Hyperopic laser in situ keratomileusis: Comparison of femtosecond laser and mechanical microkeratome flap creation

ARTICLE

Hyperopic laser in situ keratomileusis: Comparison of femtosecond laser and mechanical microkeratome flap creation Rafic Antonios, MD, Samuel Arba Mosquera, PhD, Shady T. Awwad, MD

PURPOSE: To evaluate and compare the refractive predictability and stability of laser in situ keratomileusis (LASIK) flap creation performed with a femtosecond laser and with a mechanical microkeratome to correct mild to moderate hyperopia. SETTING: American University of Beirut Medical Center, Beirut, Lebanon. DESIGN: Retrospective case series. METHODS: Patients who had hyperopic LASIK treatment using the Amaris excimer laser were included. Eyes in which the LDV femtosecond laser was used for flap creation were compared with eyes in which the Moria M2 microkeratome was used. RESULTS: The microkeratome group comprised 53 eyes and the femtosecond laser group, 72 eyes. Baseline characteristics were similar between groups (P > .05). The mean spherical equivalent (SE) deviation from target 1 week postoperatively was 0.08 diopter (D) G 0.58 (SD) in the femtosecond laser group and 0.06 G 0.87 D in the microkeratome group (P Z .92). Thereafter, the mean SE deviation from target increased gradually and by 6 months postoperatively was C0.30 G 0.50 D and C0.70 G 0.71 D, respectively (P Z .001). The correlation between the achieved and the attempted SE refraction was better in the femtosecond laser group (R2 Z 0.806) than the microkeratome group (R2 Z 0.671). CONCLUSIONS: Using the same nomogram, the short-term refractive outcomes of hyperopic LASIK with flap creation performed with the femtosecond laser were comparable to those for the microkeratome; however, the femtosecond group showed significantly better stability over the 6-month follow-up and better predictability, as reflected by a lower standard deviation and stronger Pearson correlation. Financial Disclosure: Dr. Arba Mosquera is an employee of Schwind eye-tech-solutions GmbH and Co. KG. No other author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2015; 41:1602–1609 Q 2015 ASCRS and ESCRS

Among the techniques used to correct low to moderate hyperopia, including laser in situ keratomileusis (LASIK), photorefractive keratectomy, conductive keratoplasty, and radial keratotomy, LASIK is the preferred modality of treatment.1 Hyperopic eyes present additional challenges to the refractive surgeon, who must integrate the accommodative effect of the eye and a larger angle a as part of the planned excimer laser treatment.1 Despite detailed planning to address these challenges, visual and refractive outcomes of LASIK treatment are less predictable in hyperopic eyes than in myopic eyes1,2 because with hyperopic 1602

Q 2015 ASCRS and ESCRS Published by Elsevier Inc.

eyes there is a greater tendency toward regression and a heightened risk for losing corrected distance visual acuity (CDVA).3–6 Another cause of regression of the effect of hyperopic refractive treatments, apart from the postoperative loss of accommodative spasm, are mechanical structural instability to the corneal stroma and/or irregular epithelial regeneration surrounding the zone of refractive ablation; yet, the ability to anticipate confounding biological responses at the level of the individual patient remains limited.7 In addition to applying advancements in laserdelivery platforms, several techniques have been http://dx.doi.org/10.1016/j.jcrs.2014.11.049 0886-3350

FEMTOSECOND LASER VS MICROKERATOME FOR LASIK FLAP CREATION

explored to improve the outcomes of hyperopic LASIK treatments. These include nomogram refinement, increasing the size of the optical zone and the size of the flap, altering the centration of the ablation treatment (ie, line of sight versus visual axis), and using wavefront technology.1,8,9 A poorly investigated topic is whether flap creation in hyperopic treatments using the femtosecond laser and the microkeratome results in different outcomes. Results in 1 study3 suggested that femtosecond laser–assisted flap creation resulted in more predictable, stable LASIK outcomes than microkeratome-created flaps after 3 months. This study compared the 6-month outcomes of LASIK performed with femtosecond laser flap creation and those performed with mechanical microkeratome flap creation for the correction of low to moderate hyperopia. PATIENTS AND METHODS This retrospective review comprised the charts of all consecutive patients who had hyperopic LASIK at the American University of Beirut Medical Center from January 1, 2011, to December, 31, 2012. Patients who had femtosecond laser–assisted LASIK using the LDV femtosecond laser (Ziemer Ophthalmic Systems AG) were compared with patients who had microkeratome-assisted LASIK performed using the Moria M2 microkeratome (Moria SA); all treatments were performed using the same excimer laser (Amaris, Schwind eye-tech-solutions GmbH and Co. KG) by the same surgeon (S.T.A). This study was approved by the Institutional Review Board, American University of Beirut, and adhered to the tenets of the Declaration of Helsinki. Inclusion criteria included a postoperative follow-up of at least 6 months and a difference between the manifest refraction and the cycloplegic refraction of no more than 0.50 diopter (D). This last criterion reflected a routine practice in the clinic whereby patients with more than a 0.50 D difference were given progressively higher correction over time to break their accommodative spasm and

Submitted: July 10, 2014. Final revision submitted: November 8, 2014. Accepted: November 28, 2014. From the Department of Ophthalmology (Antonios, Awwad), American University of Beirut, Beirut, Lebanon; the Department of Research and Development (Arba Mosquera), Schwind eye-techsolutions GmbH and Co. KG, Kleinostheim, Germany; the Recognized Research Group in Optical Diagnostic Techniques (Arba Mosquera), University of Valladolid, Valladolid, and the Department of Ophthalmology and Sciences of Vision (Arba Mosquera), University of Oviedo, Oviedo, Spain. Corresponding author: Shady T. Awwad, MD, Department of Ophthalmology, American University of Beirut Medical Center, Cairo Street, PO Box 110236, Beirut, Lebanon. E-mail: shady. [email protected].

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reduce their accommodation to 0.50 D or less before treatment. Exclusion criteria included a preoperative CDVA of worse than 20/25, intraoperative or postoperative complications, previous corneal surgeries, and uveal or retinal disease.

Patient Assessments The baseline examination included uncorrected distance visual acuity (UDVA) and CDVA measured on standardized National Eye Institute Early Treatment Diabetic Retinopathy Study charts, manifest refraction spherical equivalent (MRSE) using the fogging technique, cycloplegic refraction, slitlamp evaluation, Placido and dual-Scheimpflug corneal tomography using the Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems AG), and a dilated-fundus evaluation. Patients were informed about the characteristics of laser flap creation using the mechanical microkeratome and the femtosecond laser. Then, patients were allowed to choose between the techniques because the cost of them was different. Postoperative examinations were performed at 1 week and 1, 3, and 6 months and included UDVA, CDVA, MRSE, slitlamp examination, corneal tomography, and a dilated fundus evaluation.

Surgical Technique In the microkeratome group, the flap was created using suction rings of C2, C1, 0, or 1 and cutting heads of 90 mm or 110 mm, depending on the preoperative corneal curvature and according to the manufacturer’s nomogram to target a flap of at least 9.0 mm. The 90 mm and 110 mm microkeratomes both yield variable flap thicknesses that are much thicker peripherally than centrally; the 90 mm microkeratome flaps have a mean central thickness of 115 to 128 mm, and the 110 mm microkeratome flaps have a mean central thickness of 135 to 140 mm.10,11 In the femtosecond laser group, a 9.0 mm diameter flap was created at an attempted depth of 110 mm according to the following parameters: tightly focused spots less than 2 mm  2 mm  2 mm in dimension, a pulse width of 250 fs, a pulse rate of 5 MHz, and a bed energy level of 100 nJ. The femtosecond laser yields flaps less variable in thickness, with a mean thickness of 105 mm centrally and 120 mm peripherally.12 In both groups, after flap creation, the refractive error was corrected using the excimer laser. The treatment was centered on the corneal vertex when the difference between the pupil centroid and corneal vertex was less than 0.30 mm in cord length as measured by the Scout topographer (Keratron Scout, Optikon 2000 SpA). For differences of more than 0.30 mm, the treatment was centered at three quarters of the distance, closer to the vertex. A small nomogram adjustment was applied to the sphere component in both groups. In many patients, partial monovision or full monovision in the nondominant eye was performed. Any change to the actual treatment was input into the laser as the target refraction. For example, a nomogram adjustment of a 0.25 D boost to the actual treatment of C3.00 D was made by typing C3.00 D for the patient refraction and inputting 0.25 D for the target refraction, which made the actual laser treatment C3.25 D. For partial monovision, targeting a final 1.50 D postoperative refraction requires setting the target refraction to 1.50 D plus the nomogram adjustment

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(ie, 1.5 0.25 Z 1.75 D); thus, the total laser treatment becomes C3.00 C 1.75 Z C4.75 D.

men to women preoperatively. A P value less than 0.05 was considered statistically significant.

Main Outcome Measures

RESULTS

The main outcome measures were CDVA, CDVA, MRSE, manifest refractive cylinder, and simulated keratometry. The outcomes were recorded preoperatively and 1 week and 1, 3, 6, and 12 months postoperatively. The spherical equivalent (SE) deviation from target was defined as the actual postoperative MRSE minus the target SE refraction and was calculated at every postoperative visit in each group. The target SE refraction incorporated both nomogram and monovision adjustments. The SE deviation from target represents the laser treatment error and/or refractive drift, independent of confounding factors such as nomogram or monovision adjustments, and is more accurate than using the postoperative SE refraction only. An intended monovision of 1.50 D and a nomogram boost of 0.25 D would require a target SE refraction of 1.75 D. An actual final SE refraction of plano would imply an SE deviation from target of C1.75 D. Stability was defined as the repeatability of initial postoperative results (first week) over the 6 months of follow-up. Predictability was directly associated with a lower standard deviation (SD) and standard error of the SE deviation from the target at any point and a better tightness of fit of the attempted SE versus the achieved SE plot (reflected by the correlation coefficient R2).

Statistical Analysis Clinical data were analyzed using SPSS software (version 20.0, International Business Machines Corp.). Descriptive analysis with SDs for means was performed. The independent-sample t test with Bonferroni correction was used to compare the 2 groups at different timepoints. The chi-square test was used to compare the proportions of

The study included 125 eyes, 53 in the microkeratome group and 72 in the femtosecond laser group. No statistically significant differences were noted in the preoperative baseline characteristics between groups (Table 1). There was no statistically significant difference in the ratio of sex between the 2 groups (P Z.573; c2 test). Refractive Results and Stability Table 2 shows the postoperative refractive results and vision outcomes. Eyes with femtosecond lasercreated flaps had a statistically significant and clinically significant lower MRSE than eyes with microkeratome-created flaps (P ! .001). Because many surgeries were designed to overcorrect the hyperopic refractive error and target some form of monovision, evaluation of the SE deviation from the target was more appropriate (see Patients and Methods for calculation detail). Figure 1, A, plots the change in SE deviation from target versus time. One week postoperatively, the SE deviation from target was close to zero in both groups, indicating that the laser ablation was effective in achieving the target SE in each group (Figure 1, A). However, 6 months postoperatively there was an overall undercorrection of refractive error in the 2 groups, although less so in the femtosecond group (Table 2) (P Z .001). This finding further indicates that regression rather

Table 1. Baseline characteristics of patients by group. Study Group Microkeratome (19 Men, 34 Women)

Femtosecond (23 Men, 49 Women)

Parameter

Mean G SD

Range

Mean G SD

Range

P Value*

Age MRSE (D) Cycloplegic refraction SE (D) Cycloplegic refraction cylinder (D) Target SE (D) Simulated keratometry average (D) UDVA (logMAR) CDVA (logMAR)

44.67 G 12.09 2.25 G 1.06 2.66 G 1.03 0.62 G 0.68

19, 61 C0.75, 5.00 C1.25, 5.50 0.00, 3.00

46.11 G 10.20 2.24 G 0.95 2.36 G 0.95 0.49 G 0.63

18, 66 C0.50, 4.75 C1.00, 5.25 0.00, 3.00

.371 .962 .102 .273

0.48 G 0.49 43.20 G 0.86

1.50, 0.00 41.20, 45.16

0.62 G 0.63 43.07 G 1.30

2.00, 0.00 37.29, 45.50

.175 .526

0.34 G 0.21 0.01 G 0.03

0.00, 0.70 0.00, 0.10

0.31 G 0.16 0.00 G 0.02

0.00, 0.70 0.00, 0.10

.363 .254

CDVA Z corrected distance visual acuity; MRSE Z manifest refraction spherical equivalent; SE Z spherical equivalent; UDVA Z uncorrected distance visual acuity *Analysis performed using the independent t test (age, MRSE, cycloplegic refraction SE, , simulated keratometry average, UDVA)

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than ineffective laser ablation was culprit in the eyes that had mechanical microkeratome flap creation (Figure 1, C). More postoperative regression was observed in eyes in the microkeratome group than in the femtosecond laser group. The differences in SE deviation from target between the 2 groups at 1 week, 1 month, and 3 months were not statistically significant (P Z .92, P Z .02, and P Z .02, respectively; P ! .01 is statistically significant with Bonferroni correction). However, at 6 months, the difference of 0.40 D between the 2 groups became statistically significant (P Z .001).

Predictability The consistently smaller standard error for the SE deviation from target in the femtosecond laser group than in the microkeratome group over the postoperative follow-up period (Figure 1, A) signifies less divergence from the intended level of correction by the excimer laser and indirectly indicates a more predictable refractive correction. Overall, at 6 months, 47 eyes (65.3%) in the femtosecond laser group and 23 eyes (43.4%) in the microkeratome group were within G0.50 D of the SE deviation from target (Figure 1, B). Also, 17 eyes (32.1%) in the microkeratome group had an SE deviation from target of C1.00 D or more compared with 8 eyes (11.1%) in the femtosecond laser group. In addition, for the attempted SE versus the achieved SE, the femtosecond laser group had a better correlation (R2 Z 0.806) than the microkeratome group (R2 Z 0.671), which is reflected in the better data clustering and less scatter around the best-fit line in Figure 1, C.

Nomogram Adjustment Figure 1, D, plots the change in SE deviation from target at 6 months against the attempted correction. There was a very low correlation between the 2 variables in both study groups. The best-fit line in the femtosecond laser group was a mean of C0.30 D above the zero line (range C0.20 to C0.46 D for attempted corrections of C1.00 to C6.00 D, respectively). The best-fit line in the microkeratome group was a mean of C0.70 D above the zero line (range C0.57 to C0.80 D for attempted corrections of C1.00 to C6.00 D, respectively). Figure 2, A and B, shows the preoperative and 6-month postoperative refractive astigmatism. Forty-two eyes (58.3%) the femtosecond laser group and 26 eyes (49%) in the microkeratome group had 0.25 D or less of refractive astigmatism at 6 months (P O .05). Efficacy and Safety In the subset of eyes corrected for distance vision (39 for microkeratome and 47 for femtosecond laser), more achieved a UDVA of 20/20 in the femtosecond laser group than in the microkeratome group (Figure 2, C and D). No patient lost lines of CDVA (Figure 3). DISCUSSION A smoother optical surface is assumed to result in better visual and refractive outcomes after laser refractive surgery.13 Flaps created with a femtosecond laser can result in a smoother optical surface than flaps created with a microkeratome14; however, this femtosecondlaser advantage is not clear in the literature because

Table 2. Characteristics 6 months postoperatively. Study Group Microkeratome

Femtosecond

Parameter

Mean G SD

Range

Mean G SD

Range

P Value

MRSE (D) SE deviation from target (D) MR sphere (D) MR cylinder (D) UDVA (logMAR) CDVA (logMAR) Simulated keratometry average (D)

0.22 G 0.75 0.70 G 0.71 0.40 G 0.73 0.36 G 0.35 0.04 G 0.12 0.01 G 0.02 45.33 G 1.40

1.25, C1.75 0.63, C2.25 1.25, C2.00 1.50, C0.50 0.00, 0.54 0.00, 0.10 42.02, 48.43

0.32 G 0.76 0.30 G 0.50 0.19 G 0.82 0.26 G 0.39 0.03 G 0.08 0.00 G 0.01 44.99 G 1.17

2.13, C1.50 0.78, C1.63 2.50, C1.50 1.25, C0.75 0.00, 0.48 0.00, 0.10 42.99, 47.34

!.001 .001 !.001 .129 .633 .18 .577

CDVA Z corrected distance visual acuity; MR Z manifest refraction; MRSE Z manifest refraction spherical equivalent; SE Z spherical equivalent; UDVA Z uncorrected distance visual acuity

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Figure 1. Mechanical microkeratome flap versus femtosecond flap in terms of change in mean SE deviation from target over time after LASIK for hyperopia (A), SE deviation from target 6 months postoperatively (B), predictability of attempted MRSE versus achieved change in MRSE 6 months postoperatively (C), and attempted SE versus SE deviation from target (D). The solid black line denotes the SE deviation from target of zero in A and D and indicates where achieved SE was equal to attempted SE in C. The green and blue solid lines denote the best-fit lines for the femtosecond and microkeratome scatter values, respectively.

studies of the 2 techniques in myopic treatments have conflicting results.15–18 In a metaanalysis of 15 papers describing 3679 myopic eyes,19 LASIK with femtosecond laser flap creation showed no advantage over LASIK with mechanical microkeratome flap creation in terms of safety, efficacy, or change in higher-order aberrations, except for a potential advantage of being more likely to achieve a postoperative refraction within G0.50 D of the target.

Regarding hyperopic LASIK treatment, only 1 study3 reported better refractive results at 3 months with femtosecond laser flap creation than with mechanical microkeratome flap creation. Evaluating refractive results in hyperopic LASIK can be challenging. One important factor in this challenge is that reporting refractive data as percentage of eyes within G0.50 D of the manifest refraction can be misleading20; many hyperopic eyes undergo some sort of monovision treatment with

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Figure 2. Mechanical microkeratome flap versus femtosecond flap in terms of refractive astigmatism at baseline compared with 6 months postoperatively (A and B) and preoperative CDVA compared to UDVA (C and D) in the subset of eyes that were intended for distance correction (CDVA Z corrected distance visual acuity; UDVA Z uncorrected distance visual acuity).

purposeful overcorrection or are overcorrected in anticipation of regression. For this reason, the SE deviation from the target is better than the simple postoperative MRSE for quantifying the true effectiveness of a hyperopic treatment in an individual. Another important

Figure 3. Mechanical microkeratome flap versus femtosecond flap in terms of change in CDVA 6 months postoperatively (CDVA Z corrected distance visual acuity).

factor is that interpreting refractive stability can be difficult because manifest refraction can increase with age as a result of the reduced ability to compensate for latent hyperopia, falsely simulating regression. Limiting the treatment to hyperopic eyes with a difference between the cycloplegic refraction and manifest refraction of 0.50 D or less, as opposed to 1.00 D or less, as suggested by Gil-Cazorla et al.,3 minimizes this confounding variable. Femtosecond laser–created flaps have several characteristics that could affect the efficacy of the laser ablation and the postoperative refractive stability. They are planar and predictable in terms of achieving the intended flap thickness, have a more uniform thickness,21–23 and are of a size that is precisely created independent of corneal anatomy.24,25 Microkeratomecreated flaps, on the other hand, are less predictable,21,22 are meniscus shaped (ie, thinner in the center and thicker in the periphery),21,23 and have more variable diameters and peripheral depths than femtosecond flaps because of the dependence on corneal diameter and keratometry.24

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Figure 4. According to the model of Dupps and Roberts,26 the nonuniform (meniscal) microkeratome flap creation would result in deeper peripheral disruption of collagen lamellae than the more uniform femtosecond flap, which could lead to more lamellar retraction, peripheral steepening, and central flattening.

A larger flap allows adequate peripheral laser ablation, leading to a more accurate and potentially more stable ablation. A planar flap results in less undermining of peripheral stromal fibers. A meniscal flap, on the other hand, results in deeper cutting of peripheral fibers and a potential biomechanical response in which the peripheral fibers, attached to the sclera from 1 side and loose on the other side, pull up, causing the central corneal bed to flatten and resulting in a hyperopic shift (Figure 4).26 In addition, the stroma is more hydrated after microkeratome flap creation than after femtosecond laser flap creation, which limits the efficacy of laser ablation, resulting in undercorrection.27,28 We believe this factor is more important in hyperopic treatments because the peripheral stromal bed tends to be more exposed than the central stromal bed to intraoperative hydration after flap lift. Finally, femtosecond laser–created flaps show greater fibrotic scarring in the flap edges,29 which could explain the stronger flap adhesion in the late postoperative period and the stronger biomechanical flap stability compared with microkeratome-created flaps. Whether some of this stability is transferred to the overall cornea is debatable. In the present study, both modalities of flap creation were safe and effective in correcting eyes with low to moderate hyperopia in the first week; however, eyes with a femtosecond laser–created flap had statistically and clinically significant better refractive outcomes at 6 months, which is consistent with the previous study by Gil-Cazorla et al.3 Unike Gil-Cazorla et al.,3 however, we did not detect differences in achieving the target SE at 3 months; the small sample size might be a factor. The results in the femtosecond laser group were more accurate and predictable than in the microkeratome group at all postoperative visits, as reflected by a mean SE deviation from target closer to 0 and a smaller SD, and were more stable over time. On the other hand, the nomogram adjustment for hyperopic femtosecond laser flap creation for the Amaris excimer laser would be 0.30 D, as opposed to 0.70 D for the microkeratome. More important, the nomogram adjustment would be more predictable in hyperopic

femtosecond laser flap creation than in microkeratome flap creation because the postoperative variability in laser effectiveness (ie, the SE deviation from target) was statistically and clinically less (as shown by the increased scatter of values around the best-fit line in Figure 1, C and D) and the refractive stability was better over time. Across the board, hyperopic LASIK treatments have had a greater tendency toward undercorrection and regression2,4,6,30 despite adjusting the nomogram, increasing the size of the flap and the optic zone, altering the centration of treatment (ie, line of sight versus visual axis), and advancements in laser delivery platforms.1,9 Femtosecond laser flap creation could be an important modality to enhance the predictability and improve the postoperative refractive stability in hyperopic LASIK treatments. WHAT WAS KNOWN  The predictability and stability of LASIK for hyperopic treatment are not on par with those of myopic treatments. WHAT THIS PAPER ADDS  Performing hyperopic LASIK with femtosecond laser flap creation as opposed to mechanical microkeratome flap creation improved the predictability and stability of the refractive results over a 6-month follow-up.

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5. Kanellopoulos AJ, Conway J, Pe LH. LASIK for hyperopia with the WaveLight excimer laser. J Refract Surg 2006; 22:43–47 6. Kermani O, Schmiedt K, Oberheide U, Gerten G. Hyperopic laser in situ keratomileusis with 5.5-, 6.5-, and 7.0-mm optical zones. J Refract Surg 2005; 21:52–58 7. Dupps WJ Jr, Wilson SE. Biomechanics and wound healing in the cornea. Exp Eye Res 2006; 83:709–720 8. Durrie DS, Smith RT, Waring GO IV, Stahl JE, Schwendeman FJ. Comparing conventional and wavefrontoptimized LASIK for the treatment of hyperopia. J Refract Surg 2010; 26:356–363 9. Kermani O, Oberheide U, Schmiedt K, Gerten G, Bains HS. Outcomes of hyperopic LASIK with the NIDEK NAVEX platform centered on the visual axis or line of sight. J Refract Surg 2009 10. Xu Y, Zhou X, Wang L, Xu H. A morphological study of corneal flap after thin-flap laser-assisted in situ keratomileusis by anterior segment optical coherence tomography. J Int Med Res 2010; 38:1952–1960. Available at: http://imr.sagepub.com/ content/38/6/1952.full.pdf. Accessed May 11, 2015 11. Du S, Lian J, Zhang L, Ye S, Dong S. Flap thickness variation with 3 types of microkeratome heads. J Cataract Refract Surg 2011; 37:144–148 12. Ahn H, Kim JK, Kim CK, Han GH, Seo KY, Kim EK, Kim TI. Comparison of laser in situ keratomileusis flaps created by 3 femtosecond lasers and a microkeratome. J Cataract Refract Surg 2011; 37:349–357 13. Vinciguerra P, Azzolini M, Radice P, Sborgia M, De Molfetta V. A method for examining surface and interface irregularities after photorefractive keratectomy and laser in situ keratomileusis: predictor of optical and functional outcomes. J Refract Surg 1998; 14:S204–S206 14. Kymionis GD, Kontadakis GA, Naoumidi I, Kankariya VP, Panagopoulou S, Manousaki A, Grentzelos MA, Pallikaris IG. Comparative study of stromal bed of LASIK flaps created with femtosecond lasers (IntraLase FS150, WaveLight FS200) and mechanical microkeratome. Br J Ophthalmol 2014; 98:133–137 15. Tran DB, Sarayba MA, Bor Z, Garufis C, Duh Y-J, Soltes CR, Juhasz T, Kurtz RM. Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes; potential impact on wavefrontguided laser in situ keratomileusis. J Cataract Refract Surg 2005; 31:97–105 16. Patel SV, Maguire LJ, McLaren JW, Hodge DO, Bourne WM. Femtosecond laser versus mechanical microkeratome for LASIK; a randomized controlled study. Ophthalmology 2007; 114:1482–1490 17. Lim T, Yang S, Kim MJ, Tchah H. Comparison of the IntraLase femtosecond laser and mechanical microkeratome for laser in situ keratomileusis. Am J Ophthalmol 2006; 141:833–839 18. Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg 2004; 30:804–811 19. Chen S, Feng Y, Stojanovic A, Jankov MR II, Wang Q. IntraLase femtosecond laser vs mechanical microkeratomes in LASIK for myopia: a systematic review and meta-analysis. J Refract Surg 2012; 28:15–24

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First author: Rafic Antonios, MD Department of Ophthalmology, American University of Beirut Medical Center, Beirut, Lebanon