Linda Roach

June 14, 2013

SAN FRANCISCO, California — Cutting-edge wavefront aberrometry can be used to help refractive surgeons predict the biomechanical stability of the cornea after LASIK surgery.

"We all want to identify corneas that are weaker, even if they don't have any topographic abnormalities," Marcony Santhiago, MD, from the Federal University of Rio de Janeiro, Brazil, told Medscape Medical News.

In their study, Dr. Santhiago and colleagues identified 8 mathematical variables that correlate with easily measured structural parameters of the pre- and postoperative cornea.

If the proportion of the corneal depth altered by LASIK is kept low enough, LASIK eyes might be protected from ectasia because they would have the same biomechanical integrity as eyes that have undergone photorefractive keratectomy, Dr. Santhiago reported here at the American Society of Cataract and Refractive Surgery 2013 Symposium.

"A femtosecond LASIK flap that is an average of 114.0 µm and that goes slightly further than the anterior 30% of stroma behaves in a manner similar to that of surface ablation for similar levels of correction," Dr. Santhiago explained.

If this finding is confirmed, it would give refractive surgeons an objective way to identify LASIK candidates who are at risk of developing ectasia, even though their corneas have normal topographies, Dr. Santhiago explained.

Dr. Santhiago and colleagues used the Optiwave Refractive Analysis (WaveTec Vision) intraoperative wavefront aberrometer to examine 156 eyes before and 1 and 3 months after a LASIK (n = 104) or photorefractive keratectomy (n = 52) procedure. For each eye, they derived 17 novel mathematical descriptors of the cornea's biomechanical behavior from the wavefront data, and then tracked how each changed after surgery.

"We know that when we make a flap, we change the biomechanical properties of the cornea, so we used the photorefractive keratectomy eyes as a reference to compare the biomechanical effects of a LASIK flap with surface ablation, Dr. Santhiago explained. This initial work provided baseline reference values for the 17 variables in normal eyes before and after LASIK and photorefractive keratectomy procedures.

The researchers then looked at the correlation of variation between the customized variables and the corneal dimensional parameters that are thought to be biomechanically relevant, including preoperative central corneal thickness, residual stromal bed, amount of tissue ablated, and the total percentage of preoperative stromal depth altered by the flap and the ablation.

There were strong correlations between the change in pre- to postoperative thickness and the proportion of stromal depth altered by the refractive surgery for 8 of the variables (< .0001), Dr. Santhiago reported. Furthermore, for all 8 variables, the change in stromal thickness was similar after the LASIK and photorefractive keratectomy procedures (31% vs 13%; < .0001).

Michael Knorz, MD, from Heidelberg University in Mannheim, Germany, who was asked by Medscape Medical News to comment on the study, called it "very significant, because they looked into how the surgery weakens the cornea."

He explained that "rather than using absolute values, they correlated relative values of the tissue removed with the thickness of the preoperative cornea."

The existing ectasia-risk scoring system for screening LASIK candidates with normal topographies is based on the premise that the residual stromal bed should be at least 250 µm thick, although many surgeons prefer 300 µm. However, he noted that in one meta-analysis, only about 35% of eyes that developed ectasia had abnormal preoperative corneal topographies before surgery (Ophthalmology. 2008;115:37-50).

This study offers a new and possibly more accurate method of assessing the risk for ectasia before LASIK surgery in this population, Dr. Knorz said.

The study received no external funding. The authors have disclosed no relevant financial relationships. Dr. Knorz is a consultant for Alcon Laboratories.

American Society of Cataract and Refractive Surgery (ASCRS) 2013 Symposium. Paper session 1-E. Presented April 20, 2013.

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