[Current state of wavefront guided corneal surgery to correct refraction disorders]

[Corneal topography. Analysis of the corneal surface]

Various diagnostic procedures have been developed for analyzing the corneal surface. Corneal topographical measurements can be performed by classic Placido disc topographers and tomographers creating three-dimensional corneal models from cross-sectional images. Using a slit scanning system or the Scheimpflug technique additional information about the anterior segment can be obtained. New systems give us a wide range of diagnostic tools. Beside the measurement of the corneal anterior and posterior surfaces and point-to-point-pachymetry they enable analysis of corneal aberrations and measurement of the whole anterior segment, including the anterior chamber and the position of implants, pupil diameter, horizontal white-to-white and the kappa angle. Densitometry, statistical programs for the evaluation of corneal pathology and assistance with intraocular lens calculation following excimer laser treatment are further options available. This article is intended to give an overview of the principles and limitations of the various diagnostic systems.

Basic knowledge of refractive surgery: correction of refractive errors using modern surgical procedures

INTRODUCTION

Refractive ophthalmic surgery allows refractive errors to be corrected permanently in a safe, effective, and reliable way with few complications.

METHODS

Selective literature review with special reference to the guidelines of the German Commission for Refractive Surgery.

RESULTS

With a total of almost 18 million treatments performed to date, laser in-situ keratomileusis (LASIK) is the most commonly used refractive surgical procedure worldwide. Alternatives to LASIK include surface ablation procedures (PRK, LASEK, Epi-LASIK) and phakic intraocular lens implantation. If ocular accommodation is lost, removal of the crystalline lens and implantation of modern multifocal intraocular lenses (refractive lens exchange) provide an alternative means of correcting myopia, hyperopia and presbyopia.

DISCUSSION

The treatment effect is maximized and complications kept to a minimum if strict inclusion criteria are applied and a high technical standard maintained during the procedure.

[Application of wavefront analysis in clinical and scientific settings. From irregular astigmatism to aberrations of a higher order--Part II: examples]

In recent years, wavefront analysis has ceased to be purely a laboratory application and emerged as a method used in ophthalmological diagnosis. This development has been promoted mainly by the widespread use of wavefront-guided LASIK (laser in situ keratomileusis). However, aberrometry is still not a common diagnostic technique, and for many ophthalmologists interpretation of the results is difficult. The second part of this serial paper reviews findings that are relevant for the ophthalmological community and highlights current scientific applications in this area.

Topographic- and Wavefront-guided Customized Ablations With the NIDEK-EC5000CXII in LASIK for Myopia

PURPOSE

To assess refractive outcomes, changes in the total higher order root mean square (RMS) aberration, and changes in higher order wavefront aberrations after LASIK for myopia and myopic astigmatism with the NIDEK Advanced Vision Excimer Laser platform (NAVEX) using either an aspheric or topography-based or whole eye wavefront ablation algorithm.

METHODS

This was a retrospective study of 1459 eyes that underwent LASIK for myopia and myopic astigmatism. The mean preoperative spherical equivalent refraction was -4.68 diopters (D) (range: -0.50 to -9.63 D) with astigmatism up to -4.50 D. Treatments were classified into three categories depending on the type of ablation algorithm used--optimized aspheric transition zone (OATz) denoted eyes that underwent aspheric treatment zones; customized aspheric treatment zone (CATz) denoted eyes that underwent customized ablations based on corneal topography; and OPDCAT denoted eyes that underwent customized ablation based on the whole eye wavefront profile. Follow-up data are reported at 3 months (69%) and 12 months (17%) postoperatively.

RESULTS

Three months after LASIK, the predictability (10.5 D from target refraction) was 80% for OATz, 91% for CATz, and 76% for OPDCAT. Of all eyes, 96% were within +/- 1.0 D of intended refraction 3 months postoperatively and 100% after 12 months (87% +/- 0.5 D). In the aspheric and custom groups, a notable improvement of uncorrected visual acuity was noted between 3 and 12 months after LASIK. No eye lost >1 line of best spectacle-corrected visual acuity. Mean higher order RMS increased in all groups.

CONCLUSIONS

The data support that the treatment of myopia and myopic astigmatism is safe and effective with NAVEX. Customized ablation based on corneal topography rather than on total wavefront error was more predictable.

[Optical quality after refractive corneal surgery]

Correction of myopia, hyperopia and astigmatism within its indicated margin by means of refractive corneal surgical procedures such as LASIK and surface ablation (e.g. PRK) is one of the standard procedures in ophthalmology. Now that advances in the fields of surgical techniques and the technical devices employed have further progressed in terms of safety and predictability, research also focuses on optical quality. "Optical quality" is not a clearly defined parameter, but can be captured indirectly by means of directly measured data. One has to start with the anatomical properties of the eye, which determine the optical images on the retinal level. The quality of the retinal image influences the eye's function, i.e. acuity and contrast perception. Finally, there is the subjective perception of the image we receive. "Optical quality" as such is reflected by the patient's evaluation of this image perception. Three phenomena are especially responsible for deterioration of the quality of the retinal image: diffraction, aberrations and dispersion. Some of the methods for measuring optical quality are subjective questionnaires, functional testing procedures for measuring visual acuity and contrast sensitivity, optical measuring procedures for the determination of optical quality, as well as biomicroscopy, aberrometry and corneal topography for assessing anatomical changes.