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Polyfocal optics and its consequences for human vision

Poster Details

First Author: R.Chaloupka CZECH REPUBLIC

Co Author(s):    V. Stoy   J. Kopriva           

Abstract Details



Purpose:

The aim of this paper is to elucidate one of the vision science puzzles: polyfocal lens (having wide depth of focus due to controlled amount of negative spherical aberration) has inherently low Modulation Transfer Function (MTF) and, therefore, forms a relatively poor quality, low contrast image. And yet many young, healthy humans with excellent visual acuity have natural crystalline lens (NCL) that is polyfocal both due to its geometry and its refractive index gradient (GRIN). Eye and brain has apparently to convert the poor quality image on retina into a perception of image with good resolution and contrast sensitivity. This paper will show that excellent visual acuity and contrast sensitivity can be achieved even with a synthetic polyfocal IOL that is constructed to be an analogue of NCL. This bioanalogic IOL is made of an optically homogeneous hydrogel and its polyfocality is achieved strictly by geometry of its optical surfaces. The presented clinical, in vitro and ex vivo results on NCL and its synthetic analogue support our view that eye is well adjusted to interpret the blurred image formed on retina by the polyfocal optics regardless the method how the polyfocality was achieved (GRIN or external geometry of optical surfaces).

Setting:

1) Medicem Institute laboratories: Experimetal in vitro and ex vivo study of optical properties of natural crystalline lens and their comparison with corresponding optical properties of synthetic bioanalogic lenses with polyfocal optics of various profiles. 2) Czech Eye Clinics participating on National Registry: Analysis of clinical results obtained with bioanalogic polyfocal WIOL-CF IOLs and meta-analysis comparing clinical outcomes obtained with multifocal intraocular lenses and polyfocal WIOL-CF.

Methods:

1) Description of the NCL optical properties (MTF, GRIN, amount of spherical aberration) based on computer simulation using data from literature (information of NCL geometry via Scheimpflug camera, MRI and cadaver studies) and aberrometry. 2) Comparison of NCL optical properties with experimental data obtained from model cadaver lenses on optical bench and optical wave-front analyzer. 3) Measurement of optical properties of selected mono-, multi- and poly- focal IOLs, particularly synthetic polyfocal analogues of NCL, using optical bench and wave-front analyzer. 4) Meta-analysis: Comparison of clinical outcomes especially contrast sensitivity at photopic and mesopic conditions with multifocal intraocular lenses and polyfocal WIOL-CF.

Results:

In vitro measurements confirmed that synthetic polyfocal lenses have optical characteristics (particularly MTF as a measure of contrast transfer by an optical system and large negative spherical aberration) similar to optical properties of the natural crystalline lens, both obtained from the literature data and from the laboratory measurements. The MTF values for both natural and synthetic polyfocal lenses were distinctly lower than MTF for monofocal lenses. MTF values for polyfocal lenses were even lower than MTF values of multifocal lenses in their respective focal points, although higher than MTF outside their discrete focal points. These results are indicative of blurred, low-contrast retinal image and lead one to expect poor contrast sensitivity and poor visual acuity. The clinical data analysis provides an unexpected result that is contrary to the measured MTF values: patients with polyfocal WIOL-CF IOLs have excellent sensitivity at both photopic and mesopic conditions, better than with multifocal IOLs with higher MTF values and sometimes even higher than population norm for younger patients. All patients have excellent BCVA at all distances between near and far indicating that the expected higher-order aberrations can be compensated by a monofocal lens.

Conclusions:

The polyfocal lens can be regarded as a special case of multifocal lens possessing, instead of 2-3 foci, infinite number of foci within a large focal range due to high negative spherical aberration, and consequently presents wide depth of focus. From technical perspective, if we compare monofocal, multifocal and polyfocal IOLs using a standard camera detector such as CCD, we have to conclude that the satisfactory results can be achieved using monofocal lens only. The possibility to focus at different distances from far to near is, in case of polyfocal lens, redeemed by poor optical properties such as MTF (optical component of clinical contrast sensitivity). These theoretical expectations were confirmed by laboratory measurements of both synthetic and natural polyfocal lenses. However, clinical results suggest that the polyfocal IOLs perform very well in eye in the terms of contrast sensitivity and visual acuity in a wide range of distances. Contrary to our intuitive expectation, patients with WIOL-CF also have lower occurrence of disturbing optical phenomena than multifocal IOLs. These results are rather counterintuitive until we consider the similarity of WIOL-CF with natural human crystalline lens in its 'optical design' and optical parameters such as MTF. This apparent discrepancy between optical theory and clinical outcome can be explained exclusively by the retinal and neural image processing system adaptation to the polyfocal optics of NCL. Hence, the retinal and neural image processing system is able to cope with an infinite amount of images at different distances within a focal interval formed by artificial polyfocal IOL resembling in this regard the polyfocal NCL. It seems that the even synthetic polyfocal IOLs (WIOL-CF) are the most compatible with the retinal and neural image processing system needs. We suggested that bioanalogic approach attempting to mimic the NCL optics is a promising path to improving IOL performance. FINANCIAL INTEREST: One of more of the authors... is employed by a forNONEprofit company with an interest in the subject of the presentation

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