EyeWorld Asia-Pacific September 2014 Issue

59 EWAP rEfrActivE September 2014 Biomechanics following ReLEx SMILE and femto LASIK: Which cornea is stronger? by Segai KL, van tassel SH, Kim c, Pena JtG, ryder S, chapman K, coombs PG, Parlitsis G, Klufas MA The EyeWorld Journal Club reviews a prospective comparative case series I ntroduction Laser-induced extraction of a refractive lenticule (ReLEx) offers a compelling alternative to the current refractive surgery standards, laser in situ keratomileusis (LASIK) and photorefractive 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 challenging than LASIK. Currently, the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) is the only platform configured to perform ReLEx. Laser spots of 1-µm diameter are placed 2 to 5 µm 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 continued on page 60 SMILE), the lenticule is removed via a 2- to 4-mm superior corneal incision. 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—preserves 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 production postoperatively. 3 Early reports suggest that visual and safety outcomes after 500 kHz femtosecond laser SMILE are comparable 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 postoperative 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 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 preoperatively 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 thickness 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. Top row: John Pena, MD, Peter Coombs, MD, Michael Klufas, MD, Steven Ryder, MD, and Charles Kim, MD Bottom row: Sara Van Tassel, MD, and Kira Segal, MD Source: STAAR Surgical