The introduction of the femtosecond laser has been a major advancement in corneal refractive surgery. The femtosecond laser has been widely adopted in LASIK as an alternative to mechanical microkeratome to create a corneal flap.1 Despite the reported success of LASIK worldwide,2–4 the two-step surgical approach with flap creation and subsequent stromal excimer ablation could potentially be improved further in terms of surgical time, corneal wound healing, flap-related problems, and biomechanical strength. Recently, a one-step, femtosecond laser-only refractive procedure was introduced in the form of refractive lenticule extraction (ReLEx). In ReLEx, the femtosecond laser is programmed to accurately sculpt a refractive lenticule of pre-determined power within the cornea stroma, which is then removed by the surgeon.5–8
Two types of ReLEx procedure have been described.5–8 First, femtosecond lenticule extraction (FLEx) describes a procedure similar to LASIK, in that a hinged corneal flap akin to the LASIK flap is created by the femtosecond laser, the flap is then opened, and the exposed refractive lenticule is removed.5,6 Second, small incision lenticule extraction (SMILE) represents a flap-less procedure using a pocket incision approach, whereby the refractive lenticule is extracted through a small sub-3 mm incision.7,8 Both procedures have been demonstrated to be safe, with clinical results comparable to LASIK.5,7–10 Recently, we have shown that femtosecond laser-based refractive corrections deliver significantly less laser energy to the cornea compared to excimer laser-based treatment, and ReLEx results in reduced early wound healing and inflammatory responses, especially after moderate-high myopic corrections, as compared to LASIK.6
Although ReLEx surgery has been successful in achieving emmetropization, similar to LASIK surgery, some patients may have a residual refractive error from undercorrection, overcorrection, induced astigmatism, or regression as a consequence of variability in the corneal wound healing process and biomechanical properties.11,12 If the residual refractive error is significant enough, these patients may require further refractive correction or enhancement, and although flap re-lifting and refractive enhancement is easily achieved in LASIK surgery, the lack of a full flap in SMILE poses a unique challenge when laser re-treatment is indicated. Re-treatment rates following LASIK reported in the literature range from 5% to 28%.13–17 In patients who have had LASIK, the original flap can simply be re-lifted for a re-treatment or, alternatively, a surface ablation can be performed. Santhiago et al.18 recently showed that the time between the original surgery and LASIK retreatment is the main factor influencing ease of flap re-lifting and rate of conversion to surface ablation. A similar flap re-lifting technique can be employed for FLEx re-treatment, as we have previously shown in a study using a rabbit model, where we reversed the refractive procedure by cryopreserving the refractive lenticule and re-lifted the corneal flap after 28 days to re-implant the lenticule.19 However, in a flap-less procedure such as SMILE, a different approach needs to be employed in cases of re-treatment.
There are five possible strategies that can be employed for SMILE re-treatment. First, photorefractive keratectomy or laser epithelial keratomileusis (LASEK) can be performed by surface ablation if the refractive correction required is small. Second, LASIK could possibly be performed anterior to the previous SMILE cap. Third, another SMILE procedure could possibly be performed just anterior to the previous one. Fourth, an implantable contact lens could be inserted intraocularly to correct the residual refractive error. The fifth possible strategy would be to convert the previous SMILE pocket incision into a full flap, so that this can be lifted and stromal ablation performed with an excimer laser, identical to LASIK enhancement. This could be achieved by performing a peripheral laser incision to intersect with the previously created SMILE lamellar dissection plane, creating a hinged flap similar to LASIK that could then be simply lifted for enhancement; this is the approach described in the current study.
We investigated four different VisuMax Circle patterns (Carl Zeiss Meditec, Jena, Germany) programmed to create a corneal flap for refractive enhancement after a SMILE procedure using a rabbit model. The femtosecond laser pattern is programmed to create an incision plane adjoining the original SMILE lamellar interface, a side cut, and a hinge. The intrastromal incisions produced by the patterns were visualized by anterior segment optical coherence tomography (AS-OCT) and in vivo confocal microscopy. After the flap was lifted, the difficulty of lift was graded and the stromal bed quality was analyzed by scanning electron microscopy (SEM).
Materials and Methods
Six 12- to 15-week-old New Zealand White rabbits (3 to 4 kg body weight) were divided into four groups consisting of 3 eyes each that underwent either Circle pattern A, B, C, or D. The flowchart of the design and variables of the study is depicted in Figure 1. Temporary tarsorraphy was not performed in any eye after SMILE; hence, the procedure was performed bilaterally. Anesthesia, recovery from sedation, and euthanasia of the rabbits were performed as described previously.20 All animals were treated according to the guidelines of the Association for Research in Vision and Ophthalmology’s Statement for the Use of Animals in Ophthalmic and Vision Research. The protocol was approved by the Institutional Animal Care and Use Committee of SingHealth, Singapore.
Figure 1. Flowchart showing the experimental design of the study. ReLEx SMILE = refractive lenticule extraction (small incision lenticule extraction variant). AS-OCT = anterior segment optical coherence tomography; IVCM = in vivo confocal microscopy; SEM = scanning electron microscopy
ReLEx SMILE Procedure
SMILE was first performed using the VisuMax femtosecond laser system (Carl Zeiss Meditec). All rabbits underwent a −6.00 diopter (D) spherical correction. Once suction was applied, the main refractive and non-refractive femtosecond incisions were performed in the following automated sequence: the posterior surface of the lenticule (spiral in pattern) and the anterior surface of the lenticule (spiral out pattern),21 followed by a 3-mm vertical incision to the corneal surface placed superiorly. The diameter and depth of the cap were set at 7.5 mm and 120 μm, respectively. The diameter of the lenticule (equating to the optical zone) was 6.5 mm. This set-up resulted in a 0.5 mm-wide clearance zone on each side (zone between the circumference of corneal cap and optical zone). The following femtosecond laser parameters were used: 200 nJ power for lenticule, lenticule side cut, cap and cap side cut, and side cut angle of 90 degrees. The spot distance and tracking spacing were set at 3 μm/3 μm for the lenticule, 2 μm/2 μm for the lenticule side cut, 3 μm/3 μm for the cap, and 2 μm/2 μm for the cap side cut.
Following completion of the laser sequence, a Seibel spatula (Rhein Medical Inc., Petersburg, FL) was inserted into the vertical pocket incision to access the lamellar plane of dissection. A proprietary lamellar dissector (Asico, Westmont, IL), which we designed for SMILE lamellar dissection, was used to separate first the anterior portion of the lenticule from the overlying stroma and then the posterior surface of the lenticule from the underlying stroma. Once the lenticule was free from both surfaces, a co-axial Tan DSAEK forceps (Asico) was used to grasp the lenticule and extract it from within the cornea. Finally, a 24-gauge cannula was used to flush the pocket insertion with balanced salt solution.
Flap Creation After ReLEx SMILE Procedure
Flap creation was performed 28 days after the initial SMILE procedure in all rabbits, which allowed for sufficient corneal wound healing to occur prior to flap creation. Four different VisuMax Circle patterns were studied. The Circle option permits the creation of three basic components: lamellar ring, side cut with hinge, and junction cut (Figure 2A). The user-selectable femtosecond laser parameters (Figures 2B and 2C) to produce these three basic components are adjustable. The femtosecond laser parameters used in patterns A, B, C, and D are shown in Table 1. The laser energy level to perform the Circle pattern has been preset by the manufacturer and therefore is not user adjustable. Pattern A creates a side cut to meet the cap cut within the clearance zone (outside of the optical zone); pattern B creates a lamellar ring posterior to the cap to meet the cap cut in the clearance zone with the help of a junction cut; pattern C produces a lamellar ring anterior to the cap to meet the cap cut in the clearance zone with the help of a junction cut; and pattern D creates a lamellar ring at the same depth as the cap to meet the cap cut in the clearance zone with the help of a junction cut. The cross-sectional and front views of the incisions are illustrated in Figure 3.
Figure 2. Illustrations of intrastromal incision permitted by VisuMax Circle option (Carl Zeiss Meditec, Jena, Germany). (A) The Circle option permits the creation of three basic components: a side cut with hinge, a lamellar ring, and a junction cut. (B) Cross-sectional view showing the adjustable femtosecond laser parameters to create the intrastromal incision. (C) Front view showing the user-selectable parameters to create the intrastromal incision. OD = outer diameter; JD = junction diameter; SD = side cut depth; SA = side cut angle; HA = hinge angle; JU = junction upper depth; JL = junction lower depth
Table 1: Femtosecond Laser Parameters Used in the VisuMax Circle Patterns Tested in the Current Study
Figure 3. Illustrations of the tested VisuMax Circle patterns (Carl Zeiss Meditec, Jena, Germany) and the incision parameters selected for respective pattern. Pattern A creates a side cut within the cap cut (CC; in blue). Pattern B creates a lamellar ring posterior to the cap cut. Pattern C creates a lamellar ring anterior to the cap cut. Pattern D creates a lamellar ring adjacent to the cap cut. The parametric settings of the patterns tested in the current study can be found in Table 1. OD = outer diameter; JD = junction diameter; SD = side cut depth; SA = side cut angle; HA = hinge angle; JU = junction upper depth; JL = junction lower depth
Following completion of the laser sequence and subsequent corneal imaging, a Sinskey hook (Rhein Medical Inc.) was used to create a small incision along the furrow of the flap side cut near the hinge. A Seibel spatula (Rhein Medical Inc.) was inserted under the flap edge through the incision to create access from the lamellar ring to the original cap–stromal bed interface. A lamellar dissector (Asico) was then inserted through the intrastromal tunnel to gently release the flap-bed and remaining flap-lamellar ring adhesions. A Seibel spatula was re-inserted under the flap, flap adhesions were released completely by sweeping under the flap, and the flap was finally lifted.
AS-OCT and In Vivo Confocal Microscopy
AS-OCT was performed using the RTVue Fourier-domain OCT (Optovue, Fremont, CA) prior to flap creation to measure the depth of the laser resection. After the intrastromal Circle incision was made and before the flap was lifted, the cross-section images of the side cut and lamellar ring were visualized. The depth of the lamellar ring was measured using standard measurement tools provided by the Optovue software. En-face images of the side cut, lamellar ring, and junction cut were captured using an in vivo confocal microscope (Heidelberg retina tomography HRT3; Heidelberg Engineering GmbH, Heidelberg, Germany). A carbomer gel (Vidisic; Mann Pharma, Berlin, Germany) was used as immersion fluid. In vivo confocal micrographs were analyzed with the Heidelberg Eye Explorer version 1.5.1 software (Heidelberg Engineering GmbH).
After the flap was lifted and the rabbits were killed, the cornea was excised from the whole globe. The quality of the exposed stromal bed was analyzed by SEM. Corneas were fixed in 2% paraformaldehyde (Sigma, St. Louis, MO), 2% glutaraldehyde (Sigma), and 0.1M sodium cacodylate (pH 7.4) overnight at 4°C. The corneas were then washed twice in distilled water for 10 minutes each before being immersed in 1% osmium tetraoxide (FMB, Singapore) for 2 hours at room temperature. Following this, the samples were dehydrated in 25%, 50%, 75%, 95%, and 100% ethanol, with 95% and 100% concentrations being performed twice. The samples were then dried in a critical point dryer (Bal-Tec, Balzers, Liechtenstein) and mounted on stubs using carbon adhesive tabs. They were then sputter coated with a 10-nm–thick layer of gold (Bal-Tec) and examined with a JSM-5600 scanning electron microscope (JEOL, Tokyo, Japan).
Grading of Difficulty of Flap Lift
Flap lifting was performed and graded by a senior corneal surgeon experienced in LASIK, SMILE, and anterior lamellar corneal transplantation surgery (JSM). The treatment groups were not masked from the surgeon when evaluating the difficulty of flap lift. The difficulty of flap lift was graded on a scale of 1 to 5: grade 1 was the easiest, similar to microkeratome created flap lift, grade 2 was graded as similar to normal femtosecond laser-created flap lift, grade 3 had at least two areas of adhesion, grade 4 had at least three areas, and grade 5 was the most difficult and almost impossible to lift flap, with a presence of sticky interface due to excessive remnant tissue bridges.
Cross-sectional Visualization of the Cornea
The previously performed cap–stromal bed interface was barely detectable by AS-OCT 28 days after the SMILE procedure. In pattern A-treated corneas, only the side cut was visible (Figure 4A). In pattern-B treated corneas, lamellar ring with an average depth of 131.33 ± 4.04 μm and a side cut could be observed (Figure 4B). A lamellar ring with a depth of 113.67 ± 3.21 μm and a side cut were seen in pattern C-treated corneas (Figure 4C). In pattern D-treated corneas, a lamellar ring with a depth of 122.00 ± 3.46 μm and a side cut were visible (Figure 4D). The junction cut created by all patterns was not detectable on AS-OCT.
Figure 4. Visualization of the incision created by VisuMax Circle patterns (Carl Zeiss Meditec, Jena, Germany). Images were obtained after the incision and before flap lifting, using anterior segment optical coherence tomography (AS-OCT). Lamellar ring (arrowheads) was characterized by light reflective layer, indicative of femtosecond laser disrupted lamellae. The original refractive lenticule extraction small incision lenticule extraction (SMILE) cap cuts were not visible on AS-OCT by day 28 after SMILE. The presumed cap cut was marked in blue. (A) Pattern A created a side cut within the cap cut. (B) Pattern B created a lamellar ring at a measured depth of 127 μm, placing it posterior to the cap cut. (C) Pattern C created a lamellar ring at a depth of 115 μm, placing it anterior to the cap cut. (D) Pattern D created a lamellar ring adjacent to the cap cut, which depth was measured at 124 μm. Arrows indicate the side cut created by patterns A, B, C, and D. In vivo confocal micrographs of the intrastromal cuts appeared similar in all treatment groups. (E) The appearance of side cut at the depth of 34 μm from the superficial epithelium. (F) The junction between side cut and the lamellar ring at the depth of 114 μm. The femtosecond laser disrupted layer of lamellar ring is discernible by its light reflective layer. (G) The junction between the junction cut and the lamellar ring. (H) The junction between the cap cut (blue in illustration) and the junction cut.
The appearance of intrastromal cuts and lamellar ring was similar on in vivo confocal micrographs in all treatment groups. Therefore, we selected the images from pattern C-treated corneas to represent the in vivo confocal microscopy results. En-face images showed the side cut through the anterior cornea (Figure 4E) and the junction between the side cut and the lamellar ring at the depth of 114 μm (Figure 4F). The femtosecond laser-disrupted lamellar ring was marked by a light reflective layer. In vivo confocal images also showed the intersection between the junction cut and lamellar ring (Figure 4G) and between the junction cut and cap interface (Figure 4H).
SEM showed the vertical side cut after the flap was lifted (Figure A, available as supplemental material in the PDF version of this article) and the undisrupted collagen fibrils at the center of the flap bed (Figure A). The difficulty of flap lift was grade 2 for all 3 eyes, similar to the difficulty of femtosecond laser-created flap lift.
Following AS-OCT, we attempted to lift the flap by inserting a Seibel spatula under the flap edge near the hinge to create access from the lamellar ring to the original cap–stromal bed interface. Because the lamellar ring was placed posterior to the cap cut, we had difficulty in accessing the intended plane and inadvertently damaged the lamellae under the edge of the optical zone during the intrastromal manipulation with the spatula, as illustrated (Figure B, available as supplemental material in the PDF version of this article) and revealed by the SEM image (Figure B). Once the correct plane (cap cut) was reached, the flap lift was straightforward and no further significant tissue damage was observed at the opposite quadrant of the optical zone (Figure B). An approximately 10-μm elevation (arrow) could be seen from the lamellar ring to the cap cut (Figure B). SEM also revealed the vertical side cut (Figure B) and the undisrupted central corneal flap bed (Figure B). Due to the difficulty in accessing the original cap–stromal bed interface and resultant tissue damage, the difficulty of flap lift was grade 4 for all 3 eyes treated with pattern B.
SEM images showed no lamellar damage in pattern C-treated corneas. The slight depression (arrow) from the lamellar ring to the cap–stromal bed interface was barely visible (Figure C, available as supplemental material in the PDF version of this article). The vertical side cut and surface of lamellar ring are shown in Figure C. Undisrupted central corneal flap bed is shown in Figure C. The difficulty of flap lift was grade 3 in all 3 eyes because there was a minor resistance when attempting to access the original cap cut, most probably due to the difference in depth between the lamellar ring and the cap cut.
The transition between the lamellar ring and the optical zone was not discernible in the SEM image (Figure D, available as supplemental material in the PDF version of this article). SEM also revealed the vertical side cut (Figure D). Similar to corneas treated with patterns A, B, and C, the central corneal flap bed created by pattern D was also undisrupted after flap lifting (Figure D). The difficulty of flap lift after pattern D treatment was grade 2 in all 3 eyes, similar to the difficulty in lifting a standard femtosecond laser-created LASIK flap.
In this study, we assessed four new VisuMax Circle patterns (Carl Zeiss Meditec) that are programmed to create a series of intrastromal incisions, leading to the creation of a corneal flap for SMILE re-treatment. Overall, based on the ease of flap lift and the resultant good flap bed quality, we found that Circle pattern D, a lamellar ring at the same depth as the cap to meet the cap cut in the clearance zone with the help of a junction cut, was the most optimal approach for SMILE enhancement.
In pattern A, where no lamellar ring was formed, the side cut had to be made within the clearance zone of the cap cut, which results in a suboptimal reduction in retreatment area. In addition, a hyperopic enhancement limited by the smaller flap diameter would be more prone to result in an undercorrection. Flaps formed by Circle pattern A were easy to lift, with difficulty similar to femtosecond laser-created flap. On SEM, the resulting flap bed was smooth and undisrupted both centrally and peripherally. Different from pattern A, pattern D places a lamellar ring adjacent to and at the same depth as the cap cut. The resultant flaps were as easy to lift as those created by pattern A and SEM revealed similar stromal bed quality as pattern A-treated corneas and indiscernible transition between lamellar ring and original cap cut.
A significant difference in intrastromal dissection resistance was experienced when manipulating the lamellae in the attempt to access the cap cut from the lamellar ring, which was placed either posteriorly in pattern B or anteriorly in pattern C. Flaps created by pattern B were the most difficult to lift compared to the other patterns. Furthermore, the tissue damage resulting from the flap lifting process, as observed by SEM, suggests that the pattern B approach should not be attempted for SMILE re-treatment. Although no tissue damage was inflicted on the stromal bed, the flap lift after pattern C was not as easy and straightforward as after pattern D. Due to the difference in the depths of lamellar ring and cap cut, the barely visible transition between these planes could still be seen on SEM.
The measurement of the actual depth of the original interface is necessary to determine the parameters of the Circle patterns to ensure that the femtosecond laser incisions intersect the intended cap cut and thus create an easy to lift corneal flap. However, on day 28 after SMILE in rabbits we could hardly detect the original cap cut using AS-OCT, which normally appears as a highly reflective layer at the flap interface.6,22 Hall et al.23 noticed that with time, the original interface also becomes more difficult to differentiate in human patients; at early measurements (ie, those at 1 month), the interface is more defined than that at 1 year. This should not be a concern because many studies have shown that the measured thickness of flaps created by the VisuMax femtosecond laser system is not significantly different than the programmed value.23,24 When the flap interface has become indiscernible and its depth is not measurable, the programmed depth of cap cut from the initial SMILE procedure can be used to configure the Circle patterns. We have previously reported that the standard deviation of the measured thickness of the corneal caps ranged between 9.5 and 10.9 μm.23 This range is still within the span of the junction cut (set at 20 μm in depth in our study and could be increased further if necessary), which further ensures the lamellar ring would intersect the original cap cut. The junction cut also ensures the presence of postoperative epithelial hyperplasia, reported to be 9 ± 7 μm in patients after LASIK,25 does not pose any problem in selecting the depth of the lamellar ring.
Depending on the time between refractive enhancement and the initial SMILE, lifting of Circle-created flaps may become more challenging over time. If Circle-created flap lift is difficult and the surgeon decides not to proceed because of concern that the flap will be torn or damaged, surface ablation with mitomycin C may become the next viable option. Prior studies have shown the efficacy and safety of this treatment strategy, despite patient discomfort and longer visual recovery.26,27 The other possible option for SMILE re-treatment would be insertion of implantable contact lens or creation of refractive lenticule anterior to the original cap–stromal bed interface. Insertion of an implantable contact lens would not be a common technique used for re-treatment because the residual refractive error is generally small. Performing another SMILE after the primary refractive procedure could be another viable option. This technique bears resemblance to the re-cutting of flap technique for LASIK enhancement. Several studies have demonstrated similar refractive outcome, safety, and effectiveness between re-treated eyes that underwent flap re-lift and flap re-cutting.28,29 A study on the feasibility and effectiveness of performing another SMILE for enhancement after primary SMILE is currently underway in our institution. Alternatively, LASIK could possibly be performed anterior of the previous SMILE cap if the patient seeks re-treatment in an eye center that does not operate a VisuMax femtosecond laser system.
Because of the relatively small sample size (n = 3 in each treatment group), we were unable to perform an extensive and meaningful statistical analysis. It would be of great value to present the mean value of the grading of the flap lift difficulty or the percentage of cases presenting each grading. However, we found that the degree of flap lift difficulty was consistent in all 3 eyes in every treatment group, which means the standard deviation of the grading would be 0. Our SEM, AS-OCT, and in vivo confocal microscopy results were also consistent in all 3 eyes in every treatment group.
We have shown the stromal bed quality and difficulty in lifting flaps created with four Circle patterns for SMILE enhancement. We found that pattern D, which created the lamellar ring at the same depth as the original cap–stromal bed interface, appears to be the most optimal for clinical use. Accessing the correct plane (the cap–stromal bed interface) from the lamellar ring was uneventful, and the subsequent lifting of the flap was relatively straightforward and akin to lifting a femtosecond laser-created LASIK flap. In addition, the resultant flap bed was undisrupted, and the transition between the lamellar ring and the cap–stromal bed interface was smooth and indiscernible. Further studies are ongoing to ascertain the clinical feasibility of this form of SMILE enhancement.
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- Shah R, Shah S, Sengupta S. Results of small incision lenticule extraction: all-in-one femtosecond laser refractive surgery. J Cataract Refract Surg. 2011;37:127–137 doi:10.1016/j.jcrs.2010.07.033 [CrossRef] .
- Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol. 2011;95:335–339 doi:10.1136/bjo.2009.174284 [CrossRef] .
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Figure A. Scanning electron microscopy (SEM) images of the corneal stromal bed treated with pattern A. (A) SEM showing the side cut and peripheral flap bed after the flap was lifted. (B) SEM showing the central corneal flap bed.
Figure B. Scanning electron microscopy (SEM) images of the corneal stromal bed treated with pattern B. (A) SEM showing the lamellar damage under the edge of the cap cut created by the Seibel spatula. (B) The area in the opposite quadrant of that shown in (A). Arrow indicates an elevation of the cap cut from the lamellar ring. (C) The side cut and surface of lamellar ring. (D) The central corneal flap bed. (E) Illustration depicting a Seibel spatula, inserted into the lamellar ring incision, penetrates and damages the lamellae under the edge of the cap cut as shown in SEM image (A), due to the difficulty in accessing the correct plane (the cap cut), as the lamellar ring is placed posterior to it.
Figure C. Scanning electron microscopy (SEM) images of the corneal stromal bed treated with pattern C. (A) The transition between lamellar ring and cap cut is barely visible. Arrow shows a slight depression of the cap cut from the lamellar ring. (B) The side cut and surface of lamellar ring. (C) The central corneal flap bed.
Figure D. Scanning electron microscopy (SEM) images of the corneal stromal bed treated with pattern D. (A) The transition between lamellar ring and cap cut is barely visible. (B) The side cut and surface of lamellar ring. (C) The central corneal flap bed.
Femtosecond Laser Parameters Used in the VisuMax Circle Patterns Tested in the Current Study
|Pattern||Lamellar and Side Cut||Junction Cut|
|Outer Diameter (mm)||Side Cut Depth (μm)a||Side Cut Angle||Hinge Position||Junction Diameter (mm)||Junction Upper Depth (μm)||Junction Lower Depth (μm)|