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Investigators are finding that the percentage of tissue altered can create issues for LASIK patients.
This article was reviewed by Marcony R. Santhiago, MD, PhD
Ectasia most likely represents a reduction in the mechanical integrity below the threshold required to maintain the corneal shape and curvature, according to Marcony R. Santhiago, MD, PhD, who cited the 21-year-old landmark publication of Theo Seiler, MD, who recommended a residual stromal bed thickness of at least 250 Î¼m (J Refract Surg 1998;14:312-7).
An important caveat in the publication and one that is often forgotten is that the authors emphasized that in order to use the residual stromal bed thickness.
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The assumption was that “the biomechanical parameters are constant throughout the corneal thickness,” said Dr. Santhiago, professor of ophthalmology, University of Sao Paulo, Rio de Janeiro, Brazil.
However, he said, that thickness does not remain constant because the corneal tensile strength is not uniform throughout the central cornea, with progressive weakening of the posterior two-thirds of the tissue.
Traversing the percent tissue altered highway
Considering the recognition of this structural defect, Dr. Santhiago and colleagues proposed a new metric, the percent tissue altered (PTA), that they believe is a more individual metric that takes into account the deficiencies in the biomechanical properties of the corneal tissue.
Dr. Santhiago also proposed a formula.
“For LASIK, the metric would be the combination of the flap thickness plus the ablation depth divided by the preoperative central corneal thickness or the thinnest point in the tissue if it is available,” he said.
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As early as 2012, Dr. Santhiago and colleagues reported that the PTA was the variable that had the most impact on the changes in the biomechanical parameters after LASIK for myopia performed in normal eyes (J Cataract Refract Surg 2012;38:1222-31 and 2014;40:918-28). These studies showed that PTA increased the role in the weakening of the cornea.
Their next step was to investigate if there was a correlation between a high PTA and the development of ectasia after LASIK. They included eyes in their study that had normal corneal topography and went on to develop ectasia.
“We found that the study group that developed ectasia had significantly higher PTA, but that was not sufficient to establish a relationship,” he said.
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They then determined that a PTA value of 40% or greater (with an odds ratio of 223) was a risk factor for ectasia after LASIK (J Cataract Refract Surg 2014;158:87-95).
When they investigated the role of PTA in eyes with abnormal topography, the results clearly showed that the more abnormal the Placido disc topography, the lower the PTA values were that were associated with ectasia (J Cataract Refract Surg 2015;31:258-65).
In another investigation (J Cataract Refract Surg 2015;DOI:10.1016/j.jcrs.2015.05.023), Dr. Santhiago and colleagues studied two patient groups that were matched for high PTA; one group developed ectasia and the other did not.
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“The group that developed ectasia had significantly thicker flaps,” Dr. Santhiago said. “The same study also had another group with thick flaps and ectasia developed in these eyes only if it was followed by a significant ablation depth with a resultant high PTA. The flap thickness was insufficient to cause ectasia by itself.”
Another step in their investigations was to attempt to determine the limits of agreement between the PTA measured at the central and thinnest tissue points.
Dr. Santhiago pointed out that in normal eyes, the mean difference between the PTA measured at the central and the thinnest point is 0.2%, considering that in normal eyes the PTA is 26.2% (range, 26.2%-26.4%).
“The highest difference in our study was only 1.2% (26.2%-27.4%),” he said. “To reach a difference of 2%, a 40-Î¼m difference was needed between the thinnest and central points. Our recommendation is that if that difference is found in the PTA at the central point that it is equal to a value over 2%, perhaps the topography is not as normal as previously considered.”
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The effect of the volumes, which is not considered in the PTA, of the flap diameters and thickness also were considered.
Dr. Santhiago and associates studied two theoretical models with varying optical zones and found that a 1% variation in the PTA would facilitate detecting a change in volume as low as 0.3 mm3.
A 1-mm variation in the optical zone means a 0.5-mm3 decrease in volume and a 2% variation in PTA. In addition, a 1-mm variation in flap diameter means an increase in the volume of 2.07 mm3.
A recent study (Ophthalmology 2019;126:908-9) validated the original data that a PTA of 40% or higher was the risk factor cut-off point for ectasia. Interestingly, a look-back at the literature on the subject showed numerous reports on ectasia development after LASIK in normal subjects; however, all studies had patients with PTA exceeding 40%, which validates Dr. Santiago’s data.
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Another validation was seen in a study of surgically induced strain and the associated variables after myopic correction; the study found the PTA to have the strongest correlation with change after refractive surgery.
Based on all of this combined evidence, a high PTA is definitely a risk factor for ectasia after LASIK, Dr. Santhiago concluded.
Read more by Lynda Charters
Marcony R. Santhiago, MD, PhD
This article is based on Dr. Santhiago's presentation at the American Academy of Ophthalmology 2019 annual meeting. Dr. Santhiago is a consultant for Alcon, Opto, and Zeiss, Ziemer, and a speaker for Adapt, Alcon Opto, Staar Surgical, Zeiss, and Ziemer.