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EW RESIDENTS 52 June 2014 the later study, but ablation depth was not controlled. Kamiya et al also did not report any cases of ectasia in the 6-month follow-up period. 10,11 Although case reports have sug- gested an increased risk of ectasia following LASIK compared with PRK in fellow eyes of the same patients, prospective studies with long-term follow-up are needed to clarify whether this association is more than anecdotal. The removal of corneal stromal tissue in both procedures must weaken the cornea to some degree, and the fact that SMILE seems to alter these properties to a lesser extent is both interesting and prom- ising. Why this delta value only reaches significance for CRF and not CH remains unclear. The real test will be whether results from this investigation pan out long term on a clinical level. If ReLEx SMILE can limit post-refractive surgery ectasia and still achieve an excellent visual outcome, it may inspire a shift in refractive surgery patterns moving forward. EW References 1. Blum M, Kunert KS, Vobmerbaumer U, Sekundo W. Femtosecond lenticule extraction (ReLEx) for correction of hyperopia – first results. Graefes Arch Clin Exp Ophthalmol. 2013;1:349–55. 2. Hjortdal JO, Vestergaard AH, Ivarsen A, Ragunathan S, Asp S. Predictors for the out- come of small-incision lenticule extraction for myopia. J Refract Surg. 2012;28:865–71. 3. Vestergaard AH, Gronbech KT, Grauslund J, Ivarsen AR, Hjortdal JO. Subbasal nerve morphology, corneal sensation, and tear film evaluation after refractive femtosecond laser lenticule extraction. Graefes Arch Clin Exp Ophthalmol. 2013;251:2591–600. 4. Sekundo W, Kunert KS, Blum M. Small inci- sion corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astig- matism: results of a 6-month prospective study. Br J Ophthalmol. 2011;95:335–9. 5. Hjortdal JO, Vestergaard AH, Ivarsen A, Ragunathan S, Asp S. Predictors for the out- come of small-incision lenticule extraction for myopia. J Refractive Surg. 2012;25:865–71. 6. Vestergaard A, Ivarsen AR, Asp S, Hjortdal JO. Small-incision lenticule extraction for moderate to high myopia: Predictability, safety, and patient satisfaction. J Cataract Refract Surg. 2012;38:2003–10. 7. Kamiya K, Igarashi A, Ishii R, Sato N, Nishimoto H, Shimizu K. Early clinical out- comes, including efficacy and endothelial cell loss of refractive lenticule extraction using a 500 kHz femtosecond laser to correct myopia. J Cataract Refract Surg. 2012;38:1996–2002. 8. Wu D, et al. Corneal biomechanical effects: Small-incision lenticule extraction versus femtosecond laser-assisted laser in situ keratomileusis. J Cataract Refract Surg. 2014; 40:954–962. 9. Ortiz D, Pinero D, Shabayek MH, Arnalich- Montiel F, Alio JL. Corneal biomechanical properties in normal, post-laser in situ keratomileusis and keratoconic eyes. J Cataract Refract Surg 2007;33:1371–1375. 10. Hamilton DR, Johnson RD, Lee N, Bouria N. Differences in the corneal biomechanical effects of surface ablation compared with laser in situ keratomileusis using a microker- atome or femtosecond laser. J Cataract Refract Surg. 2008;34:2049–56. 11. Kamiya K, Shimizu K, Ohmoto F. Compari- son of the changes in corneal biomechanical properties after photorefractive keratectomy and laser in situ keratomileusis. Cornea. 2009;28:765–9. Contact information Starr: drstarr@gmail.com Biomechanics continued from page 51 inclusion in statistical analysis. No data was provided in regard to the variation of the three measure- ments for each patient, and if signif- icant, could represent a major limitation. If variation in CH or CRF for each measurement approached the value of delta between the two groups, then no conclusion can really be drawn from the data. Postoperative CH and CRF decreased in both groups, and values for the SMILE group were higher at each postoperative time point. Baseline CH and CRF values were not provided; however, Figure 2 in the paper suggests that the lenticule extraction group started off with slightly higher CH and CRF, making comparison of postoperative differ- ence between groups difficult. The delta CRF and CH per patient seems to be the most relevant comparison for the paper, as less of a change from baseline would be preferred no matter the value or specific applica- tion of the property. Only the delta CRF values reached significance in this comparison and came out in favor of SMILE (i.e., less of a de- crease in CRF postoperatively). The mechanism proposed by the authors is the greater weight on P1 (initial applanation event) in calculating CRF and potential contribution of the anterior stroma to this value. Correlation between residual stromal thickness (RST) index and CRF and CH was also examined. A statistically significant correlation was demonstrated between RST index and delta CRF and CH in the lenticule extraction group. In con- trast, the correlations were not sig- nificant in the femtosecond LASIK group. No explanation was provided for this difference in the discussion, but perhaps with flap creation, corneal biomechanics are no longer as closely integrated to the anterior stroma due to the redistribution of forces elsewhere. With stromal preservation in SMILE, corneal biomechanics may more closely resemble preoperative paradigms. Interestingly, two studies have explicitly sought to examine the change in corneal biomechanical properties following LASIK and PRK. While Hamilton et al found CH and CRF decreased similarly after PRK and LASIK (both microkeratome and laser-assisted flap creation) when controlling for ablation depth, Kamiya et al concluded that biome- chanical change (CH and CRF) is larger after LASIK (microkeratome). Of note, PRK and LASIK eyes had similar average refractive errors in Top row: John Pena, MD, Peter Coombs, MD, Michael Klufas, MD, Steven Ryder, MD, and Charles Kim, MD Bottom row: Sarah Van Tassel, MD, and Kira Segal, MD Source: Mark Rosenblatt 51-54 Residents_EW June 2014-DL_Layout 1 6/3/14 12:39 PM Page 52

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