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 μm (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.
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.
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.