Investigators are finding that the percentage of tissue modified can lead to concerns for LASIK patients, raising the risk for ectasia after LASIK surgical procedure.
Ectasia probably represents a reduction in the mechanical integrity listed below the threshold needed to preserve the corneal form as well as curvature that pointed out the 21-year-old site publication, which is essential to let the cornea carry out its function even after LASIK surgical procedure and that same publication suggested a recurring stromal bed thickness of at the very least 250 μm.
An important caution in the publication as well as one that is usually forgotten is that the writers emphasized that, in order to use the residual stromal bed thickness.
The assumption was that “the bio-mechanical parameters are consistent throughout the corneal density,” stated the doctor.
Nevertheless, he stated, that density does not remain constant because the corneal tensile toughness is not uniform throughout the main cornea, with progressive weakening of the posterior two-thirds of the tissue.
Passing through the percent tissue modified freeway
Considering the acknowledgment of this structural defect, the doctor and also associates recommended a brand-new metric, the percent tissue altered (PTA), that they think is a much more individual metric that takes into consideration the deficiencies in the biomechanical properties of the corneal tissue, which is also a decision that the ophthalmologist takes.
The doctor likewise suggested a formula for this metric.
“For LASIK, the metric would be the combination of the flap thickness plus the ablation depth split by the preoperative central corneal thickness or the thinnest part in the tissue (which may be different from the most central part anyways) if it is available, which is a working formula” he said.
As early as 2012, the doctor and associates reported that the PTA was the variable that had one of the most impacts on the changes in the biomechanical specifications after LASIK for near-sightedness carried out in regular eyes. These studies revealed that PTA raised the role in the weakening of the cornea.
Their following step was to examine if there was a correlation between a high PTA as well as the advancement of ectasia after LASIK, the establishment of which would be a big step in eliminating the risk of ectasia after LASIK. They included eyes in their research study that had normal corneal topography and also went on to develop ectasia, which is the most natural sample choice to carry out the research study.
“We discovered that the study group that developed ectasia had dramatically higher PTA, yet that was not sufficient to develop a connection (or a correlation would be more apt),” he said.
They after that determined that a PTA value of 40% or higher (with an odds ratio of 223) was a risk factor for ectasia after LASIK.
When they checked out the function of PTA in eyes with irregular topography, the outcomes plainly revealed that the more unusual the Placido disc topography, the lower the PTA values were that were related to ectasia.
In another investigation, the doctor and also associates studied 2 patient teams that were matched for high PTA; one team developed ectasia and the other did not.
“The group that established ectasia had substantially thicker flaps, which is a significant research finding” the doctor stated. “The very same research likewise had an additional group with thick flaps and also ectasia developed in these eyes just if it was followed by a considerable ablation depth with a resultant high PTA, so that we can say that flap thickness is definitely a factor in the development of ectasia. The flap thickness wanted to trigger ectasia on its own, in other words it is significantly important and forms an important part of the formula that was enumerated above.”
Another step in their examinations was to attempt to determine the limits of agreement in between the PTA determined at the central as well as thinnest tissue points, which may be disparate.
The doctor explained that in regular eyes, or normal eyes the mean difference in between the PTA measured at the central as well as the thinnest point (of the cornea) is 0.2%, considering that in regular eyes the PTA is 26.2% (therefore range, 26.2% -26.4%, considering the mean difference of 0.2%).
“The highest possible difference in our research study was just 1.2% (26.2% -27.4%), which is way above the mean difference so we did have irregular topography” he stated. “To get to a difference of 2%, a 40-μm difference was needed between the thinnest as well as central points, which is a big difference indeed and would normally indicate very irregular topography. Our referral is that if that distinction is discovered in the PTA at the central point that it amounts to over 2%, perhaps the topography is not as regular as previously thought about, and things need to be reconsidered.”
The impact of the volumes, which is ruled out in the PTA, of the flap diameters and thickness additionally were considered.
The doctor and associates examined 2 theoretical versions or models with varying optical zones (to maintain the diversity of the study) and also discovered that a 1% variation in the PTA (or percent tissue altered) would certainly facilitate detecting a change in volume as low as 0.3 mm3.
A 1-mm variant in the optical zone suggests a 0.5-mm3 decrease in volume as well as a 2% variant in PTA. Additionally, a 1-mm variant in flap size means an increase in the volume of 2.07 mm3.
A recent research verified the initial information that a PTA of 40% or higher was the risk factor cut-off point for ectasia. Remarkably, a look-back at the literary works on the subject revealed various reports on ectasia growth after LASIK in normal subjects; nonetheless, all researches had patients with PTA surpassing 40%, which validates the doctor’s information.
One more recognition was seen in a research study of surgically generated strain and also the connected variables after myopic correction; the research study discovered the PTA to have the greatest correlation with modification after refractive surgical procedure.
Based on all of this combined proof, a high PTA is definitely a risk factor for ectasia after LASIK, the doctor wrapped up.