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LASIK ablation centered on corneal vertex has good results

Invest Ophthalmol Vis Sci. 2008;49(12):5287-5294.

Issue: January 10, 2009

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LASIK with ablation centered on the corneal vertex produced visual, optical and refractive outcomes comparable to those of LASIK centered on the pupil.

The study included two groups of eyes with myopic astigmatism: 24 eyes of 16 patients underwent LASIK with eyes centered on the corneal vertex, and 29 eyes of 19 patients underwent LASIK with eyes centered on the pupil center.

At 6-month follow-up, 88% of corneal vertex (CV) eyes and 97% of pupil center (PC) eyes had uncorrected visual acuity better than 20/20. In addition, 58% of CV eyes were within ±0.25 D and 88% were within ±0.50 D of target refraction; 48% of PC eyes were within ±0.25 D and 83% were within ±0.50 D of targeted refraction. Differences between refractive outcomes were not statistically significant.

Induced ocular coma averaged 0.17 µm in the CV eyes and 0.26 µm in the PC eyes (P = .01), induced ocular spherical aberration averaged +0.01 µm in the CV group and +0.07 µm in the PC group (P = .05), and change in asphericity averaged +0.56 in the CV group and +0.76 in the PC group (P = .05).

"Despite this, an absolute optimum centration reference could not be determined," the study authors said. "Centering on the pupil offers the opportunity to minimize the optical zone size, whereas centering in the CV offers the opportunity to use a stable morphologic axis and to maintain the corneal morphology after treatment."

This is a beautifully performed study. The take-home message is that we all have to rethink the way we perform LASIK. We have all been trained to center laser treatments on the pupil. This study found that there were significantly more higher-order aberrations in the group whose ablation was centered on the pupil vs. the corneal vertex. It makes a compelling case that for myopes, like hyperopes, the ablation should be centered on the corneal vertex, not the pupil.

How to center the corneal vertex:

1. Using topography, measure the X and Y offset values from the pupil center to the center of the topography map (under light conditions similar to your laser being used).
2. On your laser, add these offset values to the pupil offset in the opposite directions to match the ablation to the corneal vertex.
3. Verify you have entered the correct direction of offset by looking at the upside down topography to compare it to eye/ablation image under the laser. If you have entered the wrong offset directions, ablation will be significantly de-centered.

If your laser system does not allow for offsetting the pupil, you can simply center the ablation over the corneal fixation light reflex that will be very close to the corneal vertex as well.

– Brian S. Boxer-Wachler, MD
Director, Boxer-Wachler Vision Institute, Beverly Hills
Founder, www.GrowsYourPractice.com