Despite the great advances in the field of refractive surgery, correcting myopia higher than −10 diopters (D) is still considered one of the hot topics among refractive surgeons. Various treatment options are available: either intraocular procedures, such as refractive lens exchange (RLE) and phakic intraocular lens (IOL), or laser vision correction (LVC), such as laser in situ keratomileusis (LASIK) and surface ablation. Retinal breaks, loss of corneal endothelial cells, cataract, and glaucoma are the potential risks of RLE and IOLs.1,2
RLE is additionally associated with the risk of retinal detachment and loss of accommodation, and so, it is generally not considered in patients with prepresbyopia.3,4 LASIK is no longer the best option for the treatment of myopia over −10 D because of a less predictable outcome when compared with phakic IOLs5 and its effect on corneal biomechanics, carrying the risk of ectasia.6 Haze has been reported to be a significant long-term hazard in eyes with high myopia treated with photorefractive keratectomy.7 Regression is another issue that should be taken into consideration.8
When small incision lenticule extraction (SMILE) was introduced into the refractive surgery field, it offered the advantages of less dry eye symptoms9 and better corneal biomechanics through cutting less of the strong anterior corneal stroma and avoiding flap-related complications.10,11 This offers a new solution to solve the dilemma of high myopia.12,13 One of the unique additional advantages of SMILE for high myopia is the duration of the surgery. SMILE takes about the same time to treat all refractive errors (about 30 seconds), so the effect of flap bed hydration on ablation, a concern in high myopic LASIK treatment, is negligible.14 The aim of this study is to investigate the efficacy, predictability, stability, and safety of the SMILE technique for patients with high myopia of more than 10 D.
PATIENTS AND METHODS
A retrospective noncomparative analysis of the records of 495 eyes of 270 patients treated by the ReLEx SMILE technique for a mean spherical myopic error of −12.84 2.2.47 D (range: −10.0 to −14.0 D) combined with a mean astigmatism of −1.17 ± 1.34 D (up to −4.0 D). The mean LogMAR corrected distance visual acuity (CDVA) was 0.2 ± 0.6. SMILE was conducted in both eyes of 225 patients and in one eye of 45 patients.
The study included patients with more than 10 D of myopia who underwent the SMILE surgery at the Roayah Vision Correction Center in Alexandria, Egypt, from February 2011 to December 2017. The study protocol adhered to the tenets of the Declaration of Helsinki.
Inclusion and Exclusion Criteria
To be enrolled in the study, patients had to be at least 21 years old, have a myopic error of more than 10 D that has been stable for at least 1 year and a best-corrected visual acuity of 20/50 or better, and demonstrate the ability to attend postoperative assessments. Only patients who completed 3 years of follow-up were included in this study. Patients who missed the follow-up visits for at least 3 years were not included in the statistical analysis.
SMILE was performed on corneas having a minimum thickness of 500 μm at the thinnest location, with at least 250 μm of the residual stromal bed. Calculated lenticule thickness ranged from 120 to 168 μm, with an average thickness of 141 μm. Patients were excluded if they had any ocular conditions other than myopia and/or astigmatism. This is an off-label use for SMILE (investigator-initiated study), so all patients were informed about the off-label nature of the procedure with a full explanation of other alternative options for the management of high myopia. All the guidelines for the residual stromal bed thickness and percentage of tissue ablation that apply for the LASIK procedures were respected. The study group contains consultants for the Carl Zeiss Meditec company, and special software for the investigational purpose was provided by the company.
The lenticule diameter (ie, optical zone) was 6.0 mm according to the scotopic pupil diameter and residual stroma. Therefore, there should be a balance between the residual stromal bed thickness, pupil diameter, and lenticule diameter.
Because the machine's settings had a tendency for undercorrection in high myopia, the nomogram was adjusted to add about 10% of the manifest refraction. However, some patients were intentionally undercorrected because of planned monovision in patients older than 40 years, high numerical sum of sphere and cylinder (higher than 14), or insufficient corneal thickness (ie, our intended correction was not always emmetropia).
All patients had a thorough eye examination, with the assessment of cycloplegic and manifest refraction, UDVA, CDVA, pupil size, intraocular pressure measurement, keratometric measurement, anterior segment assessment using slit-lamp examination, complete posterior segment evaluation, corneal topography, and Pentacam (Oculus) at each postoperative follow-up visit, and patients were assessed regarding best-corrected distance visual acuity (CDVA), uncorrected distance visual acuity (UDVA) (both were measured using the ETDRS visual acuity chart and expressed in LogMAR visual acuity), manifest refraction, slit-lamp examination, and Pentacam follow-up.
All surgeries were planned using research software and performed by 3 experienced surgeons (A.E., O.I., and M.A.). After the application of topical anesthesia, standard sterile draping, and insertion of the speculum, the patient's eye was centered and docked with the appropriate size curved interface cone chosen according to patient corneal diameter. Small-sized cones were chosen for patients with corneal diameter less than 12 mm, whereas medium-sized cones were chosen for larger corneas.
Once appropriate centration was achieved, the surgeon initiated the automatic suction. The femtosecond laser platform (VisuMax; Carl Zeiss Meditec, Inc, Dublin, CA) was used to create the lenticule and incision of all cases. The femtosecond laser produces ultrashort pulses of light at a repetition rate of 500 kHz with typical pulse energy of 125 nJ, which are focused at a precise depth in the corneal tissue. A plasma state develops with an 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 and track distance of 3 μm for the lamellar cuts and 2 μm for the vertical side cuts resulting in cleavage of tissue planes.
In the first step, the femtosecond laser cuts the posterior surface of the lenticule, which is followed by the side cut of the lenticule. In the third step, the anterior surface of the lenticule is created. Finally, a side-cut incision is made. The SMILE procedure had the following parameters: 100 μm cap thickness, 7.9 to 7.3 mm anterior-plane (cap) cut diameter, and from 5.5- to 6.5-mm optical zone of the lenticule. In our study, we have standardized the optical zone to be 6.0 mm with a cap diameter of 7.5 mm.
After the suction was released, the patient was moved toward the observation position under the VisuMax integrated surgical microscope. A thin spatula was inserted through the side cut over the roof of the refractive lenticule dissecting this plane followed by the bottom of the lenticule. The lenticule was subsequently grasped with a modified serrated McPherson forceps.
After the removal of the lenticule, the patient was asked to blink his/her eyes, and the alignment of the cap was again inspected. The postoperative standard treatment protocol included topical steroids, antibiotics, and lubricating eye drops 5 times daily for 1 week followed by lubricating eye drops 5 times daily for 1 month. Patients were recruited for follow-up after 1 day, 1 week, 1 month, 6 months, 1 year, and 3 years for the assessment of visual acuity and refraction. Follow-up topographic examinations were performed using the Pentacam, with comparison between different follow-up periods for detection of any possible ectatic changes. Only 1-month and 3-year follow-up data were included in the statistical analysis denoting early and late follow-up findings.
The data were entered into an Excel spreadsheet (Microsoft Corp, Redmond, WA). It was converted into a spreadsheet for SPSS (version 23 for Windows; SPSS, Inc, Chicago). Quantitative data were described using range, mean, and standard deviation. The Kolmogorov–Smirnov test was used for checking the normality of distribution. Comparison between different periods was assessed using the paired t test between the preoperative and postoperative normally distributed data. Differences were considered to be statistically significant when the associated P value was <0.05 at 95% confidence interval.
Eyes with deviation from target refraction were calculated. The χ2 test was used to compare between different percentages. Standard figures for reporting the outcomes in refractive surgery, according to the protocol and its modification, were used for displaying and summarizing the refractive outcomes of this study for each group postoperatively.15,16
The refractive outcome was expressed in efficacy, predictability, safety, and stability. The efficacy index was calculated as postoperative UDVA/preoperative CDVA. The safety index was calculated as postoperative CDVA/preoperative CDVA.
We had 4 eyes with intraoperative suction loss that have been postponed to have another SMILE procedure later. The 4 eyes were treated successfully on the next day using the same SMILE parameters. One case had a difficult lenticule dissection in one eye. Apart from these incidences, no intraoperative complications were encountered (Tables 1 and 2).
At 1 month after surgery, the mean refractive error was −0.72 ± 0.88 D (range: +1 to −1.5 D), and the mean postoperative astigmatism was −0.83 ± 1.04 D (up to −1.75 D). The mean LogMAR UDVA was 0.2 ± 0.34 at the last follow-up at 3 years.
At 3 years, visual acuity and other parameters had only minimal nonsignificant changes at the end of the 3-year follow-up period. However, the spherical error had a significant change from −0.72 ± 0.88 D at 1 month to −1.17 ± 1.01 D, but this change had an insignificant visual impact (Fig. 1). Cases that have not completed the 3-year follow-up were excluded from the statistical analysis. All eyes included in the study had completed the 3-year follow-up.
At the end of the follow-up period, approximately 94% of patients had unchanged CDVA or gained one or more lines, 6% lost one line of CDVA, and 1% lost 2 lines. We have not reported cases of ectasia or irregular astigmatism. The efficacy index was calculated from postoperative UDVA/preoperative CDVA and showed a value of 0.9 ± −0.8 at the 1-month follow-up and 1.1 ± 1 at the 3-year follow-up (Fig. 2).
Many corneal biomechanics experts have hypothesized that SMILE has no effect on the anterior stroma; hence, it maintains the corneal tensile strength postoperatively better than LASIK because the strongest anterior stroma remains uncut. This is of particular importance for very high myopic treatments because of the large amount of corneal tissue to be removed, which may carry the risk of postoperative corneal ectasia.17
Refractive surgery experts are relatively conservative regarding adopting cornea-based refractive surgeries for high myopia for fear of regression and ectasia. In our study, there were no cases of ectasia reported, and the refractive outcome seemed to be constant in most cases during the 3-year follow-up.18 Ectasia can occur anytime after corneal LVC for up to 10 years. Regarding the reported cases of ectasia after SMILE, most of them occurred from 1 to 3 years after the procedure.19 The functional optical zone in SMILE is measured to be larger than that in LASIK with a wider topographic flattening after surgery than the flattening achieved by the same optical zone when performed with excimer laser ablation, and this can allow correction of greater myopic degrees with less tissue removal and allows for performing surgeries at a smaller optical zone without compromising the postoperative visual performance.20,21
Comparing the results at 1 and 36 months in this study, there were no significant differences in the efficacy, predictability, and safety. This supports the results of other studies that reported similar 6- and 12-month visual outcomes reflecting refractive stability.22
A recently published study of a 5-year outcome for SMILE in high myopia showed 37 eyes of 37 patients with a mean attempted SE of −7.47 ± 1.10 D (range: −6.00 to −10.00 D). At the 5-year visit, the mean difference between achieved and attempted SEs was −0.43 ± 0.47 D (0.50 to −1.25 D); the efficacy index was 0.89 ± 0.26, and the safety index was 1.16 ± 0.20 at the final follow-up visit. Our series is considered a larger one with inclusion of higher degrees of myopia that is more challenging for LVC.23 A prospective cohort study was conducted in China on 53 eyes of 53 patients with a maximum myopic meridian exceeding 10.00 D were corrected and revealed that the efficacy and safety indices were 0.91 ± 0.25 and 1.15 ± 0.18, respectively, with 72% of eyes within ±0.50 D and 89% within ±1.00 D of the attempted correction during the 15-month follow-up period.24
In the light of our current results, we believe that higher errors up to −14 D or even more might be corrected with the same safety and predictability. Because of the novel nature of the high correction, we kept the residual stroma over 250 μm similar to LASIK.
SMILE is still an evolving procedure, and further experimental investigations of post-SMILE biomechanics and optics will shed light and help define the safe boundaries of corneal refractive correction. Finally, SMILE can be considered a reversible procedure. The lenticule removed may be stored and replaced into the cornea at a later time. This adds the advantage of reversing the refractive procedure if the patient develops presbyopia or when the refractive outcome was not satisfactory.
One of the limitations of the current study is its retrospective nature, which may represent a source of selection bias. Other measures for visual function rather than visual acuity as contrast sensitivity and higher order aberrations were not assessed in our series because of the lack of baseline preoperative data of early cases and will be published in an upcoming prospective short-term study. In conclusion, the SMILE technique in high myopic treatment shows promising outcomes regarding the efficacy, predictability, stability, and safety with a low complication rate that may have a major role in this category of patients seeking refractive surgery provided that adequate corneal thickness and normal corneal topography exist.
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