In the field of refractive surgery, the excimer laser has been the reliable workhorse for corneal surgery, with a large number of satisfied patients for more than three decades. In 2008, Sekundo et al.1–3 reported a new procedure using femtosecond laser technology called femtosecond lenticule extraction, which still required lifting of a flap to remove tissue (the refractive lenticule) from the anterior part of the cornea. The technique soon developed further to a minimally invasive procedure called small incision lenticule extraction (SMILE), which avoided using a corneal flap and all of its associated complications.4 There was a strong assumption that SMILE had a better retention of biomechanical strength, less dry eye, and better and faster recovery of corneal sensation.5 Continuous advances in laser design and software, surgical performance, and energy settings have led to SMILE being accepted worldwide.6–10 With more than 2 million surgeries performed and a growing number of studies and peer-reviewed publications, there is clear evidence in the scientific literature that SMILE is a safe, accurate, and effective refractive procedure.11–14
Nevertheless, keeping in mind the average age of patients who undergo refractive surgery, every new procedure has to prove long-term stability in the correction of the refractive error. Some patients might need retreatments over the course of a lifetime, and in the long-term perspective cataract surgery is most likely to be required 20 to 40 years down the road. Therefore, this follow-up study was designed to investigate the long-term results in the first eyes treated with the SMILE procedure 10 years ago. To the best of our knowledge, this is the first report covering this period of time.
Patients and Methods
This study was approved by the Ethics Committee of the Chamber of Physicians of Thuringia, Germany (DE/CA93/KP/07/001). All patients had been treated in the initial prospective study in 2008–2009 and had met the inclusion and exclusion criteria of this first study. They received a written invitation for reevaluation after 10 years on a voluntary basis. An informed consent was obtained from each patient. A total of 31 patients (9 men and 22 women) volunteered and 56 eyes were able to be reevaluated. The average age was 46 years (range: 31 to 66 years; the minimum age in the original study was 21 years). This means that almost half of the study population is now coming close to presbyopia. The 10-year results of 56 eyes that had been part of the original study with 91 eyes published in 2011 are reported herein.4
The SMILE treatment in eyes of this follow-up study was described in detail in our first publication.4
At the 10-year reevaluation visit, the following parameters were obtained: uncorrected (UDVA) and corrected (CDVA) distance visual acuity using different Early Treatment Diabetic Retinopathy Study charts at each visit and for each eye; objective and manifest refraction; pachymetry (AC Master; Carl Zeiss Meditec, Jena, Germany); Goldmann's applanation tonometry; and slit-lamp examination with inspection of the interface and corneal surface to record long-term side effects (eg, corneal scars, corneal ectasia, persistent dry eye symptoms, or cataract formation). Other unrelated ocular pathology was excluded (glaucoma or macular disorders).
All measured data were collected on standardized study spread sheets for descriptive statistics of all eyes. Values are presented as means. Statistical analysis was performed using Excel for Windows 2016 software (version 16.0; Microsoft Corporation, Redmond, WA). The Wilcoxon signed-rank test was used to compare data and P values of less than .05 were considered statistically significant.
The 10-year visit was accomplished by 56 eyes of 31 patients. The results 10 years after the first SMILE study are shown in the “Nine Standard Graphs for Corneal Refractive Surgery” in Figure 1.
Nine standard graphs for corneal refractive surgery. UDVA = uncorrected visual acuity; CDVA = corrected distance visual acuity; D = diopters
As shown in Figure 1, 16 (29%) of 56 eyes gained one or two lines of CDVA after 10 years, 32 eyes (57%) were stable, 8 (14%) eyes lost one line, and no eye lost two or more lines. Figure 2 presents the CDVA over 10 years in logMAR units, giving the number of eyes at the examinations at 1 and 6 months and 1, 5, and 10 years after surgery. There were only small changes over time from the beginning of the study to 10 years after surgery. Statistical testing did not prove any significant changes from preoperatively to 10 years (P = .08) and 5 to 10 years postoperatively (P = .093). Figure 3 presents the same for UDVA over 10 years. UDVA remained stable during the postoperative period from 1 month to 10 years (P = .75).
Corrected distance visual acuity (CDVA) (logMAR) 10 years after small incision lenticule extraction for myopia. MV = mean value; SD = standard deviation
Uncorrected distance visual acuity (UDVA) (logMAR) 10 years after small incision lenticule extraction for myopia. MV = mean value; SD = standard deviation
Attempted versus achieved correction of 56 eyes after 10 years is shown in Figures 1D–1E. After 10 years, 64.3% of eyes (36 eyes) were within ±0.50 diopters (D) and 82.1% (46 eyes) were within ±1.00 D of target refraction.
The 10-year refraction was −0.35 ± 0.66 D. This corresponds to a regression of 0.30 D within the 10-year period only (Figure 1F). However, statistical testing did prove significant changes from 1 month to 10 years (P = .001).
Data on astigmatism are shown in Figures 1G–1I.
All patients were routinely treated for dry eye symptoms within the first 3 months after SMILE. None of them needed further treatment after 3 months. No signs of corneal ectasia were found in any of the eyes. Furthermore, no signs of glaucoma were diagnosed. Three patients (aged 57, 65, and 66 years) had the first signs of cataract formation. One patient (age 66 years) was diagnosed as having a slight epiretinal membrane. Pachymetry showed an increase of 32 microns over 10 years (Figure 4). Statistical testing proved a significant increase from 1 month to 10 years (P = .001) and from 5 to 10 years (P = .001).
Pachymetry 10 years after small incision lenticule extraction. P = .001 from 1 month to 10 years and P = .001 from 5 to 10 years. MV = mean value; SD = standard deviation postoperatively
Because the SMILE procedure is a relatively new surgical approach, the change in CDVA still represents the most important value. Besides safety of the surgery itself, two parameters are of prime importance for the patients: predictability and long-term stability. To the best of our knowledge, up to now only 4- to 5-year results have been published.15,16
In laser in situ keratomileusis (LASIK), approximately 50% of eyes remain within ±1.00 D of the intended correction after 6 to 10 years, largely correlated to the degree of correction.17–19 In the first eyes treated by refractive lenticule extraction eyes in 2006, the flap was still lifted and predictability was not as good, with LASIK being the gold standard.2,3 Advances in laser technology, energy settings, laser scan patterns, and surgical performance translated into better outcomes. A 500-kHz VisuMax femtosecond laser results in less tissue damage and inflammation in animal studies, supporting clinical impressions.7 Meanwhile, some studies reported refractive outcomes superior to the first results with the 2008–2009 technology presented here and comparable to LASIK outcomes.13,14 Predictability is no longer a major concern.
Once the nomograms were established, theoretical models of SMILE surgery showed fewer biomechanical effects on the cornea and the potential for better biomechanical strength of the cornea.20,21 In the long run, clinical data have to prove this assumption because there are reports of corneal ectasia after SMILE.22,23 The 10-year data reported herein demonstrate good stability, low regression, and no signs of corneal ectasia.
In LASIK, a mean regression of 0.63 to 0.97 D has been reported after 6 to 7 years.17–19 Other reports of photorefractive keratectomy and LASIK report it to be as long as 15 to 18 years.24,25 In photorefractive keratectomy, a significant increase in spherical equivalent was found, specifically in patients younger than 40 years at the time of surgery.24 In LASIK, a regression rate of −0.12 ± 0.15 D per year with stabilization from 2 to 5 years on in moderate myopia was reported.25
Historically, the treatment of myopia has been more predictable compared with the correction of astigmatism. This has been a special concern in SMILE surgery and the results in Figures 1G–1I clearly need improvement.26 There are articles in the literature addressing this question, but all of them lack long-term follow-up. Although the undercorrection is in line with other reports, our detailed analysis of the regression revealed a stable result from 5 to 10 years. The latter aspect implies that a new nomogram has an important impact rather than a true regression on the corneal level.27
Similar to patients treated with LASIK, all patients treated with femtosecond lenticule extraction and SMILE had dry eye symptoms within the first 3 months after the procedure. No further treatment was required thereafter in the group presented here. This is in line with published data on corneal sensitivity: postoperative corneal sensitivity was less altered after SMILE surgery compared to femtosecond laser–assisted LASIK.7,28
There are limitations in the presented data. We were unable to recruit more than 56 of the 91 eyes of the original study after 10 years. Furthermore, any corneal laser refractive procedure will certainly decrease the biomechanical stability of the cornea. Because technologies to test the biomechanical stability of the cornea were not used in the original study, we cannot offer hard data on this important issue. In the original study, the first eyes were treated in a “proof of principle” prospective study (6 months). Having seen the 6-month results, the ethics committee agreed to continue this study as a longitudinal study with measuring points after 12 months, 5 years, and 10 years. However, due to this we can only report data that have been measured in the original study and no other measurements (eg, coma and higher order aberrations).
Some of the patients have now reached the age where cataract surgery might be necessary. This fact has certainly had some impact on our long-term refractive results, explaining a moderate degree of “regression” seen in the current study. In reality, at least some degree of this regression is attributable to the early nuclear sclerosis and associated myopization. In terms of the intraocular lens calculation, surgery after previous laser correction is demanding. Our theoretical results on biometry after SMILE favor ray tracing over other methods of intraocular lens calculation.
Since the introduction of this flap-free technique 10 years ago, SMILE has gained worldwide popularity. These 10-year outcomes of SMILE produced results that were stable and showed no late side effects. These long-term data will alleviate the remaining concerns among surgeons in terms of stability and late complications of this new treatment modality.
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