by Guy M. Kezirian, MD, MBA, FACS

June 2014

The issues around centration of laser refractive treatments are complex, particularly in hyperopes. The complexity extends from several factors, not all of which involve the cornea. They also include the pupil size, treatment amount, the shape of the ablation made by different lasers, and concerns about regression. As a result, it is not possible to reduce the issues to a single recommendation. Optics engineers are well aware of the issues and we have been discussing them for years.

Dan Reinstein, MD is very interested in hyperopic treatments, and published the article referenced below. It is probably the best article we have in the literature on the topic, but was based on a series of eyes operated with the Zeiss MEL 80 laser. It is not valid to extrapolate findings between lasers, so read the article with caution. However, this section in the Discussion reiterates the basis for the recommendation to center treatments between the pupil center and the corneal apex, aka “split the difference”:

In the large angle kappa group, a proportion of the ablation was done outside the edge of the pupil, which would result in asymmetric aberrations inside the pupil boundary; an increase in on-axis spherical aberration will in this case result in perceived increased coma due to the cropping effect of the pupil.

Thus, an increase in corneal spherical aberration about the nascent visual axis results in an increase in coma for the retinal image. Conversely, had an eye with a large angle kappa been treated on the entrance pupil center, a new vertex would have been created, which would induce coma away from the nascent visual axis of that eye and therefore, presumably, also have a detrimental effect on vision.

Most hyperopic ablations induce considerable spherical aberration. This can work for patients who want to achieve better vision across different ranges, and especially for patients with “blended monovision” where the target refraction is set to mid-point (e.g., -1.00 D) and spherical aberration provided added depth of focus for functional near vision.

Adding spherical aberration to the final corneal shape complicates the optics, however, so the pupil must be considered in these treatments. Leaving part of the entrance pupil untreated by centering the treatment on the corneal apex in an eye with large pupils can result in significant symptoms. Dan alludes to this in the article.

Another challenge surgeons face is determining how to best identify the corneal apex, and deciding where the visual axis is in relation to the corneal apex. The first Purkinje moves with ocular rotation. The apparent corneal apex on topography varies with fixation. These are not trivial concerns, particularly in hyperopic eyes which tend to be small, often have poor fixation, and vary widely in their anatomy.

In 2008, the SurgiVision® Consultants, Inc. WaveLight Investigator Group published the four-year follow up of the eyes treated in the US FDA Trial for Hyperopic LASIK with the WaveLight laser. In that article, we observed excellent long-term refractive stability based on keratometry readings in hyperopic treatments up to 4 D spheroequivalent. Because the primary mode of regression is epithelial remodeling, and because epithelial remodeling is induced by local curvature changes, Dan Reinstein has proposed that it is possible to design ablations that go higher than 4 D and not experience regression. He has done this to some extent with the Zeiss laser. LASIK hyperopic treatments above 4 D with the WaveLight, may experience refractive regression. With PRK, hyperopic treatments probably experience regression for nearly any treatment amount.

My recommendations are based on these practical considerations and are made in the SurgiVision® DataLink Forum for Alcon WaveLight surgeons (my recommendations for Visx users are very different). Dan’s excellent work notwithstanding, my recommendations for hyperopic LASIK with WaveLight laser are:

1. Limit treatments to 4 D spheroequivalent
2. Only treat patients who can accept the full cycloplegic amount
3. Avoid
   1. Anticipated postoperative K’s over 50 D
   2. Treatments in eyes with compromised tear films
   3. Treatments in eyes with > 1 mm angle kappa
4. Center ½ between the pupil center and the corneal apex

Going to higher treatments may not only result in regression, it makes the optical issues more pronounced. Any deviation from these guidelines are likely to induce problems, which are not “binary” or on-off, but vary by proportion. The potential issues may be managed with good informed consent, but are probably better avoided altogether.

This article is based on my experience and reflects my opinions. It is not meant to be a prescription or to suggest any standard of care. Surgeon preferences vary.

References

1. Reinstein DZ, Gobbe M, Archer TJ. Coaxially Sighted Corneal Light Reflex Versus Entrance Pupil Center Centration of Moderate to High Hyperopic Corneal Ablations in Eyes With Small and Large Angle Kappa. J Refract Surg. 2013;29(8):518-525.
3. Kezirian GM, Moore CR, Stonecipher KG; SurgiVision Consultants Inc WaveLight Investigator Group. Four-year postoperative results of the US ALLEGRETTO WAVE clinical trial for the treatment of hyperopia. J Refract Surg. 2008 Apr;24(4):S431-8.