Journal of Refractive Surgery

Original Article 

Central Corneal Sublayer Pachymetry and Biomechanical Properties After Refractive Femtosecond Lenticule Extraction

Anders H. Vestergaard, MD; Jakob Grauslund, MD, PhD, DMSci; Anders R. Ivarsen, MD, PhD; Jesper Ø. Hjortdal, MD, PhD, DMSci

Abstract

PURPOSE:

To compare central corneal sublayer pachymetry and biomechanical properties after femtosecond lenticule extraction (FLEX) and small-incision lenticule extraction (SMILE).

METHODS:

A prospective, randomized, single-masked clinical trial of 35 patients treated for moderate to high myopia with FLEX in one eye and SMILE in the other. Anterior segment optical coherence tomography imaging (Heidelberg Spectralis; Heidelberg Engineering GmbH, Heidelberg, Germany) was used to measure central corneal thickness (CCT) and epithelial, flap/cap, and residual stromal bed thickness centrally. The Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY) was used to assess corneal hysteresis (CH) and corneal resistance factor (CRF). Patients were examined before and 6 months after surgery.

RESULTS:

Mean decrease in CCT was 105 μm in FLEX-treated eyes and 106 μm in SMILE-treated eyes (P = .70), which is equivalent to approximately 14 μm/diopters corrected. Mean central epithelial thickness increased 7 ± 6 μm in FLEX-treated eyes and 6 ± 5 μm in SMILE-treated eyes (P = .64). Achieved mean flap/cap thickness was 4 ± 6 μm from the expected thickness in FLEX-treated eyes and 4 ± 9 μm in SMILE-treated eyes (P = .37). CH was reduced 2.7 ± 1.3 mm Hg in FLEX-treated eyes and 3.3 ± 1.2 mm Hg in SMILE-treated eyes (P = .08). CRF was reduced 4.5 ± 1.2 mm Hg in FLEX-treated eyes and 4.6 ± 1.2 mm Hg in SMILE-treated eyes (P = .71). CH and CRF were highly correlated with CCT, but not patient age.

CONCLUSIONS:

In this paired-eye study, the flap-based FLEX and cap-based SMILE resulted in almost identical changes in central corneal sublayer pachymetry and biomechanical properties for moderate to high myopia 6 months after treatment.

[J Refract Surg. 2014;30(2):102–108.]

From the Department of Ophthalmology, Odense University Hospital, Odense, Denmark (AHV, JG); and the Department of Ophthalmology, Aarhus University Hospital, Aarhus, Denmark (AHV, ARI, JØH).

Supported by Odense University Hospitals PhD Research Grant, Fight for Sight Denmark, Bagenkop Nielsens Myopia-Foundation, The Synoptik Foundation, The A. P. Møller Foundation for the Advancement of Medical Science, The Danish Society of Ophthalmology, Institute of Clinical Research at the University of Southern Denmark, The A. J. Andersen and Wife Foundation, The Hans and Nora Buchards Foundation, and The Henry and Astrid Møllers Foundation, University of Southern Denmark. Pentacam HR and Heidelberg Spectralis OCT with anterior segment module was donated by Bagenkop Nielsens Myopia-Foundation.

The authors have no financial or proprietary interest in the materials presented herein.

Clinical Trials registration reference number: NCT01673503, at www.clinical-trials.gov

AUTHOR CONTRIBUTIONS

Conception and design (AHV, JG, ARI, JØH); data collection (AHV, ARI, JØH); analysis and interpretation of data (AHV, ARI, JØH); writing the manuscript (AHV); critical revision of the manuscript (AHV, JG, ARI, JØH); statistical expertise (AHV, JG, ARI, JØH); obtaining funding (AHV, JG, ARI, JØH); administrative, technical, or material support (JG, ARI, JØH); supervision (JG, ARI, JØH)

Correspondence: Anders H. Vestergaard, MD, Eye Department E, OUH, Sdr. Boulevard 29, Odense C 5000, Denmark. E-mail: vestergaard_anders@hotmail.com

Received: July 30, 2013
Accepted: October 24, 2013
Posted Online: January 31, 2014

Abstract

PURPOSE:

To compare central corneal sublayer pachymetry and biomechanical properties after femtosecond lenticule extraction (FLEX) and small-incision lenticule extraction (SMILE).

METHODS:

A prospective, randomized, single-masked clinical trial of 35 patients treated for moderate to high myopia with FLEX in one eye and SMILE in the other. Anterior segment optical coherence tomography imaging (Heidelberg Spectralis; Heidelberg Engineering GmbH, Heidelberg, Germany) was used to measure central corneal thickness (CCT) and epithelial, flap/cap, and residual stromal bed thickness centrally. The Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY) was used to assess corneal hysteresis (CH) and corneal resistance factor (CRF). Patients were examined before and 6 months after surgery.

RESULTS:

Mean decrease in CCT was 105 μm in FLEX-treated eyes and 106 μm in SMILE-treated eyes (P = .70), which is equivalent to approximately 14 μm/diopters corrected. Mean central epithelial thickness increased 7 ± 6 μm in FLEX-treated eyes and 6 ± 5 μm in SMILE-treated eyes (P = .64). Achieved mean flap/cap thickness was 4 ± 6 μm from the expected thickness in FLEX-treated eyes and 4 ± 9 μm in SMILE-treated eyes (P = .37). CH was reduced 2.7 ± 1.3 mm Hg in FLEX-treated eyes and 3.3 ± 1.2 mm Hg in SMILE-treated eyes (P = .08). CRF was reduced 4.5 ± 1.2 mm Hg in FLEX-treated eyes and 4.6 ± 1.2 mm Hg in SMILE-treated eyes (P = .71). CH and CRF were highly correlated with CCT, but not patient age.

CONCLUSIONS:

In this paired-eye study, the flap-based FLEX and cap-based SMILE resulted in almost identical changes in central corneal sublayer pachymetry and biomechanical properties for moderate to high myopia 6 months after treatment.

[J Refract Surg. 2014;30(2):102–108.]

From the Department of Ophthalmology, Odense University Hospital, Odense, Denmark (AHV, JG); and the Department of Ophthalmology, Aarhus University Hospital, Aarhus, Denmark (AHV, ARI, JØH).

Supported by Odense University Hospitals PhD Research Grant, Fight for Sight Denmark, Bagenkop Nielsens Myopia-Foundation, The Synoptik Foundation, The A. P. Møller Foundation for the Advancement of Medical Science, The Danish Society of Ophthalmology, Institute of Clinical Research at the University of Southern Denmark, The A. J. Andersen and Wife Foundation, The Hans and Nora Buchards Foundation, and The Henry and Astrid Møllers Foundation, University of Southern Denmark. Pentacam HR and Heidelberg Spectralis OCT with anterior segment module was donated by Bagenkop Nielsens Myopia-Foundation.

The authors have no financial or proprietary interest in the materials presented herein.

Clinical Trials registration reference number: NCT01673503, at www.clinical-trials.gov

AUTHOR CONTRIBUTIONS

Conception and design (AHV, JG, ARI, JØH); data collection (AHV, ARI, JØH); analysis and interpretation of data (AHV, ARI, JØH); writing the manuscript (AHV); critical revision of the manuscript (AHV, JG, ARI, JØH); statistical expertise (AHV, JG, ARI, JØH); obtaining funding (AHV, JG, ARI, JØH); administrative, technical, or material support (JG, ARI, JØH); supervision (JG, ARI, JØH)

Correspondence: Anders H. Vestergaard, MD, Eye Department E, OUH, Sdr. Boulevard 29, Odense C 5000, Denmark. E-mail: vestergaard_anders@hotmail.com

Received: July 30, 2013
Accepted: October 24, 2013
Posted Online: January 31, 2014

Refractive surgery can induce irreversible corneal lamellar changes and affect biomechanical properties by reducing biomechanical strength. Therefore, characterization of such changes is important in predicting outcomes and avoiding adverse events. Iatrogenic keratectasia is a rare but vision-threatening condition that has been reported to occur more frequently after LASIK than after surface ablation techniques.1 Less severe corneal changes, such as epithelial hyperplasia, may also affect visual acuity and contribute to postoperative regression.2,3

In recent years, anterior segment optical coherence tomography (AS-OCT) imaging has been used to analyze postoperative changes in corneal sublayer pachymetry and the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY) has been used to assess changes in corneal biomechanical properties.4–6

Femtosecond laser refractive surgery has been performed since 2006. In femtosecond lenticule extraction (FLEX) and small-incision lenticule extraction (SMILE), a lenticule is cut within the corneal stroma. A subsequent surface cut allows access to dissection and manual removal of the lenticule. In FLEX, a LASIK-like flap is used to access the stromal lenticule.7,8 In SMILE, only a small incision is made.9,10 Therefore, SMILE may have biomechanical advantages over FLEX because no flap is created and the stroma over the lenticule is left intact.

Until now, there have been no reports of keratectasia after FLEX or SMILE and only a few publications concerning corneal pachymetry and biomechanical changes after all femtosecond laser treatments.4,6,11,12 Also, no studies have directly compared these parameters in FLEX- and SMILE-treated eyes in a paired-eye study.

The purpose of this study was to prospectively compare central corneal sublayer pachymetry and biomechanical properties in patients with moderate to high myopia (receiving FLEX in one eye and SMILE in the other).

Patients and Methods

From May 2011 to August 2012, 35 patients (70 eyes) were included in a prospective, single-masked paired-eye study, registered at www.clinicaltrials.gov (identifier: NCT01673503). Patients were randomized after ocular dominance to receive FLEX in one eye and SMILE in the other for treatment of moderate to high myopia (with an equal number of dominant eyes in each group).

Inclusion criteria were age 25 to 45 years, stable myopia for at least 1 year, a corrected distance visual acuity of 20/25 or better, spherical equivalent refraction of −6.00 to −10.00 diopters (D) with less than 2.00 D difference between the two eyes, and refractive astigmatism of less than 2.00 D. Patients had to discontinue soft contact lenses 2 days prior to assessment. Exclusion criteria were use of hard contact lenses, central corneal thickness (CCT) less than 480 μm, a calculated postoperative residual stromal bed of less than 250 μm, and all other ocular conditions. Pregnancy and breastfeeding were also qualifications for exclusion from surgery.

Patients underwent a thorough eye examination including objective and manifest visual acuity, intraocular pressure (NIDEK TONOREF II; NIDEK, Gamagori, Japan), pupil size (NIDEK pupillometer; NIDEK), keratometric measurements, slit-lamp examination, and funduscopy. Regular topographic patterns of both the corneal front and back were confirmed with a Pentacam-HR Scheimpflug camera (Oculus, Optikgeräte, Wetzlar, Germany). This included the use of the Pentacam “Belin/Ambrósio Enhanced Ectasia” module (Oculus, Optikgeräte) to exclude subclinical keratoconus. Furthermore, CCT was measured by laser interferometry using the OLCR pachymeter (Haag-Streit, Köniz, Switzerland).

Central Corneal Sublayer Pachymetry

AS-OCT imaging with the Heidelberg Spectralis Anterior Segment Module (Heidelberg Engineering GmbH, Heidelberg, Germany) was used to measure central corneal parameters (Figure 1). The Spectralis AS-OCT (Heidelberg Engineering GmbH) has an A-scan acquisition speed of 40 kHz and a digital resolution of 3.9 μm axially by 11 μm laterally (user manual for Heidelberg Spectralis software version 5.7). High-resolution horizontal line scans (centered at the pupil) were obtained and the images were analyzed using the integrated system software. Centered on the light reflex, the images were magnified to 800% and one masked observer (AHV) measured different corneal parameters perpendicular to the anterior surface (preoperatively and 6 months postoperatively): CCT (the distance between the epithelial surface and the endothelium–anterior chamber interface), central epithelial thickness (from the epithelial surface to Bowman’s membrane), central flap/cap thickness (from the epithelial surface to the laser-cut interface), and central residual stromal bed thickness (from the laser-cut interface to the endothelium–anterior chamber interface). The mean value of two measurements for each parameter was used to minimize measurement error.

Anterior segment optical coherence tomography (AS-OCT) thickness measurements using the integrated system software 6 months after small-incision lenticule extraction.

Figure 1.

Anterior segment optical coherence tomography (AS-OCT) thickness measurements using the integrated system software 6 months after small-incision lenticule extraction.

Corneal Biomechanical Properties

The Ocular Response Analyzer (version 2.04) was used to assess corneal hysteresis (CH) and corneal resistance factor (CRF) to characterize changes in corneal biomechanics (Figure 2). The system automatically generated an average result for each eye based on four Ocular Response Analyzer measurements. Only measurements with a waveform score greater than 3.5 were included.13 Also, measurements were obtained quickly to avoid tear-film alterations affecting results. The system uses bi-directional applanation through brief air pulses to rapidly deform the cornea inward and outward, measuring intraocular pressure two times independently. An electro-optical system monitors the central 3-mm curvature of the cornea throughout the deformation process during the 20-ms measurement. The CH is calculated as the difference between the two pressure values at the two applanation points and is an indicator of corneal tissue viscous damping. The CRF is also derived from this response and calculated using a linear function of the two pressures and a proprietary algorithm. It is believed to be a measurement of the overall corneal resistance (total viscoelastic properties) of the cornea during measurement.14–16

Screen shot of Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY) measurements and outcomes.

Figure 2.

Screen shot of Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY) measurements and outcomes.

Surgical Procedure

A VisuMax 500-kHz femtosecond laser (Carl Zeiss Meditec, Jena, Germany) was used to perform FLEX and SMILE.8,10,17 Surgery was performed bilaterally with the same laser settings for the two eyes. Laser-cut energy index ranged from 125 to 170 mJ and spot spacing ranged from 2.5 to 4.5 μm. Lenticule diameter was 6.0 to 6.5 mm and the flap/cap diameter ranged from 7.9 to 8.0 mm in FLEX and was 7.3 mm in SMILE. Intended flap/cap thickness was 100 to 120 μm. The flap was superiorly placed in FLEX, as was the small incision in SMILE. Patients received one drop each of chloramphenicol and diclofenac (Voltaren Ophtha; Novartis Healthcare, Copenhagen, Denmark) after surgery.

Postoperative Treatment and Follow-up

The postoperative treatment regimen included fluorometholone (Flurolon; Allergan Pharmaceuticals, County Mayo, Ireland) and chloramphenicol drops four times a day for 1 week followed by two times a day for 1 week. Patients returned for postoperative examination 1 day and 1, 3, and 6 months after surgery. AS-OCT imaging and Ocular Response Analyzer measurements were performed only before and 6 months after surgery. One patient dropped out after completing the 3-month follow-up, but had no complaints when contacted over the telephone. After the 6-month follow-up, the eye randomization was revealed to the patients.

Statistics

Statistical analyses were performed using Microsoft Excel 2007 (Microsoft Corporation, Redmond, WA) and Systat SigmaPlot 12 (Systat Software, Inc., San Jose, CA). Reliability/intragrader agreement was evaluated by regrading 20% of AS-OCT images (randomly chosen) and intraclass correlation coefficient was calculated using Stata Intercooled software version 11.0 (StataCorp, College Station, TX). Student’s t test and paired Student’s t test were used to compare normally distributed data. Mann–Whitney rank sum test and Wilcoxon signed rank test were used for non-normally distributed data. The Kolmogorov–Smirnov test was used for normal distribution. The Pearson correlation coefficient (r) was used for correlation analyses. The coefficient of variation was calculated as the standard deviation divided by the mean value of measurements. P values less than .05 were considered statistically significant. The sample size in this study offered 92% statistical power at a 5% level to detect a difference in mean CH or mean CRF of 1 mm Hg preoperatively to 6 months postoperatively (when the expected standard deviation of the mean difference was 1.2 mm Hg). For CCT, it offered 90% statistical power at a 5% level to detect a difference of 20 μm with an expected standard deviation of 25 μm.

Ethics

This study was approved by the Danish Data Protection Agency and the local ethical committee and was conducted in agreement with the tenets of the Declaration of Helsinki. All participants were thoroughly informed and gave oral and written consent before inclusion in the study.

Results

In total, 34 of 35 patients completed 6 months of follow-up. Mean age was 35 ± 7 years (range: 25 to 45 years), 32% were male, and 67% were right-eye dominant. When comparing FLEX- and SMILE-treated eyes, there were no statistically significant differences in baseline measurements (P > .05). The same was true when comparing the 6-month results (P > .05). Baseline and 6-month results are presented in Table 1.

Results Before and 6 Months After Surgery

Table 1:

Results Before and 6 Months After Surgery

Central Corneal Sublayer Pachymetry

Intraclass correlation coefficient of measurements were 99.8% for CCT, 74.8% for epithelial thickness, 86.4% for flap/cap thickness, and 97.8% for residual stromal bed thickness.

After 6 months, mean decrease in CCT was 105 μm in FLEX-treated eyes and 106 μm in SMILE-treated eyes (P = .70). This was equivalent to approximately 14-μm tissue removal per diopters corrected in FLEX- and SMILE-treated eyes. The change in CCT, as measured by AS-OCT, was highly correlated with the change measured by the OLCR pachymeter in both FLEX-treated (r = 0.62, P < .01) and SMILE-treated (r = 0.75, P < .01) eyes.

Mean central epithelial thickness increased 7 ± 6 μm (range: −5 to 23 μm) in FLEX-treated eyes and 6 ± 5 μm (range: −2 to 13 μm) in SMILE-treated eyes preoperatively to postoperatively (P < .01). There were no differences between FLEX- and SMILE-treated eyes (P = .64). The increase in mean central epithelial thickness was not correlated to the amount of corneal tissue removed or the preoperative CCT in FLEX- and SMILE-treated eyes (P > .05).

The difference between expected and measured mean flap thickness was 4 ± 6 μm (range: −8 to 19 μm) in FLEX-treated eyes 6 months postoperatively (P < .01). When the increase in mean central epithelial thickness was subtracted, the coefficient of variation in flap thickness was 7.1% and there was no longer a statistically significant difference (P = .11). The expected flap thickness was correlated with the achieved thickness (r = 0.61, P < .01). In SMILE-treated eyes, the difference between expected and measured mean cap thickness was 4 ± 9 μm (range −15 to 29 μm) with no statistically significant difference (P = .10) (and likewise when the increase in epithelial thickness was subtracted [P = .24]). The coefficient of variation in cap thickness was 10.6% when the increase in epithelial thickness was subtracted. A significant correlation between expected and achieved cap thickness was also found in SMILE-treated eyes (r = 0.52, P < .01). There was no difference in achieved flap/cap accuracy (P = .37) and in mean residual stromal bed thickness (P = .79) 6 months after surgery in SMILE- and FLEX-treated eyes. Also, no eyes had a residual stromal bed thickness of less than 250 μm.

Corneal Biomechanical Properties

In FLEX-treated eyes, CH was reduced 2.7 ± 1.3 mm Hg (P < .01) and CRF was reduced 4.5 ± 1.2 mm Hg (P < .01) 6 months after surgery. In SMILE-treated eyes, CH was reduced 3.3 ± 1.2 mm Hg (P < .01) and CRF was reduced 4.6 ± 1.2 mm Hg (P < .01). There was no difference in the induced changes in CH (P = .08) and CRF (P = .71) in FLEX- and SMILE-treated eyes.

Preoperatively and 6 months postoperatively, CH and CRF were highly correlated with CCT in FLEX- and SMILE-treated eyes (P < .01). The change in CH was not correlated with the change in intraocular pressure in FLEX-treated (r = −0.07, P = .70) and SMILE-treated (r = −0.11, P = .54) eyes. The same was true for CFR and intraocular pressure in FLEX-treated (r = 0.20, P = .27) and SMILE-treated (r = 0.22, P = .21) eyes. Also, there were no correlations between patient age and the postoperative reduction in CH or CRF (P > .05).

Discussion

Previous results from this cohort have shown that the less invasive SMILE technique is better at sparing the central corneal nerves when compared to FLEX,17 but both produce equally good results when comparing refractive predictability, visual acuity, and safety (data accepted for publication18). However, until now, corneal sublayer pachymetry and biomechanics have not been directly compared.

Corneal pachymetry can be assessed by several different techniques with different strengths and limitations. CCT results measured by AS-OCT were highly correlated with laser interferometry results, indicating reliability and accuracy of measurements, although results were not directly comparable due to differences in wavelengths and refractive indices.19 The average tissue removal of approximately 14 μm/diopters is consistent with previous findings in eyes with similar refractive errors.8,10

CCT is an important part of the preoperative and postoperative evaluations and epithelial thickness mapping has also been used as a diagnostic tool to exclude keratectasia and in patients with postoperative myopic regression.20 In our study, there was no significant regression (less than 0.10 D over 6 months) in FLEX- and SMILE-treated eyes up to 6 months after surgery. Previous results have shown that the initial increase in epithelial thickness of up to 24% after myopic LASIK or photorefractive keratectomy is maintained up to 7 years after surgery and remains stable in LASIK-treated eyes 3 months after surgery.2,3,20 Our 16% and 14% increases in epithelial thickness (in FLEX- and SMILE-treated eyes, respectively) are therefore expected to be stable over time. However, it is not yet known if the difference in treatment zone size between LASIK and FLEX/SMILE affects the changes in epithelial thickness. Further research is needed to clarify this.

High precision and repeatability in flap/cap cutting is of utmost importance to avoid intraoperative complications and long-term corneal changes (such as ectasia). Previous results from femtosecond laser-assisted LASIK, FLEX, and SMILE studies have shown that the VisuMax laser (Carl Zeiss Meditec) can produce accurate flaps with high reproducibility.6,21,22 This was also found in our study when the increase in epithelial thickness was deducted. Likewise, we expected and found no major difference when comparing FLEX and SMILE because the lenticule cut was the same in both. Also, potential postoperative corneal edema should not affect measurements 6 months after surgery. Only two previously published studies have evaluated precision of flap/cap thickness in FLEX- and SMILE-treated eyes. Here, predictability was high and comparable to results after eyes receiving femtosecond laser-assisted LASIK (with higher degrees of astigmatism) 1 to 3 months after surgery.6,12

Dry eye is a relatively common condition after surgery and gazing at the OCT camera can also affect the tear film. Therefore, we chose not to include the tear film in pachymetry measurements to avoid changes in the tear film layer affecting other sublayer measurements. However, there will always be an aspect of subjectivity in AS-OCT thickness measurements. For example, the light reflection from the AS-OCT sometimes made central measurements difficult due to image blurring and it is speculated to be partly responsible for the slightly lower epithelial thickness intragrader reliability compared to the other (and longer) measurements. Also, a few microns decrease of central epithelial thickness in some eyes is speculated to be due to measurement error in eyes with no significant change in epithelial thickness. Only central corneal measurements were performed because peripheral measurements are more sensitive to measurement error because of misalignment.

Since 2005, the Ocular Response Analyzer has been used to assess intraocular pressure and to quantify viscoelastic properties of the cornea in vivo. Reduction of CH and CRF after refractive surgery has been interpreted as biomechanical weakening and has been reported after both surface ablation techniques and flap-based intrastromal procedures.23–26 The flap creation has been related to a decrease in CH and CRF in some studies, whereas other studies found no difference between surface ablation procedures and LASIK; it was mainly the tissue removal that accounted for the induced changes.15,26,27

In our study, we found reduction in CH and CRF after FLEX and SMILE 6 months after surgery, but were not able to measure the theoretical biomechanical advantage of a small corneal incision (as compared to flap-creation). This unmeasurable change could be a result of changes in both viscous and elastic properties, thereby masking potential differences not characterized by CH and CRF as previously speculated.27 Also, as expected in this relatively short follow-up period, we found no clinical, tomographical, or biomechanical signs of postoperative ectasia.

Only one previously published study has evaluated CH and CRF in SMILE-treated eyes, in which a reduction of 1.94 mm Hg in CH and 2.96 mm Hg in CRF was found 6 months after surgery and with no difference between eyes treated with SMILE and femtosecond laser-assisted LASIK.4 In contrast to our study, these eyes had low to moderate myopia and therefore less tissue removal (average of 82.67 μm). Thus, the greater reduction in Ocular Response Analyzer values in our cohort of eyes was to be expected and was in accordance with the existing knowledge.26

Using a paired-eye design meant most sources of variability could be neglected when comparing FLEX and SMILE. However, not all sources of error could be controlled (eg, the ocular pulse amplitude [approximately 3 mm Hg], which has been reported to be a source of variability when measuring biomechanical parameters).28 Increased corneal stiffness due to aging was not expected to affect the Ocular Response Analyzer results because the patients were relatively young.29,30

More patients and statistical power would have made it easier to discriminate between FLEX and SMILE results and to possibly detect smaller differences. However, the paired-eye design made recruitment difficult.

To our knowledge, this is the first paired-eye study to directly compare corneal sublayer pachymetry and biomechanical properties after FLEX and SMILE. The study demonstrated increased epithelial thickness of approximately 15%, high precision flap/cap-cutting, and reduced viscous and total viscoelastic properties (as measured by the Ocular Response Analyzer) after both treatments. The induced changes were almost identical up to 6 months after surgery, regardless of treatment. Further analyses of differences in corneal biomechanics after FLEX and SMILE are currently ongoing at our department.

References

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Results Before and 6 Months After Surgery

ProcedurePreoperative6 Months Postoperative


FLEXSMILEFLEXSMILE
No. of eyes34343434
Spherical equivalent (D)
  Mean ± SD−7.59 ± 0.97−7.56 ± 1.11−0.12 ± 0.41−0.17 ± 0.34
  Range−6.00 to −9.75−6.00 to −9.88+0.75 to −1.25+0.50 to −0.88
Average keratometry (D)
  Mean ± SD43.37 ± 1.5243.43 ± 1.4737.93 ± 1.6337.98 ± 1.49
  Range40.74 to 46.8840.79 to 46.7235.34 to 42.3335.62 to 41.44
Central corneal thickness (AS-OCT, μm)
  Mean ± SD553 ± 28552 ± 30448 ± 34446 ± 36
  Range498 to 617501to 617395 to 529387 to 520
Central epithelial thickness (AS-OCT, μm)
  Mean ± SD44 ± 444 ± 451 ± 550 ± 5
  Range35 to 5135 to 5142 to 6739 to 59
Central flap/cap thickness (AS-OCT, μm)
  Mean ± SD119 ± 7119 ± 11
  Range104 to 137101 to 149
Central RSB thickness (AS-OCT, μm)
  Mean ± SD329 ± 32327 ± 37
  Range272 to 408262 to 405
Intraocular pressure (mm Hg)
  Mean ± SD15.8 ± 2.816.1 ± 3.08.7 ± 1.98.7 ± 1.8
  Range10.0 to 23.511.0 to 24.05.0 to 14.05.0 to 13.0
Corneal hysteresis (mm Hg)
  Mean ± SD10.8 ± 1.711.0 ± 1.78.0 ± 1.17.8 ± 1.3
  Range7.2 to 14.07.5 to 13.95.6 to 11.15.5 to 10.5
Corneal resistance factor (mm Hg)
  Mean ± SD10.9 ± 1.810.9 ± 1.96.4 ± 1.46.4 ± 1.4
  Range8.5 to 15.76.9 to 16.03.4 to 11.13.9 to 10.2

10.3928/1081597X-20140120-05

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