THE ABLATED CORNEA

A growing understanding of the anatomical and optical changes induced by corneal ablation is leading to treatment refinements that may further improve visual outcomes in refractive ablation procedures and postablation implantation of the intraocular lens (IOL), Douglas D Koch MD told the XXXI Congress of the ESCRS in the Binkhorst Medal Lecture 2013. He acknowledged his co-investigators Li Wang MD, PhD, Mitchell P Weikert MD and Bruna Ventura MD. Improved measurements of the posterior corneal surface, post-ablation epithelial thickness and anterior topography can help plan and monitor corneal changes in patients undergoing ablation procedures. This could also allow predictable manipulation of some aberrations to optimise depth of focus while minimising visually disturbing aberrations, said Dr Koch, Allen, Mosbacher and Law Chair in Ophthalmology at Baylor College of Medicine, Houston, US.

For post-refractive surgery patients undergoing cataract procedures, better measurements will help improve calculation of IOL power, added Dr Koch, who is also editor emeritus of the Journal of Cataract and Refractive Surgery.

Anatomical changes

Corneal ablation achieves its intended refractive effect by removing anterior stromal tissue. But it also stimulates remodelling of the corneal epithelial profile, producing unintended refractive changes, Dr Koch said. Recent optical coherence tomography studies confirm high-frequency ultrasound studies that show that central epithelial thickness progressively increases for about three months following myopic LASIK, adding up to 13 microns at the central cornea, resulting in a myopic shift of about -0.38 D (see figure below). Conversely, hyperopic LASIK results in central epithelial thinning of up to 8.0 microns with annular thickening of up to 24 microns, Dr Koch noted (Reinstein DZ et al. J Refract Surg, 2012;28(3):195-201. Ma XJ, Wang L and Koch DD. Cornea, in press).

Ablation changes corneal biomechanical properties, producing central flattening, peripheral corneal thickening, and an inward shift of the posterior cornea that can appear to be central steepening. This tends to regress during the first year, (Grzybowski DM et al. J Cataract Refract Surg. 2005;31:72-81. Smadja D et al. J Cataract Refract Surg. 2012;38:1222-1231). Dr Koch’s OCT data showed a minimal curvature change in the central 3.0mm zone with a mean power of -0.02 D. While these changes are small, monitoring and modulating them could improve refractive predictability and stability, and increase the accuracy and effectiveness of retreatment, Dr Koch said.

Altering optics

Altering anterior corneal curvature also changes the size of the effective optical zone. Based on anterior corneal refractive maps, standard LASIK ablations reduce this zone by about 4.9mm2, while wavefront-guided ablations actually increase it by about 3.9mm2 (Racine L Et Al. Am J Ophthalmol. 2006;142:227-232). But when higher order aberrations and the entire optical system are accounted for, only 65 per cent of patients maintain wavefront quality equivalent to 20/20 across a 6.0mm zone after ablation, compared with 99 per cent before, though nearly 100 per cent maintain zones of 4.0mm, Dr Koch said. “Some reduction is inevitable due to transition to unablated tissue. We need to better understand correlation with visual function and what is the minimal zone needed to prevent haloes and other unintended visual issues.”

Changing corneal curvature also induces higher order aberrations, with spherical aberration being the most significant. With myopic PRK, corneal SA rises from about +0.27 microns in the normal eye to a mean of about +0.47, Dr Koch said. This creates defocus but does not appear to reduce contrast sensitivity. On the upside, it increases depth of focus (Yeu E et al. Am J Ophthalmol. 2012;153:972-981).

IOL selection

IOL selection is affected in at least three ways:
1. Changing corneal spherical aberration complicates IOL selection for post-ablation cataract surgery patients, Dr Koch said. To achieve maximum polychromatic modulation transfer function performance, the amount of SA in the lens should be matched to the corneal asphericity of the patient. For those corneas with high amounts of positive SA (> 0.4 microns), this is not feasible and may not be desirable due to high sensitivity to decentration. Also, the optimal postoperative SA will depend on the refractive error. For 0.0 defocus, optical quality is optimal with no SA, whereas with -0.5 D of myopia, total SA of +0.2 microns provides sharpest uncorrected vision.

2. As all cataract surgeons have now found, post-ablation anterior corneal curvature changes have a major impact on selecting the spherical power for IOLs, Dr Koch noted. Current formulas (such as those on the ASCRS postrefractive surgery IOL calculator) are based on regression. They are at best highly educated guesses. Ideally, surgeons will apply one or more of these three options: 1) accurately measure total power preoperatively, 2) calculate IOL power intraoperatively, and 3) adjust IOL power postoperatively. Using fourier-domain OCT measurements of posterior and anterior cornea and a new formula, IOL power prediction matches the best available conventional formulae (see figure above). Dr Koch expects its accuracy will improve with further development.

3. Measurement is also critical for astigmatic correction and particularly selecting toric IOLs in post-ablation patients, Dr Koch said. In normal eyes, anterior curvature suggests the direction of the cylinder on the posterior corneal surface, allowing use of a nomogram to select toric lens power. Patients displaying with-the-rule astigmatism on the anterior surface are overcorrected by a mean of 0.5 D while those displaying against-the-rule anteriorly are undercorrected by 0.3 D. But this correlation is lost in post-ablation patients, requiring examination anterior and posterior corneal curvature to determine the power and axis of toric lenses.

Dr Koch is confident that these issues will be worked out. “We are making great progress and I am optimistic about where we are going and how we are improving what we do for patients. The field has a tremendous future ahead.”