All surgical procedures have the potential for

Laser in situ Keratomileusis Retreatment for Residual Myopia and Astigmatism Khaled M. Rashad, MD ABSTRACT PURPOSE: To evaluate the visual and refractive results of laser in situ keratomileusis (LASIK) retreatment on eyes with residual myopia with or without astigmatism. METHODS: LASIK retreatment was performed on 35 eyes of 23 patients for correction of residual myopia, with or without astigmatism, with a mean manifest spherical equivalent refraction of -2.17 ± 0.82 D (range, -1.00 to -3.87 D) and mean refractive astigmatism of -0.55 ± 0.61 D (range, 0 to -1.75 D). Retreatment was performed 3 to 18 months after primary LASIK (mean, 5.1 ± 2.6 mo). The corneal flap of the previous LASIK was lifted and laser ablation was performed using the Chiron- Technolas Keracor 116 excimer laser. Follow-up was 12 months for all eyes. RESULTS: At 1 year after retreatment, manifest spherical equivalent refraction was reduced to a mean -0.23 ± 0.28 D (range, 0 to -0.87 D), and refractive astigmatism was reduced to a mean -0.16 ± 0.25 D (range, 0 to -0.75 D). Thirty-two eyes (91.5%) had a manifest spherical equivalent refraction within ±0.50 D of emmetropia, and 33 eyes (94.3%) had 0 to 0.50 D of refractive astigmatism. Uncorrected visual acuity was 20/20 or better in 11 eyes (31.4%). Spectacle-corrected visual acuity was not reduced in any eye after retreatment. There were no significant complications. CONCLUSION: LASIK retreatment was effective for correction of residual myopia or astigmatism after primary LASIK. Refractive results were predictable with good stability after 3 months. Lifting the flap during LASIK retreatment was relatively easy to perform and did not result in visual morbidity in eyes treated from 3 up to 18 months after primary LASIK. [J Refract Surg 2000;16:170-176] From the Ophthalmology Department, Alexandria University, Egypt and the Almana General Hospital, Dammam, Saudi Arabia. The author has no proprietary interest in the materials presented. Correspondence: Khaled M. Rashad, MD, Al Mana General Hospital, PO Box 2366, Dammam 31451, Saudi Arabia. Fax: 966.3.8274747 Received: August 7, 1998 Accepted: July 21, 1999 All surgical procedures have the potential for side effects and complications, and refractive surgery is no exception. 1 Laser in situ keratomileusis (LASIK) may be the corneal refractive procedure with the fewest complications. Although the predictability of LASIK is high and better than other refractive procedures, the nature of surgery upon living tissue is such that accurate prediction of refractive outcome is not entirely possible. 2-7 Previous studies reported that refraction stabilizes between 1 and 3 months after LASIK. 8-15 Fortunately, with the current excimer laser algorithms, overcorrection rarely occurs after LASIK performed for correction of myopia and myopic astigmatism. However, undercorrection and/or regression of the achieved correction of myopia or astigmatism are inevitable in some eyes. 8-17 PATIENTS AND METHODS Between January 1995 and October 1996, 417 LASIK procedures were performed by the author at Almana General Hospital in Dammam for correction of myopia ranging from -1.00 to -17.00 diopters (D). The surgical technique and results of the first 157 consecutive eyes were reported. 16,17 Forty-six of 417 eyes (11.0%) were undercorrected by 1.00 D or more. In eleven eyes, patients were satisfied with their results and decided not to have additional treatment. Overcorrection did not exceed +0.50 D in any eye. This study reports the results of LASIK retreatment performed on 35 eyes of 23 patients for correction of residual myopia, with or without astigmatism. All these patients had undergone LASIK for correction of myopia (and astigmatism), and after at least 3 months from their primary LASIK procedure, they still had 1.00 D or more of residual myopia, with or without residual astigmatism. Before primary LASIK, manifest spherical 170 Journal of Refractive Surgery Volume 16 March/April 2000

Table 1 Characteristics of 35 Eyes Before LASIK Retreatment No. Eyes Percent Age (yr) 22 to 30 23 65.7 31 to 40 10 28.6 41 to 50 2 5.7 Spherical Equivalent Refraction (D) -1.00 to -1.50 8 22.9-1.62 to -2.00 9 25.7-2.12 to -2.50 8 22.9-2.62 to -3.00 5 14.3-3.12 to -3.50 3 8.6-3.62 to -3.87 2 5.7 Time Since Primary LASIK (mo) 3 to 5 26 74.3 6 to 8 2 5.7 9 to 11 2 5.7 12 to 14 2 5.7 15 to 18 3 8.6 equivalent refraction of these eyes ranged from -3.75 to -15.50 D (mean, -8.23 ± 3.51 D). After primary LASIK, manifest spherical equivalent refraction was reduced to a mean -0.53 ± 0.38 D at 1 week, and regressed to a mean -1.58 ± 0.63 D at 1 month after surgery. At 3 months after LASIK, mean manifest spherical equivalent refraction was -2.09 ± 0.71 D (range, -0.75 to -3.87 D). Refractive changes after 3 months were minimal and insignificant. LASIK retreatment was performed 3 to 18 months after the primary LASIK procedure (mean, 5.1 ± 2.6 mo). Manifest spherical equivalent refraction at the time of LASIK retreatment ranged from -1.00 to -3.87 D (mean, -2.17 ± 0.82 D). The refractive astigmatism (as measured by manifest refraction) ranged from 0 to -1.75 D with a mean of -0.55 ± 0.61 D. All retreatment procedures were performed by the author at Almana General Hospital in Dammam. Patient age ranged from 22 to 50 years (mean, 29.7 ± 6.2 yr). Twenty-five eyes (71.4%) were from female patients and 10 eyes (28.6 %) were from male patients. The preoperative characteristics (age distribution, manifest spherical equivalent refraction, and time since the primary LASIK procedure) are summarized in Table 1. All patients had a complete ophthalmological examination with emphasis on manifest refraction, uncorrected and spectacle-corrected visual acuity, keratometry, slit-lamp biomicroscopy, and applanation tonometry. Corneal topography was examined using the Computed Anatomy Topographic Modeling System TMS-1 (Computed Anatomy Inc., New York, NY) and corneal thickness was measured using the Teknar Ophthasonic pachymeter (Teknar Corporation, St. Louis, MO). Patients had no corneal or ocular disease other than residual myopia and astigmatism, nor systemic disease that was likely to affect corneal healing (eg, diabetes or collagen diseases). Patients with intraocular pressure higher than 20 mmhg by applanation tonometry or those having corneal thickness less than 380 µm after their primary LASIK procedure were excluded from the study. The refractive goal was emmetropia in all eyes. Both myopia and astigmatism were corrected at the same retreatment. The procedure and its possible complications were explained to the patients, who then signed an informed consent. One hour before the operation, 2% pilocarpine eye drops were instilled to constrict the pupil and help the patient to fixate on the laser beam. Five minutes prior to surgery, topical anesthesia was started with 0.4% oxybuprocaine hydrochloride (Novesin) eye drops at 1-minute intervals. The edge of the corneal flap of the previous LASIK operation was marked preoperatively at the slit lamp with a gentian violet sterile surgical pen to facilitate identification of the flap edge during surgery. In eyes in which residual myopia was accompanied by astigmatism, the axis of the flat corneal meridian was identified and marked at the slit lamp to avoid problems of cyclorotation during surgery. The position of the mark was verified by streak retinoscopy. The patient was positioned horizontally (supine position) under the operating microscope of the Chiron-Technolas Keracor 116 excimer laser (Chiron Technolas GMBH, Munich, Germany). An eyelid speculum was inserted. A T incision marker, painted with gentian violet, was used to mark the cornea to help in accurate flap repositioning at the end of the procedure. The edge of the flap was picked up by one blade of a Kelman McPherson tying forceps to enter into the interface between the corneal flap and the stromal corneal bed. Once the flap edge was separated from the corneal bed at one site, a Rycroft 27 gauge anterior chamber irrigation cannula was entered into the cleavage plane and swept gently in the interface (without irrigation) until the corneal flap was separated from its bed along the entire flap circumference. The flap was everted with forceps and the corneal stromal bed was dried with a microsponge. Alignment of the patient s head and eye were reconfirmed and the patient was asked to fixate on the red fixation light of the microscope. Laser ablation Journal of Refractive Surgery Volume 16 March/April 2000 171

was performed in the stromal corneal bed using the Chiron-Technolas Keracor 116 excimer laser. The fluence at the corneal plane was 120 mj/cm 2 and the repetition rate was 10 Hz. Sequential correction was used in which the astigmatism was corrected first followed by correction of the myopia. The size of the effective ablation zone for the astigmatic correction was 4.5 mm by 8.5 mm, and the ablation zone diameter for the myopia was 6.0 mm with a blend zone of 7.0 mm. To maintain corneal integrity and decrease the chance of future ectasia, in some patients the ablation zone size was reduced to 5.0 mm so that the remaining corneal thickness of the stromal bed after the laser ablation would be at least 200 µm, and preferably 250 µm. The amount of preoperative myopia (and astigmatism) was fully corrected as it was in manifest refraction, without correction for the vertex distance. This was done to compensate for slight undercorrection inherent to the laser algorithm based on our previous experience with the Chiron-Technolas Keracor 116 excimer laser. 16,17 After laser ablation was completed, the corneal flap was irrigated with balanced salt solution on the stromal surface and reflected back to its original position on the corneal stromal bed. The interface was irrigated gently to remove any epithelial cells or debris, and the flap was allowed to stick to the corneal stromal bed. The flap was tested for adequate adhesion, and the eye speculum was removed. A plastic eye shield was applied postoperatively for 1 day and then continued at bedtime only for 1 week. Postoperatively, patients were instructed to use Spersadexoline eye drops (0.1% dexamethasone, 0.5% chloramphenicol, 0.025% tetrahydrozoline hydrochloride; CibaVision, Hettlingen, Switzerland) four times daily for 1 week, followed by 0.1% fluorometholone (Flucon, Alcon Laboratories Inc., Ft. Worth, TX) eye drops four times daily for 2 weeks and then twice daily for another 2 weeks. Follow-up examinations were scheduled at 1 day, 1 week, 1, 3, 6, 9, and 12 months postoperatively. During each examination, the following were measured or performed: manifest refraction, uncorrected and spectacle-corrected visual acuity, keratometry, slit-lamp microscopy, applanation tonometry, and corneal topography. Any subjective symptoms were also checked using a formal questionnaire. RESULTS Identification and marking of the site of the primary LASIK flap on the slit lamp was easy in eyes where retreatment was done within 3 to 6 months after primary LASIK. Identification of the flap edge was more difficult in eyes that had retreatment later than 6 months. Nevertheless, it was always possible to identify the flap and mark its edge on the slit lamp; this took more time in eyes retreated late, as more time had passed after the primary surgery. Once the edge of the flap was marked, lifting the flap was relatively easy to perform up to 18 months after LASIK. Eyes with thin flaps (4 eyes in this series that had undergone primary LASIK with a 130-µm plate) needed more care in handling the flap. Eyes having retreatment later than 6 months required more effort to lift the flap from the corneal bed compared to eyes retreated within 3 to 6 months after the original surgery. Overall, there was no substantial difficulty encountered in lifting the flap in any eye. Postoperatively, most patients complained of discomfort, foreign body sensation, and excessive lacrimation that lasted for a period of 4 to 12 hours. Most patients reported that these symptoms lasted longer than they experienced after their primary LASIK. All eyes were re-epithelialized and quiet on the first postoperative examination 1 day after surgery. Figure 1 presents the changes in manifest spherical equivalent refraction in relation to time after LASIK retreatment. Mean manifest spherical equivalent refraction changed from -2.17 ± 0.82 D preoperatively to a mean of +0.07 ± 0.14 D on the first postoperative day. The achieved refractive results showed some regression to a mean of +0.03 ± 0.23 D at 1 week, and continued to regress to -0.12 ± 0.35 D at 1 month after LASIK retreatment. After that time, refraction showed little regression to a mean of -0.17 ± 0.25 D at 3 months after surgery and was relatively stable thereafter. The minimal regression that occurred after the third postoperative month was not statistically significant (P >.05). The mean manifest spherical equivalent refraction at 12 months after LASIK retreatment was -0.23 ± 0.28 D. Mean change in manifest spherical equivalent refraction induced by LASIK retreatment (postoperative compared to preoperative refraction) was a reduction of myopia of 1.94 D, which was statistically significant (P <.05). Figure 2 presents the final postoperative versus preoperative manifest spherical equivalent refraction for each eye. Manifest spherical equivalent refraction of all eyes 1 year after LASIK retreatment ranged from 0 to -0.87 D. Thirty-two eyes (91.5%) had a spherical equivalent manifest refraction within ±0.50 D of emmetropia (0 to -0.50 D). There were no eyes with overcorrection. The width of the 90% confidence interval for the postoperative 172 Journal of Refractive Surgery Volume 16 March/April 2000

Figure 1. Change in spherical equivalent manifest refraction over time after LASIK retreatment. Figure 3. Comparison of preoperative and 1-year postoperative refractive astigmatism after LASIK retreatment. Table 2 Preoperative and 1-year Postoperative Refractive Results of LASIK Retreatment in 35 Eyes Mean ± SD Range (D) (D) Preoperative Spherical equivalent refraction -2.17 ± 0.82-1.00 to -3.87 Astigmatism -0.55 ± 0.61 0 to -1.75 Postoperative Spherical equivalent refraction -0.23 ± 0.28 0 to -0.87 90% confidence interval 0 to -0.62 Astigmatism -0.16 ± 0.25 0 to -0.75 90% confidence interval 0 to -0.50 Figure 2. Spherical equivalent manifest refraction at 1 year after LASIK retreatment versus the preoperative spherical equivalent manifest refraction. spherical equivalent manifest refraction was 0.62 D (range, 0 to -0.62 D). Refractive astigmatism (astigmatism measured by manifest refraction) was reduced from a mean of -0.55 ± 0.61 D preoperatively (range, 0 to -1.75 D) to a mean of -0.16 ± 0.25 D at 1 year after surgery (range, 0 to -0.75 D). Reduction in refractive astigmatism after LASIK retreatment was statistically significant (P <.05). The amounts of preoperative and 1-year postoperative refractive astigmatism are shown in Figure 3. Preoperatively, 10 eyes (28.6%) had no astigmatism, and 22 eyes (62.9%) had 0 to 0.50 D of refractive astigmatism. After LASIK retreatment, 23 eyes (65.7%) had no astigmatism, and 33 eyes (94.3%) had 0 to 0.50 D of refractive astigmatism. The width of the 90% confidence interval for postoperative refractive astigmatism was 0.50 D (range, 0 to -0.50 D). Refractive results of LASIK retreatment for myopia and astigmatism are summarized in Table 2. Before primary LASIK, mean average keratometric power was 43.89 ± 1.12 D. Average keratometric power was reduced to a mean 39.83 ± 0.93 D at the time of LASIK retreatment (3 to 18 months after primary LASIK). After LASIK retreatment, average keratometric power was reduced further to a mean 38.80 ± 1.52 D at 1 year after surgery. Mean change in keratometric power induced by LASIK retreatment was a reduction of 1.03 D in contrast to 1.94 D mean change in manifest spherical equivalent refraction. Preoperative uncorrected and spectacle-corrected visual acuity and postoperative uncorrected visual acuity at 1 year after LASIK retreatment are detailed in Table 3. Before surgery, 28 eyes (80%) had spectacle-corrected visual acuity of 20/40 or better but only 10 eyes (28.6%) could see 20/40 or Journal of Refractive Surgery Volume 16 March/April 2000 173

Table 3 Uncorrected and Spectacle-corrected Visual Acuity After LASIK Retreatment in 35 Eyes Visual Acuity Preoperative 1-year Postoperative Uncorrected Spectacle-corrected Uncorrected No. % Cum% No. % Cum% No. % Cum% Eyes Eyes Eyes 20/20 0 0 0 10 28.6 28.6 11 31.4 31.4 20/25 2 5.7 5.7 8 22.9 51.4 7 20.0 51.4 20/30 1 2.9 8.6 2 5.7 57.1 2 5.7 57.1 20/40 7 20.0 28.6 8 22.9 80.0 12 34.3 91.4 20/50 to 20/70 13 37.1 65.7 7 20.0 100 3 8.6 100 20/80 to 20/100 12 34.3 100 0 0 100 0 0 100 better without correction. At 1 year after LASIK retreatment, 11 eyes (31.4%) achieved 20/20 uncorrected visual acuity and 32 eyes (91.4%) saw 20/40 or better. No patient required spectacles after LASIK retreatment. Spectacle-corrected visual acuity remained unchanged in 30 eyes (85.7%) and improved by one line in 5 eyes (14.3%). Mean preoperative intraocular pressure (IOP), as measured by applanation tonometry, was 9.9 ± 2.5 mmhg (range, 6 to 14 mmhg). After LASIK retreatment, mean IOP was reduced to 9.0 ± 2.6 mmhg (range, 6 to 13 mmhg). None of the eyes developed a secondary rise of IOP due to topical corticosteroids. Videokeratography showed proper centration of laser ablation in 34 eyes (97.1%) and documented further corneal flattening after LASIK retreatment (Fig 4). Laser ablation was decentered by 0.5 to 1.0 mm in only one eye (2.9%). There were no instances of irregular astigmatism or central islands. Complications of LASIK retreatment are listed in Table 4. There were no intraoperative complications Table 4 Complications of LASIK Retreatment in 35 Eyes Complication No. Eyes Percent Scarring at flap edge 2 5.7 Interface deposits 3 8.6 Transient tear film disturbances 6 17.1 Filamentary keratitis 1 2.9 Diminished night vision 2 5.7 Decrease of spectacle-corrected visual acuity 0 0 Rise in intraocular pressure 0 0 Corneal haze 0 0 or difficulties in lifting the corneal flap of the previous LASIK in any eye. During the first postoperative week, filamentary keratitis developed in one eye (2.9%) at the edge of the corneal flap. It responded well to debridement and treatment with artificial tears. Islands of epithelial cells or other deposits at the interface were observed in 3 eyes (8.6%) but did not affect visual acuity. Slight scarring at the edge of A B Figure 4. Videokeratography (TMS) before (A) and 1 year after (B) LASIK retreatment. Further corneal flattening occurred after retreatment. 174 Journal of Refractive Surgery Volume 16 March/April 2000

the corneal flap occurred in two eyes (5.7%) without any sequelae. During the first postoperative month, tear film disturbances in the form of decreased break-up time and areas of corneal dryness were observed in 6 eyes (17.1%). These responded well to treatment with artificial tears and returned to normal by the third month postoperative examination. None of the eyes developed corneal haze or lost any lines of spectacle-corrected visual acuity. Subjective symptoms after LASIK retreatment were few and self-limiting. Fifteen patients (42.9%) reported foggy vision and glare during the first postoperative week, but the symptoms eventually disappeared. At 1 year after surgery, only two eyes (5.7%) had diminished night vision. None of the patients reported the presence of persistent glare or starburst. DISCUSSION Refractive changes after LASIK retreatment in this study showed immediate reduction of the residual myopia with slight overcorrection during the first postoperative week, followed by some regression that occurred mainly during the first month. There was little regression that continued till the third postoperative month, with relative stability thereafter. Changes in refraction after the third month were not significant. Other studies have also reported stability of refraction between 1 and 3 months after LASIK. 10-17 Some eyes may continue to regress between 3 and 6 months after surgery, but the amount of regression is usually small. 16-18 Factors that cause regression after LASIK (or LASIK retreatment) are not clearly understood. Previous studies postulated that regression may be caused by the molecular memory in the corneal collagen fibers, stromal remodeling, the effect of IOP on the thinned cornea, and epithelial hyperplasia in response to an excessively flattened corneal curvature. 9-12,16 The refractive changes after the primary LASIK procedure for patients in this study showed regression of the achieved correction that continued until the third month after LASIK. Changes in refraction after 3 months from the primary LASIK were minimal and negligible. Therefore, in the presence of residual myopia and/or astigmatism after LASIK, one should wait at least 3 months before retreatment to allow for stabilization of refraction. Some authors have suggested that LASIK enhancement should be performed by lifting the flap within 3 to 6 months after the primary LASIK procedure and that after 6 months, the patient should have a repeat LASIK with creation of a new flap. 10,18 In this study, lifting the original corneal flap was technically feasible at all times from 3 to 18 months after the primary LASIK procedure. Even in eyes where retreatment was performed at 18 months after the primary LASIK, the corneal flap could be lifted without difficulty. This is because the flap was found to be adherent only along its edge where Bowman s layer was cut, and there was no actual adhesion within the corneal stroma central to the flap edge. None of the eyes required a repeat LASIK or creation of a new flap. There were no intraoperative complications. Preoperative marking of the edge of the corneal flap at the slit lamp was of great help in identification of the site of the flap during surgery in order to minimize manipulations. This technique of LASIK retreatment (by lifting the corneal flap and reablation) has advantages over a repeat LASIK in that it avoids subjecting the patient to potential flap complications. 10 Predictability of correction of residual myopia and astigmatism in this study was high, evidenced by the 90% confidence interval. At 1 year after LASIK retreatment, 90% of eyes had a manifest spherical equivalent refraction from 0 to -0.62 D and 90% of eyes had 0 to 0.50 D of refractive astigmatism. These results indicate the precision with which refractive outcome could be predicted. Therefore, in planning the nomograms for the primary LASIK procedure (for myopia or myopic astigmatism), it would be wise to aim at achieving emmetropia or slight undercorrection because the residual refractive error can be dealt with later on, with high predictability and accuracy. This avoids problems of overcorrection that would result in consecutive hyperopia, which is difficult to treat. Changes in average keratometric power were generally less than changes in manifest refraction after LASIK retreatment. After keratomileusis, where only the anterior corneal curvature has been altered, keratometric power is often inaccurate and may underestimate the true refractive power. This is due to the selective points measured by the keratometer and the aspheric nature of the cornea after surgery. 16,17,19 Uncorrected visual acuity was significantly improved after LASIK retreatment. At 1 year after retreatment, 91.4% of eyes attained 20/40 or better uncorrected visual acuity compared to only 28.6% of eyes that could see 20/40 or better before surgery. None of the patients required use of spectacles after LASIK retreatment because the amounts of residual Journal of Refractive Surgery Volume 16 March/April 2000 175

myopia and/or astigmatism after retreatment were negligible. None of the eyes lost any line of spectacle-corrected visual acuity. Spectaclecorrected visual acuity was improved by one line in 5 eyes (14.3%). This could be due to improvement of the refractive amblyopia, and magnification of the retinal image after retreatment (compared to the minifying effect of the myopic spectacles). 16,20 Subjective symptoms and complications after LASIK retreatment were few and minor. The discomfort reported by patients on the first postoperative day was due to the epithelial defect at the edge of the corneal flap. The discomfort lasted more than that experienced after the first LASIK surgery because of slight irregularity of the epithelium at the flap edge due to surgical manipulations during lifting the flap, compared to the clear cut edge of the flap created with the microkeratome during the primary LASIK procedure. 16,18 None of the eyes developed a rise of IOP due to the short-term use of topical corticosteroids. Corneal haze did not develop in any eye due to preservation of Bowman s layer in LASIK retreatment. Spectacle-corrected visual acuity was not reduced in any eye due to absence of corneal haze, central islands, or irregular astigmatism. The minimal subjective symptoms of diminished night vision could be caused by slight decentration of the ablation zone, and the presence of small amounts of uncorrected residual myopia or astigmatism in some eyes. LASIK retreatment by lifting the corneal flap and reablating the stromal bed is effective in correction of residual myopia and astigmatism after primary LASIK. One should wait for at least 3 months after the primary LASIK procedure to allow for stabilization of refraction before retreatment. Refractive results of LASIK retreatment are fairly predictable and refraction stabilizes by 3 months after retreatment. Lifting the corneal flap during LASIK retreatment was relatively easy to perform and did not result in visual morbidity in eyes treated within 18 months after the primary LASIK procedure. REFERENCES 1. Bores LD. Side effects and complications of refractive surgery. In: Bores LD. Refractive Eye Surgery. Boston, MA: Blackwell Scientific Publications; 1993:500-552. 2. Pallikaris IG, Siganos DS. Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia. J Refract Corneal Surg 1994;10:498-510. 3. Gimbel HV, Basti S, Kaye GB, Ferensowicz M. Experience during the learning curve of laser in situ keratomileusis. J Cataract Refract Surg 1996;22:542-550. 4. Gris O, Guell JL, Muller A. Keratomileusis update. J Cataract Refract Surg 1996;22:620-623. 5. Kanellopoulos AJ, Pallikaris IG, Donnenfeld ED, Detorakis S, Koufala K, Perry HD. Comparison of corneal sensation following photorefractive keratectomy and laser in situ keratomileusis. J Cataract Refract Surg 1997;23:34-38. 6. Kent DG, Soloman KD, Peng Q, Whiteside SB, Brown SJ, Apple DJ. Effect of surface photorefractive keratectomy and laser in situ keratomileusis on the corneal endothelium. J Cataract Refract Surg 1997;23: 386-397. 7. Pallikaris IG, Siganos DS. Laser in situ keratomileusis to treat myopia: early experience. J Cataract Refract Surg 1997;23:39-49. 8. Salah T, Waring GO III, El Maghraby A. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996;121:143-155. 9. Ghaith AA. Laser in situ keratomileusis (LASIK). Egypt J Cataract Refract Surg 1996;2:47-58. 10. Machat J. LASIK retreatment technique and results. In: Machat J. Excimer Laser Refractive Surgery: Practice and Principles. Thorofare, NJ: Slack, Inc; 1996:401-413. 11. Machat J. LASIK clinical issues and results. In: Machat J. Excimer Laser Refractive Surgery: Practice and Principles. Thorofare, NJ: Slack, Inc; 1996:415-427. 12. Bas AM, Onnis R. Excimer laser in situ keratomileusis for myopia. J Refract Surg 1995;11(suppl): S229-S233. 13. Fiander DC, Tayfour F. Excimer laser in situ keratomileusis in 124 myopic eyes. J Refract Surg 1995; 11(suppl): S234-S238. 14. Kremer FB, Dufek M. Excimer laser in situ keratomileusis. J Refract Surg 1995;11(suppl):S244-S247. 15. Guell JL, Muller A. Laser in situ keratomileusis (LASIK) for myopia from -7 to -18 diopters. J Refract Surg 1996;12: 222-228. 16. Rashad KM. Laser assisted in situ keratomileusis (LASIK) for correction of myopia. Egypt J Cataract Refract Surg 1996;2:17-29. 17. Rashad KM. Laser in situ keratomileusis for myopic astigmatism. J Refract Surg 1999;15:653-660. 18. Santonja JJ, Bellot J, Claramonte P, Ismail MM, Alio JL. Laser in situ keratomileusis to correct high myopia. J Cataract Refract Surg 1997;23:372-385. 19. Bores LD. Surgical management of astigmatism. In: Bores LD. Refractive Eye Surgery. Boston, MA: Blackwell Scientific Publications; 1993;260-323. 20. Applegate RA, Howland HC. Magnification and visual acuity in refractive surgery. Arch Ophthalmol 1993;111: 1335-1342. 176 Journal of Refractive Surgery Volume 16 March/April 2000