Choosing Intra ocular Lens -2: Optics
Continuing our series on choosing the right Intra Ocular Lens we will discuss on the OPTICS of an IOL with special focus on aspheric IOLs. Remember, in the first part of this series (https://www.quickguide.org/post/choosing-iol-for-cataract-surgery-material-part-1) we had noted that intra ocular lens (IOL) features can largely be categorized into material, design, optics and delivery. In that article, 'Choosing right intra ocular lens for cataract surgery - material ' we had in detail talked about the material aspects of an IOL. Like material, the optics of an IOL play no less an important role, particularly when today's patients do not just look forward to vision restoration, but also the quality of the vision delivered. Accordingly, a wide variety of IOLs have an array of features which is often claimed to give a better image quality to the patient. When it comes to optics, one important aspect that has been particularly important and has been considered as a 'must - have' feature in IOLs is the benefit of spherical aberration correction of cornea.
What is spherical aberration?
Spherical aberration is the phenomenon of marginal rays of light that pass through a lens falling not at the same focal point as the paraxial rays of light. When marginal rays of light passing through a lens are over refracted and fall before the point where paraxial rays of light come to focus, we call it as positive spherical aberration (fig1). Likewise, if marginal rays of light that pass through a lens are under refracted and fall beyond the point where paraxial rays of light come to focus, we term it as negative spherical aberration.
The cornea of the human eye is largely prolate in shape. This has been discussed in the article https://www.quickguide.org/post/spherical-aberration-corneal-asphericity-qfactor
where it has been explained that prolate shape refers to the cornea being steep in the middle and flatter in the periphery. On the other hand an oblate corneal shape would refer to one where the cornea is flatter in the centre and steeper in the periphery. Generally, a prolate shape of a lens, where the centre is steeper than the periphery, would lead to under refraction of the marginal rays, thus leading to negative spherical aberration. This is seen in hyper prolate cornea ( specailly in keratoconus patients ) leading to negative spherical aberration.
The average cornea though prolate in shape, has some amount of positive spherical aberration. This is defined by the term asphericity or Q factor of the cornea. A Q factor of -.50 would mean the cornea is like a parabola and thus have neither positive nor negative sphercial aberration. But the average cornea may have a Q factor of around -.26 approximately, giving rise to a small amount of postive spherical aberration of around +.28 microns.
During young age the natural crystalline lens compensates for the positive spherical aberration of the cornea, as the natural lens has negative spherical aberration (fig2). Over decades, as one ages, the natural crystalline lens becomes more positive and adds to the cornea spherical aberration. Thus total ocular spherical aberration becomes more positive as one lands up for cataract surgery at an advanced age.
Spherical aberration and non aspheric (spherical) IOLs -
Implanting a spherical IOL like the Alcon SP (SA60AT), Johnson & Johnson Sensar MP or the HOYA 150/151 IOLs will not negate the spherical aberration of the cornea. The spherical lenses have positive spherical aberration and will add to the positive spherical aberration of the cornea. In figure 2a you will see how different IOLs with different refractive index and dioptric power have an effect on spherical aberration. A 15.0 diopter IOL will induce less spherical aberration compared to a 25.0 diopter IOL with a given refractive index. An IOL with lower refractive index will induce more spherical aberration compared to an IOL with higher refractive index.
Spherical aberration and aspheric IOLs -
There are a number of IOLs that are aspheric. However it is important to classify two groups of such aspheric IOLs - one that are negative spherical aberration IOLs and thus negate the positive spherical aberration of lens, and the other group of lenses which are zero (spherical) aberration lenses and neither adds nor negates corneal aberration. Alcon AcrySof IQ (SN60WF), Tecnis (ZCB00) and Hoya Vivinex or 250,251 IOLs are lenses that incorporate negative spherical aberration in their optics thus negating the corneal spherical aberration of the cornea. Among theses lenses, the AcrySof IQ, Vivinex XY1 and 250 and 251 do not totally negate the entire positive corneal spherical aberration and leaves a little amount of residual positive spherical aberration in the eye. This is arguably, to provide not only a good image quality ( by negating positive aberration of cornea ) but also providing an acceptable depth of field ( by leaving a small amount of residual spherical aberration ). Thus both Alcon and Hoya aspheric lenses leave the patient with around .1 micron of residual positive spherical aberration for a patient who would have a corneal spherical aberration of around .28 microns as these lenses typically have a negative spherical aberration of around -.17 micron to -.20 micron. The Tecnis on the other hand negate the entire positive spherical aberration of the cornea to provide for a better image quality for the average patient who would have around .28 micron of positive spherical aberration.
Among the other group of lenses, the B&L Advanced Optics lenses like Akreos AO, enVista (MX60) or Zeiss AT LARA 929M/MP are zero spherical aberration lenses, that is, they are truly an aspheric lens. If you were to examine such lenses in air, all paraxial and marginal rays of light would come to focus at one single focal point. Thus when such lenses are placed inside the eye, they neither add nor negate the positive spherical aberration of the cornea. Such lenses would provide a very good depth of field for the patient with an average positive corneal spherical aberration, but may leave the eye with spherical aberration.
It is generally argued, that .5 mm or more decentration of negative spherical aberrations in the eye may lead to other higher order aberrations, like Coma. Pablo Merino and Susana Marcos in their study analyzed the effect of decentration on retinal image quality with negative spherical aberration lenses and found the image quality degradation with such decentration. A truly aspheric IOL may be beneficial in the sense that lens decentration or tilt may not lead to coma** and degrade image quality. On the other hand, it may also be argued that most hydrophobic acrylic C-loop lenses are found to be stable inside the eye, and incidence of decentration beyond .5 mm may be rare. The hydrophillic IOLs with a lower overall diameter***, run the risk of decentration and therefore aberration nerutral optics may be helpful for such lenses.
Spherical aberration of cornea and choice of IOLs -
If you have an option to measure the spherical aberration of the cornea with aberrometry like iTrace, or your corneal topography machine gives you the amount of corneal spherical aberration, then the following decision(fig3) tree may help you in the choice of lenses. Keep in mind, it is important to see spherical aberration in the central 6.0 mm of the cornea only, and not of the whole eye.
Chromatic aberration and its effect on eye-
Another area of focus in IOL optics is the issue of chromatic aberration and possible chromatic aberration correction in IOL optics. What is chromatic aberration ? Chromatic aberration unlike other aberrations like spherical, coma, trefoil, etc, is not a monochromatic aberration. Monochromatic aberrations happen even with a single wavelength of light, that is say a green light of 550 nm or blue light of 450 nm, passes through the lens. However, chromatic
aberration, is caused when white light of many different wavelengths pass through the lens, causing the blue light with higher frequency and refractive index to fall befere the green light, and green light with relatively higher frequency and refractive index fall before the red light(Fig4).
The deleterious effect of chromatic aberration is seen in pictures taken by camera wherein the chromatic aberration has not been corrected. In such pictures you may notice colur fringing or colour dispersion. This colour fringing is noticed in the ages of the photograph.
In the eye, chromatic aberration occurs as lens acts as a prism, causing light to bend or refract. As white light passes through the lens wavelengths of light with different frequency and refractive index refract differently, giving rise to dispersion of light. also called chromatic aberration. In optics the effect of chromatic aberration can be minimized by using an achromatic lens or achromat, in which materials with differing dispersion are assembled together to form a compound lens. The most common type is an achromatic doublet, with elements made of crown and flint glass.
What effect does chromatic aberration have on the eye? The effect of longitudinal chromatic aberration on human vision and contrast is rather limited as the eye has its own mechanism to negate. Following four factors contribute to limiting the effects of chromatic aberration on human eye:
The human crystalline lens (fig 6) filters out the short wavelenth blue light, especially wavelength of light lower than 450 nm are increasingly blocked. Thought there is no sharp cut off for blue light filtration in human eye, the below figure explains the light transmission of a 53 year
old human lens that filters out over 50 percent of blue light below 450 nm of wavelength of light.
2. The macula pigment, consisting of three caratenoids, lutein, Zeaxanthin, and meso Zeaxanthin, is thought to filter out blue light. This is important to consider as after the cataract surgery and replacement by IOLs, a UV filtering IOL no
longer filters out the short wavelength blue light ( see the figure on the right wherein the XCI (Vivinex IOL with UV filtration property only) does not filter out blue light in the 400 nm to 500 nm zone). Wooten and Hammond reported that transmission of visible short-wavelength (blue) light decreases substantially with increasing MP optical density****.
3. The macula is dominated by three types of cones cells ( S- M- L- ) each containing different opsins that alters the absorption properties of macula pigment , each therefore responding to short, medium and long wavelength of light. There is markedly less concentration of blue light sensitive S-cones in the macula, which helps the eye to be relatively insensitive to short wavelength blue light, thus reducing the effects of chromatic aberration in the eye.
4. The peak photopic and mesopic human spectral sensitivity is from 500 nm to 550 nm wavelength of light. Thus the effect of chromatic aberration is reduced in the eye, as the eye's sensitivity shifts more towards green and red zone. Some studies have also indicated neural compensation for longitudinal chromatic aberration.
It has been debated by some IOL manufacturing companies, that a lower refractive index with a higher abbe number reduces the lens chromatic aberration and dispersion of light. While in camera optics, and microscope, a higher abbe number is helpful to reduce dispersion of light, the benefit of such concepts for the pseudophakic eye is yet to be conclusively proved.
The objective of this article was to describe the many aspects of optics that may be carefully considered while choosing an IOL for a patient. The choice of IOL ultimately depends on the four broad characteristics embedded in the IOL - material (see Part I of this article) , design, optics and delivery.
** Effect of Intraocular Lens Tilt and Decentration on Visual Acuity, Dysphotopsia and Wavefront Aberrations Zahra Ashena 1, Sundas Maqsood 2, Syed Naqib Ahmed 3, Mayank A Nanavaty, Vision (Basel) 2020 Sep 14;4(3):41. doi: 10.3390/vision4030041
****Wooten BR, Hammond BR . Macular pigment: influences on visual acuity and visibility. Prog Retin Eye Res 2002; 21: 225–240.