Femtosecond lasers for laser in situ keratomileusis: a systematic review and meta-analysis

Introduction

Laser-assisted in situ keratomileusis (LASIK) is the most commonly used refractive surgery technique for the correction of myopia, hyperopia, and astigmatism.^1,2 The first phase of LASIK, the creation of a corneal flap, is the most critical step of LASIK, and it affects the visual outcome of the whole procedure. The flap creation is followed by excimer laser ablation of the exposed stroma after which the flap is repositioned. The technological evolution of flap creation has emerged from mechanical manually guided microkeratomes to automated microkeratomes and single-use microkeratomes, and most recently to femtosecond (FS) laser technology.^3,4 In the FS laser technology, FS laser photodisrupts tissue at a preset depth and produces microcavitation bubbles consisting of water and carbon dioxide.⁵ The expansion of these bubbles separates the corneal lamellae and forms a resection plane.⁵

There are several FS lasers on the market. IntraLase, now produced by Abbott Medical Optics Inc. (Santa Ana, CA, USA), was the first FS laser keratome introduced in the USA in 2001.⁶ Technolas Femtosecond Workstation, formerly known as Femtec, by Technolas Perfect Vision (Munich, Germany), was introduced immediately after the market launch of IntraLase.⁷ FEMTO LDV by Ziemer Ophthalmic Systems (Port, Switzerland) was introduced in the late 2005, and Visumax by Carl Zeiss Meditec AG (Jena, Germany) in the fall 2006.⁷ Wavelight FS200 by Alcon Laboratories, Inc. (Fort Worth, TX, USA) received the US Food and Drug Administration clearance for marketing in the USA in the late 2010.⁸

FS laser technology has been increasingly used in LASIK. According to a poll conducted in 2006, >30% of the LASIK flaps were created by FS laser,⁹ while according to a MarketScope’s second-quarter survey for the year 2010, ~70% of the LASIK flaps were created using an FS laser.¹⁰ As the published meta-analysis studies for FS lasers for LASIK have been concentrated only on IntraLase,^10,11 the current study was undertaken to review and meta-analyze whether there are differences between reported FS lasers for LASIK in terms of efficacy, predictability, and safety as primary outcomes and corneal flap thickness measurements (difference from the target flap thickness and combined standard deviation [SD]) and complications as secondary outcomes.

Materials and methods

Literature search strategy

Comprehensive literature searches of PubMed and Science Direct databases were first conducted on December 2, 2013. The literature search of Scopus database was conducted on March 10, 2014, and another literature search of the Cochrane CENTRAL Trials database on October 4, 2014. After the review process, all the databases were rechecked on December 28, 2015, for newer publications. The literature searches were conducted by using the following terms: “femtosecond laser lasik clinical controlled randomized” and “femtosecond laser lasik flap thickness controlled randomized” without date limitations. Language was restricted to English. Citations initially selected were first retrieved as titles by one reviewer (AH). After the initial screening, potentially relevant articles were retrieved as abstracts and screened by all authors. After this step, relevant articles were retrieved as complete papers and assessed for compliance.

Quality scoring

The quality of each study was assessed using the Jadad et al¹² score with a scale of 0–5. Each study was assessed by three main aspects of study design: randomizing, masking, and participant withdrawals/dropouts. One point was given for the presence of randomizing, masking, and participant withdrawals/dropouts. If randomizing and blinding were appropriate, one additional point was added to each. Studies scoring <3 points were considered to be of low quality.

Outcome measures

The primary outcome measures were efficacy, predictability, and safety. The efficacy measure was the proportion of eyes achieving an uncorrected distance visual acuity (UDVA), an UDVA of 20/20 or better. The predictability measure was refraction within ±0.5 diopters (D) of mean target spherical equivalent refraction. The safety measure was a loss of ≥2 Snellen lines of CDVA, a corrected distance visual acuity. Secondary outcome measures were flap thickness measurements (mean flap thickness and flap predictability as SD), and intraoperative and postoperative complication rates. For primary outcome measures, hyperopic eyes were excluded from the analysis.

Statistical methods

The data were pooled together for different FS lasers. Percentage values were calculated back to basic count values in order to make nonparametric analysis possible. Statistical significance for efficacy, predictability, safety, and intra- and postoperative complications was evaluated with the chi-square test. Corneal flap thickness measurements were also pooled together and analyzed with the one-way analysis of variance (ANOVA) (SigmaPlot; Systat Software Inc., San Jose, CA, USA). In this meta-analysis, which included several previously published studies, P-value <0.001 was considered significant.

Results

Results of the literature search

Figure 1 shows the flowchart of the studies from the initial literature search to the final inclusion. Based on the full paper review, 21 controlled randomized trials and ten prospective or retrospective nonrandomized studies were included in the meta-analysis.

Figure 1 Flowchart of study selection.

Primary outcome measures

Table 1 shows the main preoperative characteristics of the 31 studies describing a total of 5,404 eyes included in the meta-analysis. Studies written in bold (nine studies) have been included in the previously published meta-analysis studies^10,11 concerning FS laser for LASIK. Postoperative characteristics for primary and secondary outcomes are presented in Table 2.

Table 1 Characteristics of the studies included in the meta-analysis of femtosecond lasers for LASIK Notes: aMean manifest defocus refraction as reported by the authors. Studies written in bold have been included in the previously published meta-analyses.10,11 Abbreviations: D, diopters; LASIK, laser-assisted in situ keratomileusis; SD, standard deviation; SE, refractive spherical equivalent; mo, months; WF, wave front; wk, weeks; preop, preoperative; intraop, intraoperative.

Table 2 Postoperative outcome measures in the studies included in the meta-analysis of femtosecond lasers for LASIK Notes: –, not reported; aestimated from a figure; bhyperopic eyes were excluded; crefraction within the ±0.25 D of target refraction; donly astigmatism correction. Studies written in bold have been included in the previously published meta-analyses.10,11 Abbreviations: CM, confocal microscopy; LASIK, laser-assisted in situ keratomileusis; OBL, opaque bubble layer; OCP, optical coherence pachymetry; OCT, optical coherence tomography; SD, standard deviation; SOP, subtraction online pachymetry, difference between preoperative corneal thickness and the thickness of corneal bed after flap lifting; SUP, subtraction ultrasound pachymetry; VHF, very-high-frequency digital ultrasound; WF, wave front; postop, postoperative; intraop, intraoperative; mo, month.

Efficacy

Among the different IntraLase types, the efficacy ranged from 85.1% (IntraLase FS 60 kHz, seven studies included^13–19) to 100% (IntraLase FS 10²⁰ and 30 kHz,²¹ Figure 2). For the most commonly reported IntraLase FS 60 kHz, the average efficacy was 85.1%. In one study, IntraLase FS 60 or 150 kHz²² was used and its efficacy was 98.3%. For IntraLase 150 kHz,^19,23–25 the average efficacy was 93.7%. For FEMTO LDV,^26,27 the average efficacy was 91.3%. For Visumax^18,24,28 and Wavelight FS200,^29,30 the average efficacy rates were 79.1% and 83.6%, respectively.

Figure 2 Efficacy, the proportion of eyes within UDVA ≥20/20 after FS laser for LASIK. Abbreviations: χ2, chi-square statistic; CI, confidence interval; df, degrees of freedom; FS, femtosecond; LASIK, laser-assisted in situ keratomileusis; UDVA, uncorrected distance visual acuity.

Predictability

The average predictability for the different IntraLase types ranged from 91.0% (IntraLase FS 60 kHz^14–19) to 95.9% (IntraLase FS 30 kHz;^21,31 Figure 3). FEMTO LDV^26,27 had the average predictability of 89.4%. For Visumax^18,24 and Wavelight FS200,^29,30 the average predictabilities were 87.1% and 90.5%, respectively.

Figure 3 Predictability, the proportion of eyes within ±0.5 D of target refraction after FS laser for LASIK. Abbreviations: χ2, chi-square statistic; CI, confidence interval; D, diopters; df, degrees of freedom; FS, femtosecond; LASIK, laser-assisted in situ keratomileusis; ns, nonsignificant.

Safety

The average safety percentage was zero or very close to zero for all the studied FS laser types (Figure 4). For IntraLase FS 15^32–36 and 30 kHz,^21,31,37 the average safety was 0.4% and 0.7%, respectively. For IntraLase FS 60 kHz,^13–18 it was 0.2%. For Wavelight FS 200,^29,30 it was 0.5%.

Figure 4 Safety, the proportion of eyes losing two or more Snellen lines of CDVA after FS laser for LASIK. Abbreviations: CDVA, corrected distance visual acuity; χ2, chi-square statistic; CI, confidence interval; df, degrees of freedom; FS, femtosecond; LASIK, laser-assisted in situ keratomileusis; ns, nonsignificant.

Secondary outcome measures

Flap thickness measurements

The average difference from the target flap thickness for IntraLase FS 30^31,37 and 60 kHz^{13,14,16,38–40} was +0.8 and +0.6 μm, respectively (Figure 5). For IntraLase FS 15 kHz,^33,34,41 the average difference from the target flap thickness was much bigger, +6.7 μm. For FEMTO LDV,^26,40,42 the average difference from the target flap thickness was 5.3 μm less than intended. For Visumax,^28,40 the difference from the target was 10.6 μm more than intended. Wavelight FS200^29,30 produced corneal flaps that were close to the intended value (difference from the target flap thickness was +0.03 μm). The average SD for IntraLase FS 15 and 30 kHz was 15.5 and 13.8 μm, respectively. For the most studied IntraLase FS 60 kHz, the average SD was 12.4 μm. For FEMTO LDV, the reproducibility, the average SD was 8.2 μm. For Visumax, the average SD was 23.3 μm and for Wavelight FS200 14.3 μm.

Figure 5 Corneal flap measurements after FS laser for LASIK. Abbreviations: F, F factor; FS, femtosecond; LASIK, laser-assisted in situ keratomileusis; ns, nonsignificant; SD, standard deviation.

Complication rates

Among the different IntraLase types, IntraLase FS 10 kHz²⁰ had no intraoperative complications, but postoperative complication rate was 37.5% (one study was included; Figures 6 and 7). IntraLase FS 15 kHz^33–35,41 had no intraoperative complications, but the average postoperative complication rate was 18.0%. IntraLase FS 30 kHz^31,37 had no intraoperative complications, while the percentage for postoperative complication was 2.6%. IntraLase FS 60 kHz^13–19 had the average intraoperative and postoperative complication rates of 1.4% and 3.2%, respectively. IntraLase 150 kHz^19,24 had the average intraoperative complication rate of 0.9% and postoperative complication rate of 0%. FEMTO LDV^26,27 had the average intraoperative complication rate of 11.8%. The postoperative complication rate for FEMTO LDV averaged 9.9%. For Visumax,^24,28 intraoperative and postoperative complication rates averaged 3.3% and 0.2%, respectively. Wavelight FS200^29,30 had the lowest intraoperative and postoperative complication rates of 0.4% and 0%, respectively.

Figure 6 Intraoperative complications after FS laser for LASIK. Abbreviations: χ2, chi-square statistic; CI, confidence interval; df, degrees of freedom; FS, femtosecond; IO, intraoperative; LASIK, laser-assisted in situ keratomileusis.

Figure 7 Postoperative complications after FS laser for LASIK. Abbreviations: χ2, chi-square statistic; CI, confidence interval; df, degrees of freedom; FS, femtosecond; LASIK, laser-assisted in situ keratomileusis; PO, postoperative.

Discussion

This meta-analysis compared different types of FS lasers for LASIK. In the literature search, we found 109 potentially relevant abstracts for review, but only 47 were suitable for a full paper review. Additionally, we included one study³³ from the previously published meta-analyses.^10,11 From these 48 articles, we excluded 17 papers based on lacking or duplicate data, and the final analysis included 31 articles. For primary outcome measures, hyperopic eyes were excluded. No studies were excluded from the analysis due to the low-quality scoring by Jadad et al system.¹²

The different FS laser systems can be classified into two groups: one group is characterized with high pulse energy–low pulse frequency (such as IntraLase and Femtec) and the other with low pulse energy–high pulse frequency (such as FEMTO LDV and Wavelight FS200).⁷ In the FS laser technology in the high pulse energy–low pulse frequency group, pulse energies are in the range of 1 μJ and repetition rates on the order of kilohertz.⁷ The low pulse energy–high pulse frequency system delivers only pulse energy on the order of nano-joule and uses megahertz repetition rates.⁷ From all FS lasers on the market, IntraLase was the first introduced and different IntraLase FS types (10, 15, 30, 60, and 150 kHz) are the most commonly reported.

The pooled primary outcome results showed that there was a statistically significant difference in the proportion of eyes within a UDVA of 20/20 or better (efficacy). Based on efficacy, IntraLase and FEMTO LDV gave the best results. There were also statistically significant differences in the mean spherical equivalent refraction within ±0.5 D of target refraction (predictability). Based on predictability, IntraLase types FS 15, FS 30, and FS 60 kHz were the best. There was no statistically significant difference in the loss of ≥2 Snellen lines of CDVA (safety) between different FS lasers.

It was difficult to combine the results of randomized controlled trials because of different follow-up times. In the 31 studies chosen for this meta-analysis, the follow-up time was ≤1 month in five studies.^{14,32,37,39,42} The most commonly reported follow-up times were 3 months (ten studies) ^{18,20,22,24,30,31,33,36,38,50} and 6 months (eight studies).^{13,15,21,26–29,41} The follow-up time was ≥1 year only in seven cases.^{16,17,19,23,25,34,35} Long follow-up times should be recommended for reporting refractive results, especially in controlled randomized studies. In the meta-analysis, different excimer laser choices used in LASIK made it also more difficult to compare refractive results. Another drawback of this meta-analysis is that due to limited reporting, the results were pooled together from standard, wavefront-guided, and wavefront-optimized treatments, and there was no compensation for this.

Mechanical microkeratomes typically create meniscus-shaped flaps that are thinner in the center and thicker in the periphery, whereas FS laser flaps have been found to be typically more uniformly planar.^43–47 Preliminary studies with the IntraLase FS laser have demonstrated that free flaps, irregular flaps, microperforations, decentered flaps, epithelial defects, and abrasions were significantly reduced or eliminated.^48–50 The SD of achieved flap thickness with FS lasers has also been found to be narrower than with mechanical systems.^4,46

Although in this meta-analysis flap thickness measurements and complication rates were classified as secondary outcomes from the surgeons’ point of view, they are in fact the areas of major concern in LASIK. In the meta-analysis, the pooled secondary outcome results showed some variations between different FS lasers. Based on the SD of the measured flap thicknesses, FEMTO LDV reproduced the most accurate flap thicknesses.

Certain complications have been shown to be unique to the FS laser, such as transient opaque bubble layer (OBL),^51–55 especially with the IntraLase, transient light sensitivity syndrome,^56,57 increased corneal backscatter,⁴¹ and rainbow glare.^58–60 The incidence of transient OBL, transient light sensitivity syndrome, and rainbow glare has reduced with lower energies used. In this meta-analysis, IntraLase and Wavelight SF200 had the fewest intraoperative complications. IntraLase FS 60 kHz, Visumax, and Wavelight FS200 had the most seldom postoperative complications. In the meta-analysis, it was difficult to compare the complications based on the percentage of complications reported in the studies. In general, there were very few intraoperative complications reported. The most common intraoperative complications were a loss of suction, OBL, and adhesions. The most frequently reported postoperative complications were diffuse lamellar keratitis and microstriae. No ectasia was found in these studies. However, there seemed to be big differences between authors in reporting complications in LASIK. For instance, in the first FEMTO LDV study that reported the results of a preproduction FS laser system, its intraoperative complication rate was 17.1% and postoperative complication rate was 14.4%.²⁶ Furthermore, there was a mild epithelial sloughing in 11.8% of the eyes. Yet, in another FEMTO LDV study, there were no complications.²⁷ Therefore, we suggest a standardized system for reporting complications in refractive surgery. Intraoperative side effects that do not have any effects on the refractive outcome should also be reported, such as OBL, decentered, incomplete or free flaps, and flap adhesion. Bleeding from the limbal vessels should also be reported.

Conclusion

In conclusion, there were dissimilarities between different FS lasers based on efficacy and intraoperative and postoperative complications. All FS lasers were predictable and safe for making corneal flaps in LASIK.

Acknowledgment

Dr Matti Kataja is acknowledged for his statistical and graphical expertise.

Disclosure

Dr Pietilä has financial interest in the Ziemer Ophthalmic Systems. The other authors report no conflicts of interest in this work.

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