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Association of percentage of tissue altered with post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography

Poster Details

First Author: M.Santhiago BRAZIL

Co Author(s):    D. Smadja   G. Mello   B. Fiuza Gomes   S. Wilson   B. Randleman  

Abstract Details


To investigate the association of a novel metric, the Percentage of Tissue Altered, with the occurrence of ectasia after Laser in situ keratomileusis (LASIK) in eyes with normal corneal topography and to compare this metric with other recognized risk factors.


University of Sao Paulo, Sao Paulo, Brazil; Cole Eye Institute, Cleveland Clinic, Cleveland OH, USA; University Center Hospital of Bordeaux, Bordeaux, France; Emory University School of Medicine, Atlanta, Georgia, USA.


This retrospective comparative case-control study included 30 eyes from 16 patients that developed ectasia after LASIK for myopia and myopic astigmatism with bilateral normal preoperative Placido disk-based corneal topography (Ectasia group) and a contemporaneous population comprised of 174 eyes from 88 consecutive patients that underwent uncomplicated LASIK for myopia and myopic astigmatism without developing ectasia with at least 3 years of postoperative follow-up (Control group). The main outcome was the percentage of tissue altered, obtained from the equation: Percentage of tissue altered (PTA) = [(Flap Thickness + Ablation Depth)/preoperative Central Corneal Thickness]. Analysis of receiver operating characteristic (ROC) curve revealed a cut-off of 40.0 for PTA as the value with the maximized sum of sensitivity (97%) and specificity (89%). Specific cut-off values of previously recognized risk factors were identified for comparative purposes. These included individual metrics used in the Ectasia Risk Score System (ERSS): Age (<30 years), central corneal thickness (CCT) ≤ 510μm, residual stromal bed (RSB) ≤ 300μm and myopia (≥ 8Diopters). Mean values and prevalence were calculated. To investigate the association between factor and event (ectasia), the factors had odds ratios calculated. Variables were further compared through logistic stepwise regression to investigate the significance as predictors of ectasia.


Percentage of tissue altered (PTA) was significantly higher in patients with ectasia and normal topography compared to controls. In the ectasia group, percentage of tissue altered (PTA) ≥ 40 was the most prevalent evaluated variable (97%), followed by age < 30 (63%), RSB ≤ 300 (57%), and ERSS ≥ 3 (43%). When comparing the prevalence of risk factors between ectasia and control groups, percentage of tissue altered (PTA) ≥ 40 (p < 0.0001), Age < 30 (p < 0.0001), RSB ≤ 300μm (p < 0.0001), ERSS values ≥ 3 (p = 0.0002) and ERSS values ≥ 4 (p = 0.001) prevalence were significantly different. Prevalence of CCT ≤ 510μm (p = 0.065) and myopia ≥ 8 Diopters (p = 0.547) were not significantly different between the groups. Percentage of tissue altered (PTA) presented the highest odds ratio (223) followed by RSB ≤ 300μm (74), ERSS ≥ 4 (8), age < 30 (5), ERSS ≥ 3 (5), CCT ≤ 510μm (2.9) and myopia ≥ 8 Diopters (1.5). Stepwise logistic regression revealed the PTA ≥ 40%, as the most significant remained independent variable (P < 0.0001).


This aggregate analysis provides evidence that the percentage of tissue altered (PTA) is a relevant factor in the development of post-LASIK ectasia in eyes with normal preoperative Placido disk-based topography and therefore should be taken into account as a screening parameter for refractive surgery candidates. Compared to other variables, PTA had higher prevalence, higher odds ratio and higher predictive capabilities for ectasia risk than moderate to high ERSS values, RSB, CCT, high myopia, ablation depth, and age. PTA ≥ 40 was a more robust indicator of risk than other variables in patients with normal preoperative topography, being even more sensitive than the absolute value of the RSB itself that influenced on the risk of ectasia the most. FINANCIAL INTEREST: NONE

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