Francisco Alba-Bueno

Energy Distribution between Distance and Near Images in Apodized Diffractive Multifocal Intraocular Lenses

PURPOSE To determine the energy distribution between the distance and near images formed in a model eye by spherical and aspheric apodized diffractive multifocal intraocular lenses (IOLs). METHODS The IOL was inserted in a model eye with an artificial cornea with positive spherical aberration (SA) similar to that of the human cornea. The energy of the distance and near images, as a function of the pupil size, was experimentally obtained by image analysis. The level of SA on the IOL, which is pupil-size-dependent, was determined from simulations. The influence of the SA was deduced from results obtained in monofocal IOLs and by comparison of the experimentally obtained energy efficiency to theoretical results based solely on the diffractive profile of the IOL. RESULTS In contrast with theoretical predictions, the energy efficiency of the distance image strongly decreased for large pupils, because of the high level of SA in the IOL. The decrease was smaller in the apodized diffractive multifocal lens with aspheric design. As for the near image, since the diffractive zone responsible for the formation of this image was the same in the spherical and aspheric lenses and the apertures involved were small (and so the level of SA), the results turned out to be similar for both designs. CONCLUSIONS For large pupils, the energy efficiency of the distance image is strongly affected by the level of SA, although aspheric IOLs perform slightly better than their counterparts with a spherical design. For small pupils, there are no differences between the spherical and aspheric IOLs.

Corneal shape changes induced by first and second generation silicone hydrogel contact lenses in daily wear

PURPOSE To compare the corneal topographical changes induced by two first and second generation silicone-hydrogel (SiH) contact lenses after 3 months of daily wear (DW). METHODS Prospective, consecutive case-series in which patients wore one of 3 different contact lenses (either the first generation SiH Focus Night & Day, the second generation SiH Acuvue Oasys or the monthly disposable Soflens 38 hydrogel lens as control group) on a DW basis for 3 months. Over-refraction, visual acuity, mean keratometry, corneal astigmatism, corneal eccentricity, superficial regularity and superficial asymmetry indices were monitored over the 3-month period. RESULTS Nineteen eyes of 10 patients completed the study. Seven Focus Night & Day, 7 Acuvue Oasys and 5 Soflens 38 contact lenses were fitted. There were no significant changes between any of the parameters measured at the 3-month visit in any of the SiH groups (non-parametric Wilcoxon test, p>0.05). However, the control group (Soflens 38) showed statistically significant changes regarding mean keratometry, corneal astigmatism and corneal eccentricity (p<0.05). Three patients wearing the first generation SiH showed adverse events of different degree related to their high modulus of elasticity. CONCLUSIONS After 3 months of DW, wearers of first and second generation SiH lenses showed greater corneal stability than hydrogel monthly disposable contact lenses users regarding commonly used topographic corneal shape indices. However, complications related to the mechanical properties of first generation SiH were seen in three patients in the follow-up time.

Halo and Through-Focus Performance of Four Diffractive Multifocal Intraocular Lenses

PURPOSE. To compare, as a function of pupil size, the through-focus performance and halo features of four diffractive intraocular lenses (IOLs). METHODS. Three diffractive bifocal IOLs (ReSTOR þ2.5 D SV25T0, Tecnis þ2.75 D ZKB00, and AT LISA þ3.75 D 809M) and a diffractive trifocal IOL (AT LISA tri þ3.33 D, þ1.66 D 839MP) were tested in vitro in a modified International Organization for Standardization eye model. The modulation transfer function (MTF) at the IOLs’ foci was obtained with pupils ranging from 2.0 to 5.0 mm. Through-focus MTF curves (at 50 cycles/mm) were compared among all the IOLs. The halo formation and characteristics were obtained from image analysis. RESULTS. The multifocal IOLs studied in this work showed, at their foci, secondary out-of-focus images, which originate halos and whose characteristics depend on the lens design and pupil size. The smallest halo occurred for the distance focus of the SV25T0. The distance and near foci of the SV25T0 yielded, respectively, the best and lowest optical quality among the studied IOLs. The distance focus of the ZKB00, AT LISA, and AT LISA tri were of similar quality, but the near focus of the ZKB00 outperformed the near foci of the rest of the IOLs. The IOLs’ optical performance gradually deteriorates as pupil increases. CONCLUSIONS. Differences in the design of the diffractive IOLs translate into differences in optical quality at their foci, through-focus performance, and halo features, which can offer further information to surgeons when selecting which IOL to implant.

Design of a Test Bench for Intraocular Lens Optical Characterization

The crystalline lens is the responsible for focusing at different distances (accommodation) in the human eye. This organ grows throughout life increasing in size and rigidity. Moreover, due this growth it loses transparency through life, and becomes gradually opacified causing what is known as cataracts. Cataract is the most common cause of visual loss in the world. At present, this visual loss is recoverable by surgery in which the opacified lens is destroyed (phacoemulsification) and replaced by the implantation of an intraocular lens (IOL). If the IOL implanted is mono-focal the patient loses its natural capacity of accommodation, and as a consequence they would depend on an external optic correction to focus at different distances. In order to avoid this dependency, multifocal IOLs designs have been developed. The multi-focality can be achieved by using either, a refractive surface with different radii of curvature (refractive IOLs) or incorporating a diffractive surface (diffractive IOLs). To analyze the optical quality of IOLs it is necessary to test them in an optical bench that agrees with the ISO119679-2 1999 standard (Ophthalmic implants. Intraocular lenses. Part 2. Optical Properties and Test Methods). In addition to analyze the IOLs according to the ISO standard, we have designed an optical bench that allows us to simulate the conditions of a real human eye. To do that, we will use artificial corneas with different amounts of optical aberrations and several illumination sources with different spectral distributions. Moreover, the design of the test bench includes the possibility of testing the IOLs under off-axis conditions as well as in the presence of decentration and/or tilt. Finally, the optical imaging quality of the IOLs is assessed by using common metrics like the Modulation Transfer Function (MTF), the Point Spread Function (PSF) and/or the Strehl ratio (SR), or via registration of the IOLs wavefront with a Hartmann-Shack sensor and its analysis through expansion in Zernike polynomials.

Stereo-Acuity in Patients Implanted with Multifocal Intraocular Lenses: Is the Choice of Stereotest Relevant?

Abstract Purpose: A randomized and double-blinded study design was implemented to assess the stereo-acuity in patients symmetrically implanted with four types of multifocal intraocular lenses (MIOLs), compared to a monofocal lens (control group). In addition, the influence of the type of test employed for the evaluation of stereo-acuity was explored. Materials and Methods: Six months after cataract intervention, stereo-acuity was measured with the Titmus and TNO stereotests in 143 patients implanted with one of the following MIOL lens types: hybrid spherical SN60D3, hybrid aspheric SN6AD1, diffractive aspheric ZMA00 and refractive spherical NXG1. A control group implanted with the monofocal aspheric ZA9003 (in which stereo-acuity was measured with a near addition) was also included in the study. Results: Statistically significant better stereo-acuity was found in the monofocal group with both stereotests (except for the SN60D3 group with the Titmus test) (all p < 0.001). No significant differences in stereo-acuity between MIOLs were found using the Titmus test. However, with the TNO, patients implanted with hybrid diffractive MIOLs exhibited statistically significant worse stereo-acuity than those with the refractive design (SN60D3, p < 0.001; SN6AD1, p = 0.006). Conclusions: Patients implanted with MIOLs have worse stereo-acuity than those implanted with monofocal IOLs due to the decrease in retinal image contrast originating in the simultaneous presence of two images. A wavelength-based stereotest such as the TNO induces large differences in image contrast between fellow eyes implanted with diffractive-based MIOLs, which may result in an underestimation of the real stereo-acuity of the patient.

New trends in intraocular lens imaging

As a result of modern technological advances, cataract surgery can be seen as not only a rehabilitative operation, but a customized procedure to compensate for important sources of image degradation in the visual system of a patient, such as defocus and some aberrations. With the development of new materials, instruments and surgical techniques in ophthalmology, great progress has been achieved in the imaging capability of a pseudophakic eye implanted with an intraocular lens (IOL). From the very beginning, optical design has played an essential role in this progress. New IOL designs need, on the one hand, theoretical eye models able to predict optical imaging performance and on the other hand, testing methods, verification through in vitro and in vivo measurements, and clinical validation. The implant of an IOL requires a precise biometry of the eye, a prior calculation from physiological data, and an accurate position inside the eye. Otherwise, the effects of IOL calculation errors or misplacements degrade the image very quickly. The incorporation of wavefront aberrometry into clinical ophthalmology practice has motivated new designs of IOLs to compensate for high order aberrations in some extent. Thus, for instance, IOLs with an aspheric design have the potential to improve optical performance and contrast sensitivity by reducing the positive spherical aberration of human cornea. Monofocal IOLs cause a complete loss of accommodation that requires further correction for either distance or near vision. Multifocal IOLs address this limitation using the principle of simultaneous vision. Some multifocal IOLs include a diffractive zone that covers the aperture in part or totally. Reduced image contrast and undesired visual phenomena, such as halos and glare, have been associated to the performance of multifocal IOLs. Based on a different principle, accommodating IOLs rely on the effort of the ciliary body to increase the effective power of the optical system of the eye in near vision. Finally, we present a theoretical approach that considers the modification of less conventional ocular parameters to compensate for possible refractive errors after the IOL implant.

Defocus correction in the optical system of the eye: unconventional degrees of freedom

Despite the interest in developing improved formulas for intraocular lens power calculation, there are several sources of uncertainty that may well give rise to a significant residual refractive error. Those concerning the estimation of the corneal power are reviewed. In addition, we explore the possibility of introducing changes in some unconventional parameters of the eye to compensate for defocus and illustrate their effectiveness in two cases: a natural eye and an eye that has undergone previous surgical actions (anterior refractive surgery and cataract surgery with an intraocular lens implant). The results show that changes in the refractive index, thickness, or posterior radius of the cornea have relatively little effect on the overall refractive error. However, small changes in the refractive indexes of the aqueous or the vitreous humors are highly effective, much more so than a similar amount of change in the anterior curvature of the cornea. This fact opens new and attractive possibilities to compensate for refractive error through the introduction of changes in degrees of freedom so far considered unconventional.

Optical Characterization of Intraocular Lenses

espanolLa caracterizacion optica de las lentes intraoculares (IOLs del ingles Intraocular Lenses) proporciona una informacion objetiva y cuantitativa que es necesaria para comprender su funcionamiento como implante que sustituye al cristalino en el sistema visual humano. Ademas, permite predecir el rendimiento de los nuevos disenos. Dicha caracterizacion se debe llevar a cabo mediante pruebas in vivo en pacientes ya implantados, asi como con pruebas in vitro en banco optico o mediante simulacion teorica. La tesis comienza con un analisis de las fuentes de error en el calculo de la potencia de las lentes intraoculares y explora la posible compensacion de los errores refractivos residuales mediante el uso de grados de libertad no convencionales. Se centra, fundamentalmente, en la caracterizacion in vitro de una variedad de lentes comercialmente disponibles (monofocales, multifocales, esfericas, asfericas, apodizadas, no-apodizadas, y distintos materiales, potencias y adiciones) en un banco optico. Para ello se han disenado y puesto a punto los metodos de medida y un montaje experimental que reproduzca las condiciones en las que las lentes se implantan en el ojo de acuerdo con la normativa internacional. Se ha desarrollado e implementado un metodo para la cuantificacion experimental de la eficiencia energetica de los distintos modelos de IOLs. Este metodo ha servido para explicar algunos resultados clinicos obtenidos al evaluar la vision estereoscopica con dos tests con diferente principio de funcionamiento. Se ha medido la calidad optica basicamente a traves de la Funcion de Transferencia de Modulacion. Se ha desarrollado e implementado un metodo para caracterizar el halo que perciben algunos pacientes implantados con IOLs multifocales. Finalmente, los resultados experimentales se han utilizado para comparar el rendimiento de las IOLs entre ellas. EnglishThe optical characterization of an intraocular lens (IOL) provides objective and quantitative information that is essential to fully understand its performance as an implant that replaces the crystalline lens in the human visual system. Additionally, it can be used to predict the performance of the new IOL designs. This characterization has to be carried out using in vivo tests in implanted patients and in vitro, by means of an optical bench or theoretical simulations. This thesis begins with an analysis of the main sources of uncertainty in the calculation of the IOL power and explores the compensation of the residual refractive errors by using some unconventional degrees of freedom. The thesis focuses on the in vitro characterization of a variety of commercially available IOLs (monofocal, multifocal, spherical, aspherical, apodized, full-aperture, and of different materials, powers and additions) in optical bench. To this end, we have designed and implemented the necessary methods of measurement and an experimental setup that, according to the international standard regulation, reproduces the conditions of such implants in the human eye. We have developed a method to measure the energy efficiency of IOLs. This method has allowed us to explain the clinical results obtained in the evaluation of the stereoscopic acuity when using two tests based on different principles. The optical imaging quality of IOLs has been basically quantified through the experimental measurement of the modulation transfer function. We have developed and implemented a method to characterize some artefact, named halo, which can be perceived by those patients implanted with multifocal IOLs. Finally, all the experimental results have been used as a basis for the inter-class comparison of the IOL performances

Halo and Through-Focus Performance of Four Diffractive Multifocal Intraocular LensesHalo and Through-Focus of Four Diffractive MIOLs

PURPOSE. To compare, as a function of pupil size, the through-focus performance and halo features of four diffractive intraocular lenses (IOLs). METHODS. Three diffractive bifocal IOLs (ReSTOR þ2.5 D SV25T0, Tecnis þ2.75 D ZKB00, and AT LISA þ3.75 D 809M) and a diffractive trifocal IOL (AT LISA tri þ3.33 D, þ1.66 D 839MP) were tested in vitro in a modified International Organization for Standardization eye model. The modulation transfer function (MTF) at the IOLs’ foci was obtained with pupils ranging from 2.0 to 5.0 mm. Through-focus MTF curves (at 50 cycles/mm) were compared among all the IOLs. The halo formation and characteristics were obtained from image analysis. RESULTS. The multifocal IOLs studied in this work showed, at their foci, secondary out-of-focus images, which originate halos and whose characteristics depend on the lens design and pupil size. The smallest halo occurred for the distance focus of the SV25T0. The distance and near foci of the SV25T0 yielded, respectively, the best and lowest optical quality among the studied IOLs. The distance focus of the ZKB00, AT LISA, and AT LISA tri were of similar quality, but the near focus of the ZKB00 outperformed the near foci of the rest of the IOLs. The IOLs’ optical performance gradually deteriorates as pupil increases. CONCLUSIONS. Differences in the design of the diffractive IOLs translate into differences in optical quality at their foci, through-focus performance, and halo features, which can offer further information to surgeons when selecting which IOL to implant.

Experiment design for through-focus testing of intraocular lenses

Eye models to test intraocular lenses (IOLs) in an optical bench are commonly designed in agreement with the ISO 11979-2 and 11979-9 standard requirements. However, modifications to the ISO eye model have been proposed to test IOLs in conditions closer to real human eye. Wavefront analysis and aberration characterization, wavelength dependence, efficiency, off-axis performance and imaging degradation under certain amount of misalignment can thus be measured in vitro. The main parts of the system to test IOLs are: the illumination system and object test, the eye model including the IOL immersed in a wet cell and a microscope assembled to a sensor that magnifies and captures the aerial image of the object formed by the eye model. A problem concerning the simultaneous variation of defocus and magnification arises when using the microscope to capture out-of-focus images in a through-focus study. Using the eye model, we study the problem of implementing a through-focus measurement of the imaging quality of an IOL. We find a solution based on geometrical optics and compare it with other proposals reported in the literature. The effects on the measurement of the Modulation Transfer Function and the Point Spread Function are predicted. Experimental results are obtained and discussed.