The concept of a proactive intervention involving in situ

CLINICAL SCIENCE Epithelial Remodeling After Femtosecond Laser-assisted High Myopic LASIK: Comparison of Stand-alone With LASIK Combined With Prophylactic High-fluence Cross-linking Anastasios J. Kanellopoulos, MD,* and George Asimellis, PhD* Purpose: The aim of this study was to evaluate the possible topographic epithelial profile thickness changes (remodeling) after high myopic femtosecond laser in situ keratomileusis (LASIK) with concurrent prophylactic high-fluence cross-linking (CXL) in comparison with standard femtosecond LASIK. Methods: Preoperative and 6-month postoperative 3-dimensional epithelial thickness distribution maps were investigated through clinical spectral domain anterior-segment optical coherence tomography in 2 groups of femtosecond laser assisted myopic LASIK cases. Group A represented 67 eyes treated additionally with concurrent prophylactic CXL (LASIK-Xtra); group B represented 72 eyes subjected to stand-alone femtosecond LASIK. Optical coherence tomography measurements of the epithelial thickness over the center 2-mm-diameter disk, mid-peripheral 5-mm rim, and overall (the entire 6-mm-diameter disc area) were investigated. Results: The comparison of matched myopic correction subgroups indicated statistically significant differences in the epithelial thickness increase specifically between high myopia subgroups. For example, in group A (LASIK-Xtra), the mid-peripheral epithelial thickness increase was +3.79 and +3.95 mm for the 28.00 to 29.00 diopter and 27.00 to 28.00 diopter subgroups, which compare with increased thickness in group B (stand-alone LASIK), of +9.75 mm (P = 0.032) and +7.14 mm (P = 0.041), respectively, for the same subgroups. Conclusions: Application of prophylactic CXL concurrently with high myopic LASIK operation results in a statistically significant reduced epithelial increase in comparison with stand-alone LASIK. This comparison is observed between matched high myopic correction subgroups. This difference may correlate with higher Received for publication August 7, 2013; revision received January 8, 2014; accepted January 9, 2014. Published online ahead of print March 11, 2014. From the *LaserVision.gr Eye Institute, Athens, Greece; and Department of Ophthalmology, Langone Medical Center, NYU Medical School, New York, NY. A. J. Kanellopoulos is a consultant and holds advisory positions with Alcon/ WaveLight, Allegran, Avedro, and i-optics. G. Asimellis has no funding or conflicts of interest to disclose. Design and conduct of the study (A.J.K.); collection (A.J.K. and G.A.), management (A.J.K.), analysis (G.A.), and interpretation of the data (A.J.K. and G.A.); manuscript preparation (G.A.), review (A.J.K. and G.A.), and approval (A.J.K.). Reprints: A. John Kanellopoulos, MD, Medical Director, the Laservision.gr Eye Institute, 17 Tsocha Street, Athens, Greece 115 21 (e-mail: ajk@ brilliantvision.com). Copyright 2014 by Lippincott Williams & Wilkins regression rates and/or may depict increased biomechanical instability in stand-alone LASIK. Key Words: anterior segment optical coherence tomography, epithelial imaging, epithelial thickness distribution, epithelial layer topography, myopic LASIK, femtosecond LASIK, LASIK-Xtra, high-fluence cross-linking, prophylactic cross-linking (Cornea 2014;33:463 469) The concept of a proactive intervention involving in situ cross-linking (CXL) application concurrent with laser in situ keratomileusis (LASIK) surgery has been introduced with the term LASIK-Xtra. 1,2 The concept has been based on successful treatments and femtosecond laser created intrastromal pocket CXL implementation 3,4 in a patient population who seems to have a high rate of keratoconus. The rationale for such a prophylactic action in routine LASIK is to strengthen the cornea, particularly in high myopic cases with thin residual stroma and younger patients who may have not yet exhibited ectasia risk factors. 5,6 Our experience with high myopic corrections without any such preventive application is suggestive of a long-term corneal steepening trend (refractive regression toward a myopic shift). 7 Because of this troublesome finding, and the high incidence of keratoconus in our patient populace, in situ CXL may be justified in high myopic LASIK cases. Refractive regression has been associated with substantial postoperative epithelial thickness increase 8,9 after LASIK myopic correction. 10 12 Specifically, postoperative epithelial evaluation after myopic LASIK has demonstrated a topographically nonuniform increase in the epithelial thickness, 13 dependent on the extent of myopia corrected. We have recently investigated epithelial remodeling after femtosecond laser assisted LASIK. 14 The study, conducted with Fourier-domain anterior-segment optical coherence tomography (AS-OCT), in large myopic corrections [eg 28.00 diopter (D)] indicated 1-year postoperative central epithelial thickness increase of up to +6 mm and localized increase at the 5-mm mid-peripheral rim of up to +9 mm. This present study aimed to comparatively investigate the potential differences in epithelial remodeling between 2 groups, a LASIK-Xtra and a stand-alone LASIK group, in which no concurrent CXL is applied. The epithelial study was facilitated by 3-dimensional epithelial thickness maps produced by a clinically available spectral domain AS-OCT system. Cornea Volume 33, Number 5, May 2014 www.corneajrnl.com 463

Kanellopoulos and Asimellis Cornea Volume 33, Number 5, May 2014 MATERIALS AND METHODS This prospective, observational longitudinal study received approval by the ethics committee of our institution (Laservision.gr Eye Institute) and adhered to the tenets of the Declaration of Helsinki. Informed written consent was obtained from each subject at the time of the first study visit. Inclusion and Exclusion Criteria One hundred thirty-nine consecutive eyes enrolled for primary myopic LASIK correction constituted this study. Group A (LASIK-Xtra) consisted of 67 eyes in which concurrent prophylactic CXL was applied, whereas group B consisted of 72 eyes in which no such additional intervention was implemented (stand-alone LASIK). All cases enrolled in the study had no other previous ocular surgery. Before the intervention, a complete preoperative ophthalmologic evaluation ensured no present or past ocular pathology other than refractive error, no epithelial defects, and no irritation or dry eye disorder. Visual acuity and keratometry were also assessed preoperatively. All operations were performed by the same surgeon (A.J.K.). Surgical Technique In both groups, the FS200 femtosecond laser (Alcon/ WaveLight, Fort Worth, TX) was used to provide a corneal flap of 110-mm thickness and 8.00-mm diameter. 15 The average pulse energy for flap bed cut was 0.75 mj, the side cut angle was 70 degrees, and the hinge position was inferior. The myopic ablation (6.50-mm ablation zone) was completed with by the EX500 excimer laser (Alcon/WaveLight). 16 Both lasers are contiguous and operate on their own ethernet network, which allows the import of diagnostic data from screening devices into the software planning tools. For example, Placido topography or Scheimpflug topometry data may be imported into the treatment planning mode, and accordingly customize the excimer corneal treatment (eg, for topographyguided treatments). Specifically in group A, after excimer laser ablation completion, and with the flap folded onto itself and protected with a dry Weck Cel sponge, 1 drop of VibeX Rapid (Avedro Inc, Waltham, MA) consisting of 0.10% saline-diluted riboflavin (very slightly hypotonic mixed with hydroxypropyl methyl cellulose (HPMC), a dextran substitute) was placed on the exposed stromal bed and carefully spread over the bed only with an irrigating cannula. Special attention was applied so that the drop did not soak onto the folded flap and its hinge (Fig. 1 in Ref. 1). After 60 seconds of soaking time, the flap was repositioned into place, the residual riboflavin irrigated and the flap secured in place; then, UV-A fluence of 30 mw/ cm 2 for a total of 80 seconds provided by the KXL system (Avedro Inc, Waltham, MA) was applied. According to our clinical protocol during the last 6 years, in this study, the myopic LASIK treatments were randomized to receive LASIK-Xtra if they had high myopia (more than 25 D spherical equivalent) and/or high keratometric preoperative astigmatism (more than 1.5 D on the Scheimpflug-derived simulated keratometry). The combination of the above parameters has led to the fact that no eye with myopia larger than 210 D has been treated with standalone LASIK during the course of the study. Postoperative evaluation included slit-lamp examination, clinical evaluation of dry eye, indications of epithelial ingrowth, 17 corneal haze or ectasia, and measurement of the best-spectacle corrected distance visual acuity (CDVA). Instrumentation The Fourier-domain AS-OCT system RTVue-100 (Optovue Inc, Fremont, CA), running on software version A6 (9,0,27), was used in the study. Data output included total corneal and epithelial thickness maps covering a 6 mm-diameter area. In all cases, to minimize testing variations, OCT imaging was conducted by the same trained investigator and preceded ocular clinical examination. Two consecutive acquisitions were obtained in each case to ensure data validity. The scans were centered on the pupil center, using the on-line view of the patient s pupil image during the scan. 18 The settings were: L-Cam lens, 8 meridional B-scans per acquisition, which consisted 1024 A-scans each, example of which is shown in Figure 1A. The system software produces, by interpolation, the 3- dimensional thickness maps. Preoperative imaging was conducted the day preceding LASIK operation; postoperative data included in this study were based on the sixth month postoperative examination. Data Collection and Analysis The main analysis report produced by the AS-OCT system displayed total corneal (reported as pachymetry) and epithelial thickness maps covering the 6-mm-diameter corneal area. As shown in Figure 1B, each pachymetry map is divided in 17 sectors. Specifically, these are the 2-mm-diameter pupil center disk of 12.56 mm 2 area, 8 octants within the annulus between 2 and 5 mm, each of 8.24 mm 2 area, and 8 sectors within the annulus of 5- to 6-mm zones, each of 4.32 mm 2 area. For each sector, the average thickness is displayed numerically in steps of 1 mm over the corresponding area. In this study, the data reported as center corresponded to the integer value representing the average thickness over the 2-mm center disk. The overall epithelial thickness was calculated by the average of the 17 segment thickness values, and the mid-peripheral epithelial thickness was calculated by the average of the thickness values corresponding to 18 equally spaced points along the 5-mm radius rim. The topographic thickness variability corresponded to the SD over the entire imaged area of all 17 segments. To specifically investigate the possible correlation with myopic ablation and to compare spherical-equivalent matched cases, the 2 groups were subdivided on the basis of the amount of ablation, grouped per 1 D increments, based on the spherical equivalent of the programmed correction. In addition, we focused the comparative investigation on the mid-periphery epithelial changes, which has been shown to display the morepronounced postoperative epithelial thickness changes, when compared with the 2-mm center and overall. 464 www.corneajrnl.com Ó 2014 Lippincott Williams & Wilkins

Cornea Volume 33, Number 5, May 2014 Topographic Epithelial Profile Thickness Changes FIGURE 1. A, AS-OCT high-resolution cross-sectional meridional image of a right eye treated with LASIK-Xtra for 28.00 D of sphere and 20.25 D of astigmatism, and was it imaged 6 months postoperatively. There is a clear depiction of the central corneal epithelial layer, Bowman membrane, anterior stroma, Descemet membrane, and anterior chamber. Deep stromal hyperreflective lines may correlate with the depth of the CXL-effect achieved with the LASIK-Xtra procedure according to our previous reported findings.b,detailfromoftheanalysisand report software main report, showing corneal and epithelial 3-dimensional pachymetry maps over the 6-mm corneal diameter. The symbol * indicates thickness minimum (both corneal and epithelial maps), and the symbol + thickness maximum (epithelial map only). Descriptive statistics, linear regression analysis to seek possible correlations, paired analysis t tests, and analysis of variance were performed by Minitab version 16.2.3 (MiniTab Ltd, Coventry, UK) and OriginLab version 9 (OriginLab Corp, Northampton, MA). Paired analysis P values that were less than 0.05 were indicative of statistically significant results in this study. For the correlation with the amount of myopia correction, the spherical equivalent of myopic ablation, as programmed in the excimer laser system, was used. RESULTS The 67 eyes included in group A (LASIK-Xtra) belonged to 37 female and 10 male patients; 35 eyes were right and 32 were left. The mean patient age at the time of operation was 27.5 6 6.1 (range, 19 39) years. The average preoperative refractive error was sphere 26.58 6 2.31 (range, 22.50 to 211.50) D, and cylinder 21.39 6 1.41 (range, 0.00 to 25.25) D. The average preoperative central corneal thickness was 545.96 6 33.93 (range, 474 595) mm. Six months postoperatively, the average central corneal thickness was 441.75 6 28.35 (range, 405 508) mm. Group A (LASIK-Xtra) had a postoperative average refractive error 20.20 6 0.28 D, compared with 26.58 6 2.31 D preoperatively. All eyes in this group A had a postoperative CDVA of 20/20, and 64 of 67 eyes were within 60.25 D of the intended correction. The 72 eyes in group B (stand-alone LASIK) belonged to 41 female and 31 male patients; 33 eyes were right and 39 were left. The mean patient age at the time of operation was 24.2 6 5.8 (range, 18 42) years. The average preoperative refractive error was sphere 25.13 6 1.59 (range, 22.50 to 28.75) D and cylinder 20.86 6 0.77 (range, 0.00 to 23.50) D. The average preoperative central corneal thickness was 553.51 6 19.11 (range, 503 592) mm. Six months postoperatively, the central corneal thickness was 453.15 6 31.11 (range, 415 500) mm. Group B (stand-alone LASIK) had an average postoperative average manifest refractive spherical equivalent 20.20 6 0.30 D, compared with 25.13 6 1.59 D preoperatively. All patients in this group B had a CDVA of 20/20, and 69 of 72 cases were within 60.25 D of the intended correction. Epithelial Thickness Results As shown in Table 1, for group A, preoperative central epithelial thickness was 52.53 6 2.85 mm, overall (the entire 6-mm average) was 52.61 6 2.75 mm, mid-peripheral was 52.95 6 2.65 mm, and the topographic thickness variability was 1.85 6 0.93 mm. Six months after the LASIK-Xtra operation, the center epithelial thickness was 54.54 6 3.11 mm (change +2.01 mm, P = 0.0031), mid-peripheral was 57.25 6 3.21 mm (change +4.30 mm, P, 0.001), and overall was 56.92 6 3.13 mm (change +4.31 mm, P, 0.001); finally, the topographic thickness variability was 2.99 6 0.71 mm. For group B, preoperatively, the central epithelial thickness was 51.65 6 2.21 mm, mid-peripheral was 51.79 6 2.53 mm, overall was 51.74 6 2.33 mm, and the topographic thickness variability was 1.20 6 0.59 mm. Six months after the Ó 2014 Lippincott Williams & Wilkins www.corneajrnl.com 465

Kanellopoulos and Asimellis Cornea Volume 33, Number 5, May 2014 TABLE 1. Descriptive Statistics for Preoperative Epithelial Thickness for the Center (2-mm Disk), Overall (Entire 6-mm Diameter), Mid-Peripheral (5-mm Rim), and Topographic Variability (SD of Thickness Over the 6-mm Diameter) Center Overall Mid- Peripheral Topographic Variability LASIK-Xtra group A Mean 52.53 52.61 52.95 1.85 SD 62.85 62.75 62.65 60.93 Max 62 61.58 61.50 4.30 Min 46 48.33 48.40 0.80 Stand-alone LASIK group B Mean 51.65 51.74 51.79 1.20 SD 62.21 62.33 62.53 60.59 Max 55 55.33 55.70 3.30 Min 45 44.58 44.50 0.60 Group A, n (A) = 67 eyes; Group B, n (B) = 72 eyes. Data are represented in micrometers. LASIK operation, the central epithelial thickness was 53.85 6 3.23 mm (change +1.70 mm, P = 0.00125), mid-peripheral was 55.23 6 2.75 mm (change +3.44 mm, P, 0.001), and overall was 54.78 6 3.65 mm (change +3.04 mm, P, 0.001); finally, the topographic thickness variability was 2.61 6 0.90 mm. Correlation of Epithelial Changes With Refractive Correction As shown in the statistical results of the postoperative epithelial thickness changes presented in Table 2, there was no statistically significant difference in epithelial thickness changes between groups A and B within the staged myopia subgroups up to 27.00 D. However, as illustrated in Figure 2 and reported in the data in Table 2, a significant increase in the epithelial thickness change in the high myopic subgroups specifically in the stand-alone LASIK group B is noted. For example, the mid-peripheral epithelial increase for the 28.00 to 29.00 D subgroup (n = 6 eyes) was on average +9.75 mm and for the 27.00 to 28.00 D subgroup (n = 8 eyes) was +7.14 mm. However, in group A (LASIK-Xtra) no such correlation is identified. The data suggest that the epithelial thickness increase in this group was on average between +3 and +4 mm, which, however, was not significantly increased in the high myopic subgroups. For example, the corresponding increase was +3.79 mm (n = 13 eyes) and +3.95 mm (n = 6 eyes) for the 28.00 to 29.00 D subgroup and the 27.00 D to 28.00 D subgroup, respectively. DISCUSSION Clinical in vivo corneal epithelial thickness evaluation has used in the past scanning high-frequency ultrasound biomicroscopy (HF-UBM) 19 and confocal microscopy through-focusing (CMTF). 20 AS-OCT is a recent entry in this field. 21,22 The RTVue-100 system offers epithelial thickness imaging over the cornea (currently at 6-mm), which facilitates in vivo epithelial thickness quantitative and qualitative investigation in normal, 18 keratoconic eye, 23 dry eye population, 24 post-cataract, 25 and post-lasik cases. 14 A number of previous reports have investigated postoperative epithelial changes in myopic LASIK, using ultrasound 11,13,26,27 or confocal imaging. 8,9,28 In these studies, besides the difference in imaging technology, flap-creation and sample size were different compared with those of this study. Specifically, LASIK cases with microkeratome-flap creation or a small number of individuals have been investigated (eg, n = 11 cases in Ref. 26 ). The known topographic irregularity in flap thickness 29 of the mechanical microkeratome-created flaps in comparison with those created with current femtosecond lasers may have introduced additional fluctuating parameters. Technical limitations of such imaging technologies regarding sensitivity and specificity can be attributed to the contact nature/saline immersion, or longer acquisition time. For example, scanning HF-UBM 27 requires repetitive sequential manual re-positioning by the examiner of the meridian scan orientation, resulting in a long, overall acquisition time in comparison with the OCT measurement. TABLE 2. Comparative Statistical Results for the Postoperative Minus Preoperative Epithelial Thickness, per Matched Subgroups of Attempted Refractive Correction Group A (LASIK-Xtra) Group B (Stand-alone LASIK) 2-Sample t test Analysis A vs. B Degree of Myopia n Center Overall Mid-Peripheral n Center Overall Mid-Peripheral P Value for Mid-Peripheral 210.00 and larger 5 6.54 7.04 7.25 0 29.00 to 210.00 13 2.23 5.47 6.27 0 28.00 to 29.00 13 2.85 3.43 3.79 6 6.41 9.32 9.75 0.032 27.00 to 28.00 6 3.71 3.87 3.95 8 4.15 6.73 7.14 0.041 26.00 to 27.00 7 2.69 4.13 4.28 12 3.77 4.43 3.47 0.068 25.00 to 26.00 7 2.45 4.27 4.33 9 3.31 3.87 4.21 0.431 24.00 to 25.00 8 1.27 2.25 3.05 14 3.14 3.61 3.81 0.135 23.00 to 24.00 3 2.62 2.93 3.15 17 1.27 2.81 3.47 0.980 22.00 to 23.00 5 21.20 0.25 0.54 6 21.25 20.75 20.23 0.347 Epithelial thickness changes reported for center (calculated over the 2-mm disk), overall (calculated over the entire 6-mm diameter), and mid-peripheral (calculated over the 5-mm rim). Data are represented in micrometers. 466 www.corneajrnl.com Ó 2014 Lippincott Williams & Wilkins

Cornea Volume 33, Number 5, May 2014 Topographic Epithelial Profile Thickness Changes FIGURE 2. Correlation of increase in epithelial thickness at the 5-mm mid-peripheral zone, 6 months postoperatively, in comparison for the 2 groups, with refractive correction spherical equivalent. In this study, we investigated 6-month postoperative corneal epithelial thickness changes in 139 LASIK eyes with a clinically available AS-OCT system, using raw data obtained from the system report and analysis software, and as such, it presents a novel comparative benchmark study. The 2 groups in the study were matched by all other means such as ablation zone, flap thickness, surgeon, lasers used, and postoperative medication and treatment. We have reported significant postoperative epithelial thickness increase in a previous investigation of 61 stand-alone myopic LASIK eyes. 14 Specifically, 1 year postoperatively, the epithelial thickness was increased centrally by +1.42 mm (P = 0.146), overall by +2.90 mm(p, 0.001), and mid-peripherally by +3.19 mm(p, 0.001). More importantly, the noted change in the central, overall, and mid-peripheral epithelial thickness appeared to be in correlation with the amount of myopic correction. It seems that this epithelial hyperplasia may be responsible for perceived regression, particularly in higher myopic corrections. 7 We have also had preliminary confirmation of similar such epithelial postoperative thickness increase with R. R. Krueger, MD (personal communication, September 2013, Cleveland Clinic foundation, Cleveland, OH). The data in this work support that high myopic cases with stand-alone LASIK treatments (group B) demonstrate significant epithelial hyperplasia, of average magnitude between +8 and +9 mm. There are a number of factors that may explain this increased epithelial hyperplasia after high myopic stand-alone LASIK. For example, there has been the explanation that the amount of epithelial thickening is determined by the rate of change of the curvature of the stromal surface. 30 Another possible factor 19,31 is that a thinned cornea (especially in large myopic ablation) might become biomechanically less stable, thus oscillate more because of external factors and/or be more susceptible to aqueous pulsing, and as a result, induce epithelial hyperplasia. The association between apical corneal displacement and intraocular pressure (IOP) change and the cardiac cycle is already established, 32 and possibly supports the above argument. Comparison with group A, in which prophylactic highfluence CXL is incorporated in the primary LASIK procedure (LASIK Xtra), indicates that this hyperplasia in the high myopic cases appears to be moderate, approximately +3 to +4 mm, almost similar to the levels encountered in the lower myopic corrections. We therefore believe that this moderate epithelial thickness increase can no longer be justified only by the differences in corneal curvature, because they correspond to similar myopic ablation patterns, and therefore can only be attributed to the difference between the 2 groups, which is the preventive CXL application. In addition, the postoperative evaluation in group A has not indicated any clinical or topographic evidence of complications in comparison with the stand-alone group B. Visual rehabilitation between the 2 groups, as expressed by CDVA, was determined in similar levels in comparison with the stand-alone LASIK, without inducing any side effects or compromising visual safety. It is worth noting that the cylinder treated in the LASIK Xtra group A (mean, 21.39 D; max 25.25 D) was significantly greater than that in group B (mean, 20.86 D; max, 23.50 D). Astigmatic ablation profiles are significantly different from spherical profiles particularly in the mid-periphery because the ablation is much less in 1 meridian for cases with higher cylinder. Although the epithelial response has not been studied in high astigmatism cases specifically in this work, it is likely that the differences in the epithelial thickness remodeling indicated in this study may have been influenced by this aspect. We believe, though, that the higher astigmatism treated in group A would have only contributed to the direction of mid-periphery increase, and thus the observed reduced midperiphery epithelial changes can be suggestive that postoperative epithelial hyperplasia is indeed less in the LASIK-Xtra treated eyes. Ó 2014 Lippincott Williams & Wilkins www.corneajrnl.com 467

Kanellopoulos and Asimellis Cornea Volume 33, Number 5, May 2014 In view of the expressed skepticism by colleagues regarding possible regression of the refractive effect in cases of large myopic corrections 33 with the risk of progressive ectasia, extreme caution has been recommended when myopic LASIK is performed in high myopic cases. Although there are reports expressing predictable and long-term stable results in refractive and visual outcomes in correcting moderate-tohigh myopia, 34 there are also other reports, such as the work by Alió et al, 35 which report 1-in-5 or specifically the compelling percentage of 20.8% of high myopic cases requiring retreatments because of, or a combination of, over-correction, under-correction, and regression. Our team has performed a large number of successful LASIK-Xtra procedures, and we view this prophylactic treatment as a pivotal biomechanical enhancement of a LASIK procedure in a young adult, any patient who is younger than 30 years, with myopia more than 6 D, with astigmatism more than 1 D, and any patient with a difference in the amount of the astigmatism between the 2 eyes of more than 0.5 D. Certain limitations of our study include possible effect of a difference in the corneal thickness, corneal curvature, or IOP between the 2 groups. Residual corneal thickness could be a contributing factor, and similarly, differences in IOP might affect the response of the cornea after tissue removal in corneas of equivalent biomechanical strength. Corneal curvature may also have an effect on laser fluence, although this aspect is addressed by the current implementation in the EX500, therefore there might have been slightly reduced midperipheral ablation in steeper corneas. It is also possible that corneal curvature changes may have an influence on epithelial remodeling beyond the 6-mm corneal diameter currently imaged by the device used in this study. Further studies, possibly involving a larger corneal diameter imaging, up to 9-mm, which would cover the entire area affected by the LASIK flap and the corresponding epithelial remodeling, would be highly desired to investigate the entire affected area of epithelial remodeling. CONCLUSIONS High myopic LASIK combined with prophylactic higher-fluence CXL (LASIK-Xtra) intervention seems to induce less postoperative epithelial increase in large myopic cases. This may be an indication of enhanced cornea stability and may reduce the incidence of future myopic regression and/or the potential for ectasia. The prospective clinical relevance of the theories presented herein is quite important in our opinion. Myopic regression after LASIK has been traditionally treated over the past 2 decades by retreating the cornea with a new excimer laser ablation, and potentially exaggerating the theoretical response we described in this work. REFERENCES 1. Kanellopoulos AJ. Long-term safety and efficacy follow-up of prophylactic higher fluence collagen cross-linking in high myopic laser-assisted in situ keratomileusis. Clin Ophthalmol. 2012;6:1125 1130. 2. Kanellopoulos AJ, Kahn J. Topography-guided hyperopic LASIK with and without high irradiance collagen cross-linking: initial comparative clinical findings in a contralateral eye study of 34 consecutive patients. J Refract Surg. 2012;28(suppl 11):S837 S840. 3. Kanellopoulos AJ. Collagen cross-linking in early keratoconus with riboflavin in a femtosecond laser-created pocket: initial clinical results. J Refract Surg. 2009;25:1034 1037. 4. Krueger RR, Ramos-Esteban JC, Kanellopoulos AJ. 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Correlation between epithelial thickness in normal corneas, untreated ectatic corneas, and ectatic corneas previously treated with CXL; is overall epithelial thickness a very early ectasia prognostic factor? Clin Ophthalmol. 2012;6:789 800. 20. Møller-Pedersen T, Vogel M, Li HF, et al. Quantification of stromal thinning, epithelial thickness, and corneal haze after photorefractive keratectomy using in vivo confocal microscopy. Ophthalmology. 1997;104: 360 368. 21. Francoz M, Karamoko I, Baudouin C, et al. Ocular surface epithelial thickness evaluation with spectral-domain optical coherence tomography. Invest Ophthalmol Vis Sci. 2011;52:9116 9123. 22. Rocha KM, Perez-Straziota E, Stulting RD, et al. SD-OCT analysis of regional epithelial thickness profiles in keratoconus, postoperative corneal ectasia, and normal eyes. J Refract Surg. 2013;29:173 179. 23. Kanellopoulos AJ, Asimellis G. 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Cornea Volume 33, Number 5, May 2014 Topographic Epithelial Profile Thickness Changes 26. Reinstein DZ, Archer TJ, Gobbe M. Change in epithelial thickness profile 24 hours and longitudinally for 1 year after myopic LASIK: threedimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg. 2012;28:195 201. 27. Reinstein DZ, Srivannaboon S, Gobbe M, et al. Epithelial thickness profile changes induced by myopic LASIK as measured by Artemis very high-frequency digital ultrasound. J Refract Surg. 2009;25:444 450. 28. Moilanen JA, Holopainen JM, Vesaluoma MH, et al. Corneal recovery after lasik for high myopia: a 2-year prospective confocal microscopic study. Br J Ophthalmol. 2008;92:1397 1402. 29. Kanellopoulos AJ, Asimellis G. Three-dimensional LASIK flap thickness variability: topographic central, paracentral and peripheral assessment, in flaps created by a mechanical microkeratome (M2) and two different femtosecond lasers. Clin Ophthalmol. 2013;7:675 683. 30. Reinstein DZ, Archer TJ, Gobbe M. Refractive and topographic errors in topography-guided ablation produced by epithelial compensation predicted by three-dimensional Artemis very high-frequency digital ultrasound stromal and epithelial thickness mapping. J Refract Surg. 2012;28:657 663. 31. Kanellopoulos AJ, Asimellis G. Introduction of quantitative and qualitative cornea optical coherence tomography findings induced by collagen cross-linking for keratoconus: a novel effect measurement benchmark. Clin Ophthalmol. 2013;7:329 335. 32. Knox Cartwright NE, Tyrer JR, Marshall J. Age-related differences in the elasticity of the human cornea. Invest Ophthalmol Vis Sci. 2011;52: 4324 4329. 33. Magallanes R, Shah S, Zadok D, et al. Stability after laser in situ keratomileusis in moderately and extremely myopic eyes. J Cataract Refract Surg. 2001;27:1007 1012. 34. Liu Z, Li Y, Cheng Z, et al. Seven-year follow-up of LASIK for moderate to severe myopia. J Refract Surg. 2008;24:935 940. 35. Alió JL, Muftuoglu O, Ortiz D, et al. Ten-year follow-up of laser in situ keratomileusis for myopia of up to -10 diopters. Am J Ophthalmol. 2008; 145:46 54. Ó 2014 Lippincott Williams & Wilkins www.corneajrnl.com 469