Below we will try to provide you with a basic explanation of the types of intraocular lenses (IOL) available.
What is an Intraocular Lens? Why would I need an intraocular lens?
First of all, we must take into account that the image we see reaches our retina (posterior part of the eye), similar to how it does on the negative of a photographic camera, due to “two main lenses”: the cornea (the outermost transparent layer of the eye) and the crystalline lens (the interior lens).
The focusing power of the cornea (measured in dioptres) is constant whilst that of the crystalline lens is, until the appearance of presbyopia, variable due to accommodation (variation of the shape of the crystalline lens thanks to the contraction of the muscles inside the eye). This process of accommodation takes place because the crystalline lens has sufficient flexibility to change shape and thereby increase its capacity to focus close up.
When an object is successfully focused on the retina, the quality and clarity with which this image is projected must be assessed as well as how this clarity varies according to light conditions. The clarity/quality of an image may vary according to high order aberrations in these two lenses (to understand we can use distortion as a comparison to aberration). Of these high order aberrations, it is worth highlighting the impact of spherical aberration.
To obtain the ideal image, total spherical aberration must be minimal. The fact that the shape of the cornea itself causes a positive spherical aberration, requires the crystalline lens to compensate for this spherical aberration so that the combination of “the two lenses” focuses the image with as much quality as possible. When the crystalline lens is healthy, transparent and flexible, it naturally tends to compensate for the positive spherical aberration generated by the cornea, but over time this stops happening.
The crystalline lens must therefore be transparent to allow the image to pass through and must be flexible to be able to vary its shape, to enable the person to see close up and in the distance as clearly as possible (focus capacity + influence of spherical aberration).
If the crystalline lens starts to opacify, becoming fully or partially cloudy, we are faced with a cataract, with the resulting loss of visual acuity. When the crystalline lens hardens, losing its flexibility and with it its ability to change shape, we are faced with what is known as presbyopia or tired sight. Whilst there are several causes for the appearance of a cataract, it is not a condition which affects all people with the same impact or following the same course. Presbyopia or tired sight meanwhile, is a natural ageing process of the crystalline lens which does affect everyone starting at around the age of 40-45.
When, either due to a cataract or the effects of presbyopia, a decision is made to replace the crystalline lens, it is substituted for an Intraocular Lens. Replicating the characteristics of a healthy natural crystalline lens is still not something which the industry has achieved, but thanks to research and constant advances many different solutions are available with the objective of achieving visual capacity that is as similar as possible to that enjoyed with the natural crystalline lens.
What are the differences between the types of Intraocular Lenses available to me?
Provided below is a simplified presentation of the principles and characteristics of the different types of intraocular lenses available on the market.
This is the most basic type of intraocular lens on the market, which does not have any additional features. It is a transparent spherical lens (therefore it does not compensate for the spherical aberration induced by the cornea) with a single focusing power (Monofocal), made from biocompatible material with a design which allows for it to be implanted in the position occupied by the natural crystalline lens. In Spain, these lenses are implanted for free in public health centres.
Monofocal IOLs are lenses which, as their name suggests, have a single focus point. They do not allow the eye in which they are implanted to see “well” in the distance and up close at the same time. For this reason, patients normally choose to see in the distance without the need for spectacles. The ophthalmologist therefore selects the lens with the appropriate dioptric power for the patient to have the best possible visual acuity for distance without correction and use spectacles for near vision.
This single focus means that they are not (* monovision aside) the intraocular lenses used to correct presbyopia.
As regards their spherical shape, when we talk about a monofocal lens alone or a standard monofocal IOL, we are referring to a spherical monofocal lens. As indicated above, for the image to reach the retina with the maximum quality, the spherical aberration of the crystalline lens must have a negative value to compensate for the asphericity of the cornea which is positive, and thereby minimise this total ocular aberration. If we replace the crystalline lens with a lens which does not correct this positive asphericity but increases it, we generate an image of a poorer quality than desired.
As explained above, they are lenses which are used in cataract surgery in public health services.
This type of intraocular lens offers additional features compared to Spherical Monofocal IOLs. As we will see below, these additional features may stem from:
- The design of their central part: To enable focusing at different distances (multifocal), distribution of the area according to focus.
- Presence of filters: Ultraviolet or blue light
- Asphericity of the lens (spherical or negative aspherical).
- Shape- Structure of the lens (guarantee stability in positioning)
- More (Advanced search for ophthalmologists interested in more in-depth information)
As we have already said on this page, to be able to see objects in an optimum way, it is not enough to simply project the image onto the retina, as we also need to ensure that its quality is optimum. This quality is largely determined by the presence of spherical aberration. If we take into account that the cornea naturally induces a positive spherical aberration, we can conclude that to achieve a minimal total spherical aberration ( cornea+crystalline lens combined) , we need the intraocular lens (like the healthy natural crystalline lens) to present a negative spherical aberration to compensate for this.
Aspherical Intraocular Lenses are IOLs precisely designed to induce the necessary asphericity for the image projected on the retina to have the best possible clarity and quality.
This clarity translates into better definition and contrast of the visual image as well as less halo effect and glare at night.
Bifocal Multifocal Intraocular Lenses
As their name suggests these are Intraocular Lenses which present two focuses, one for seeing in the distance and another with an addition for near vision.
The young and healthy natural crystalline lens offers the patient its full transparent surface for projection of the image on the retina. The Bifocal Multifocal intraocular lens, meanwhile, achieves focus at multiple distances by dividing the effective surface of the intraocular lens into areas. Some areas of the surface of the lens are used for near vision and others are used for distance.
The human brain is capable of processing the visual information projected without the patient noticing and at the end of the neuroadaptation process, the patient will find their dependency on spectacles significantly reduced.
Trifocal Multifocal Intraocular Lenses
The principle is the same as for bifocal multifocal intraocular lenses. The difference obviously lies in the addition of a third focus for enhancing visual quality at intermediate distances.
The surface of the lens is divided into several areas, each allocated to one of the three focuses. By adding an extra focal distance, the surface of the lens available for projection of the image for near and distance is reduced.
Thanks to technological advances, work is being carried out to produce as clear as possible an image at each of the distances and reduce dependency on spectacles to a minimum.
These Intraocular Lenses imitate the accommodative process of the natural crystalline lens to achieve focus on near images. Their design enables them to partially emulate the crystalline lens and vary their refractive power thanks to the action of the muscles themselves (the ciliary muscles) which act on the crystalline lens in natural accommodation.
What makes this type of lens different from other intraocular lenses is that as with the natural crystalline lens, 100% of the surface of the lens is used for projection on the retina and it also slightly changes position within the eyeball, to focus at different distances.
The main characteristic of this type of intraocular lens is that it can be “moulded” after it has been implanted thanks to the application of ultraviolet rays. This subsequent moulding is carried out when the post-operative result is not the best result which can be achieved. This may be due to minor changes during the healing process, inaccuracy in the initial calculation or the presence of chronic astigmatism.
These lenses allow for a subsequent correction of up to two dioptres. This range, given the levels of precision in current calculations, should be more than enough to eliminate any residual defect and achieve the best possible visual result that the lens allows.
They are single focus IOLs which can also correct astigmatism (Toric, see below). As they do not offer multifocality, but do offer high precision, they can be used for monovision strategies with every guarantee.
In basic terms, we can consider astigmatism as a lack of spherical symmetry in our ocular system, as a result of which vision becomes blurred. The defect of astigmatism is compensated for with a cylindrical shaped lens aligned on the “main axis” of this asymmetry. When the cylindrical lens is put in place and the symmetric defect is compensated for, the image is projected more clearly on the back of the eye. As a result, the degree of astigmatism will be the power required by this lens to project the best image possible on the retina.
In the same way that there are spectacles to correct astigmatism, there are also types of intraocular lenses which can correct this defect thanks to their design, when they are positioned in a specific direction. This type of intraocular lenses which enable correction of astigmatism are known as Toric lenses.
According to the degree of astigmatism and the characteristics of the cornea, your ophthalmologist may consider it appropriate (normally with high astigmatisms or when they prefer not to operate on the cornea) for the astigmatism to be corrected using an intraocular lens.
If, for example, you have cataracts and astigmatism, it makes sense that whilst replacing the cataract by an IOL, you take the opportunity to correct the refractive defect too.
Intraocular Lenses with Filters
As we already know, when “we see” we do so because the image is projected on the retina. To reach the retina, the image has crossed the entire optical system, from the cornea to the vitreous passing through the natural crystalline lens.
The light which comes from outside the eye is formed of a wide range of frequencies, some of which are potentially harmful for the retina. Another of the functions and characteristics of the natural crystalline lens which we have not mentioned is that it is capable of filtering and preventing the rays of these harmful frequencies from reaching the retina.
When we replace the crystalline lens with an intraocular lens, we have to take into account that the natural protection of the crystalline lens is no longer present and that this function must be performed to a certain extent by the implanted intraocular lens.
The retina is an especially sensitive part of the eye and its exposure to light, be it natural or artificial, is a risk factor for the appearance of conditions such as AMD.
Therefore, the function of protecting and filtering light components is important for protecting the retina as much as possible. Of the potentially harmful frequencies found in light rays (or light) the main ones are blue light and ultraviolet light.
By using the lens search tool you will find many yellow coloured lenses. These lenses block blue light as a preventative measure to ensure the least possible damage is caused to the macula. As for ultraviolet light, practically all intraocular lenses, to a greater or lesser extent, now incorporate a certain degree of protection for the macula.
These lenses do not replace the natural crystalline lens, but as their name suggests, are placed in the sulcus (between the iris and the capsular sac which surrounds the crystalline lens) as an addition to the intraocular lens which has replaced it.
In the case of a crystalline lens which is clear or with a cataract which has been replaced by an IOL in a small number of cases, it can happen that there is a residual refractive defect after the operation. This residual refractive error can be compensated for with an add-on lens to achieve optimum visual results for the patient.
The designs of Add-On lenses are practically the same as we find in intraocular lens solutions which replace the crystalline lens (these are known as pseudophakic) in terms of the number of distances in focus, correction of aberrations, use of filters, etc.
Phakic Intraocular Lenses
Phakic intraocular lenses are lenses which are not implanted inside the eye to replace the crystalline lens but are added to it without modifying any of the eye’s structures. Their shape and position varies according to their type. They offer a good solution for solving refractive conditions in patients who do not want to remove their natural crystalline lens or who for some reason are not candidates for corneal surgery (high number of dioptres to correct, fine or irregular cornea, etc.).
The letters ICL are an acronym for “Implantable Contact Lens”. ICL is a phakic intraocular lens, meaning that it is put in place in addition to the crystalline lens. It is implanted in the posterior chamber, between the iris and the crystalline lens. As it is made from a flexible material, the incision required to implant it is very small.
It can correct up to 18 dioptres of myopia and 10 of hypermetropia. Toric models are also available which can correct astigmatism.
Verisyse intraocular lenses are phakic lenses (the natural crystalline lens is retained) which are anchored to the iris in the anterior chamber (the space between the cornea and the iris).
Their purpose is to correct myopia and they cover a range of corrections from -5 to 20 dioptres. There are also (toric) models available which correct astigmatism of up to 2.5 dioptres.