Marco A. Rosales

Advanced Surface Ablation for Presbyopia Using the Nidek EC-5000 Laser

PURPOSE To present 1 to 6-month follow-up results of laser in situ keratomileusis (LASIK) using multizone presbyopic advanced surface ablation (PASA) with a peripheral near zone. METHODS LASIK was performed on 28 eyes of 17 patients (10 men and 7 women; mean age 49.8 years with a range of 37 to 62 years). Eyes had primary or enhancement treatments with the Nidek EC-5000 excimer laser. Three techniques were used: 1) total transepithelial ablation, 2) surface ablation for far vision ametropia correction, and 3) concentric peripheral near zone presbyopia correction (technique developed by Dr. A. Telandro with a modified nomogram by Dr. R. Cantú for surface ablation). One surgeon (RC) performed all surgery. RESULTS We present the preoperative and postoperative measurements for far and near uncorrected visual acuity, total high order aberrations, spherical aberration (Z-12), asphericity Q index, eccentricity corneal shape factor, and total coma and trefoil aberrations. Increases occurred in negative spherical aberration, negative asphericity index, and positive eccentricity corneal shape factor. CONCLUSIONS Advanced surface ablation for presbyopia with a concentric peripheral near zone is a promising approach for surgical correction of presbyopia and potentially could be used with any advanced surface ablation procedure. Increases in negative spherical aberration and asphericity/ eccentricity indices seemed to increase the depth of focus of the eye, improving the near vision.

Objective quality of vision in presbyopic and non-presbyopic patients after pseudoaccommodative advanced surface ablation.

PURPOSE To analyze the objective quality of vision at 6 months postoperatively after pseudoaccommodative (presbyopic) advanced surface ablation (PASA). METHODS The study comprised 62 eyes of 35 patients with 6-month follow-up that underwent primary or secondary treatments using PASA. Pre- and postoperative results of distance and near uncorrected visual acuity (UCVA), spherical aberration (coefficient of the Z12 Zernike polynomial), and the asphericity (Q) index were reviewed. The corresponding wavefront maps (total, low, and high order aberrations) and the corresponding point spread function and modulation transfer function (MTF) were also calculated. RESULTS Our results show that PASA improves distance and near mean UCVA, increases negative spherical aberration and negative asphericity index, and improves the corresponding MTF. CONCLUSIONS Pseudoaccommodative advanced surface ablation is a promising approach for the surgical correction of presbyopia with distance refractive error (myopia and hyperopia with or without astigmatism). This PASA technique could theoretically be used in non-presbyopic patients with refractive error or post cataract patients with monofocal intraocular lenses. The increase in negative spherical aberration and asphericity/eccentricity index seems to increase the depth of focus of the eye, improving the near vision and compensating the age-related lens changes. Rather than creating a multifocal cornea, PASA appears to create an improved aspheric (prolate) ablation profile.

Anterior corneal profile with variable asphericity.

We present a corneal profile in which the eccentricity, e(Q=-e(2)), has a nonlinear continuous variation from the center outwards. This nonlinear variation is intended to fit and reproduce our current experimental data in which the anterior corneal surface of the human eye exhibits different values of e at different diameters. According to our clinical data, the variation is similar to an exponential decay. We propose a linear combination of two exponential functions to describe the variation of e. We then calculate the corneal sagittal height by substituting e in the first-order aspherical surface equation to obtain the corneal profile. This corneal profile will be used as a reference to analyze the resultant profiles of the customized corneal ablation in refractive surgery.

PSF and MTF comparison of two different surface ablation techniques for laser visual correction

It is well known that the Zernike expansion of the wavefront aberrations has been extensively used to evaluate the performance of image forming optical systems. Recently, these techniques were adopted in the field of Ophthalmology to evaluate the objective performance of the human ocular system. We have been working in the characterization and evaluation of the performance of normal human eyes; i.e., eyes which do not require any refractive correction (20/20 visual acuity). These data provide us a reference model to analyze Pre- and Post-Operated results from eyes that have been subjected to laser refractive surgery. Two different ablation techniques are analyzed in this work. These techniques were designed to correct the typical refractive errors known as myopia, hyperopia, and presbyopia. When applied to the corneal surface, these techniques provide a focal shift and, in principle, an improvement of the visual performance. These features can be suitably described in terms of the PSF and MTF of the corresponding Pre- and Post-Operated wavefront aberrations. We show the preliminary results of our comparison.

A new model for the anterior corneal surface using higher-order aspheric surfaces and variable eccentricity

Contact lenses manufacturers and ophthalmologists who perform laser surgery to correct visual problems depend on an accurate model of the anterior corneal surface. Several models have been suggested in the past, going from Gullstrands initial idea to the aspheric profiles and to Bonnets profile based on anatomical data. Clinical evidence shows, however, that the anterior corneal surface is characterized by a variable eccentricity, contrary to the hypothesis on which the current models are based. We present, in this work, a new model for the anterior corneal surface in terms of higher-order aspheric surfaces in which the eccentricity at a given point of the cornea is a continuous function of its distance to the optical axis. We also establish the conditions under which the different conic and Bonnets profiles are recovered from our model. Finally, we present our preliminary results using this model.

The inversion potential for NH3 and its isotopic species

Usingab initio data of the potential surface for the ground state of NH3 a function is obtained to represent, with a very high precision, the potential along the inversion coordinate. The inversion spectra of NH3, ND3, NH2D and NHD2 are found by numerically solving the Schrödinger equation for this potential. Comparison is made of the calculated inversion frequencies and the experimental values, and the molecular constants of NH3 are also compared to those found in the literature.

PSF and MTF analysis of the visual performance in undilated Mexican normal virgin whole eyes (UCVA greater than or equal to 20/20, 20/30, and 20/40)

ABSTRACT In the area of ophthalmic refractive surgery research, the Zernike expansion of the Wavefront aberrations has been the key factor in the measurement, representation and evaluation of the human eye aberrations in many different clinical situations. The Wavefront Aberrations described by the Zernike expansion can be translated as the polynomial expansion of the exit pupil of an optical system that represents the total Wavefront aberrations (corneal + internal elements) of the eye. In this direction, we can use this exit pupil to calculate both the incoherent Point Spread Function (PSF) and the Optical Transfer Function (OTF) of this system. With either of these two functions (PSF or OTF), we can easily calculate the output image of a certain object. This information can be used to evaluate the visual performance of the eyes with a set of pre-determined objects and their corresponding images. We apply these results to characterize the behavior of the aberrations of human eyes that in principle do not need any type of refract ive compensation or correction, in order to have a reference which may be ethnical-dependent. We are interested in describing the set of aberration characterizing the Normal Mexican Eye (NME). To achieve our goal, we start our study with a group of Mexican Males with an Uncorrected Visual Acuity (UCVA) of 20/20, 20/30, and 20/40. We present the preliminary results of our characterization. Keyword : Ocular wavefront aberrations, Visual performance of human eye, Zernike Polynomials, PSF, MTF.

Comparison between two different methods to obtain the wavefront aberration function

The analysis and measurement of the wavefront aberration function are very important tools that allow us to evaluate the performance of any specified optical system. This technology has been adopted in visual optics for the analysis of optical aberrations in the human eye, before and after being subjected to laser refractive surgery. We have been working in the characterization and evaluation of the objective performance of human eyes that have been subjected to two different surface ablation techniques known as ASA and PASA1. However, optical aberrations in the human eye are time-dependent2 and, hence, difficult to analyze. In order to obtain a static profile from the post-operatory wavefront aberration function we applied these ablation techniques directly over hard contact lenses. In this work we show the comparison between two different methods to obtain the wavefront aberration function from a reference refractive surface, in order to generalize this method and being able to fully characterize hard contact lenses which have been subjected to different ablation techniques typically used in refractive surgery for vision correction. For the first method we used a Shack-Hartmann wavefront sensor, and in the second method we used a Mach-Zehnder type interferometer. We show the preliminary results of this characterization.

Refractive power maps of the anterior surface of the cornea according to different models

In order to explore and analyze the effect of an ablation performed on the anterior corneal surface, it is useful to calculate the refractive power maps of the original and the treated corneas. The optical characteristics of the anterior corneal surfaces are typically simulated with different models, according to different degrees of simplification. To predict which ablation would improve the refractive power of such cornea, which is directly related with the spherical aberration associated with the shape of the anterior corneal surface, it is important to analyze those simplifications. Such information is displayed in a refractive power map, which yields the true refractive power of the corneal surface, point by point, expressing this power in diopters. The aim of the present work is twofold: different corneal models are simulated so as to compare the spherical aberration produced by each one. On the other hand, simulations are made in such a way that permits to foresee how the visual performance of an eye can be achieved by modifying the anterior surface of its cornea through the corresponding power maps.

Phase retrieval from a single interferometric pattern to determine the profile caused by laser ablation on spherical surfaces

We have been working in the interferometric analysis of the ablation profile obtained with different techniques of refractive surgery, applied directly on hard contact lenses. We have demonstrated qualitatively that different ablations produce different fringe patterns; implying different focal shifts1. These results were obtained by means of a Mach- Zehnder type interferometer, where we used a similar unablated contact lens as a reference. Due to the size of each sample, it is difficult to get different fringe patterns with different phase factors. Therefore, the typical phase shifting methods are not suitable in our case. To determine the corresponding profile caused by the different ablation techniques we applied in this work the interpolation method that provide an analysis of static fringe patterns. This method of phase retrieval allows us to obtain the PSF and MTF related to each profile. The advantage of this procedure is that we can obtain a time invariant performance of the resulting ablated surface.