Corneal vertex centration a benefit for hyperopic LASIK

July 15, 2007

Hyperopic LASIK ablation centration on the entrance pupil center versus on the corneal vertex was compared in a prospective randomized study. The results favored the corneal vertex centration technique because it induced a more prolate profile and was associated with less induction of coma-like aberrations.

Key Points

Dr. Alio and colleagues at the Vissum Instituto Oftalmologico de Alicante, Spain, compared the two methods for hyperopic LASIK centration in a prospective, randomized clinical trial that enrolled 20 patients undergoing bilateral surgery for up to +6.0 D of cycloplegic spherical equivalent (SE). Analyses of outcomes at 3 months after surgery showed no significant differences in refractive, visual acuity, or predictability outcomes between the two groups. Measurement of corneal asphericity, however, showed that centration on the corneal vertex induced a more prolate profile that correlated with less induction of coma-like aberrations.

"When performing refractive surgery, centration on the visual axis represents the ideal. However, determination of the visual axis is not possible in clinical practice, and so it is necessary to use an alternate site that is easy to locate and near the visual axis," said Dr. Alio, chairman and medical director of the institute.

All of the treatments used an aspherical ablation design performed with custom ablation software (ORK-CAM, Schwind eye-tech-solutions). For the procedure centered on the corneal vertex, the coordinates were obtained from a topographer (Compagnia Strumenti Oftalmici) and were referenced to the mydriatic entrance pupil center.

Analysis of refractive data showed no significant differences between the vertex and pupil centration groups in mean preoperative SE (+2.92 D versus +2.26 D) or mean postoperative SE (–0.28 D versus +0.17 D). Mean uncorrected visual acuity postop was 0.70 in the vertex group and 0.78 in the pupil group, and those values also were not significantly different.

In the vertex group, the postoperative SE was within 1.00 D of target in 70% of eyes in the vertex group and 88.9% of eyes in the pupil group. The efficacy and safety ratios were 0.69 and 0.94, respectively, in the vertex group and 0.78 and 0.98, respectively, in the pupil group. None of those values was significantly different in comparisons of the two different centration groups.

Corneal asphericity (Q) was measured over an area of both 4.5 and 8.0 mm, and the results showed a significant tendency to prolateness (Q became more negative) after the hyperopic ablation with either centration technique. The profile, however, was more prolate in patients treated with the vertex centration technique compared with centration on the pupil entrance center, and the difference in Q values measured over 8.0 mm was statistically significant (vertex –1.02; pupil center –0.64).

Analyses of wavefront aberrations showed no significant change in spherical-like aberrations at 3 months after surgery compared with baseline in either group, and no significant difference was seen in postoperative total spherical-like aberration root mean square (RMS) between treatment groups.

In both study groups, total coma-like aberration RMS was increased after surgery. Although mean total RMS for coma-like aberrations was not significantly different between groups at 3 months after surgery, the increase from baseline was statistically significant only in the group that had the pupil-centered treatment.

Dr. Alio mentioned that in addition to the benefits demonstrated in this study for centration on the corneal vertex, other advantages of that technique are that the corneal vertex can be determined independent of corneal transparency and whether or not the epithelium is intact. In addition, it should be a precise reference for re-treatment procedures in addition to primary surgery.