 100 % were within ±1.0 D, and at 1 year 97% of eyes were within ±0.50 D and 100 % were within ±1.0 D (SE). The difference between the two groups as regards refractive outcome and predictability after 1 year was statistically insignificant (Table 1 and Fig. 1). Table 1: Refractive predictability for LASIK vs PRK. 1 3M 6M 12 M Parameters Month ±0.5D ±1D ±0.5D ±1D ±0.5D ±1D ±0.5D ±1D LASIK 97 100 96 100 97 100 97 100 PRK 85 97 92 95 96 98 96 99 Fig. 1: Refractive predictability for Lasik versus PRK groups. Volume-4 Issue-4 May-2014 Page 7

Patients in the LASIK group reached a stable refraction after 1 month while those in the PRK group reached a stable refraction at 3 months post-operatively (Table 2). The difference in average refraction between the two groups at one month was in favor of the LASIK group (Table 3), with a statistically significant less residual refractive error (P< 0.01) in spherical as well as astigmatic diopteric power as well as spherical equivalent. At 3 months postoperative this difference has disappeared with respect to spherical diopteric power and spherical equivalent but persisted with respect to residual astigmatic error (P=0.03) and was still in favor of LASIK over PRK. Table 2: Average preoperative and postoperative refraction for LASIK Versus PRK groups. Parameters preoperative 1 month 3 months 6 months 12 months LASIK PRK LASIK PRK LASIK PRK LASIK PRK LASIK PRK Sphere (D) -4.92± 1.76 (D) Range (-2.5 to -6.5 D) Astigmatism (D) Spherical equivalent (D) +0.85±0.62 (D) Range (0 to +2 D) -4.21± 1.73 (D) Range (1.75 to -6.1 D) -3.66±1.4D Range (-2 to -5 D) +0.72±0.68 D Range (0 to +1.5 D) -3.13± 1.2D Range (-1.25to- 4.5D) -0.22 ±0.25 +0.24 ±0.22-0.18 ±0.21-0.61 ±0.3 7 +0.5 7 ±0.3 4-0.33 ±0.3 1-0.25 ±0.26 +0.21 ±0.19-0.19 ±0.17-0.23 ±0.28 +0.33 ±0.29-0.23 ±0.24-0.29 ±0.23 +0.20 ±0.21-0.21 ±0.18-0.27 ±0.31 +0.29 ±0.25-0.21 ±0.21-0.32 ±0.29 +0.22 ±0.21-0.22 ±0.19 Table 3: Difference in refractive outcomes at 1 month post-operatively. Parameters 1 month t- LASIK PRK test p-value Sphere (D) -0.22 ± -0.61 ±0.37 5.5 <0.01* 0.25 * Astigmatism (D) +0.24 ± +0.57 ± 5.15 <0.01* 0.22 0.34 * Spherical equivalent -0.18 ± -0.33 ± 2.5 0.014* (D) 0.21 0.31 At 6 and 12 months no statistically significant difference was noted between the two groups as regards spherical diopteric power, astigmatism or spherical equivalent (Table 4). Table 4: Difference in refractive outcomes at 12 months post-operatively. - 0.34 ±0.3 1 +0.2 5 ±0.2 3-0.21 ±0.2 1 Volume-4 Issue-4 May-2014 Page 8

Parameters 12 months t- p- LASIK PRK test value Sphere (D) -0.32 ± -0.34 ± 0.3 0.7 0.29 0.31 Astigmatism (D) +0.22 ± +0.25 ± 0.6 0.5 0.21 0.23 Spherical equivalent -0.22 ± -0.21 ± 0.22 0.82 (D) 0.19 0.21 At 1 month, uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), induction of higher-order aberrations (HOAs), and subjective symptoms of visual clarity, vision fluctuation, ghosting, and overall self-assessment of vision were worse in the PRK group (P<0.05). By 3 months, most of these differences had resolved. Higher order aberrations (coma, trefoil and spherical aberrations) were also statistically analyzed and no significant difference was found between the two groups except for Trefoil aberration (Fig. 2) which was significantly higher in the PRK group at 1, 3 and 6 months (P<0.05) but there was no significant difference at 12 months (P=0.23). Table 5 shows high order aberrations at designated intervals for LASIK and PRK groups. Table 5: High order aberrations at designated intervals for LASIK and PRK groups. Param preoperative 1 month 3 months 6 months 12 months eters LASIK PRK LASIK PRK LASIK PRK LASIK PRK LASIK PRK Coma 0.16 ±0.09 0.21 ±0.12 0.22 ±0.19 0.19 ±0.1 0.22 ±0.17 0.17 ±0.13 0.24 ±0.14 0.22 ±0.11 0.23 ±0.12 0.22 ±0.13 Trefoil 0.17 ±0.09 Spherical aberrations 0.1 ±0.13 0.21 ±0.11 0.09 ±0.12 0.13 ±0.11 0.11 ±0.16 1 0.18 ±0.0 9 0.08 ±0.1 3 0.12 ±0.08 0.1 ±0.13 0.17 ±0.09 0.09 ±0.12 0.12 ±0.06 0.12 ±0.16 0.16 ±0.08 0.12 ±0.18 0.12 ±0.07 0.13 ±0.14 0.14 ±0.08 0.12 ±0.15 Volume-4 Issue-4 May-2014 Page 9

Fig. 2: Difference between HOA s in both groups at designated intervals. DISCUSSION In this study, both PRK and LASIK were effective in treating simple myopia with and without astigmatism. Both techniques demonstrated stable and predictable long-term results and significant improvement in UCVA at 1 year. In addition, both techniques demonstrated excellent safety profiles with no loss of BSCVA >2 lines, contrast sensitivity loss, or incidents of corneal haze, scarring or flap complications. When comparing PRK versus LASIK, we found an initial delay in optimal visual recovery in the PRK group. This is consistent with other previous studies evaluating lamellar surgery versus surface ablations in fellow eyes (Moshirfar et al., 2010 and Slade, 2009). In a study done by Edward Manche and Weldon Haw (2011) comparing the two procedures in an eye to eye fashion; LASIK-treated eyes outperformed the fellow PRK-treated eyes in several subjective and objective categories: subjectively, patients experienced less visual clarity during both day and night, more vision fluctuation, and more ghosting/double vision in the PRK group (P <0.05) as compared to the LASIK-treated group. In addition, patients rated their overall self-assessment of vision as poorer in the PRK group at 1 month (P=0.02). Many of these symptoms were consistent with the residual lower-order aberrations, such as spherocylinder refractive error in the PRK-treated eyes at one month. These results were also similar to what we found in our study at one month postoperatively as residual sphero-cylinder refractive errors were statistically significantly higher in the PRK group than in the LASIK group. This study also yielded similar results to the present study at 12 months postoperatively where we both found no statistically significant difference in visual outcomes, BSCVA and/or HOA s between the two groups. However, the results within hand were not similar to those of a study done back in 2006 (Shortt et al., 2006) which found that LASIK appears to have efficacy and safety superior to those of PRK. However, the data examined in this study was from procedures conducted more than 10 years ago so it is therefore unclear how these findings can relate to present-day methods and outcomes. Wallau and Campos (2009) evaluated the use of mitomycin C as an adjunctive treatment to PRK in comparison to LASIK treated fellow eyes. In contrast to our study- which did not use mitomycin- they found that eyes undergoing PRK with adjunctive intraoperative mitomycin C performed better than LASIK-treated eyes in many objective parameters, such as UCVA, BSCVA, contrast sensitivity, refractive outcomes, astigmatism and spherical aberrations at the 1-year follow-up. However, the investigators cautioned that further studies are indicated before widespread use of mitomycin C in keratorefractive surgery is encouraged. A recent study concluded that LASIK and PRK are comparably safe, effective, and predictable procedures for excimer laser correction of high astigmatism of greater than 3 D in myopic eyes. Predictability of the correction of the cylindrical component is lower than that of the spherical equivalent (Katz et al., 2013). In summary, both PRK and LASIK are safe and effective procedures at reducing myopia with and without astigmatism. The decision of which procedure to pursue should be based on Volume-4 Issue-4 May-2014 Page 10

an individual basis and should take into consideration many factors unique to the individual patient. The advantages of each technique should be balanced by the potential risks of each individual procedure. LASIK provides a more comfortable and rapid visual recovery in terms of many subjective and objective criteria, although by 3 months postoperatively, these differences have largely dissolved. PRK provides the advantages of a flap-free procedure (reduced potential for ectasia and flap complications) at the cost of a slower, more uncomfortable visual recovery. Conclusion Both PRK and LASIK seem to be equally effective refractive surgical options to treat mild to moderate myopia with or without astigmatism. However, LASIK offers faster symptom recovery, an initially lower incidence of high order aberrations and faster refractive stability. At six months almost no statistically significant difference existed between the two groups except for trefoil aberrations which persisted in the PRK group. No statistically significant difference was found in visual and/or refractive outcome after 12 months follow up in both groups including those of high order aberrations. In conclusion, both PRK and LASIK demonstrate promise in the correction of myopia with and without astigmatism. Financial disclosure: None of the authors has any financial interest in any material or method mentioned in this study. REFERENCES 1. Chan, A. and Manche E.E. 2011. Effect of preoperative pupil size on quality of vision after wavefront-guided LASIK. Ophthalmology, 118(4): 736 741. 2. Edward, E. Manche and Weldon W. Haw. 2011. Wavefront-Guided Laser in Situ Keratomileusis (LASIK) versus Wavefront-Guided Photorefractive Keratectomy (PRK): A Prospective Randomized Eye-to-Eye Comparison. Trans Am Ophthalmol Soc. Dec., 109: 201 220. 3. El Danasoury, M.A., ElMaghraby A., Klyce S.D. and Mehrez K. 1999. Comparison of photorefractive keratectomy with excimer laser in situ keratomileusis in correcting low myopia (from 2.00 to 5.50 diopters); a randomized study. Ophthalmology, 106: 411 420; discussion by JH Talamo, 420-421. 4. El-Maghraby, A., Salah T., Waring GOIII et al. 1999. Randomized bilateral comparison of excimer laser in situ keratomileusis and photorefractive keratectomy for 2.50 to 8.00 diopters of myopia. Ophthalmology, 106: 447-457. 5. Fiander, D.C. and Tayfour F. 1995. Excimer laser in situ keratomileusis in 124 myopic eyes. J. Refract. Surg., 11(3 suppl): S234 S238. 6. Goes, F.J. 1996. Photorefractive keratectomy for myopia of 8.00 to 24.00 diopters. J. Refract. Surg., 12: 91 97. 7. Golas, L. and Manche E.E. 2011. Dry eye after laser in situ keratomileusis with femtosecond laser and mechanical keratome. J Cataract Refract Surg., 37(8): 1476 1480. Volume-4 Issue-4 May-2014 Page 11

8. Hamberg-Nystro m, H., Fagerholm P., Tengroth B. and Sjo -holm C. 1996. Thirty-six month follow-up of excimer laser photorefractive keratectomy for myopia. Ophthalmic Surg. Lasers, 27(5suppl): S418 S420. 9. Hersh, P.S., Brint S.F, Maloney R.K, et al. 1998. Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia; a randomized prospective study. Ophthalmology, 105: 1512 1522; discussion by J. H. Talamo, 1522-1523. 10. Hersh, P.S., Stulting R.D., Steinert R.F. et al. 1997. Results of phase III excimer laser photorefractive keratectomy for myopia; the Summit PRK Study Group. Ophthalmology, 104: 1535 1553. 11. Katz, T., Wagenfeld L., Galambos P., Darrelmann B.G., Richard G. and Linke S.J. 2013. LASIK versus photorefractive keratectomy for high myopic (> 3 diopter) astigmatism. J Refract Surg., 29(12): 824-31. 12. Knorz, M.C., Liermann A., Seiberth V. et al. 1996. Laser in situ keratomileusis to correct myopia of 6.00 to 29.00 diopters. J. Refract. Surg., 12: 575 584. 13. Moshirfar M., Schliesser J.A., Chang J.C. et al. 2010. Visual outcomes after wavefront-guided photorefractive keratectomy and wavefront-guided laser in situ keratomileusis: prospective comparison. J. Cataract Refract. Surg., 36(8): 1336 1343. 14. Russell, N. Van Gelder, Karen Steger-May, Susan H. Yang et al. 2002. Comparison of photorefractive keratectomy, astigmatic PRK, laser in situ keratomileusis, and astigmatic LASIK in the treatment of myopia. J Cataract Refract Surg - VOL 28, MARCH, 462-476 15. Shortt, A.J., Bunce C. and Allan B.D. 2006. Evidence for superior efficacy and safety of LASIK over photorefractive keratectomy for correction of myopia. Ophthalmology, 113(11): 1897-908. 16. Slade, S.G., Durrie D.S. and Binder P.S. 2009. A prospective, contralateral eye study comparing thin-flap LASIK (sub-bowman keratomileusis) with photorefractive keratectomy. Ophthalmology, 116(6): 1075 1082. 17. Steinert, R.F. and Hersh P.S. 1998. Spherical and aspherical photorefractive keratectomy and laser in-situ keratomileusis for moderate to high myopia: two prospective, randomized clinical trials; Summit Technology PRK-LASIK Study Group. Trans. Am. Ophthalmol. Soc., 96: 197 221; discussion, 221-227. 18. Stephenson, C.G.; Gartry D.S.; O Brart D.P. et al. 1998. Photorefractive keratectomy; a 6- year follow-up study. Ophthalmology, 105: 273 281. 19. Van De Pol, C., Greig J.L., Estrada A., Bissette G.M. and Bower K.S. 2007. Visual and flight performance recovery after PRK or LASIK in helicopter pilots. Aviat. Space Environ. Med., 78: 547 53. 20. Wallau, A.D. and Campos M. 2009. One-year outcomes of a bilateral randomised prospective clinical trial comparing PRK with mitomycin C and LASIK. Br. J. Ophthalmol., 93(12): 1634-8. 21. Waring, GOIII, O Connell M.A., Maloney R.K. et al. 1995. Photorefractive keratectomy for myopia using a 4.5-millimeter ablation zone. J. Refract. Surg., 11: 170 180. Volume-4 Issue-4 May-2014 Page 12

22. Yu, J.H., Hu F.R., Chang S.W. and Wang I.J. 1996. Excimer laser photorefractive keratectomy for myopia. J. Formos. Med. Assoc., 95: 225 230 Volume-4 Issue-4 May-2014 Page 13