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Comparison of visual outcomes and flap morphology using 2 femtosecond-laser platforms

Garcia-Gonzalez, Montserrat MD, PhD*; Bouza-Miguens, Carmen OD, PhD; Parafita-Fernandez, Alberto MD; Gros-Otero, Juan MD, PhD; Cañones-Zafra, Rafael MD, PhD; Villa-Collar, Cesar OD, PhD; Teus, Miguel A. MD, PhD

Author Information
Journal of Cataract & Refractive Surgery: January 2018 - Volume 44 - Issue 1 - p 78-84
doi: 10.1016/j.jcrs.2017.10.041
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Abstract

Laser in situ keratomileusis (LASIK) is considered a technique of choice for flap creation in the surgical correction of myopia, hyperopia, and astigmatism.1,2 The critical step of creation of the corneal flap can be accomplished with a mechanical microkeratome or a femtosecond laser.

Although several femtosecond laser platforms have been specifically developed for LASIK flap creation and other corneal procedures, the majority of published studies provide the visual outcomes and the corneal flap characteristics with the Intralase platform (Abbott Medical Optics, Inc.).3–7 In 2000, it became the first clinical femtosecond laser approved by the U.S. Food and Drug Administration for creating lamellar corneal flaps, and its results in terms of less pulse of energy, higher pulse repetition rate, and shorter flap-cutting time have improved from the earliest 6 kHz model to the latest 60 kHz and 150 kHz models. In contrast, limited data have been published on the other newer femtosecond laser devices.8–10

Similar to a mechanical microkeratome, the Intralase femtosecond laser induces a high intraocular pressure (IOP) increase caused by the use of a suction ring,11 and it requires an applanation of the cornea to create a perfect cut parallel to the corneal surface. Nevertheless, the femtosecond laser creates a more predictable, thin, planar corneal flap, which is thought to have less biomechanical effect on the cornea than the thicker meniscus-shaped flap usually obtained with a mechanical microkeratome.10

More recently, new femtosecond laser platforms have been developed to create corneal flaps in LASIK and incisions and capsulotomies in cataract surgery with the same device.4 As opposed to the Intralase and the other femtosecond lasers exclusively designed for performing corneal procedures that use a flat patient interface, new dual femtosecond-laser platforms require a curved patient interface. Theoretically, a curved contact surface could make the creation of a planar flap more technologically challenging3; however, to date, only 1 study12 has evaluated the corneal flap morphology when a dual femtosecond laser device (Lensx, Alcon Laboratories, Inc.) was used.

Given that a thick flap is a major risk factor for corneal ectasia in eyes with preoperative normal corneal topography13 and that flap thickness homogeneity could influence LASIK outcomes,14,15 we compared the 3-month postoperative visual and refractive results and flap thickness homogeneity using 2 femtosecond-laser platforms to correct myopia.

Patients and methods

This prospective nonrandomized observational single-center pilot study comprised consecutive patients who had LASIK in which Intralase (ie, a well-documented, conventional femtosecond laser that performs corneal procedures such as a LASIK flap exclusively and that uses a planar corneal interface) (Group 1) and Victus (software version TFW V3.2, Bausch & Lomb, Inc.), a new dual femtosecond laser device that performs LASIK flaps and femtosecond laser–assisted cataract surgery, and uses a curved patient interface (Group 2) was used to create the flap for the correction of myopia, with or without astigmatism. All the surgeries were performed at the same clinic from January 2015 to June 2015. All patients provided written informed consent, and institutional review board approval was obtained. The study was performed in accordance with the tenets of the Declaration of Helsinki.

Myopic patients (up to −6.0 diopters [D]) with low astigmatism (<−1.50 D) and a preoperative corrected distance visual acuity (CDVA) of at least 1.0 (decimal notation) were included. During the surgical evaluation, patients with unstable refraction, previous ocular surgery (refractive or other surgical procedures), topographic suspicion of keratoconus (defined as even mild localized steepening seen on Placido corneal topography or slight bowing of the posterior corneal surface detected by corneal tomography), ocular disease, and systemic disease that could interfere with the wound-healing process (eg, diabetes mellitus and connective tissue disorders) were excluded.

For all patients, a masked observer performed the same full preoperative examination, which included measuring uncorrected distance visual acuity (UDVA), CDVA, and the manifest and cycloplegic refractions as well as Placido-based corneal topography (Allegro Topolyzer, Alcon Laboratories, Inc.), corneal pachymetry and tomography (Pentacam, Oculus Optikgeräte GmbH), mesopic infrared pupillometry (Colvard pupillometer, Oasis Medical, Inc.), slitlamp biomicroscopy, Goldmann tonometry (CT-80, Topcon Corp.), and fundoscopy evaluations.

Surgical Technique

The same experienced surgeon (M.A.T.) performed all the procedures. A povidone–iodine solution was applied to the skin and the conjunctiva, and a sterile surgical drape and a rigid eyelid speculum were positioned. All surgeries were performed using topical anesthesia of lidocaine 2.0%.

The 60 kHz femtosecond laser was used with the following parameters: a raster pattern with a 0.95 μJ bed energy level, a spot size smaller than 3 μm, a spot separation of 7 μm, an attempted flap thickness of 110 μm, and a flap diameter of 9.3 mm.

The dual femtosecond-laser platform used the following settings: a spiral pattern with a 0.78 μJ bed energy level, a spot size of 2 μm, a spot separation of 5.9 μm, an attempted flap thickness of 110 μm, and a flap diameter of 9.0 mm.

After the flap was created in both groups, it was raised with a spatula. Next, the stromal bed was dried with a sponge and the ablation was performed using the same Allegretto excimer laser (Wavelight Technologie AG, Alcon Laboratories, Inc.); the optical zone was the same as or larger than the mesopic pupil size. The stroma was then rinsed copiously with a balanced salt solution (BSS, Alcon Laboratories, Inc.), and the flap was gently put back in place with a cannula. At the end of the surgery, ciprofloxacin 3 mg/mL drops (Oftacilox) and ketorolac trometamol 5 mg/mL drops (Acular) were instilled.

Postoperative Follow-up

Postoperatively, all patients used preservative-free artificial tears as needed and were instructed to apply topical ciprofloxacine 3 mg/mL and dexamethasone 1 mg/mL (Maxidex) 4 times daily during the first week postoperatively. Artificial tears were continued thereafter as needed.

Examinations were scheduled for 1 day, 1 week, and 1 and 3 months postoperatively. Two experienced optometrists, masked to the type of surgery, refracted the patients at each postoperative visit. All the patients were refracted in the same room with the same light adjusted to mesopic conditions.

Corneal aberrations were derived from the data of the anterior surface of the cornea obtained with a tomography system. The root-mean-square (RMS) values were computed for total aberrations, higher-order aberrations (HOAs) (coefficients from 3rd- to 7th- orders), coma-like aberrations, and spherical-like aberrations. The RMS values were obtained for a 6.0 mm pupil in all cases preoperatively and 3 months postoperatively. Changes in corneal aberrations were expressed as the difference between the 3-month postoperative values and the preoperative values.

In addition, 3 months after surgery, an experienced optometrist (masked observed) performed spectral-domain anterior segment optical coherence tomography (AS-OCT) (Spectralis, Heidelberg Engineering GmbH) to measure flap thickness. Nine flap thickness datapoints were evaluated in the horizontal meridian of the cornea (ie, corneal vertex; 1.0 mm, 2.0 mm, 3.0 mm, and 4.0 mm nasally; and 1.0 mm, 2.0 mm, 3.0 mm, and 4.0 mm temporally from the corneal vertex). To evaluate the flap thickness homogeneity and the reproducibility of the flap morphology with the conventional femtosecond laser and the dual femtosecond laser, the difference between the maximum thickness and the minimum thickness point of each flap, the standard deviation (SD) for the 9 flap thickness points measured in each flap (ie, the SD intraflap), and the difference between the achieved and the targeted central flap thickness were calculated.

Statistical Analysis

Statview SE+Graphics software (Abacus Concept, Inc.) was used for the data analysis. A sample of 20 eyes per group was expected to detect intraflap differences of 3 μm 80% of the time with a P value equal to 0.05. Statistical comparisons were made using the unpaired 2-tailed Student t test. A P value less than 0.05 was considered statistically significant. The logarithm of the minimum angle of resolution values of all visual acuity tests were used for the statistical analyses and then converted to Snellen notation (decimal scale) using a visual acuity conversion chart. The data are expressed as the means ± SD.

Results

Fifty-one consecutive myopic eyes treated with femtosecond LASIK were included in the study. Group 1 comprised 31 eyes treated with the 60 kHz conventional femtosecond laser, and Group 2 comprised 20 refraction-matched eyes treated with the dual femtosecond platform. Table 1 shows the preoperative data in both groups.

Table 1
Table 1:
Preoperative data of 51 consecutive eyes.

Table 2 shows the visual and refractive results 3 months after surgery in both groups. The postoperative mean UDVA was significantly better in Group 1 than in Group 2 (P = .0001). The residual spherical equivalent was significantly lower in Group 1 than in Group 2 (P = .0001). In Group 1, 30 (96.8%) of 31 eyes had an SE within ±0.5 D of target versus 12 (60.0%) of 21 eyes in Group 2 (P = .0001), and 31 eyes (100%) in Group 1 had an SE within ±1.0 D versus 16 eyes (75.0%) in Group 2 (P = .0001). The efficacy and safety indices were significantly better in Group 1 than in Group 2 (P = .0001 and P = .02, respectively). Two eyes (6.5%) in Group 1 and 6 eyes (30.0%) in Group 2 lost 1 line of CDVA, whereas 4 eyes (12.9%) in Group 1 and 1 eye (5.0%) in Group 2 gained 1 line of CDVA. Figure 1 shows the change in lines of CDVA 3 months after surgery in both groups.

Figure 1
Figure 1:
Changes in lines of CDVA preoperatively and 3 months after femtosecond LASIK performed with the conventional femtosecond laser (Group 1) or the dual femtosecond laser (Group 2) for the correction of myopia (CDVA = corrected distance visual acuity).
Table 2
Table 2:
Three-month postoperative visual and refractive results in the 2 groups.

Regarding predictability, 30 (96.8%) of 31 eyes in Group 1 and 12 (60.0%) of 21 eyes in Group 2 were within ±0.5 D of emmetropia 3 months postoperatively (P = .0001). Figure 2 shows the predictability after 3 months of follow-up.

Figure 2
Figure 2:
Predictability 3 months after femtosecond LASIK performed with the conventional femtosecond laser (Group 1) or the dual femtosecond laser (Group 2) to correct myopia.

Table 3 shows the changes in corneal HOAs after LASIK with each femtosecond-laser device. The dual femtosecond laser induced a significantly higher increase in total corneal root mean square (RMS) (P = .007), coma aberration (P = .04), and spherical aberration (P = .0001) than the conventional femtosecond laser at the 3-month postoperative visit.

Table 3
Table 3:
Changes in corneal HOAs after LASIK was performed with both lasers.

Table 4 shows the flap thickness values obtained with the spectral-domain AS-OCT device. At the 3-month postoperative visit, the mean central flap thickness was greater in Group 1 than in Group 2 (P = .005). (The intended central flap thickness was 110 μm in both groups.) Nevertheless, although the mean difference between the achieved and the target flap thickness at the corneal vertex was significantly more accurate in Group 2 (P = .0001), the SD and the range of achieved flap thickness at the corneal vertex were significantly higher with the dual femtosecond laser (Group 2) than with the conventional femtosecond laser (Group 1).

Table 4
Table 4:
Flap thickness values obtained by spectral-domain AS-OCT in the 2 groups.

Moreover, the flap thickness homogeneity was significantly better when the conventional femtosecond laser was used. The differences between the maximum and the minimum flap thickness point in each flap and the SD between the 9 flap thickness points evaluated in each flap were significantly lower with the conventional femtosecond laser than with the dual femtosecond laser. Table 5 shows the differences in flap thickness homogeneity obtained with both femtosecond lasers. The central and all nasal flap thickness points evaluated were significantly thinner with the dual femtosecond laser than with the conventional femtosecond laser, whereas no significant differences in the temporal flap thickness points were found between the 2 devices.

Table 5
Table 5:
Differences in flap thickness homogeneity in the 2 groups.

No intraoperative or postoperative flap-related complications, such as corneal haze, wrinkles, or microstriae, were detected in any case during the follow-up.

Discussion

In the current pilot study, the Intralase femotosecond laser provided significantly better visual and refractive outcomes than the Victus dual femtosecond laser for the correction of myopia in a 3-month follow-up. In addition, flaps created with the Intralase laser were significantly more uniform in flap thickness than those obtained with the Victus laser.

Several studies evaluated the visual and refractive results of the Intralase laser for the correction of myopia,3,4,16,17 and our outcomes are comparable with those in the published literature in terms of efficacy, safety, and predictability. However, regarding flap thickness measurements, previous studies of intended versus achieved flap thickness using the Intralase system showed inconsistent findings, with differences ranging between −16 μm and +15 μm.10 Moreover, a recent metaanalysis found that the average difference from the target flap thickness for the 60 kHz Intralase laser was +0.6 μm and the average SD was 12.4 μm. In the current study, the mean central flap thickness for the conventional femtosecond laser was 123.2 μm (mean 13.2 ± 9.2 μm thicker than intended). We believe that 1 explanation for the inconsistency in flap thickness of the femtosecond laser between studies might be the measurement device used. Corneas seem to be thinner when measured with ultrasound pachymetry than with Fourier-domain AS-OCT.18 Moreover, Fourier-domain AS-OCT19 and spectral-domain AS-OCT20 (as in our study) seem to have a higher resolution and provide more consistent flap thickness measurements than those obtained with time-domain AS-OCT. However, despite these small discrepancies in flap thickness between studies, it is well accepted that the femtosecond laser creates a more predictable thin and planar corneal flap, which is thought to have less biomechanical effect on the cornea than the thicker meniscus-shaped flap usually obtained with a mechanical microkeratome.10,21–23

Another outcome that should be considered, which could explain the thicker than intended flaps obtained in the current study, is that we performed the AS-OCT examinations 3 months after surgery. It is well known that there is some degree of flap thickening in the early postoperative period of femtosecond-assisted LASIK, mainly because of epithelial thickening.24 For this reason, we do not believe that the central flap thickness values obtained in this study are reliable parameters to evaluate the precision of the devices analyzed.

Recently, new femtosecond laser platforms have been developed that create the corneal flap in LASIK and the incision and capsulotomy in cataract surgery with the same device.4 These include the Lensx Femto LDV Z8 (Ziemer Ophthalmic Systems AG), and Victus. Some of these new dual femtosecond lasers (Lensx and Victus) use a curved corneal interface that theoretically makes the docking process easier and minimizes corneal compression.3

To our knowledge, only 1 study to date has evaluated corneal flap morphology when a dual femtosecond laser device is used.12 In that study, the Lensx femtosecond laser provided good visual results (UDVA 20/25 or better in 100% of eyes at 3-month follow-up visit) and a high flap thickness predictability in the 38 eyes evaluated. However, it is noteworthy that in the current study, we targeted a thick corneal flap of 140 μm in all cases to avoid possible gas breakthrough or difficult flap lifting.25

To our knowledge, the current pilot study is the first specifically designed analysis to compare the visual and refractive results and flap thickness homogeneity of the Intralase laser (ie, the gold standard of femtosecond lasers for LASIK) versus the Victus laser (a dual femtosecond laser with no reported clinical experience). Although the dual femtosecond laser seemed to create corneal flaps that were closer to the intended value (mean difference from targeted flap thickness at corneal vertex +6.3 μm and +13.2 μm with dual femtosecond laser and the conventional femtosecond laser, respectively), the SD and the range of achieved flap thickness at the corneal vertex were remarkably higher with the dual femtosecond laser than with the conventional femtosecond laser. The dual femtosecond laser flap thickness flaps ranged from as thin as 82 μm to as thick as 141 μm, whereas the conventional femtosecond laser flap thickness flaps ranged from 94 to 137 μm. Moreover, we found that the differences between the maximum and the minimum flap thickness points and the SD between the 9 flap thickness points recorded in each flap were significantly lower in Group 1.

It is well accepted that flap thickness plays an important role in the efficacy, safety, and predictability of LASIK.14 Thinner than expected flaps are related to vision-threatening complications such as irregular astigmatism26 or haze.27 In contrast, thicker-than-expected-flaps seem to be a major risk factor for corneal ectasia in eyes with preoperative normal corneal topography28 and have been related to postoperative refractive undercorrection.29 In the current pilot study, the conventional femtosecond laser provided significantly better outcomes than the dual femtosecond laser in terms of UDVA (1.14 versus 0.76), residual SE (−0.03 D versus −0.70 D), efficacy (1.00 versus 0.68), and safety (1.00 versus 0.94) 3 months after the surgery. In addition, the predictability was clearly better when the conventional femtosecond laser was used for LASIK flap creation; 98.6% versus 60.0% of eyes were within ±0.5 D of emmetropia 3 months postoperatively with the conventional femtosecond laser and dual femtosecond laser, respectively. Given that all surgeries were performed by the same experienced surgeon using the same excimer laser for the correction of the same refractive defect (refraction-matched eyes), it is presumable that the better flap thickness homogeneity obtained with the conventional femtosecond laser would explain the remarkably better visual and refractive outcomes than those obtained with the dual femtosecond laser.

Four hypotheses could explain these differences detected in flap thickness reproducibility and flap thickness homogeneity between the 2 laser platforms. First, is the different patient interface model. During flap creation by the conventional femtosecond laser, the eye is fixated by a suction ring that induces a high IOP increase and the planar interface induces a uniform compression and applanation of the cornea. Thus, the flap is easily created parallel to the applanation surface. In contrast, during flap creation by the dual femtosecond laser, the eye is fixated by a suction ring, which induces a low IOP increase, and the cornea is minimally applanated by a curved patient interface. Thus, it could be possible that the curved interface would make the creation of a planar flap (ie, parallel to the corneal surface) more challenging.

Second, the curved patient interface used in the dual femtosecond laser has only a single curvature radius that must adapt to all corneas independent of their preoperative keratometric values. For this reason, it is possible that the compression of the cornea with a curved interface would be different in flattest and steepest corneas or that this compression would be different at different areas of the same cornea, thus explaining the more heterogeneous flap thickness profiles obtained with the dual femtosecond laser. However, because of the limited number of eyes evaluated in the current pilot study, we could not determine whether some preoperative parameters,30 such as keratometry, corneal thickness, or patient age, might influence the thickness of flaps created by the dual femtosecond laser.

Third, small amounts of eccentric docking when using a curved interface could inadvertently create nonuniform compression of the cornea, thus creating a more variable thickness in flaps created with a dual femtosecond laser. It might be possible that using a curved patient interface with low IOP increase during the suction inadvertently moves the eye, as has been hypothesized to occur when the capsulotomy is incomplete in femtosecond laser–assisted cataract surgery.

Fourth, in addition to the differences in flap thickness homogeneity, other factors could explain the worse visual results obtained (including the HOAs) in Group 2. These include differences in the ablation pattern used (ie, raster pattern with conventional femtosecond laser and spiral pattern with dual femtosecond laser); differences in spot size, the energy used, and stromal bed roughness; or problems directly related to the use of the dual femtosecond laser (improper laser calibration, laser head not optimally serviced, requirement of laser optimization, and energy setting improvement). During our study, a Bausch & Lomb technician performed the calibration of the device.

One limitation of the study is that we did not perform corneal confocal microscopy. We believe that the determination of the backscattered light would be helpful to evaluate whether the differences in flap thickness homogeneity and stromal bed pattern obtained with the 2 femtosecond platforms induce a different degree of keratocyte activation or differences in corneal transparency between flaps, thus explaining the differences observed in the induction of HOAs.

Another potential limitation of the study could be the different level of surgical experience of the surgeon with these femtosecond platforms. The surgeon who performed all the procedures in our study had used the Intralase FS60 laser since 2006; thus, he had more experience with that platform than with the newer dual femtosecond laser, which has been available at our clinic since 2014. However, because the first 12 eyes operated on with the dual femtosecond laser were not included, we believe that this potential source of error was minimized.

In conclusion, our preliminary results suggest that the conventional femtosecond laser offers higher degrees of efficacy, safety, predictability, and flap thickness homogeneity than the dual femtosecond laser for the correction of myopia. More studies with more patients and longer follow-up are required to confirm these findings and their clinical significance. Until these studies are available, it might be premature to assume that all femtosecond lasers are equally efficacious, in particular, with laser platforms that have minimal or no reported clinical experience.

What Was Known

  • It is well accepted that femtosecond laser devices create a more predictable thin and planar corneal flap, which is thought to have less of a biomechanical effect on the cornea than the thicker meniscus-shaped flap usually obtained with a mechanical microkeratome.

What This Paper Adds

  • The conventional femtosecond laser provided significantly better visual and refractive outcomes than the dual femtosecond laser for the correction of myopia at a 3-month follow-up.
  • Flaps created with the conventional femtosecond laser were significantly more uniform in flap thickness than those created with the dual femtosecond laser.

References

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Disclosures

None of the authors has a financial or proprietary interest in any material or method mentioned.

© 2018 by Lippincott Williams & Wilkins, Inc.