Small incision lenticule extraction (SMILE) and femtosecond laser–assisted laser in situ keratomileusis (FS-LASIK) not only yield equivalent results in correcting myopia and myopic astigmatism,1,2 but also require similar rates of re-treatment in cases of overcorrection/undercorrection or myopic regression.3–9 The incidence of enhancement after SMILE ranges from 2.2% to 4.4% depending on the re-treatment method reported, including surface ablation, CIRCLE cap-to-flap conversion (VisuMax; Carl Zeiss Meditec AG, Jena, Germany), or thin-flap LASIK.3–6
Because SMILE caps are usually set to a depth of 120 to 140 µm, LASIK enhancement anteriorly to the SMILE interface requires very thin flaps, which increases the risk of flap-related complications, mainly buttonholing and gas bubble breakthrough.10 Therefore, surface ablation and CIRCLE are the most widely used enhancement methods to date. We have previously provided data on the safety and efficacy of both strategies, but using different cohorts with mixed pre-enhancement mean refractive spherical equivalent (MRSE) values, which makes both options difficult to compare.5,6 Moreover, our pilot study on surface ablation after SMILE still included an aspherically optimized excimer (ASA) profile, which is now regarded as obsolete for enhancements because it induces significant overcorrection in the case of retreatment.5 The objective for this study therefore was to provide a direct comparative study of state-of-the-art surface ablation and CIRCLE re-treatment after SMILE by generating two cohorts matched by pre-SMILE and pre-enhancement spherical equivalent, visual acuity, and age.
Patients and Methods
The databases of the SMILE Eyes centers in Munich, Marburg, and Cologne, Germany, and Linz, Austria, were screened for patients who had undergone CIRCLE or surface ablation re-treatment after myopic SMILE. Eyes re-treated with the ASA profile were excluded from the analysis due to reported overcorrection.5 Respective institutional review board approval was obtained for all centers. Prior to treatment, slit-lamp biomicroscopy and funduscopy, objective, subjective, and cycloplegic refraction, and Pentacam corneal tomography (OCULUS Optikgeräte GmbH, Wetzlar, Germany) were performed to rule out contraindications for corneal refractive treatment. Subjective refraction, uncorrected distance visual acuity (UDVA), and corrected distance visual acuity (CDVA) were assessed at all visits. Informed consent was obtained from all patients. All study-related procedures adhered to the tenets of the Declaration of Helsinki.
The primary SMILE procedure was performed as described previously with a standard cap thickness of 90 to 140 µm and an optical zone of 5.75 to 7 mm, depending on the individual white-to-white diameter.5,11–13 In brief, surgery was performed under topical anesthesia with oxybuprocaine hydrochloride (Bausch & Lomb, Rochester, NY) using a 500-kHz VisuMax femtosecond laser (Carl Zeiss Meditec AG). After sterile draping, the eye of the patient was placed under the femtosecond laser's operating microscope, centered, and immobilized with a suction contact glass. While the patient was asked to fixate on a target light, the posterior surface, lateral border, and anterior surface of the lenticule were cut by the laser, followed by one incision of 40° to 60° chord length located supero-temporally. The lenticule planes were then manually dissected and the lenticule was removed through the incision. The postoperative treatment regimen consisted of levofloxacin (Oftaquix; Santen, Osaka, Japan) eye drops four times daily for 1 week, dexamethasone (DexaSine; Alcon, Hünenburg, Switzerland) eye drops four times daily in the first week, tapered by one drop per week in the following 3 weeks, and lubricant eye drops as needed.
At each follow-up visit, individual refractive and visual outcomes were reevaluated. In cases of unsatisfying refractive results, patients were thoroughly informed about the chances and risks of the re-treatment methods available. To be eligible for CIRCLE enhancement, the calculated residual stromal bed after retreatment had to exceed 250 µm. Otherwise, surface ablation was recommended as an alternative.
Surface ablation after SMILE was performed as described previously5 with plano target in all eyes. In brief, the corneal epithelium was delaminated by application of 20% ethanol for 20 seconds, thoroughly rinsed with balanced salt solution (BSS), and then manually peeled off. Ablation was performed on MEL 80 or MEL 90 excimer lasers (Carl Zeiss Meditec AG) using the topography-guided, tissue-saving (TSA), or ASA algorithm on the MEL 80, or the Triple-A algorithm on the MEL 90. Eyes re-treated with the ASA profile were later excluded from analysis. To prevent haze,14 a sponge soaked with 0.02% mitomycin C solution was applied for 20 to 25 seconds immediately after excimer laser treatment, followed by thorough rinsing with BSS. At the end of surgery, the epithelium was discarded (epi-off laser epithelial keratomileusis). Postoperative care included application of a bandage contact lens and levofloxacin eye drops four times daily until reepithelialization, dexamethasone eye drops four times daily tapered over 4 weeks as after SMILE, and lubricant eye drops as needed.
CIRCLE-Assisted Excimer Ablation
CIRCLE enhancement after SMILE using the VisuMax laser was performed as described previously6 with plano target in all eyes. In brief, the cap of the primary SMILE was converted into a full flap. For this, the outer diameter of the CIRCLE procedure was programmed to extend beyond the SMILE interface (eg, 8.2 mm over 7.9 mm or larger, depending on the white-to-white diameter), whereas the inner diameter was smaller than the lenticule (eg, 6.2 mm within 6.5 mm). Of the four patterns A to D, pattern D was used as recommended by the manufacturer because it has been shown to produce the easiest flaps to lift.15 Pattern D consists of three consecutive laser cuts. First, a lamellar ring around the original cap is cut at the same depth as the cap. Second, a side cut is created around the new incision plane except for an area used as a hinge. Third, a junction cut parallel to the side cut is created to establish a connection between the planes of the primary cap and the secondary lamellar ring around it, creating one large joint plane.
Care was taken to rotate the planned flap so that the new flap hinge area did not overlap with the former side cut incision (eg, SMILE incision at 130° and CIRCLE flap at 50°). After femtosecond laser application, the new flap was lifted using a blunt spatula like a regular LASIK flap. Excimer ablation was then performed with a MEL 90 laser on the previous lenticule zone with plano target refraction in all cases. At the end of surgery, the flap was repositioned. Postoperative care included levofloxacin eye drops four times daily until reepithelialization, dexamethasone eye drops four times daily tapered over 4 weeks as after SMILE, and lubricant eye drops as needed.
Eyes re-treated with surface ablation were matched by pre-SMILE and pre-enhancement spherical equivalent to a correlating partner eye re-treated with CIRCLE. In the case of no exactly matching partner, a difference of ±0.13 D of spherical equivalent was tolerated. After matching, 9 of 12 eyes (75%) had a partner eye with the same spherical equivalent. In 3 eyes (25%), the spherical equivalent between groups differed by 0.13 D. Individually, matched spherical equivalent and astigmatism differed by 0.25 D maximally.
All data were gathered and analyzed in Microsoft Excel spreadsheets (version 15.39 for Mac; Microsoft Corporation, Redmond, WA) according to the established graphic reporting of outcomes of refractive surgery.16 Statistical analysis was performed in SPSS Statistics 23 software (SPSS Inc., Chicago, IL). A P value of less than .05 was considered statistically significant. Normal distribution of the data was confirmed using the Shapiro–Wilk test. An independent samples t test was employed to test for significant differences in baseline parameters between both groups (eg, MRSE and age) and postoperative outcome parameters (eg, MRSE and visual acuity). Comparison of MRSE and visual acuity between multiple time points was performed with a repeated measures analysis of variance. Bonferroni's correction was applied to compensate for multiple testing. A Mann–Whitney U test was employed to compare safety and efficacy indices between both groups. Graphs were plotted in Microsoft Excel showing the standard deviation.
In the involved centers, 2,803 eyes that had undergone SMILE with a follow-up of 3 months or more were identified. Re-treatment was performed in 65 eyes (2.3%). Surface ablation was used in 43 eyes (66.2%) and CIRCLE in 22 eyes (33.8%). All eyes retreated with the ASA profile (2 in total) were excluded from analysis due to the reported problem of overcorrection.5 Of the remaining 41 surface ablation and 22 CIRCLE eyes, 12 in each group had a matching partner within the prespecified margins, equaling a total of 24 eyes in the final analysis. Within the surface ablation group included in the analysis, the Triple-A algorithm on the MEL 90 was used in 8 eyes (66.6%) and the TSA algorithm on the MEL 80 in 4 eyes (33.3%).
A comparison of pre-SMILE and pre-enhancement parameters between both groups can be found in Table A (available in the online version of this article). In brief, both groups did not differ before SMILE concerning age (P = .77), MRSE (P = .89), spherical error (P = .65), astigmatism (P = .17), CDVA (P = 1.00), or pachymetry (P = .49). Pre-enhancement, there was also no difference concerning MRSE (surface ablation: −0.91 ± 0.55 vs CIRCLE: −0.90 ± 0.61 D; P = .97), spherical error (surface ablation: −0.63 ± 0.70 vs CIRCLE: −0.63 ± 0.64, P = 1.00), astigmatism (surface ablation: −0.68 ± 0.43 vs CIRCLE: −0.54 ± 0.40 D; P = .42), CDVA (surface ablation: 0.00 ± 0.03 vs CIRCLE: 0.02 ± 0.04 logMAR, P = .56), and UDVA (surface ablation: 0.21 ± 0.15 vs CIRCLE: 0.32 ± 0.14 logMAR; P = .11). Optical zone diameters and cap thickness values of the previous SMILE procedure were not different between groups (optical zone: surface ablation: 6.5 to 6.6 mm; CIRCLE: 5.75 to 7 mm; P = .19; cap thickness: surface ablation: 90 to 140 µm; CIRCLE: 105 to 120 µm; P = .78).
Pre-SMILE and Pre-enhancement Parameters
A comparison of postoperative results can be found in Table 1 and Figure 1. In both groups, MRSE significantly improved at 1 week after enhancement (surface ablation: P = .01, CIRCLE: P = .01) and remained stable until the end of follow-up at 3 months (surface ablation: P = .0003, CIRCLE: P = .007). There was no significant difference in the target accuracy (MRSE: surface ablation: −0.07 ± 0.19 D [range: −0.50 to 0.25 D] vs CIRCLE: 0.04 ± 0.22 D [range: −0.38 to 0.25 D]; P = .18) or astigmatism (surface ablation: −0.23 ± 0.29 D [range: −0.75 to 0.00 D] vs CIRCLE: −0.25 ± 0.24 D [range: −0.50 to 0.00 D]; P = .86) at 3 months. In both groups, all eyes were within ±0.50 D of plano postoperatively (Figure 1D). All eyes in both groups had astigmatism below 1.00 D at 3 months (Figure 1E). Astigmatism of 0.50 D or less was achieved in 92% of surface ablation eyes and 100% of CIRCLE re-treated eyes.
(A) Distribution of uncorrected distance visual acuity (UDVA) postoperatively compared to corrected distance visual acuity (CDVA) preoperatively. (B) Change in lines of CDVA and UDVA. (C) Attempted vs achieved spherical equivalent refraction. (D) Distribution of manifest spherical equivalent refraction after re-treatment. (E) Distribution of refractive astigmatism before and after re-treatment. (F) Refractive stability after enhancement. (G) Time course of UDVA and CDVA after enhancement.
In both groups, UDVA improved significantly (surface ablation: P = .007, CIRCLE: P < .0001). There was no difference in the final UDVA at 3 months (surface ablation: 0.02 ± 0.09 logMAR [range: −0.1 to 0.3 logMAR] vs CIRCLE: 0.03 ± 0.03 logMAR [range: 0.0 to 0.1 logMAR]; P = .78). Figure 1A shows the cumulative Snellen visual acuity for surface ablation and CIRCLE before and after enhancement. The efficacy index (mean postoperative UDVA/mean preoperative CDVA) was comparable in both groups at 3 months (surface ablation: 0.95; CIRCLE: 1.03; P = .36).
There were no intraoperative complications in either group. All flaps created using CIRCLE could be lifted easily without interference with the previous SMILE interface. No patient after surface ablation showed haze. Figure 1B shows the change of lines of CDVA postoperatively. There was no loss of two or more lines in any group. One eye (8%) in the surface ablation group lost one line of CDVA without any anatomical correlate, whereas there were no losses in the CIRCLE group. There was no difference in CDVA before and after enhancement in both groups (surface ablation: 0.00 ± 0.03 logMAR [range: 0.0 to 0.1 logMAR] before vs −0.03 ± 0.06 logMAR [range: −0.1 to 0.1 logMAR] after; P = .34; CIRCLE: 0.02 ± 0.04 logMAR [range: 0.0 to 0.1 logMAR] before vs 0.00 ± 0.03 logMAR [range: −0.1 to 0.0 logMAR] after; P = .08). The safety index (mean postoperative CDVA/mean pre-operative CDVA) did not differ between groups (surface ablation: 1.00 vs CIRCLE: 1.06; P = .36).
Speed of Visual Recovery
Table 1 and Figure 1F show the speed of visual recovery after re-treatment. In the CIRCLE group, CDVA remained stable throughout all follow-up time points. UDVA had already improved at week 1 (pre-enhancement: 0.32 ± 0.14 vs 1 week: 0.06 ± 0.10; P = .0003) and remained stable until the end of follow-up at 3 months. In contrast, in the surface ablation group, at 1 week UDVA had not yet improved (0.21 ± 0.15 logMAR [range: 0.0 to 0.4 logMAR] before vs 0.23 ± 0.14 logMAR [range: 0.1 to 0.5 logMAR] at 1 week; P = .42), whereas CDVA had worsened (0.00 ± 0.03 logMAR [range: 0.0 to 0.1 logMAR] before vs 0.13 ± 0.08 logMAR [range: 0.0 to 0.2 logMAR] at 1 week; P = .005). At 6 weeks, CDVA again returned to baseline levels (P = .35) and UDVA improved (P = .03) as compared with baseline.
Comparing both re-treatment methods, visual recovery was significantly faster in patients who had undergone CIRCLE. UDVA at 1 week was significantly better after CIRCLE (0.06 ± 0.10 logMAR [range: 0.0 to 0.3 logMAR] vs 0.23 ± 0.14 logMAR [range: 0.1 to 0.5 logMAR]; P = .008). Additionally, at 1 week, CDVA was significantly better after CIRCLE (0.03 ± 0.05 logMAR [range: 0.0 to 0.1 logMAR] vs 0.13 ± 0.08 logMAR [range: 0.0 to 0.2 logMAR]; P = .002).
For many patients seeking refractive surgery, the prospect of a flap-free approach makes SMILE an attractive option.17 However, in the case of required enhancement, re-treatment using a second SMILE procedure is currently considered off-label and is cumbersome to perform, particularly for small refractive errors. At the moment, other techniques have to be used as substitutes, including flap-less approaches such as surface ablation and flap-based procedures such as thin-flap (FS-LASIK) or the VisuMax CIRCLE program, which converts the SMILE cap into an FS-LASIK flap for secondary excimer laser ablation.6
Thorough patient counseling about the advantages and disadvantages of each method is crucial to select the best moment and option of enhancement. Depending on the cap thickness, there might be little tissue left for re-treatment anteriorly to the SMILE interface. In higher myopic ablations, thinner caps are used to maintain a thicker residual stromal bed despite increasing lenticule thickness. However, it is these eyes with high myopia that are more prone to require enhancement due to primary undercorrection and regression.4 This makes thin-flap FS-LASIK challenging in a multitude of cases (eg, for caps of ≤ 110 µm), with the risk of intraoperative complications related to very thin flaps potentially increasing.3 In thicker caps, thin-flap FS-LASIK can yield good results,3 but requires diligent preoperative planning, including epithelial thickness measurements using very high-frequency ultrasound or high-resolution anterior segment optical coherence tomography to prevent buttonholing18 and residual stroma calculations to prevent cross-talk between the FS-LASIK and SMILE interface. On the other hand, thick FS-LASIK flaps posteriorly to the SMILE interface would cause a disproportionate impact on corneal biomechanical stability.10 For these reasons, surface ablation and the CIRCLE procedure, creating a flap from (and at the exact depth of) the cap, remain the enhancement methods of choice for most surgeons.
We have previously reported on the safety and efficacy of surface ablation and CIRCLE enhancement after SMILE, but using heterogenous cohorts and ablations. At the moment, there are no data to compare the outcomes of both methods. In this context, our study represents the first direct matched comparative study between surface ablation and CIRCLE enhancement. As shown in our results, both methods deliver almost identical, excellent results concerning predictability, safety, and efficacy. Only one eye in the surface ablation group lost one line of CDVA, whereas there were no losses of two or more lines in either group. Although MRSE had already improved at 1 week in both groups, all patients in both groups were within ±0.50 D of plano MRSE with significant gains in UDVA at 3 months, and 83% of eyes in each group achieved 20/20 or better. This is in line with data on thin-flap LASIK re-treatment after SMILE, reporting 74% within ±0.50 D of target refraction and 81% of eyes achieving 20/20 or better.3 Compared to our first report on surface ablation after SMILE, which resulted in 80% within ±0.50 D of target refraction, the results of this study are clearly superior. This can largely be attributed to the exclusion of patients re-treated with the ASA profile, which is not recommended for retreatments due to induced overcorrection.5 It is safe to say that re-treatment after SMILE with CIRCLE or surface ablation using the TSA or Triple-A profile can be safely recommended as equivalent and will yield comparable results at 3 months. However, due to the previous SMILE procedure, some aspects of surface ablation might differ from surface ablation on virgin eyes (eg, the absence of slight overcorrection at 1 week after surface ablation due to still recovering, thin epithelium).
Based on these data, the choice of re-treatment method from the surgeon's and patient's perspective might be based on two additional parameters: speed of patient recovery and biomechanical stability. As shown in our study, visual recovery defined as a significant gain in UDVA was already seen after CIRCLE at 1 week, whereas no improvement was detectable yet after surface ablation. Moreover, patients re-treated with CIRCLE showed no postoperative decrease in CDVA, whereas there was a significant worsening after surface ablation at 1 week. Because most patients undergoing SMILE are of working age, this issue might be a key limiting factor of surface ablation enhancement.19 Moreover, as reported previously, the aspect of postoperative pain and discomfort might render this technique even more unattractive for patients. However, our study lacks the objective assessment of postoperative pain and cannot determine its exact effect on decision making.
On the other hand, it might seem paradoxical to suggest a flap-based re-treatment to a patient after having chosen SMILE over LASIK due to the absence of a flap, and thus less biomechanical impact on the cornea.20 As shown by Kling et al.,21 the introduction of a flap by CIRCLE re-treatment resulted in a significantly higher effect on corneal biomechanical integrity than SMILE re-treatment and surface ablation re-treatment in ex vivo porcine eyes, resulting from the introduction of a side cut.22 These results were recently echoed by a fellow eye study in human corneas showing that the effective elastic modulus was 1.47 times higher after SMILE than LASIK.23 The deeper the flap creation, the more biomechanical weakening can be expected.22 For this reason, thin-flap LASIK might be less detrimental to corneal biomechanical stability than CIRCLE re-treatment in the presence of deep caps (eg, 150 to 160 µm),10 which would cause a disproportionate effect on corneal biomechanics.20,24 It is unclear whether these theoretical assumptions show any clinical manifest relevance if the established thresholds for refractive surgery are respected, but this should be discussed with the patient.
The limitations of our study include a small sample size and short follow-up. Moreover, the use of different surface ablation profiles on two different excimer laser platforms (MEL 80 for surface ablation and MEL 90 for CIRCLE) should be considered. Nevertheless, our data provide evidence that surface ablation and the CIRCLE procedure can be recommended as safe, effective, equivalent re-treatment options after SMILE. Additional studies with a larger sample size and longer follow-up, as well as prospective trials, are warranted for further evidence. In the future, both procedures, as well as thin-flap FS-LASIK in thicker caps, will probably also have to be compared to the results of a possible re-SMILE enhancement, once clinically established.
- Han T, Xu Y, Han X, et al. Three-year outcomes of small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) for myopia and myopic astigmatism. Br J Ophthalmol. 2019;103:565–568. doi:10.1136/bjophthalmol-2018-312140 [CrossRef]
- Zhang Y, Shen Q, Jia Y, Zhou D, Zhou J. Clinical outcomes of SMILE and FS-LASIK used to treat myopia: a meta-analysis. J Refract Surg. 2016;32:256–265. doi:10.3928/1081597X-20151111-06 [CrossRef]
- Reinstein DZ, Carp GI, Archer TJ, Vida RS. Outcomes of retreatment by LASIK after SMILE. J Refract Surg. 2018;34:578–588. doi:10.3928/1081597X-20180717-02 [CrossRef]
- Liu YC, Rosman M, Mehta JS. Enhancement after small-incision lenticule extraction: incidence, risk factors, and outcomes. Ophthalmology. 2017;124:813–821. doi:10.1016/j.ophtha.2017.01.053 [CrossRef]
- Siedlecki J, Luft N, Kook D, et al. Enhancement after myopic small incision lenticule extraction (SMILE) using surface ablation. J Refract Surg. 2017;33:513–518. doi:10.3928/1081597X-20170602-01 [CrossRef]
- Siedlecki J, Luft N, Mayer WJ, et al. CIRCLE enhancement after myopic SMILE. J Refract Surg. 2018;34:304–309. doi:10.3928/1081597X-20180308-02 [CrossRef]
- Randleman JB, White AJ Jr, Lynn MJ, Hu MH, Stulting RD. Incidence, outcomes, and risk factors for retreatment after wavefront-optimized ablations with PRK and LASIK. J Refract Surg. 2009;25:273–276.
- Mimouni M, Vainer I, Shapira Y, et al. Factors predicting the need for retreatment after laser refractive surgery. Cornea. 2016;35:607–612. doi:10.1097/ICO.0000000000000795 [CrossRef]
- Pokroy R, Mimouni M, Sela T, Munzer G, Kaiserman I. Myopic laser in situ keratomileusis retreatment: incidence and associations. J Cataract Refract Surg. 2016;42:1408–1414. doi:10.1016/j.jcrs.2016.07.032 [CrossRef]
- Moshirfar M, Shah TJ, Masud M, Linn SH, Ronquillo Y, Hoopes PC Sr, . Surgical options for retreatment after small-incision lenticule extraction: advantages and disadvantages. J Cataract Refract Surg. 2018;44:1384–1389. doi:10.1016/j.jcrs.2018.07.047 [CrossRef]
- Sekundo W, Kunert K, Russmann C, et al. First efficacy and safety study of femtosecond lenticule extraction for the correction of myopia: six-month results. J Cataract Refract Surg. 2008;34:1513–1520. doi:10.1016/j.jcrs.2008.05.033 [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]
- Ivarsen A, Asp S, Hjortdal J. Safety and complications of more than 1500 small-incision lenticule extraction procedures. Ophthalmology. 2014;121:822–828. doi:10.1016/j.ophtha.2013.11.006 [CrossRef]
- Majmudar PA, Forstot SL, Dennis RF, et al. Topical mitomycin-C for subepithelial fibrosis after refractive corneal surgery. Ophthalmology. 2000;107:89–94. doi:10.1016/S0161-6420(99)00019-6 [CrossRef]
- Riau AK, Ang HP, Lwin NC, Chaurasia SS, Tan DT, Mehta JS. Comparison of four different VisuMax circle patterns for flap creation after small incision lenticule extraction. J Refract Surg. 2013;29:236–244. doi:10.3928/1081597X-20130318-02 [CrossRef]
- Reinstein DZ, Waring GO 3rd, . Graphic reporting of outcomes of refractive surgery. J Refract Surg. 2009;25:975–978. doi:10.3928/1081597X-20091016-01 [CrossRef]
- Reinstein DZ, Archer TJ, Gobbe M. Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes. Eye Vis (Lond). 2014;1:3. doi:10.1186/s40662-014-0003-1 [CrossRef]
- Reinstein DZ, Carp GI, Archer TJ, Vida RS. Inferior pseudo-hinge fulcrum technique and intraoperative complications of laser in situ keratomileusis retreatment after small-incision lenticule extraction. J Cataract Refract Surg. 2018;44:1355–1362. doi:10.1016/j.jcrs.2018.07.051 [CrossRef]
- Luft N, Siedlecki J, Sekundo W, et al. Small incision lenticule extraction (SMILE) monovision for presbyopia correction. Eur J Ophthalmol. 2018;28:287–293. doi:10.5301/ejo.5001069 [CrossRef]
- Damgaard IB, Reffat M, Hjortdal J. Review of corneal biomechanical properties following LASIK and SMILE for myopia and myopic astigmatism. Open Ophthalmol J. 2018;12:164–174. doi:10.2174/1874364101812010164 [CrossRef]
- Kling S, Spiru B, Hafezi F, Sekundo W. Biomechanical weakening of different re-treatment options after small incision lenticule extraction (SMILE). J Refract Surg. 2017;33:193–198. doi:10.3928/1081597X-20161221-01 [CrossRef]
- Knox Cartwright NE, Tyrer JR, Jaycock PD, Marshall J. Effects of variation in depth and side cut angulations in LASIK and thin-flap LASIK using a femtosecond laser: a biomechanical study. J Refract Surg. 2012;28:419–425. doi:10.3928/1081597X-20120518-07 [CrossRef]
- Spiru B, Kling S, Hafezi F, Sekundo W. Biomechanical properties of human cornea tested by two-dimensional extensiometry ex vivo in fellow eyes: femtosecond laser-assisted LASIK versus SMILE. J Refract Surg. 2018;34:419–423. doi:10.3928/1081597X-20180402-05 [CrossRef]
- Raevdal P, Grauslund J, Vestergaard AH. Comparison of corneal biomechanical changes after refractive surgery by non-contact tonometry: small-incision lenticule extraction versus flap-based refractive surgery: a systematic review. Acta Ophthalmol. 2019;97:127–136. doi:10.1111/aos.13906 [CrossRef]
| 1 week||−0.22 ± 0.54 (−1.00 to 0.50)||−0.07 ± 0.34 (−0.75 to 0.50)||.47|
| 6 weeks||−0.10 ± 0.23 (−0.50 to 0.25)||0.04 ± 0.28 (−0.38 to 0.50)||.32|
| 3 months||−0.07 ± 0.19 (−0.50 to 0.25)||0.04 ± 0.22 (−0.38 to 0.25)||.18|
|Mean astigmatism (D)|
| 1 week||−0.56 ± 0.45 (−1.25 to 0.00)||−0.08 ± 0.20 (−0.50 to 0.00)||.04|
| 6 weeks||−0.30 ± 0.26 (−0.75 to 0.00)||−0.17 ± 0.19 (−0.50 to 0.00)||.18|
| 3 months||−0.23 ± 0.29 (−0.75 to 0.00)||−0.25 ± 0.24 (−0.50 to 0.00)||.85|
| 1 week||0.23 ± 0.14 (0.10 to 0.50)||0.06 ± 0.10 (0.00 to 0.30)||.008|
| 6 weeks||0.03 ± 0.10 (−0.10 to 0.30)||0.04 ± 0.07 (0.00 to 0.20)||.96|
| 3 months||0.02 ± 0.09 (−0.10 to 0.30)||0.03 ± 0.03 (0.00 to 0.10)||.78|
|Change in UDVA (preoperative/postoperative)||−0.19 ± 0.18 (−0.4 to 0.1)||−0.29 ± 0.15 (−0.6 to −0.1)||.08|
| 1 week||0.13 ± 0.08 (0.20 to 0.00)||0.03 ± 0.05 (0.00 to 0.10)||.002|
| 6 weeks||−0.02 ± 0.00 (0.00 to 0.10)||0.02 ± 0.04 (0.00 to 0.10)||.79|
| 3 months||−0.03 ± 0.06 (−0.10 to 0.10)||0.00 ± 0.03 (−0.10 to 0.00)||.54|
|Change in CDVA (preoperative/postoperative)||0.00 ± 0.04 (−0.10 to 0.10)||−0.02 ± 0.05 (−0.10 to 0.00)||.18|
Pre-SMILE and Pre-enhancement Parametersa
|No. of eyes (n)||12||12|
|No. of patients (n)||12||12|
|Mean age (y)||36.9 ± 10.7 (22 to 52)||38.3 ± 12.2 (22 to 52)||.77|
|Mean follow-up (mo)||3 (3 to 3)||3 (3 to 3)||1.00|
| Mean SEQ (D)||−5.97 ± 1.18 (−7.88 to −4.38)||−5.88 ± 1.95 (−9.25 to −3.13)||.89|
| Mean sphere (D)||−5.44 ± 1.16 (−7.75 to −4.25)||−5.13 ± 2.00 (−8.50 to −2.00)||.65|
| Mean astigmatism (D)||−1.06 ± 0.85 (−2.25 to −0.25)||−1.5 ± 0.67 (−2.50 to −0.50)||.17|
| Mean CDVA (logMAR)||0.00 ± 0.07 (0.00 to 0.10)||0.00 ± 0.00 (0.00 to 0.00)||1.00|
| Pachymetry (µm)||527 ± 24 (490 to 583)||548 ± 39 (511 to 616)|
| Cap thickness (µm)||115 ± 16 (90 to 140)||118 ± 6 (105 to 120)||.78|
| Optical zone (mm)||6.52 ± 0.04 (6.50 to 6.60)||6.41 ± 0.32 (5.75 to 7.00)||.19|
| Mean SEQ (D)||−0.91 ± 0.55 (−1.75 to 0.00)||−0.90 ± 0.61 (−1.88 to 0.13)||.97|
| Mean sphere (D)||−0.63 ± 0.70 (−1.50 to 0.75)||−0.63 ± 0.64 (−1.50 to 0.50)||1.00|
| Mean astigmatism (D)||−0.68 ± 0.43 (−1.50 to 0.00)||−0.54 ± 0.40 (−1.25 to 0.00)||.42|
| Mean UDVA (logMAR)||0.21 ± 0.15 (0.00 to 0.40)||0.32 ± 0.14 (0.20 to 0.60)||.11|
| Mean CDVA (logMAR)||0.00 ± 0.03 (0.00 to 0.10)||0.02 ± 0.04 (0.00 to 0.10)||.56|