Trifocal lenses

One of the most common surgeries in ophthalmology is cataract surgery, either if the lens has become cloudy (cataract) or just to get rid of glasses (clear lens). We already explained in another post how a lens is implanted inside de eye during surgery, to compensate for the graduation. Trifocal lenses have gained popularity recently… not for a good reason, but a bad reputation. How come a subset of patients are incredibly happy while others are extremelly disappointed? Let’s find out.

Physical principles

If a lens was not implanted during surgery, we would need glasses with very high graduation. We have also mentioned that there are many types of lenses, today we will focus on trifocal lenses. Unlike classic monofocal lenses, with a single focus point, trifocal lenses, as their name indicates, have three focus points: close, intermediate and far.

In order to obtain these different focus points, the lens has a design in converging circles, as you can see in the image. These circles alternate their focus distance between the three we have discussed: close, intermediate and far.

This concentric circle design causes the light to be distributed to the different focus points. Id est when light reaches the lens, a certain percentage of light gives a clear image in vision in each focus point. Therefore, we will always have a clear image at the three distances. In return, a percentage of light is sacrificed when we are not using that specific distance.

For example, while reading a novel in a well-lit living room, 50% of the light that reaches the lens will focused on the novel. On the other hand, reading the newspaper with very dim lighting, a lower percentage of the already scarce light will reach the lens. And we will not be able to read (more on this in the next section).

The three focus points

The far focus point is designed to be used beyond 2 meters away and allows us to have a comfortable vision when watching TV, walking or driving. Most common designs of the lenses have an outer ring focused for distance sight.

One in every 3 central rings is focused from far away, but the entire surface outside of the last ring is focused for distance sight. And this has a very useful practical effect.

During nighttime driving, the pupil dilates due to the low luminosity and the driver uses distance vision. Thanks to this design of the outer ring, all trifocal lenses increase the percentage of light intended for distance vision under low light conditions.

The intermediate vision is designed to make it easy to use the computer at a distance of approximately 60-80cm and to have greater brightness in conversation distance. Bifocal lenses are still an interesting option for patients who do not use the computer.

Close vision allows comfortable reading at about 40 cm. As we have already explained, the inner circles, especially the central one, are designed for short distances. So when we are in good lighting and the pupil closes, a higher percentage of light will help us focus up close. The distance is around 40 cm and quite precise. If we try to read at 20 cm, we’ll fail miserably. This behavior is typical of the myopic patient, the shifting of reading distances is unexcepted for them and we specify it before the surgery.

Disadvantages

Trifocal lenses are very demanding on the eye system. Unless everything is within strict parameters of normality, the result will be disastrous. This is why we receive many patients requesting a second opinion and complaining about trifocal lenses. We have to be very strict when choosing which eyes will benefit from these lenses and which ones will not.

First of all, these lenses usually cause halo vision at night. Especially visible during driving, patients describe halos around the headlights of cars. It usually disappears during the first weeks or few months.

Generally speaking, we cannot implant these lenses in patients with strabismus, retina disorders, very high graduations, wandering eyes, single eyes, irregular corneas, irregular pupils, uveitis…

Today, cataract surgery or transparent crystalline implant with trifocal lens, is a very interesting procedure from an economic point of view for the ophthalmologist who perform the procedure. Although they are not the majority, professionals with more greed than moral values are not scarce, as in all trades. Unfortunately, once a trifocal lens is implanted, if the patient is not satisfied, we have very little or no room for maneuver to fix the situation.

In addition, there are other limitations that are not so strict but equally important when making the decision. Trifocal lenses provide good vision, but the image quality is not excellent. Therefore, image professionals (photographers, painters, designers…) are usually not satisfied with the vision quality. On the other hand, very demanding or myopic people with 2-5 diopters are usually unsatisfied as well (they usually have pathologically excellent close vision).

Advantages

With all that has been described previously, how can it be that some patients implant these lenses? Well, because in specific patients, they provide an excelent quality of life and independence from glasses. Undoubtfully. I have many patients with excellent visual acuity from far and near. They will never need glasses again. The results are definitive and usually excellent.

Models

The most common models in our environment are Zeiss AT LISA 909MP, Alcon Panoptix and Physiol Finevision. Each of these lenses has its own characteristic.

In general, Zeiss lenses provide the highest image quality, but also more nighttime artifacts. Alcon Panoptix provides a closer intermediate distance (60 cm) that makes working with screens easier, greater independence from pupil size, and fewer aberrations. The Physiol model is an intermediate lens between the two mentioned and, in addition, it is the most permissive with small refractive errors. When calculating the lens (we will have to write about it someday), we always play with a margin of calculation error, the Finevision of Physiol has a somewhat wider margin of error than the rest.

Moreover, patients with corneal astigmatism need the toric versions of these lenses to compensate for it. The trifocal toric lenses, in addition to having three focuses, are designed with a slightly cylindrical shape, to compensate for the irregularity of the cornea.

Conclusion

Trifocal lenses offer excellent results in some specific cases. Patients get rid of glasses, forever. Although they are very attractive for some specific cases, they are far from being a panacea. As we have already seen, there are many disadvantages and limitations of these lenses. To obtain an ideal result, each case will have to be individualized and studied in depth before making the decision.