Who are Multifocal Intraocular Lenses for?
With standard monofocal lenses it is possible to achieve good visual acuity for distance after cataract surgery, but near vision becomes blurry. For this reason, reading glasses are needed with this type of surgery, and many patients are not satisfied.
Multifocal lenses are designed for anyone who would like to enjoy good vision at all distances, ranging from distant to near. With this solution patients can enjoy good vision without spectacles for all types of tasks, including driving, sports, watching television, using the computer, sewing and reading. Selection of the type of multifocal lens depends on each patient and the opinion of the ophthalmologist, as there are different models and manufacturers which cater for different situations.
What types of Multifocal Intraocular Lenses are available and how do they work?
Refractive Intraocular Lenses
Refractive intraocular lenses have an optic with different powers distributed in different areas.
An example of this type of lens is the AMO ReZoom. They provide good distance vision but the visual performance from intermediate to near vision is variable. The central area of the lens is designed for distance. Then there is an area adapted for near vision and so on. Areas for near and distance are alternated. The pupil plays an important role in this design and can become a disadvantage. If the pupil contracts significantly, the most effective area will be the centre, meaning that distance vision will remain good but near vision quality will be lost.
On the down side, there is a higher incidence of halo effect and glare than with other lens types, such as diffractive. Even so, the success rate is high and the results are satisfactory.
Trifocal Multifocal Intraocular Lenses
Trifocal lenses have concentric rings with different refractive powers which redirect the light to three different focuses: distance, intermediate and near vision. The lens has a basic power calculated for distance and the corresponding rings are on its periphery. Then, there are the rings for intermediate distance which are approximately 1.66D stronger to achieve focus. Finally, the more central rings are those responsible for near vision and are around 3.33D stronger than the refraction for distance.
This optic designed not to depend on the pupil size, and works when the pupil is contracted (high illumination) and dilated (low illumination).
The most modern lenses are aspherical and are designed to minimize unwanted optical effects after surgery, such as halos or glare, especially to improve night vision.
It is a treatment option which produces very good visual quality at all distances, and as a result, independence from spectacles. It can be implanted for both cataract surgery and for refractive lens surgery.
Diffractive Multifocal Intraocular Lenses
In the case of diffractive lenses, there are various areas with different optical powers. There is a series of concentric rings which form a “diffraction network”. This optical feature has the capacity to direct rays of light to two different focuses at the same time, thereby creating two separate focal points, one for distance and the other for near.
With diffractive lenses, the light reaches the entire surface of the lens and there is therefore no dependency on the pupil size. They can cause glare at night when driving and produce a halo effect.
The diagram on the right shows the different areas with different optical powers. In the central area, the rays focus closer to the lens (higher power) and in the outer area they focus further away (lower power)
Apodised diffractive lenses:
Apodised diffractive multifocal lenses have two areas.
The first is the central one, where the apodised area is located. This means that this area contains concentric levels of different heights, which decrease as they move from the centre to the periphery. This makes the lens diffractive. It is technology which allows for optimum light distribution to different focus distances, both distant and near. It does not depend on lighting conditions.
The second area is the periphery of the lens, which surrounds the apodised region. This part takes care of distance vision and directs the rays of light to a distant focal point.
This technology is patented by the intraocular lens AcrySofMR IQ ReSTORMR.
The aim is to reduce potential optical discomfort which may arise after cataract surgery as much as possible compared to other intraocular lenses, in addition to achieving good image quality. Thanks to this, independence from spectacles can be easily achieved.
Asymmetric Multifocal Intraocular Lenses:
All multifocal lenses available on the market today have been created based on the concept of rotational symmetry: refractive, diffractive or apodised diffractive multifocal lenses.
This means that they are composed of circles or concentric areas which provide distance and near vision on an alternate basis.
There is now an innovative multifocal lens concept, which comprises an aspherical and asymmetric area for distance vision next to an area for near vision. In other words, it is a lens with modifications in the radius of curvature to adapt to different distances.
The aim of this is to avoid “image jumps” which can happen with other designs. This optical design functions independently from the pupil size, which is an advantage.
Toric Multifocal Intraocular Lenses:
Until recently, it was not possible to correct presbyopia and astigmatism simultaneously during a single operation. There were no multifocal lenses which could cover all the requirements at the same time. Today, patients with astigmatism are now able to enjoy independence from spectacles too. It is a very recent solution which confirms the major advances on today’s market. These lenses have the ability to correct presbyopia and astigmatism at the same time. As a result, patients can enjoy good vision at all distances. Today’s lenses are also made with an aspherical design to reduce spherical aberration as much as possible and improve visual acuity in nocturnal conditions. These lenses are effective when corneal astigmatism is over one dioptre.
The multifocal optic of this lens can have different characteristics. There are designs which are refractive, apodised diffractive and even with the same characteristics as an asymmetric lens. A large visual field is obtained at all distances: distant, intermediate and near.
What are the pros and cons of multifocal lenses?
Today, innovative multifocal lenses are less invasive, and over time the brain adapts to diminish the optic phenomena caused. For many patients, the new quality of life without spectacles is better and more important than the minor optical disadvantages which arise after surgery. Certain activities which have not required the use of spectacles for a long time such as reading, sewing or using a computer can continue to be performed after surgery with a multifocal intraocular lens. People who are more demanding in this aspect are good candidates for multifocal lens implants. In any case, it is important to be aware that the use of spectacles may be required for more infrequent, specific tasks.
In some cases, depending on the lens implanted and the post-surgical result, it is true that there may be minor side effects such as glare, halo effect or increase in aberrations. Over time the neurological system will adapt to this new situation and the patient will become accustomed. Adaptation varies according to person and can take a few months. To ensure total adaptation, if possible both eyes must have been operated on under the same conditions and with the same lens.
Some form of correction may need to be performed after surgery which has produced a residual refraction. Your ophthalmologist will advise you on the need to perform a LASIK procedure, an implant of an additional intraocular lens (Add-On) or make limbal incisions to treat a minor astigmatism.