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EW REFRACTIVE SURGERY 50 April 2015 by Maxine Lipner EyeWorld Senior Contributing Writer with, for example, a 300-µm bed. "Other corneas were left with less than 250 µm posterior residual beds and never developed ectasia. Our ways of preventing or anticipating that were imperfect." Dr. Santhiago, who worked for a time at the Cole Eye Institute, had become intrigued by the rigidity of the tissue in the anterior part of the cornea, which seemed to have the most strength, Dr. Wilson recalled. "It's the anterior cornea that is the most able to resist development of ectasia," he said. Indeed, Dr. Santhiago has since substantiated this. "Corneal tensile strength is not uniform to all the central cornea," he said. "We saw progressive weakening in the deeper 60%." Also playing a role in weakening the cornea is the extent of biome- chanical alteration after refractive surgery and the stress this places on the deeper tissue, Dr. Santhiago explained. The flap thickness should also be considered. The fact is, the pure lamellar flap postoperatively does not contribute significantly to tensile strength, he said. "Based on this corneal structure, it seems logical that a ratio or a per- centage could provide more specific information than previous thickness values of total cornea or residual stromal bed," Dr. Santhiago said. With this in mind, he set out to in- vestigate an association between the percentage of tissue altered and the occurrence of ectasia after LASIK in eyes with normal preoperative Plac- ido disk-based color corneal topog- raphy. Dr. Santhiago noted that he and fellow investigators also wanted to compare this new metric dubbed PTA (percent of tissue altered) to currently recognized risk factors such as residual stromal bed, corneal thickness, high myopia, and age. Studying a new equation Included in this retrospective case-controlled study were eyes that developed ectasia after LASIK for myopia or myopic astigmatism, with normal bilateral preoperative topography, he said. This "ectasia group" was compared to a control reduction in corneal biomechanical integrity below which the shape is compromised. "This could occur when a cornea already destined to manifest ectasia has surgery, or when a preop- eratively weak but clinically stable cornea has surgery," he said. "This could [also] happen when a relative- ly normal cornea is weakened below a safe threshold." The study here concentrated on this last group. The traditional read Steven E. Wilson, MD, professor of ophthalmology, Cole Eye Institute, Cleveland, said that there are indeed some people who develop ectasia seemingly out of the blue. Tradition- al warning signs include abnormal corneal topography, such as inferior steepening, or ablations leaving less than 250 µm of posterior residual bed, he said. "Two hundred fifty microns was a magic number that was made up many years ago," Dr. Wilson said, adding that this was flawed, with ectasia cases cropping up in corneas What to take note of before LASIK I t is a continuing issue for re- fractive surgeons: Just because patients appear to have normal topography does not necessar- ily mean they will not develop ectasia, according to Marcony R. Santhiago, MD, PhD, professor of ophthalmology, Federal Univer- sity of Rio de Janeiro, Brazil, and associate professor, University of Sao Paulo. Study results published in the July 2014 issue of the American Jour- nal of Ophthalmology indicate that it is the percent of tissue altered at the time of LASIK that is the most im- portant factor in determining who with normal topography will devel- op the condition postoperatively. "The percentage tissue altered (PTA) higher or equal to 40% was by far the most prevalent risk factor when we analyzed the ectasia group," Dr. Santhiago said, adding that 97% of those who developed ectasia reached this 40% level. Dr. Santhiago views postop- erative ectasia as resulting from a Getting schooled on PTA for ectasia prevention Prevalence of individual variables investigated as possible ectasia risk factors in eyes with post-LASIK ectasia with normal preop corneal topography. The percent tissue altered ≥40 was by far the most prevalent risk factor, followed by age <30 years, residual stromal bed (RSB) ≤300 μm, preoperative central corneal thickness (CCT) ≤510 μm, and myopia ≥8 D. Source: Marcony R. Santhiago, MD, PhD group comprised of similar eyes that did not develop ectasia, with at least 3 years of postoperative follow-up. Results indicated that the PTA for eyes with ectasia was compara- tively greater than for controls, Dr. Santhiago reported. "When we compared the ectasia group to the control, the PTA was considerably higher in the ectasia group," he said. "It was on average 45.1% for the ectasia group com- pared to controls where it was only 31.9%." Investigators found that a cutoff value of 40% of tissue altered was able to provide the maximum com- bination of sensitivity and specificity in discriminating between post- LASIK ectasia cases and controls, Dr. Santhiago noted. In the ectasia group, this was overwhelmingly the most prevalent risk factor, he said. This was 97% for the ectasia group versus 57% for residual stromal beds below 300 and 16% with central corneal thickness below 510. "By stepwise logistic regression, we viewed the PTA higher than 40% [as] the single most significant inde- pendent variable, and the odds ratio analysis revealed a strong associa- tion between ectasia and PTA higher than 40%," Dr. Santhiago said. "We believe that the main explanation for this finding most likely lies in the relative percentage contribution of the anterior stroma to the total corneal strength that is modified af- ter excimer laser refractive surgery." He thinks PTA provides a more individualized measure for potential LASIK patients because it considers the relationship between flap thick- ness, tissue altered by ablation and flap creation, and also the residual stromal bed thickness. "The results of this study show that rather than an absolute value of central corneal thickness and resid- ual stromal bed, these appear more important as part of the equation that generates the percentage of tis- sue that's altered after surgery," Dr. Santhiago said. "We believe that is why absolute value and cutoff num- bers for residual stromal bed have historically failed in determining the high ectasia risk patient."

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