LASIK nomograms hide corneal biomechanical and epithelial profile changes induced by surgery

By Roibeard O'hÉineacháin

London - LASIK nomograms should factor in biomechanical and epithelial changes within the cornea, according to an ophthalmic surgeon.

Dan Z. Reinstein MD, Assistant Professor and Research Director of Refractive Surgery at the Weill Medical College of Cornell University in New York and Medical Director of the Reinstein Institute in London, UK, notes that corneal biomechanical changes influence the refractive effect of the LASIK procedure by an average of 15%.

He cited a study of 52 eyes which underwent standard LASIK treatment for -1 D to -10 D of myopia. Very high frequency (VHF) digital ultrasound scanning and Orbscan analysis showed that an average of 20% of the refractive flattening effect of laser sculpting was lost to the biomechanical and epithelial profile changes induced by the surgery.

"We found that there would have been 15% more flattening had there been no biomechanical changes. Epithelial profile changes accounted for another 5% loss of flattening.

"That is telling us that our nomograms contain on average 20% of variable information which we could in fact be taking advantage of to control shape changes within the cornea with more accuracy," he explained.

The postoperative refractive error observed statistically correlated with the amount of measured epithelial power shift (p<0.05) with an average epithelial power shift of 0.25 D accounting for each dioptre of postoperative refractive error.
Postoperative biomechanical shift also correlated to the degree of refractive error in a biphasic response (p<0.05).

To perform flap, residual stromal thickness and epithelial thickness measurements in three dimensions, Dr Reinstein used the new Artemis 2 (Ultralink LLC, St. Petersburg, Florida) VHF digital ultrasound scanning system based on technology that he and his colleagues developed at Cornell University.
Thickness mapping of the epithelium, flap, residual stromal thickness and full cornea before and after LASIK was obtained and studied.

"This system allows us to map the epithelial layer with one micron precision in three dimensions separately from the stromal layer. We can also separately map the thickness of the stromal layer preoperatively and postoperatively to determine the exact stromal change induced by LASIK in the cornea - a change that involves both tissue removal and biomechanical thickness changes," he said.

"We are able to map the residual stromal bed thickness in three dimensions, allowing determination of the thinnest point under the flap. With digital subtraction pachymetry, we can determine the exact epithelial change in three dimensions, and therefore measure the optical effect of these epithelial changes.

"Similarly, we can measure the effective stromal change and by analysing this together with curvature changes in the back surface, we are able to calculate biomechanical changes," he added.

In another study using the system, Dr Reinstein and associates showed why calculating the size and position of a phakic IOL on the basis of white-to-white measurements can be so unreliable.

White-to-white measurement is the current standard method of predicting sulcus-to-sulcus diameters for placement of posterior chamber IOLs, and for calculating angle-to-angle diameters for placement of anterior chamber IOLs, he noted.
In a study of myopic and hyperopic eyes, they measured white-to-white using digital photographic imaging and correlated this to the direct measurement of internal angle-to-angle and sulcus-to-sulcus from Artemis scans.

The Artemis is capable of imaging the entire anterior segment in one high-precision scan. In fact, the study showed no statistically significant correlation between either of the two internal measurements and white-to-white measurements in myopic eyes.
In hyperopic eyes, there was no correlation between white-to-white and sulcus-to-sulcus measurements, although there was a weak but statistically significant correlation between angle-to-angle and white-to-white.

"The irony is that such hyperopic patients are least likely to be the candidates for anterior chamber IOLs," Dr Reinstein pointed out.
"What these results explain is that we are in fact worse off using the white-to-white standard to predict the sizing of phakic IOLs than if we used just one size.

"It also explains why angle-supported anterior chamber IOLs can produce ischaemic changes in the iris when basing the sizing on non-direct angle-to-angle measurement. Hopefully we will soon get to the point where we will be measuring twice and cutting once," he added.

Dr Reinstein has a financial interest in the Artemis 2 ultrasound eye scanner.

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