Estimation of the mechanical properties of the eye through the study of its vibrational modes.

Measuring the eye's mechanical properties in vivo and with minimally invasive techniques can be the key for individualized solutions to a number of eye pathologies. The development of such techniques largely relies on a computational modelling of the eyeball and, it optimally requires the synergic interplay between experimentation and numerical simulation. In Astrophysics and Geophysics the remote measurement of structural properties of the systems of their realm is performed on the basis of (helio-)seismic techniques. As a biomechanical system, the eyeball possesses normal vibrational modes encompassing rich information about its structure and mechanical properties. However, the integral analysis of the eyeball vibrational modes has not been performed yet. Here we develop a new finite difference method to compute both the spheroidal and, specially, the toroidal eigenfrequencies of the human eye. Using this numerical model, we show that the vibrational eigenfrequencies of the human eye fall in the interval 100 Hz-10 MHz. We find that compressible vibrational modes may release a trace on high frequency changes of the intraocular pressure, while incompressible normal modes could be registered analyzing the scattering pattern that the motions of the vitreous humour leave on the retina. Existing contact lenses with embebed devices operating at high sampling frequency could be used to register the microfluctuations of the eyeball shape we obtain. We advance that an inverse problem to obtain the mechanical properties of a given eye (e.g., Young's modulus, Poisson ratio) measuring its normal frequencies is doable. These measurements can be done using non-invasive techniques, opening very interesting perspectives to estimate the mechanical properties of eyes in vivo. Future research might relate various ocular pathologies with anomalies in measured vibrational frequencies of the eye.

Optical power distribution of refractive and aspheric multifocal contact lenses: Effect of pupil size.

PURPOSE: To evaluate the power profile within the optic zone of different designs of multifocal contact lenses (CLs) and to analyze how the effect of pupil size could impact on their optical performance. METHODS: The optical power distribution within the optic zones of multifocal CLs was measured by the Nimo TR1504 (LAMBDA-X, Belgium). The multifocal CLs under study were the Acuvue Bifocal, the Acuvue Oasys for Presbyopia, the PureVision Multifocal and the PureVision 2 for Presbyopia. Each design was considered in all their available addition powers. All lenses had a nominal power of -3.0D. At the same time, three lenses of each model were considered and five consecutive readings of each lens were performed. RESULTS: The results show that the PureVision Multifocal and the PureVision 2 for Presbyopia have aspheric power profiles. Both designs showed aspheric center-near designs with a smoother progression of the optical power in the PureVision 2 for Presbyopia. The Acuvue Bifocal and the Acuvue Oasys for Presbyopia are shown to have concentric alternating near and far zones. Apart from the refractive rings, the Acuvue Oasys for Presbyopia showed an increase in negative (or less positive) values toward the periphery of the lens. CONCLUSIONS: Besides the refraction, the knowledge of the power profiles of multifocal CLs and the effect of pupil size on the optical distribution of these lenses could be crucial to understand the performance of these designs when they are fitted.

In vitro optical performance of a new aberration-free intraocular lens.

PURPOSE: To assess the optical performance of the new EnVista intraocular lens (IOL). MATERIALS AND METHODS: Four aspheric IOLs were evaluated; the new EnVista is one amoung them. This IOL, similarly to the Z-Flex HB and the Bi-Flex 1.8 667AB ones, has a neutral aspheric design, whereas the fourth IOL under test (AcrySof IQ IOL SN60WF) presents a negative spherical aberration (SA). The IOL's aberration patterns were measured in vitro, by setting them up on an optical bench. From these aberration-pattern data, the modulation transfer function (MTF), the average modulation values, and the points spread function (PSF) were calculated. Furthermore, in order to assess the potential optical quality that these IOLs would yield once they are implanted, an average corneal-aberration pattern was juxtaposed to the in-vitro profiles and the same parameters were calculated again. RESULTS: For the IOL-only scenario (ie, without including the corneal factor), it was the EnVista IOL, which is aberration-free that showed the higher MTF, PSF values. This was followed by the other two aberration-free IOL models. However, when the effect of an average corneal pattern was also taken into consideration, the AcrySof IQ IOL SN60WF always outperformed the other neutral-asphericity IOLs. CONCLUSIONS: The in-vitro optical performance of the EnVista IOL was good, but it decreases substantially in a whole-eye scenario, when the wavefront profile of an average cornea is added. Other designs with different degrees of SA should be considered for this IOL in order to surpass these results.

The posterior chamber phakic refractive lens (PRL): a review.

Implantation of phakic intraocular lenses (pIOLs) is a reversible refractive procedure, preserving the patient's accommodative function with minimal induction of higher order aberrations compared with corneal photoablative procedures. Despite this, as an intraocular procedure, it has potential risks such as cataracts, chronic uveitis, pupil ovalization, corneal endothelial cell loss, pigmentary dispersion syndrome, pupillary block glaucoma, astigmatism, or endophthalmitis. Currently, only two models of posterior chamber pIOLs are commercially available, the implantable collammer lens (STAAR Surgical Co.) and the phakic refractive lens (PRL; Zeiss Meditec). The number of published reports on the latter is very low, and some concerns still remain about its long-term safety. The present article reviews the published literature on the outcomes after PRL implantation in order to provide a general overview and evaluate its real potential as a surgical refractive option.

Contrast sensitivity function in children: normalized notation for the assessment and diagnosis of diseases.

The purpose of this article is to describe a normalized notation for the assessment of the contrast sensitivity in children. This notation is obtained dividing the log contrast sensitivity value found in a patient by the corresponding normative contrast sensitivity data. The ratio obtained describes the contrast sensitivity of a patient facilitating its understanding, assessment and consequently effective communications. This article shows the normalized notation developed for children aged from 3 to 7 years old. An evaluation of the contrast sensitivity in healthy and amblyopic patients has been shown in order to explain the procedure to follow. The use of normalized notation in clinical procedures will provide to the clinician a better understanding of the results and the changes over time as well by comparison in the assessment of an ocular disease.

Distribution of refractive errors in Spain.

The aim of this work was to characterise the refractive state of a Spanish population. Special attention to myopia prevalence was carried out. At this stage 7621 subjects, 3692 males and 3929 females, between 3 and 93 years were examined by means of static retinoscopy and subjective refraction. The most prevalent was emmetropia with a mean percentage of 43.2% and ranging from 60.6% (3-8 years) to 29.0% (66-93 years). The prevalence of hyperopia (35.6%) increased with age, from 36.9% (3-8 years) to 55.8% (66-93 years). Myopia appeared to be more prevalent in the 30-35 age group (30.1%), agreeing with the more near-work demanding age range. Mean refractive error obtained was +0.19 +/- 3.42 D, ranging from +0.05 +/- 3.18 D (20-35 years) to +0.96 +/- 2.42 (66-93 years). Reading addition value obtained was similar to respective values of age of other populations. The whole Spanish population evaluated was emmetrop. No differences in prevalence from other populations were found, obtaining greater values of myopia in groups with more near work and a greater percentage of hyperopia versus age.