Eyeworld

of SMILE and could have been a question of adjustment of the optical configurations of the lenticule. Efficacy was good for both SMILE and WFG-LASIK, and there was no better efficacy with SMILE compared to WFG- LASIK at 12 months postop (Figure 2). From various stud- ies, the mean logMAR UDVA was 0.02339 to –0.1712 for SMILE and –0.0447 to –0.1855 for WFG-LASIK, while the percentages of eyes with post- operative UDVA 0.00 logMAR or better ranged from 60.00% to 100% for SMILE and 83.80% to 99.40% for WFG-LASIK. Control of higher-order aberrations, however, was better with WFG-LASIK than SMILE, with mean changes of +0.03±0.10 μm HOA RMS and +0.05±0.08 μm primary SA reported after WFG-LASIK in naval aviators 4 against chang- es of 0.15 μm HOA RMS, 0.14 μm SA, and 0.33 μm coma RMS reported after SMILE. 5 High levels of HOAs (0.503 μm SA, 0.706 μm coma RMS, 0.427 μm HOA RMS) were also reported after SMILE. 6 Studies report no significant levels of HOAs (coma RMS: 0.28±0.14 [150 kHz FS laser], 0.29±0.03 [60 kHz FS laser]; SA: 0.22±0.19 [150 kHz FS laser], 0.21±0.17 [60 kHz FS laser]) after WFG-LASIK. 7 There was no clear dif- ference between SMILE and WFG-LASIK in terms of safety, although visual recovery with SMILE seems to be delayed in the initial postoperative period (Figure 3). 8 Structural changes The delay in visual recovery with SMILE may be due to the production of microdistortions of Bowman's layer—88.5% with SMILE vs. only 42.1% with WFG-LASIK—which appear to be associated with lenticule thickness. 9 These microdistor- tions may explain the increased backscattered light intensity in the anterior stroma as demon- strated through analysis by in vivo confocal microscopy. 10 One of the theoretical ad- vantages of SMILE is a higher total stromal tensile strength compared to LASIK and PRK procedures. 11 However, this estimation is based on a mathematical model not vali- dated experimentally, based on the assumption that the anteri- or stroma is completely intact. Clinically, however, there is no evidence of better biome- chanical behavior with SMILE vs. LASIK, with studies using the ORA system (Alcon, Fort Worth, Texas) and Corvis ST (Oculus, Wetzlar, Germany) showing comparable corneal hysteresis and corneal resis- tance factor values between the two procedures. 12,13 Advantages related to structure touted by SMILE proponents are the reduction of corneal sensitivity and poten- tially lower incidence of dry eye symptoms as well as the lack of flap-related complications with the procedure. However, other complications such as dif- fuse lamellar keratitis, 14 decen- trations, 15 and ectasia 16 have been reported after SMILE. Moreover, when complica- tions do occur, there is to date little scientific evidence about the results of SMILE retreat- ments and the procedures to do them. Ocular surface health One thing true to both SMILE and any LASIK procedure is how the status of the ocular surface affects outcomes. Studies looking at subjective dry eye questionnaires have shown that ocular surface disease index (OSDI) worsens after both SMILE and femto LASIK procedures, returning to preop values after 1 month postop. 17 Meanwhile, the McMonnies questionnaire scores of patients who under- went SMILE and 90-μm flap LASIK recovered to their preop values by 3 months postop. 18 Conclusion Considering all the evidence thus far, what is the real benefit of SMILE over WFG-LASIK? Future controlled randomized comparative studies are neces- sary to determine whether such a benefit exists, but for now, whether one procedure can replace the other is not some- thing that we can say from a scientific point of view. References 1. Zhang J, et al. Vector analysis of low to moderate astigmatism with small incision lenticule extraction (SMILE): results of a 1-year follow-up. BMC Ophthalmol. 2015;15:8. 2. Blum M, et al. Femtosecond lenticule extraction (ReLEx) for correction of hyperopia – first results. Graefes Arch Clin Exp Ophthalmol. 2013;251:349–55. 3. Pinero DP, et al. Clinical outcomes of small-incision lenticule extraction and femtosecond laser-assisted wavefront-guided laser in situ ker- atomileusis. J Cataract Refract Surg. 2016;42:1078–93. 4. Tanzer DJ, et al. Laser in situ keratomileusis in United States Naval aviators. J Cataract Refract Surg. 2013;39:1047–58. 5. Vestergaard AH, et al. Efficacy, safety, predictability, contrast sensi- tivity, and aberrations after femtosec- ond laser lenticule extraction. J Cata- ract Refract Surg. 2014;40:403–11. 6. Lin F, et al. Comparison of the visu- al result after SMILE and femtosec- ond laser-assisted LASIK for myopia. J Refract Surg. 2014;30:248–54. 7. Yu CQ, et al. Comparison of 2 femtosecond lasers for flap creation in myopic laser in situ keratomileusis: one-year results. J Cataract Refract Surg. 2015;41:740–8. 8. Lin F, et al. Comparison of the visu- al results after SMILE and femtosec- ond laser-assisted LASIK for myopia. J Refract Surg. 2014;30:248–54. 9. Yao P, et al. Microdistortions in Bowman's layer following fem- tosecond laser small incision lenticule extraction observed by Fourier-Domain OCT. J Refract Surg. 2013;29:668–674. continued on page 8 Sponsored by Abbott Medical Optics 7

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