|Year : 2015 | Volume
| Issue : 2 | Page : 50-57
Macular and retinal nerve fiber layer thickness changes after small-incision lenticule extraction and femto-LASIK
Asaad A Ghanem1, Salah A Mady MD 2, Tarek N Attia2, Tamer I Salem2
1 Mansoura Ophthalmic Center, Faculty of Medicine, Mansoura University, Mansoura, Egypt
2 Department of Ophthalmology, Faculty of Medicine, Banha University, Banha, Egypt
|Date of Submission||01-Apr-2015|
|Date of Acceptance||09-May-2015|
|Date of Web Publication||28-Oct-2015|
Salah A Mady
Ophthalmology Department, Faculty of Medicine, Banha University, 35511, Banha
Source of Support: None, Conflict of Interest: None
Femtosecond laser flap creation exerts less intraocular pressure (IOP) fluctuation but requires more procedural time compared with microkeratome flap creation. This IOP elevation during suction may affect the macular and retinal nerve fiber layer thickness that can be assessed with optical coherence tomography.
The aim of this study was to compare the influence of the transient elevation of IOP during suction on the macular and retinal nerve fiber layer thickness after small-incision lenticule extraction (SMILE) and femtosecond laser-assisted in-situ keratomileusis (femto-LASIK).
Patients and methods
A total of 80 and 75 eyes that received SMILE and femto-LASIK procedures for myopia and myopic astigmatism, respectively, were enrolled in this study. Spectral-domain optical coherence tomography was used to measure macular and peripapillary retinal nerve fiber layer thickness preoperatively, at 1 week, and 1, 3, and 6 months postoperatively.
The study included 155 eyes. In both the SMILE group and the femto-LASIK group, the mean foveal, parafoveal, and perifoveal retinal thickness did not change significantly from the preoperative to any postoperative timepoint (P > 0.05). The mean foveal retinal thickness was significantly greater in the femto-LASIK group than in the SMILE group (P<0.05). The retinal nerve fiber layer thickness did not change significantly from the preoperative to any postoperative timepoint in either group (P > 0.05). The differences in the retinal nerve fiber layer between the SMILE group and the femto-LASIK group were not statistically significant at any follow-up visit (P > 0.05).
Both SMILE and femto-LASIK procedures had no significant effects on the macular and retinal nerve fiber layer thickness postoperatively.
Keywords: femtosecond laser-assisted in-situ keratomileusis, macular, optical coherence tomography, retinal nerve fiber layer, small-incision lenticule extraction
|How to cite this article:|
Ghanem AA, Mady SA, Attia TN, Salem TI. Macular and retinal nerve fiber layer thickness changes after small-incision lenticule extraction and femto-LASIK. Delta J Ophthalmol 2015;16:50-7
|How to cite this URL:|
Ghanem AA, Mady SA, Attia TN, Salem TI. Macular and retinal nerve fiber layer thickness changes after small-incision lenticule extraction and femto-LASIK. Delta J Ophthalmol [serial online] 2015 [cited 2017 Aug 19];16:50-7. Available from: http://www.djo.eg.net/text.asp?2015/16/2/50/168528
| Introduction|| |
Great advances in refractive surgery for correcting myopia and myopic astigmatism have been made with the laser-assisted in-situ keratomileusis (LASIK) procedure by means of refractive lenticule extractions: femtosecond lenticule extraction, small-incision lenticule extraction (SMILE), and pseudo-SMILE  .
Femtosecond machines have revolutionized the creation of flaps for LASIK using a neodymium laser at 1.053 nm to dissect corneal tissue through microphotodisruption. This has increased the ability to create very uniform and thin flaps with greater biomechanical stability. These machines have been shown to elevate the intraocular pressure (IOP) to a lower extent compared with traditional mechanical microkeratomes  . Femtosecond laser offers the following advantages: increased precision, a reduced incidence of flap complications, and the ability to cut thinner flaps without the risk of the buttonhole formation , .
With the introduction of the VisuMax femtosecond laser in 2006  , keratorefractive surgery was revolutionized and femtosecond intrastromal keratomileusis was reconsidered as a refractive lenticule extraction  .
During LASIK flap creation using a microkeratome, IOP increases to levels exceeding 65 mmHg. Femtosecond laser flap creation exerts less IOP fluctuation but requires more procedural time compared with microkeratome flap creation , . Vetter et al.  showed an IOP elevation up to 150 mmHg with flat corneal applanation interfaces and up to 65 mmHg with curved corneal interfaces.
A common approach to understand retinal anatomic variation is to use optical coherence tomography (OCT). OCT is a noninvasive quantitative objective method that provides a real-time in-vivo image of the retina  . It is based on the backscattering of low-coherence laser light as it passes through layers of differing index of refraction and is recorded by an interferometer and amplified to construct a high-resolution, cross-sectional image of the tissue  .
Spectral-domain OCT uses a diode laser wavelength of 840 nm with a scan rate of at least 20 000 axial measurements per second, thus providing better resolution compared with earlier time-domain OCT. Therefore, better image acquisition with fewer artifacts becomes feasible with the possibility of generating three-dimensional images , .
The aim of this prospective study was to evaluate and compare the influence of the transient elevation of IOP during suction on the macular and peripapillary retinal nerve fiber layer thickness after SMILE and femtosecond laser-assisted in-situ keratomileusis (femto-LASIK) using spectral-domain OCT.
| Patients and methods|| |
This was a prospective, nonrandomized, comparative study. After explaining the details of the study, written informed consent was obtained from all patients before enrollment. The study was approved by Al Nour Femto-Laser Center, Trust Ethics Committee, and was carried out in accordance with the Declaration of Helsinki (1989) of the world medical association.
This prospective study included eyes with myopia and/or myopic astigmatism scheduled to have SMILE (the SMILE group) or femto-LASIK (the femto-LASIK group).
Inclusion criteria were as follows: age, 21 years or older; a stable refractive error for at least 6 months, with manifest spherical equivalent of −1.00 to −10.00 D and manifest cylinder of −0.50 to −5.00D; a sufficient corneal thickness that was greater than 500 μm; a minimum calculated residual corneal stromal bed thickness greater than 300 μm; IOP of 21 mmHg or less; and discontinuation of soft contact lens wear at least 2 weeks before surgery.
Exclusion criteria were as follows: previous ocular, refractive surgeries; presence of glaucoma; IOP higher than 21 mmHg; evidence of glaucomatous optic nerve damage; cup-disc ratio greater than 0.4; presence of severe dry eye, progressive corneal degeneration, keratoconus, or corneal scars; pregnancy or breast feeding; or presence of diseases that affect the regenerative process of the cornea (diabetes mellitus, collagen-related disease).
Patients underwent eye examination, including slit-lamp examination (Slit Lamp BM900; Haag-Streit, Bern, Switzerland), dilated fundus examination using a +90 D lens and manifest and cycloplegic refraction, and best corrected visual acuity using a Snellen chart. Uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) were assessed using an automated refractometer (Nidek Co. Ltd, Gamagori, Japan), and IOP was assessed using a noncontact tonometer (CT-80, Non-Contact Computerized Tonometer, Topcon, Tokyo, Japan). The preoperative central corneal thickness, keratometry, and anterior and posterior corneal elevation were measured using a Scheimpflug topography camera (Pentacam HR; Oculus optikegerate Gmbh, Wetzlar, Germany).
Optical coherence tomography imaging
A spectral-domain OCT instrument (Cirrhus-HD OCT; Carl Zeiss Meditec AG, Jena, Germany) was used. The optical resolution of this machine is ~5 μm axially and ~25 μm transversely with a scan speed of 27 000 axial scans per second.
Macular thickness was determined using the macular cube protocol (128 horizontal line raster with 512 A-scans each, within a 6 × 6 mm 2 area). Examinations were performed in the foveal, parafoveal, and perifoveal zones according to the region determined in the Early Treatment Diabetic Retinopathy Study (ETDRS), and the mean thickness was presented as numerical values. The thickness map represents the mean thickness of the foveal region (1.0 mm diameter central circle area), 3.0 mm parafoveal region (ring area between 1.0 and 3.0 mm in diameter, resulting in a 2-5-mm-wide parafoveal ring), and 6.0 mm perifoveal region (ring area between 3.0 and 6.0 mm in diameter, resulting in a 3.0-mm-wide perifoveal ring divided into the following quadrants: superior (316-45°), nasal (46-135°), inferior (136-225°), and temporal (226-315°) [Figure 1].
|Figure 1: The nine macular areas defined by the Early Treatment Diabetic Retinopathy System (ETDRS) research group.|
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The retinal nerve fiber layer thickness was determined using the optic nerve head protocol (five horizontal line scans consisting of 4096 A-scans each, within a 6 × 1 mm 2 area), with all scans having a signal strength of at least 7 (range: 8.22-10.81). The protocol uses 13 concentric ring scans of 1.3-4.9 mm in diameter centered in the optic disc. The retinal nerve fiber layer thickness results were shown as a thickness map of 16 regions.
The examinations were performed preoperatively, at 1 week, and 1, 3, and 6 months postoperatively. The OCT measurements were performed by means of pupil dilation with tropicamide 1% and phenylephrine 2.5%. All measurements were taken between 11 a.m. and 1 p.m. to eliminate the effect of diurnal variations in IOP, and any possible effect on retinal nerve layer (RNFL) thickness. Good-quality scans, defined as scans with image quality 40 or more (default good image quality is 30), without RNFL discontinuity or misalignment, involuntary saccade or blinking artifacts, and absence of algorithm segmentation failure on careful visual inspection, were used for analysis.
The VisuMax (Carl Zeiss Meditec AG) femtosecond laser system was used to perform the surgical refractive corrections for patients in the SMILE and the femto-LASIK group with a repetition rate of 500 kHz. All procedures were performed under topical anesthesia (preservative-free benoxinate hydrochloride 0.4% eye drops) in all cases.
In the SMILE technique, after sterile draping and insertion of the speculum, the patient's eye is centered and docked with the curved interface cone before suction fixation is applied under the VisuMax surgical microscope. Afterwards, the table is moved to the laser treatment position under an illuminated and curved suction contact glass (so-called treatment pack). While the patient focuses on an internal target light for centration, the cornea is partly applanated by moving the table upward toward the curved contact glass. The surgeon observes this motion through the operating microscope and controls the movement with a joystick. Once an appropriate centration, 'center of the pupil', is achieved, the surgeon initiates the automatic suction. The patient continues to observe the blinking target green light even when the suction is being applied. The VisuMax femtosecond laser produces ultrashort pulses of light, at a repetition rate of 500 kHz, that are focused at a precise depth in the corneal tissue.
A plasma state develops with optical breakdown, and a small gas bubble is formed from the vaporization of tissue. A series of bubbles are created in a spiral manner with a typical spot distance of 3-5 μm, resulting in cleaving of tissue planes.
Four subsequent femtosecond incisions are performed: the posterior surface of the refractive lenticule; the lenticule border; the anterior surface of the refractive lenticule; and the side-cut incision. After the suction has been released, the patient is moved toward the observation position under the microscope. A thin spatula is inserted through the side cut over the roof of the refractive lenticule dissecting this plane, followed by the bottom of the lenticule. The lenticule of the intrastromal corneal tissue is dissected through the 2-3 mm tunnel side-cut opening incision (usually superotemporal). The lenticule is subsequently grasped with modified serrated McPherson forceps (Geuder GmbH, Heidelberg, Germany; design M. Blum) and removed.
Laser cut energy was approximately from 130 to 160 nJ and spot spacing ranged from 2.5 to 4.5 μm. Thereafter, a 40-60° incision located at 12 O'clock position was created to allow the lenticule extraction. The intended thickness of the upper tissue arcade was 100 μm, and its intended lenticule diameter was 7.5 mm, which was 1 mm larger than the diameter of the refractive lenticule (6.5 mm). The side cuts made for access to the lenticule were set 90° apart at a width of 4.5 mm. At the end of the procedure, any redundant portions of the cap need to be distributed evenly to the periphery using a dry microspear to avoid mud-crack microfolds in the cap, which was achieved through the slit-lamp of the VisuMax.
In the femto-LASIK technique, after standard sterile draping and insertion of the speculum to keep the eye open, the patient's eye was positioned under the VisuMax femtosecond laser surgical microscope. The patient fixated on an internal target light in the microscope for centration, and the cornea was applanated by moving the table upward toward the contact glass. The patient continued to observe the blinking target green light even when the suction was applied. Femtosecond laser pulses with a typical pulse energy of ~110 nJ were delivered with a pulse repetition rate of 500 kHz. The pulses were focused at a precise depth in the corneal tissue, and the laser pulses created microphotodisruption or an expanding bubble of carbon dioxide gas and water that in turn cleaved the tissue and created a plane of separation. The track distance and spot distance were 3.0 μm during flap creation and 1.5 μm during flap side-cutting. The created flap diameter was 8.0 mm, and flap thickness was set to 100-110 μm. Side-cut angle and hinge angle were 90 and 50°, respectively. The hinges were set in a superior orientation with a hinge length of 4.0 mm. The flaps were created by laser scanning in spirals from the periphery to the center of the pupil superior hinge 4 mm.
After completion of the procedure, a spatula was inserted under the flap near the hinge and the flap was lifted. The corneal stroma tissue ablation was performed with a scanning-spot excimer laser (Allegretto, Ex500 Excimer Laser; Wavelight Laser Technologie AG, Erlangen, Germany) using a tissue-saving function with a repetition rate of 250 kHz and a pulse energy of 150 nJ. After completion of the procedure, a spatula was inserted under the flap near the hinge and the flap was lifted. Finally, the flap was repositioned and the interface flushed.
After surgery for both procedures, all patients wore bandage soft contact lenses (Johnson & Johnson Vision Care, Inc. Florida, USA) until the next day of the operation. Postoperative topical medication regimens were identical for each eye and consisted of the administration of an ophthalmic solution of Vigamox eye drops (moxifloxacin hydrochloride 0.5%; Alcon Inc., Alcon Aliso Viejo, USA) four times per day for 7 days, Pred Forte eye drops (prednisolone acetate 1%; Allergan Co., Mayo, Ireland) six times per day with a taper over the course of 2 weeks, and a nonpreservative tear supplement Optive eye drop (carboxy-methylcellulose sodium eye drops; Allergan Inc., Irvine, California, USA) four times per day for 2 months.
The following parameters were evaluated in all patients before surgery and 1 week, 1, 3, and 6 months after surgery: mean foveal, parafoveal, and perifoveal retinal thickness, IOP, UDVA, CDVA, and retinal nerve fiber layer thickness. Each visit also included slit-lamp examination, tonometry, corneal topography, and OCT.
All data were analyzed with SPSS (version 15; SPSS Inc., Chicago, Illinois, USA). Parameters were summarized as mean and SD of the percentage difference from baseline. Comparisons of continuous variables were examined using independent Student's t-test or Mann-Whitney U-test as appropriate, and the χ2 test was used for statistical analysis of categorical variables at the baseline. The Pearson correlation coefficient (r) was used to evaluate the correlation between variables. In addition, we evaluated the mean differences between the SMILE group and the femto-LASIK group before surgery and at each postoperative timepoint for each variable. The Bonferroni-corrected post-hoc test was conducted to adjust the observed significant level for multiple comparisons. A P value of 0.05 or less was considered statistically significant.
| Results|| |
A total of 155 eyes of 155 patients, age ranging from 21 to 36 years, were enrolled in the present study according to the inclusion/exclusion criteria described. A total of 80 eyes underwent SMILE (the SMILE group), and 75 eyes underwent femto-LASIK (the femto-LASIK group).
[Table 1] summarizes the number of eyes evaluated at each examination point in both groups. More than 95% of patients were followed up for 6 months.
|Table 1 The number and percentage of eyes evaluated at each examination point in both groups|
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[Table 2] shows the demographic and clinical features in the studied groups. In total, 80 eyes of 80 patients (35 male, 45 female) who received SMILE procedure were included, and 75 eyes of 75 patients (35 male, 40 female) who underwent femto-LASIK procedure were included. There were no statistically significant differences between the SMILE group and the femto-LASIK group as regards age, sex, manifest spherical equivalent, manifest cylinder, logMAR UDVA, and logMAR CDVA (P = 0.64, 0.43, 0.56, 0.37, 0.18, and 0.52, respectively). The mean preoperative IOP, K reading, and central corneal thickness were not statistically significantly different between the two groups (P = 0.29, 0.63, and 0.64, respectively).
|Table 2 Comparison of preoperative demographic and clinical characteristics between the small-incision lenticule extraction group and the femtosecond laser-assisted in-situ keratomileusis group|
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At 6 months postoperatively, UDVA and CDVA were −0.07 ± 0.03 logMAR (Snellen equivalent, 20/15.3) and −0.07 ± 0.04 logMAR (Snellen equivalent, 20/14.6) in the SMILE group and −0.07 ± 0.06 logMAR (Snellen equivalent, 20/16) and −0.07 ± 0.06logMAR (Snellen equivalent, 20/16) in the femto-LASIK group. There was no significant difference between the two groups in postoperative UDVA (P = 0.17) and CDVA (P = 0.27).
[Table 3] shows preoperative and postoperative macular thickness parameters in both groups. The mean foveal, parafoveal, and perifoveal retinal thickness did not change significantly from the preoperative to any postoperative timepoint (P > 0.05). The mean foveal retinal thickness was significantly greater in the femto-LASIK group than in the SMILE group (P = 0.04). However, the differences in the parafoveal and perifoveal retinal thickness between the two groups were not statistically significant at 1 week postoperatively (P > 0.05). The differences in foveal, parafoveal, and perifoveal retinal thickness between the two groups were not statistically significant at 1, 3, and 6 months (P > 0.05).
|Table 3 Comparison of preoperative and postoperative macular thickness (¦Ìm) parameters between the small-incision lenticule extraction group and the femtosecond laser-assisted in-situ keratomileusis group|
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[Table 4] shows preoperative and postoperative peripapillary retinal nerve fiber layer thickness parameters in both groups. The retinal nerve fiber layer thickness postoperatively, including the mean thickness, temporal, nasal, superior, and inferior thickness, was less than that preoperatively in both the SMILE and femto-LASIK groups (P > 0.05). The differences in the mean retinal thickness, temporal, nasal, superior, and inferior thickness between the two groups were not statistically significant at any follow-up visit (P > 0.05).
|Table 4 Comparison of preoperative and postoperative peripapillary retinal nerve layer thickness (¦Ìm) between the small-incision lenticule extraction group and the femtosecond laser-assisted in-situ keratomileusis group|
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| Discussion|| |
Since femtosecond lasers were first introduced into refractive surgery, the ultimate goal has been to create an intrastromal lenticule that can then be manually removed as a single piece, thereby circumventing the need for incremental photoablation by an excimer laser. A precursor to modern refractive lenticule extraction was first described in 1996 using a picosecond laser to generate an intrastromal lenticule that was removed manually after lifting the flap , . However, significant manual dissection was required, leading to an irregular surface.
The switch to femtosecond laser improved the precision  , and studies were performed in rabbit eyes in 1998  , and in partially sighted eyes in 2003  , but these initial studies were not followed up with further clinical trials. Following the introduction of the VisuMax femtosecond laser in 2007  , the intrastromal lenticule method was reintroduced in a procedure called femtosecond lenticule extraction.
Since 2006, the VisuMax femtosecond laser has been used to make the refractive cut in the SMILE procedure  . Studies have shown that SMILE is more precise and accurate in treating refractive errors with fewer complications compared with standard laser in-situ keratomelieusis , .
Refractive lenticule extractions are gaining popularity over laser in-situ keratomelieusis. Studies , have reported refractive lenticule extraction to be safe and effective and to have a decreased incidence of dry eye, better long-term stability of the cornea, and no chance of flap removing or folding after surgery. Because SMILE is not affected by external factors such as temperature, humidity, and draft, this increases its accuracy.
Refractive lenticule extractions are performed with femtosecond laser, unlike femto-LASIK surgery, which uses excimer laser ablation after femtosecond laser flap creation. In SMILE surgery, the patients need to be transferred to a separate machine; moreover, it is a flapless procedure and offers corneal biomechanical integrity compared with LASIK  .
The ability of the OCT to evaluate retinal damage with measurements of the optic nerve head, retinal nerve fiber layer thickness, and macular thickness has been reported , . Spectral-domain OCT may be a good machine for retinal nerve fiber layer thickness and macular thickness assessment for detecting retinal damage after LASIK. Macular thickness measurements and their reproducibility have been studied extensively in the past for Stratus OCT, whereas few studies have examined measurements for the new spectral/Fourier domain instruments , .
In the present study, a new generation of SD-OCT (Cirrhus-HD OCT; Carl Zeiss Meditec AG) instrument was used for its higher sensitivity and better resolution in studying the macular and RNFL thickness. Advances with liquid optical interfaces with no applanation at all are being developed to avoid excessive IOP elevation.
In the current study, spectral-domain OCT was used to evaluate and compare the effect of applied suction with transient elevation of IOP during SMILE and femto-LASIK procedures on macular and peripapillary RNFL thickness in 155 myopic and myopic astigmatism eyes.
In both SMILE and femto-LASIK, the mean foveal, parafoveal, and perifoveal retinal thickness did not change significantly from the preoperative to any postoperative timepoint (P > 0.05). The mean foveal retinal thickness was significantly greater in the femto-LASIK group than in the SMILE group (P = 0.04). This may be explained by the fact that the suction during the procedures might have caused localized edema early after surgery mainly in the foveal region, whereas the perifoveal and parafoveal retina were not affected. However, the differences in the parafoveal and perifoveal retinal thickness between the two groups were not statistically significant at 1 week postoperatively (P > 0.05). The differences in foveal, parafoveal, and perifoveal retinal thickness between the two groups were not statistically significant at 1, 3, and 6 months (P > 0.05).
The effect of LASIK on retinal nerve fiber layer thickness remains a concern of several studies , . In the current study, the retinal nerve fiber layer thickness after the surgery was less than before surgery in both SMILE and femto-LASIK procedures, although the changes were not statistically significant. This is consistent with the findings of Aristeidou et al.  and Sharma et al.  . In the femtosecond laser VisuMax surgery, because the suction is applied to the edge of the cornea and limbus, the increase in IOP is less than 35 mmHg  . Thus, this procedure reduces the complications associated with sudden increase in IOP during suction.
In the current study, we found no statistically significant effect of the transient elevation of IOP during suction on the macular thickness and retinal nerve fiber layer thickness. However, the exposure to suction during SMILE is twice as long as in eyes that had femto-LASIK. However, the increase in IOP during SMILE was similar to that in eyes having femto-LASIK. SMILE did not have an adverse effect on the macular thickness and retinal nerve fiber layer thickness; therefore, the prolonged vacuum time is unlikely to have an adverse effect on the visual outcome.
Other studies assessing the visual field stability, biomechanical waveform analysis, and visual quality such as higher-order aberrations, and contrast sensitivity would be necessary to evaluate the safety of this procedure.
The authors thank Taha Baker for his care and diligence during writing the paper.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4]