One of the questions that most frequently arises in our consultation is the one referring to what type of multifocal intraocular lens (IOL), we will implant when operating the Cataracts.
You have to remember that in the cataract operation we remove the natural lens of the eye, the crystalline, which over time or for other reasons, became cloudy and by not letting the light pass into the eye, towards the retina, was reducing the vision. That natural lens must be replaced by another artificial lens, the intraocular lens or IOL, as we usually call it.
What is the lens
The lens is a lens of approximately 22 diopters and has a biconvex structure, with a size somewhat larger than a lentil. If we extract it, we have to supply that graduation and reoccupy the empty space that we have left. This last point is interesting because it was the one that gave rise to intraocular lenses.
The lens, in addition to a lens, acts as a septum that separates the eye into two compartments, the anterior segment and posterior segment, as shown in the figure.
The lack of this septum causes the internal structures to be more "loose" when that space is left empty and substances from one or another segment can also pass, with toxic effects on the retina, causing alterations on the taint or even favoring the appearance of a retinal detachment.
To avoid all this and re-separate the eye when extracting the cataract, it was thought to place a structure similar to the crystalline lens, in the shape of a lens, to sequester and to incorporate the graduation that that eye needs, that is why we always recommend implanting the intraocular lens , although the graduation of this is zero diopters since the purpose is to re-separate the anterior and posterior segment, recover the physiological septation of the eye.
Types of multifocal intraocular lenses
Once it is clear why we implanted the IOL, the question is what type of lens to choose. When the first IOLs appeared, they were of the monofocal type, to correct far or near vision, as in glasses. Over time IOLs were appearing that incorporated mixed far-near correction systems, such as progressive lenses. Since the 80s, designs have been refined but it was not until a decade ago that lenses that offer very good results have been achieved.
At the present time there are several models based on different optical systems. This variety of lenses must be interpreted not so much as that there is still not a fully satisfactory lens for everyone but that each patient needs different optical conditions. It is true that science always evolves and every time we will have more perfect lenses but, at this moment, there are already IOLs that offer a very good optical quality for a large majority of patients, the issue is knowing how to choose the most appropriate in each case or, advise against this type of lens in those patients who we suspect that the desired benefit will not be achieved.
Optically, there are four basic types of multifocal lenses:
- Those that have a design with diffractive rings
- Those that the design is of type symmetric refractive, with a central zone and a peripheral zone
- Our asymmetric refractives, with the upper part for distance vision and the lower part for near vision
- Our pseudoacomodatives, based on the displacement of the lens within the eye
In all four cases, far, intermediate and near vision is improved.
Diffractive multifocal intraocular lens
Diffractive intraocular lenses have rings based on Fresnel prisms, alternate foci near and far and their aspheric design, improves intermediate vision. The result is that all the foci arrive at the retina, far, intermediate and near, simultaneously and it is the brain that must choose the focus that it needs at each moment, depending on the distance to which the object is located. What are we watching.
This simultaneous arrival of foci has the advantage that each focus offers a very good quality of vision, especially in near vision and, as all the foci arrive simultaneously, the change of focus far-near or near-far is very fast.
On the other hand, they have the drawback that the ring design makes some patients see reflections of halos, especially around lights, which can complicate activities such as driving at night. This is especially evident in patients with large pupils, with a tendency for the pupil is dilated, minimizing in those who tend to have small, miotic pupils.
Symmetric refractive multifocal intraocular lens
Symmetric refractive IOLs have double optics, a central lens and a peripheral ring, as shown in the figure. In most cases the central zone has the near graduation and the peripheral one the far one, although some commercial houses opt for the inverse situation. The fact is that the focus of the retina is reached from far and near simultaneously, but only one focus for near and another for far, not as in the diffractives that several bulbs arrived, one for each ring of the lens. The intermediate distance is also solved with the aspheric design of the IOL.
In this case, the brain must also choose the appropriate focus for each distance, but since only two main focus sources arrive, there is no longer a sensation of halos around the lights and makes them more suitable for people who have to drive at night or are very sensitive to light to suffer glare.
Asymmetric refractive multifocal intraocular lens
The asymmetric refractive intraocular lens, like the one in the figure, where the upper part focuses the distant objects and the lower one the near ones, in a more physiological way than the eye does in normal conditions. This type of lens, the last to appear on the market, are those that perhaps best adapt to a greater number of patients, offering good optical quality at all distances, although not as high as in the other designs but with the advantage of that the sensation of halos and reflections in the light are inferior with respect to the other IOLs.
Pseudo-accommodative multifocal intraocular lens
Pseudo-accommodative intraocular lenses are lenses that are based on internal displacement. When we want to see closely, the ciliary muscle contracts, dilates and compresses the vitreous, the gel that fills the eye, moving it forward, a movement that presses the intraocular lens causing it to also move forward, as shown in the figure . In an optical system like that of the eye, moving the IOL forward increases its power, which is just what we need to focus closely. When looking again from afar, the pressure of the vitreous on the IOL is released and the IOL returns to its initial position, recovering the distant focus.
This mechanism of far-near vision would have the advantage that each focus arrives separately, never overlapping on the retina, the basis of light discomfort, the problem is that it has not been possible to demonstrate its operation and in most cases it does not a sufficiently good near vision is achieved, for this reason we do not usually recommend these lenses in patients who are very demanding with near tasks.
Conclusion and summary
Although there are other models or even the combination of different lenses, seeking to enhance one eye more for far and the other for near, we can say that diffractive lenses are a good option when we want to prioritize near and far vision; Symmetric refractive lenses are very good to prioritize distant vision, especially for driving, asymmetric refractive lenses are the ones of choice for those patients who are sensitive to light, with large pupils and who have to drive at night or do sports such as tennis or skiing and, finally, pseudo-accommodative lenses would be recommended in patients who are very sensitive to light and who seek good distance and intermediate vision but are not very fond of reading.
In any case, it is important to consult a cataract specialist with experience in multifocal lenses, so that they can assess each patient and decide which is the best option.
And in cases where there is innovation in surgery for cataracts, what is your opinion of the new technique that Dr. Laureano Álvarez-Rementería comments on the femtosecond laser in La Razón today? It seems that it is very advantageous because of the decrease in manual maneuvers by the surgeon. This doctor says that he is going to start implanting it, in a pioneer way, in December; Will it be safe and tested?
Indeed, femotosecond technology is very promising for cataract surgery and although there is already a history of lasers in this type of operation, the truth is that they did not work well at all, I myself was one of the pioneers in this campoy a few years ago I published the results of that experience (J Cataract Refract Surg 2003;29:1339-45). Now it is different and the laser has improved, it is less traumatic and more precise, which makes us think that it will be a step forward to improve results.
At the present time we can not say that the application of the laser supposes a differential fact with respect to the ultrasound techniques, the reality is that the femtosecond laser helps us in the preparation of the surgery, in the incision and in the capsulorexis but the emulsification of the cataract and its aspiration, is still done with conventional systems. The laser still does not surpass the micropulsed ultrasound in this phase of the surgery, for that reason we think that it is a very promising technology but with little advantages today, in addition to the increase in the cost of its use.
Good afternoon, I would like to ask about multifocal intraocular lenses.
In my case, three years ago I suffered a traumatic cataract as a result of which a monofocal IOL was implanted. My VA is 100%, but being pseudoaphakic, it bothers me in near vision. Well, the other eye I have my natural lens, and at the moment, I do not have presbyopia.
I would like to know if it is advisable to replace my current monofocal IOL with a multifocal type, to improve my near vision. Bearing in mind that it would only be implanted in one eye, since the other is accommodated thanks to its natural lens. Would my bifocal vision be worse?
Thanks for your reply
Our eyes are very important, that is why we must take care of them very carefully.
In fact, Irma, once cataract surgery is done, we are going to live with the intraocular lens for the rest of our lives and, therefore, its correct choice is very important.
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Thank you very much for your comment Oscar.