Refractive surgery has come a long way since its inception, and the innovations continue to occur. In some of the initial FDA trials of the conventional excimer laser (Nidek EC-5000; Nidek) for the treatment of low myopia (-2.00 D to -5.50 D) without astigmatism, a distance UCVA of at least 20/20 at 1 year was reported in 62.5% and 79.2% of eyes treated with PRK and LASIK, respectively.1 This was considered a satisfactory outcome at the time, but results have improved with each development, including wavefront-guided and -optimized treatments, femtosecond lasers for flap creation, better eye tracking systems, and faster ablation times.
Conventional LASIK in the early FDA trials involved the creation of a microkeratome flap and eliminated lower-order aberrations (myopia, hyperopia, and astigmatism) but left higher-order aberrations (HOAs) uncorrected. Treatment of HOAs has resulted in an improvement not only in the percentage of eyes that achieve 20/20 but also in patients’ quality of vision. With recent technological advances, patient expectations have risen exponentially, and we strive to provide the best refractive outcomes. This review discusses the most recent LASIK and PRK data and explores a newer refractive procedure, small-incision lenticule extraction (SMILE), which is not yet available in the United States.
CURRENT EXCIMER LASER VISUAL OUTCOMES
Two of the most commonly used excimer lasers for LASIK and PRK are the Visx Star S4 IR (Abbott Medical Optics) and the WaveLight Allegretto Wave Eye-Q (Alcon).
Visx Star S4 IR. The FDA approval studies for the Visx Star S4 IR using wavefront-guided technology were numerous. In a study of patients with myopia from -0.50 D to -6.00 D and astigmatism of -3.00 D or less, 88.4% of eyes (n = 281 of 318) had a UCVA of 20/20 or better at 12 months and 0.3% of eyes (n = 1 of 318) lost 2 or more lines of BCVA at 6 months.2 The FDA approval results for less than +6.00 D of hyperopia with or without less than +5.00 D of astigmatism showed a UCVA of 20/20 or better in 72% of eyes (n = 85 of 118) and a loss of BCVA of 2 or more lines at 6 months in 0% of eyes (n = 0 of 137).3
Recent studies with the Visx laser showed significant improvement in refractive outcomes. A prospective, noncomparative study of 255 patients undergoing LASIK with myopia from -0.50 D to -9.00 D and astigmatism of -5.00 D or less showed a UCVA of 20/20 or better at 1 year and 4 years in 96.1% (n = 242) and 98.8% (n = 249) of patients, respectively; no loss of BCVA of 2 or more lines was observed at 1 year and 4 years in either group.4 More recently, a retrospective study of 90 eyes with myopia of -10.00 D or less and astigmatism of -5.00 D or less that were treated with the most updated wavefront system (iDesign Advanced Aberrometer) achieved a UCVA of at least 20/20 at 3 months in 94.4% of eyes (n = 85 of 90), and no eyes lost BCVA.5
WaveLight Allegretto Wave Eye-Q. The FDA approval studies for WaveLight Allegretto Wave Eye-Q conventional LASIK for myopia of less than 14.00 D with or without astigmatism of less than -6.00 D showed a UCVA of at least 20/20 at 12 months in 87.4% of eyes (n = 682 of 780) and a loss of BCVA of 2 or more lines at 12 months in 0.5% of eyes (n = 4 of 813).6 The FDA studies for hyperopia of less than +6.00 D with or without astigmatism of less than +6.00 D showed a UCVA of at least 20/20 at 6 months in 67.5% of eyes (n = 143 of 212) and a loss of BCVA of 2 or more lines at 6 months in 1.5% of eyes (n = 4 of 260).7
With software upgrades and the development of the wavefront-optimized method, outcomes continued to improve. A retrospective chart review using the newer 400-Hz ablation laser with wavefront-optimized treatment looked at 136 LASIK patients (ranging from -13.00 D to +4.50 D) and 64 PRK patients (less than 13.00 D) and found a UCVA of at least 20/20 at 3 months in 92% of eyes (n = 125 of 136) and 80% of eyes (n = 51 of 64), respectively, with dry eye being the most common complaint in both groups.8 In a recent retrospective review of PRK in 222 eyes with myopia of 5.50 D or less with or without astigmatism of 4.50 D or less, a mean UCVA of 20/20 or better at 3 months and 6 months was observed in 86.5% and 86.6% of eyes, respectively; no patient in either group had lost 2 or more lines of BCVA at 3 months or 6 months.9
Wavefront-guided and wavefront-optimized comparison in LASIK and PRK.
He et al10 conducted a prospective, randomized, fellow-eye-controlled study to compare wavefront-guided LASIK with the Visx CustomVue S4 IR and wavefront-optimized LASIK with the Allegretto Wave Eye-Q 400-Hz excimer laser. The investigators found a UCVA of 20/20 or better at 12 months in 87% of eyes (n = 54) in the wavefront-guided group and 78% of eyes (n = 54) in the wavefront-optimized group. A loss of BCVA of 2 or more lines at 12 months was found in 0% of eyes (n = 54) in the wavefront-guided group and 1.9% of eyes (n = 54) in the wavefront-optimized group. Additionally, there was no difference in HOAs between groups. Another recent study comparing LASIK with the variable spot-scanning ablation pattern of the Visx Star S4 IR and wavefront-optimized LASIK with the Allegretto Wave Eye-Q 400-Hz excimer laser showed no difference in UCVA at 3 months and significantly more coma and spherical aberrations at 3 months in the Allegretto Wave Eye-Q group.11
He et al12 also recently conducted a prospective, randomized, fellow-eye controlled study comparing wavefront-guided PRK with the Visx CustomVue S4 IR and wavefront-optimized PRK with the Allegretto Wave Eye-Q 400-Hz excimer laser. The investigators observed a UCVA of 20/20 or better at 12 months in 93% of eyes (n = 66) in the wavefront-guided group and 94.4% of eyes (n = 67) in the wavefront-optimized group. No loss of BCVA of 2 or more lines at 12 months was observed in either group. Additionally, there was significantly more coma at 12 months in the wavefront-optimized group but no significant difference in other HOAs.12
LASIK AND PRK: POTENTIAL COMPLICATIONS
Despite the excellent visual results achieved with LASIK and PRK, certain complications can arise in these refractive procedures. A Cochrane review of LASIK and PRK—including only cases performed with the femtosecond laser—showed potential serious complications such as postoperative haze (average of 3.3% in PRK), flap-related complications (average of 4% in LASIK), ectasia, and infections. However, these outcomes are quite infrequent, and reports from the review showed fewer complications in studies conducted after 2000.13 Dry eye; visual fluctuations and disturbances such as glare, halos, and light sensitivity; and foreign body sensations are some of the most common patient complaints after these procedures. This is likely due to the ablation of the nerve plexus and change in tear architecture. In many patients, these visual complaints and dry eye decrease after 3 to 6 months.14
SMILE: EARLY OUTCOMES
SMILE is a newer refractive procedure that uses the VisuMax femtosecond laser (Carl Zeiss Meditec) to create four sequential photoablative incisions to create a lenticule and then a 2- to 3-mm corneal incision that is used for dissection of the intrastromal lenticule.15 No flap is required, and there is potentially less destruction of the corneal nerve plexus. Refractive outcomes have been reported to be similar to those with LASIK and PRK. A review by Reinstein et al16 showed that previous studies have reported a postoperative distance UCVA of 20/20 or better in a range of 37% to 100% of eyes at follow-up visits between 3 and 12 months. In a study by Chansue et al17 (n = 347 eyes), 95% of eyes achieved a distance UCVA of 20/25 or better after 1 year and no eyes lost 2 or more lines of distance BCVA.
SMILE: POTENTIAL COMPLICATIONS
Given the small incision used with SMILE, the manipulation of the lenticule is initially difficult. Ivarsen et al18 reported 194 perioperative complications associated with 1,500 SMILE procedures. The most frequent complications were minor epithelial abrasions. However, there were more serious complications reported, including four cap perforations, one major tear, one impossible extraction of the lenticule, and five cases of infiltrates or keratitis. This was reported to be similar to the frequency of complications in LASIK.18
SMILE VERSUS LASIK AND PRK OUTCOMES
In a recent prospective, randomized, controlled comparative study by Liu et al,19 SMILE was performed on 113 eyes and femtosecond LASIK was performed on 84 eyes. At 6 months postoperatively, 96% of SMILE patients and 99% of femtosecond LASIK patients had a UCVA of 20/20 or better. There was only a statistically significant difference on postoperative day 1 favoring femtosecond LASIK. Two eyes in the SMILE group and no eyes in the LASIK group lost more than 2 lines of distance BCVA at 6 months; however, there was no significant difference in logMAR visual acuity. Additionally, the contrast sensitivity of SMILE patients was statistically significantly lower on postoperative day 1 and postoperative week 1 but became similar at subsequent visits. There was no difference in HOAs at the 4-mm zone; however, there was a statistically significant decrease in spherical aberrations at the 6-mm zone in the SMILE group.19
In another prospective, randomized, controlled comparative study of dry eye in SMILE versus LASIK (n = 30 eyes in each group), Denoyer et al20 reported that mild to moderate dry eye disease was statistically significantly higher at 6 months in the LASIK group and noted decreased corneal nerve density and more frequent use of tear substitutes in these patients. Additionally, some studies have found SMILE eyes to have statistically more corneal sensation at all time points compared with LASIK eyes, especially in the first 3 months, which is most likely due to the preservation of corneal nerve fibers, as no corneal flap is needed.16
CONCLUSION
Compared with the long-term follow-up data available for LASIK And PRK, outcomes for SMILE are still early, and the procedure is not yet approved in the United States. However, the SMILE procedure shows great promise in its comparable refractive outcomes and potential advantages of less dry eye disease, improved biomechanical structure, and possibly reduced HOAs. Reportedly, a steep learning curve is required for the delicate removal of the lenticule from the small incision. Additionally, SMILE is indicated only for myopia (less than 10.00 D) and moderate myopia (less than 6.00 D), whereas LASIK and PRK have a much wider range of treatment from myopia and hyperopia to mixed astigmatism.14 Retreatments for residual refractive error or regression in LASIK and PRK have been more time-tested than SMILE as well—there is no established enhancement procedure for SMILE. Time and further studies will tell if this procedure will become as prevalent in refractive surgery as LASIK and PRK.
1. El Danasoury MA, El Maghraby A, Klyce SD, Mehrez K. Comparison of photorefractive keratectomy with excimer laser in situ keratomileusis in correcting low myopia (from -2.00 to -5.50 diopters). A randomized study. Ophthalmology. 1999;106(2):411-420; discussion, 420-421.
2. FDA Summary of Safety and Effectiveness Data, Visx Star S4 Excimer Laser System and WaveScan WaveFront System. https://www.accessdata.fda.gov/cdrh_docs/pdf/P930016S016b.pdf. Accessed April 1, 2016.
3. FDA Summary of Safety and Effectiveness Data, Visx Star S4 Excimer Laser System with Variable Spot Scanning (VSS”) and WaveScan WaveFront System. https://www.accessdata.fda.gov/cdrh_docs/pdf/P930016S017b.pdf. Accessed April 1, 2016.
4. Shaheen MS, Massoud TH, Ezzeldin H, Khalifa MA. Four-year visual, refractive, and contrast sensitivity outcomes after wavefront-guided myopic LASIK using an advanced excimer laser platform. J Refract Surg. 2013;29(12):816-822.
5. Smadja D, Santhiago MR, Tellouck J, et al. Safety and efficacy of wavefront-guided myopic laser in situ keratomileusis using a new wavefront sensor technology: first 100 cases. J Cataract Refract Surg. 2015;41(8):1588-1593.
6. FDA Summary of Safety and Effectiveness Data, WaveLight Allegretto Wave Excimer Laser System. https://www.accessdata.fda.gov/cdrh_docs/pdf2/P020050b.pdf. Accessed April 1, 2016.
7. FDA Summary of Safety and Effectiveness Data, WaveLight Allegretto Wave Excimer Laser System. https://www.accessdata.fda.gov/cdrh_docs/pdf3/P030008b.pdf. Accessed April 1, 2016.
8. George MR, Shah RA, Hood C, Krueger RR. Transitioning to optimized correction with the WaveLight Allegretto Wave: case distribution, visual outcomes, and wavefront aberrations. J Refract Surg. 2010;26(10):S806-813.
9. Costa E, Franqueira N, Rosa AM, et al. Photorefractive keratectomy for myopia and myopic astigmatism correction using the WaveLight Allegretto Wave Eye-Q excimer laser system. Int Ophthalmol. 2014;34(3):477-484.
10. He L, Liu A, Manche EE. Wavefront-guided versus wavefront-optimized laser in situ keratomileusis for patients with myopia: a prospective randomized contralateral eye study. Am J Ophthalmol. 2014;157(6):1170-1178.e1.
11. Khalifa MA, Mossallam EF, Massoud TH, Shaheen MS. Comparison of visual outcomes after variable spot scanning ablation versus wavefront-optimized myopic LASIK. J Refract Surg. 2015;31(1):22-28.
12. He L, Manche EE. Contralateral eye-to-eye comparison of wavefront-guided and wavefront-optimized photorefractive keratectomy: a randomized clinical trial. JAMA Ophthalmology. 2015;133(1):51-59.
13. Shortt AJ, Allan BD, Evans JR. Laser-assisted in-situ keratomileusis (LASIK) versus photorefractive keratectomy (PRK) for myopia. Cochrane Database Syst Rev. 2013;1:CD005135.
14. Murakami Y, Manche EE. Prospective, randomized comparison of self-reported postoperative dry eye and visual fluctuation in LASIK and photorefractive keratectomy. Ophthalmology. 2012;119(11):2220-2224.
15. Moshirfar M, McCaughey MV, Reinstein DZ, Shah R, Santiago-Caban L, Fenzl CR. Small-incision lenticule extraction. J Cataract Refract Surg. 2015;41(3):652-665.
16. 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.
17. Chansue E, Tanehsakdi M, Swasdibutra S, McAlinden C. Efficacy, predictability and safety of small incision lenticule extraction (SMILE). Eye Vis (Lond). 2015;2:14.
18. Ivarsen A, Asp S, Hjortdal J. Safety and complications of more than 1500 small-incision lenticule extraction procedures. Ophthalmology. 2014;121(4):822-828.
19. Liu M, Chen Y, Wang D, et al. Clinical outcomes after SMILE and femtosecond laser-assisted LASIK for myopia and myopic astigmatism: a prospective randomized comparative study. Cornea. 2016;35(2):210-216.
20. Denoyer A, Landman E, Trinh L, Faure JF, Auclin F, Baudouin C. Dry eye disease after refractive surgery: comparative outcomes of small incision lenticule extraction versus LASIK. Ophthalmology. 2015;122(4):669-676.
