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EW RESIDENTS June 2014 51 Segal KL, Van Tassel SH, Kim C, Pena JTG, Ryder S, Chapman K, Coombs PG, Parlitsis G, Klufas MA Biomechanics following ReLEx SMILE and femto LASIK: Which cornea is stronger? Corneal biomechanical effects: Small-incision lenticule extraction versus femtosecond laser-assisted laser in situ keratomileusis Di Wu, PhD, Yan Wang, MD, PhD, Lin Zhang, MD, Shengsheng Wei, MD, Xin Tang, MD, PhD J Cataract Refract Surg (June) 2014; 40:954–962 Purpose: To compare the biomechanical properties of the cornea after small-incision lenticule extraction (lenticule extraction group) with those after femtosecond laser-assisted laser in situ keratomileusis (femtosecond LASIK group). Setting: Tianjin Eye Hospital & Eye Institute, Tianjin Key Laboratory of Ophthalmology and Visual Science, Tianjin Medical University, Tianjin, China. Design: Prospective comparative case series. Methods: Corneal hysteresis (CH), the corneal resistance factor (CRF), and 37 other biomechanical waveform parameters were quantitatively assessed with the Ocular Response Analyzer preoperatively and 1 week and 1, 3, and 6 months postoperatively. Results: Each group comprised 40 eyes. The decrease in CH and the CRF was statistically significant 1 week postoperatively compared with preop- eratively in both groups (P<.0001). However, the CH and CRF values in the lenticule extraction group were significantly higher than in the femtosecond LASIK group 3 months and 6 months postoperatively (P<.032). The residual stromal thickness index versus the CRF and CH and the planned lenticule thickness versus the change in central corneal thick- ness were statistically significant in the lenticule extraction group (r=0.388 to 0.950, P<.018); no significant correlation was found in the femtosecond LASIK group. In the waveform analysis of the lenticule extraction group, 28 of the 37 biomechanical waveform parameters differed significantly between preoperative values and postoperative values (P<.035). Conclusions: Both small-incision lenticule extraction and femtosecond laser-assisted LASIK can cause biomechanical changes in the cornea. However, changes in the cornea's viscoelastic properties were less after lenticule extraction than after LASIK. Financial disclosure: No author has a financial or proprietary interest in any material or method mentioned. One potential advantage of ReLEx (laser-induced extraction of a refractive lenticule) is greater corneal biomechanical strength relative to LASIK or PRK. I invited the Cornell residents to review a study on this very subject in this month's JCRS. –David F. Chang, MD, chief medical editor Introduction Laser-induced extraction of a refrac- tive lenticule (ReLEx) offers a com- pelling alternative to the current refractive surgery standards, laser in situ keratomileusis (LASIK) and pho- torefractive keratectomy (PRK). The next generation of lenticule-based refractive surgery, ReLEx SMILE, has many theoretical benefits over LASIK owing to its flapless technique and preservation of the anterior stroma. In the June issue of the Journal of Cataract & Refractive Surgery, Wu et al compared postop biomechanical properties of the cornea following ReLEx SMILE and LASIK. The surgical procedure of ReLEx is generally considered more chal- lenging than LASIK. Currently, the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) is the only platform configured to perform ReLEx. Laser spots of 1-micrometer diameter are placed 2 to 5 microme- ters apart in a spiral configuration. The posterior surface of the lenticule is cut first, followed by a slightly larger anterior cut. In ReLEx FLEx, the lenticule is removed using a LASIK type flap, whereas in small incision lenticule extraction (ReLEx SMILE), the lenticule is removed via a 2 to 4 mm superior corneal inci- sion. A pair of forceps is placed through the small incision to remove the lenticule after tissue bridges are broken with a blunt spatula. 1,2 By this technique, SMILE minimizes corneal trauma and— unlike ReLEx FLEx and LASIK—pre- serves the integrity of the anterior corneal stroma, which is thought to provide the bulk of corneal strength. By the same principle, SMILE may spare corneal nerves, optimizing corneal sensitivity and tear produc- tion postoperatively. 3 Early reports suggest that visual and safety outcomes after 500 kHz femtosecond laser SMILE are compa- rable to those achieved with LASIK. Currently, the VisuMax is powered to treat up to 10 D of myopia and up to 5 D of astigmatism. 4 In the largest study of 670 myopic eyes, the mean postoperative refraction was –0.25 +/–0.44 D and remained within +/–0.50 and +/–1.00 D of plano in 80% and 94% of eyes, respectively. 5 Regression of –0.15 D was observed during the first postop- erative month in a study of 279 eyes, although 95% of these patients were satisfied enough to recommend the procedure to a friend. 6 One study examining the effects of SMILE on the endothelium reported no change in cell density, and the initial concern surrounding delayed visual recovery has been minimized by adjusting laser settings. 7 Both surgeon learning curve and specific laser parameters have been found to influence visual outcome, and although early results are promising, clinical studies evaluating the ReLEx smile procedure are few in number and limited to small cohorts. In a prospective study of 80 myopic patients, Wu et al examined biomechanical corneal properties following ReLEx SMILE and com- pared these results with femtosec- ond laser-assisted LASIK (FS-LASIK). The hypothesis of the paper, and presumed benefit of flapless in- trastromal surgery, is that SMILE provides superior corneal biome- chanics postoperatively. Theoreti- cally, a well-preserved stroma should minimize long-term risk of ectasia. 8 To measure corneal biomechan- ics, the authors enlisted the Ocular Response Analyzer (ORA, Reichert Technologies, Depew, N.Y.), cur- rently the only device available that allows for direct analysis of corneal biomechanics in the clinic setting. The main parameters of the ORA included corneal hysteresis (CH) and corneal resistance factor (CRF). Corneal hysteresis, from a mathe- matical perspective, reflects the difference in air pressure between force-in (P1) and force-out (P2) ap- planation but can also be derived from corneal properties including corneal thickness (CCT), rigidity, intraocular pressure (IOP), and hy- dration. Clinically, abnormally low CH is demonstrated in a variety of pathologic states, most notably in keratoconus. 9 Corneal resistance fac- tor is a derived measurement using P1, P2 and k, a constant developed based on relationships between P1, P2 and CCT. CRF measures corneal resistance theoretically isolated from the effects of IOP. Patients were divided evenly in the two groups such that 40 patients were treated with lenticule extrac- tion and 40 underwent femtosecond LASIK. No explanation was provided as to how and why patients were selected for either procedure. Lack of randomization in this prospective study limits validity of conclusions, although differences in gender, age, baseline manifest refraction, mean K and CCT between the two groups were not significant. Thirty-seven versus 34 eyes reached the 6-month follow-up mark in SMILE and LASIK groups, respectively. The fact that three additional patients—represent- ing nearly 10% of the cohort—com- pleted follow-up for SMILE may bias results in favor of SMILE. Measurements of corneal biome- chanics were taken preoperatively and at postoperative week 1, and months 1, 3 and 6. Three measurements for each patient were obtained and then averaged for Christopher Starr, MD, director of ophthalmic education, Weill Cornell Medical College, New York continued on page 52 EyeWorld journal club 51-54 Residents_EW June 2014-DL_Layout 1 6/3/14 12:39 PM Page 51

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