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tive corneal deformation amplitude was correlated with postoperative CCT in both treatment groups. The correlations between postoperative change in deformation amplitude and stromal ablation/removal depth, percentage of tissue ablated/ removed, optical zone, and patient age were also examined. The femtosecond laser platform used for SMILE and for flap creation was the VisuMax 500 (Carl Zeiss Meditec, Jena, Germany), and the excimer laser platform was the Allegretto excimer laser (Alcon, Fort Worth, Texas). Results: A total of 40 eyes of 20 patients were involved in the study. Femto-LASIK was performed in 20 eyes and SMILE in 20. Mean preop- erative corneal deformation ampli- tude of all eyes was 0.97±0.15 mm. Mean age, preoperative CCT, corneal deformation amplitude and abla- tion/tissue removal profile were sim- ilar in both groups. A statistically significant increase in deformation amplitude occurred following femto- LASIK of 18.9%±3 and the SMILE group showed an increase of 4.7%±2. A weak correlation was found between postoperative decrease in deformation amplitude and the parameters examined. EW REFRACTIVE SURGERY July 2014 47 Best in refractive G View Dr. Berdahl's presentation at Clinical.EWrePlay.org. Scan to watch video! Conclusion: Corneal deformation amplitude increased following femto-LASIK much more signifi- cantly in comparison with the SMILE group. Postoperative defor- mation amplitude increase did not correlate with the amount or per- centage of corneal tissue removed, nor with optical zone or patient age. Dr. Berdahl: Dr. Abdalla and colleagues looked at corneal rigidity following small incision lenticular extraction (SMILE). They used deformation amplitude to compare corneal strength following the SMILE procedure or femtosecond LASIK. In this study, they found that eyes having the SMILE procedure had significantly less deformation than LASIK eyes. The authors conclude that the SMILE proce- dure may have more stable corneas. Role of percentage of tissue altered as risk factor for ectasia after LASIK in eyes with normal preoperative topography Marcony R. Santhiago, MD, PhD, and colleagues Purpose: To investigate the role of the percentage of tissue altered (PTA) on ectasia after LASIK in eyes with normal preoperative corneal topography. Methods: This retrospective compar- ative case-control study included 30 eyes that developed ectasia after LASIK for myopia with bilateral normal preoperative corneal topog- raphy (ectasia group) and 174 eyes that underwent uncomplicated LASIK for myopia without develop- ing ectasia with at least 3 years of postoperative follow-up (control group). Prevalence of risk factors and odds ratio were determined and variables were further submitted to logistic stepwise regression. The percentage of anterior tissue depth altered (PTA) during LASIK was obtained from the equation PTA = (FT + AD)/CCT where FT = flap thickness, AD = ablation depth (AD), and CCT = preoperative central corneal thickness (CCT). Residual stromal bed (RSB), manifest refrac- tive spherical equivalent (MRSE), age, and Ectasia Risk Score System (ERSS) values were also investigated. Results: In the ectasia group, PTA ≥40% was the most prevalent risk factor (97%), followed by age ≤30, RSB ≤300, and ERSS ≥3. The PTA ≥40% presented the higher odds ratio values followed by RSB ≤300 μm, CCT ≤500 μm, ERSS ≥4, ERSS ≥3, MRSE ≥8 D, and age ≤25 (years). Stepwise logistic regression revealed the PTA ≥40% as the most important independent variable (P<0.0001). Conclusion: The study provides scientific evidence of the role of PTA on ectasia after LASIK in eyes with normal corneal topography and therefore should be incorporated in the preoperative screening. Dr. Berdahl: Dr. Santhiago and colleagues looked at a metric known as percentage of tissue altered (PTA) to determine if it may be predictive of eyes that could develop ectasia. The retro- spective study … looked at eyes that de- veloped ectasia after having a normal preop topography. The PTA was defined as flap thickness plus ablation depth divided by the central corneal thickness. If the PTA was greater than 40%, patients had a much higher likelihood of having developed ectasia. When compared to other risk factors such as residual stromal bed, age, and corneal thickness, PTA stood out as more predictive. This study provides refractive surgeons another metric to potentially use in refractive surgery screening. EW Editors' note: Dr. Berdahl has no finan- cial interests related to his comments. Contact information Berdahl: john.berdahl@vancethompsonvision.com 43-47 Refractive_EW July 2014-DL2_Layout 1 6/30/14 8:45 AM Page 47

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