One of the inconveniences experienced by those who reach the presbyopic stage in their vision (age 40 years and more), is having to cope with reading glasses. For this reason many people go about their daily routine with glasses around their necks, or they have multiple pairs of readers all around the house and office, or they wear bifocal or multifocal glasses on a permanent basis.
Intra-ocular lens implants (IOLs) are used on a routine basis to treat several eye conditions, such as refractive errors, keratoconus and cataracts. In the case of cataract surgery, the process is relatively simple: The natural crystalline lens in the eye is removed and an IOL is implanted into the eye according to the specifications for that eye. The most common IOLs fitted today are fixed monofocal lenses that are matched to distance vision. When a monofocal artificial lens replaces the natural lens, you may no longer require spectacles or contact lenses for distance vision, but will surely need spectacles for close-up activities such as reading or computer work, since the implanted lens will only correct distance vision. However, other refractive options with monofocal IOLs are available (apart from balanced distance vision), which are described elsewhere on this website.
Nowadays various types of IOLs are available to suit each person's visual needs. The different types of IOLs are mentioned in their respective categories and discussed below:
These IOLs provide the patient with two images - one for near and one for distance, which are simultaneously focused on the retina.
Light entering the eye from a distant image is refracted by the two optical elements in the lens so that the light entering the eye through the distance optical element is in focus, whereas the light passing through the near element from the same distant object is a complete blur.
The optical elements in the lens are either arranged in a circular configuration or in two separate sections separated by a horizontal transition area. The lenses with a circular arrangement may further be categorized as an apodized optic, consisting of concentric circular rings with differing refractive powers, or as a refractive optic where adjacent refractive zones are blended into their neighbouring zones. In the latter case, this leads to a smooth transition between zones, while they are also arranged in a concentric fashion.
For lenses with a circular arrangement, the central part of the lens (whether it is an apodized or refractive optic) is usually used for near vision, whereas the peripheral part is used for distance vision. For example, when the person concentrates on a distant image, light passing through the distance optical element is focused, while light from the same object through the near element remains out of focus. The situation switches around for a close-up object. Therefore, for both near and far, there will always be a focused and an unfocused image projected simultaneously onto the retina of the eye. Since the brain prefers a focused image, the unfocused portion of the image is subconsciously suppressed. This type of "multifocal vision" may also be called "simultaneous perception".
The main advantages:
- The principal reason why one would choose this option is to be relatively independent of glasses. Sometimes glasses are still required for intermediate vision, such as working on a computer. Although this option provides reduced dependency on glasses, glasses may at times still be required.
- Since both eyes have near and distance vision, depth perception (or "stereopsis") remain intact.
The main disadvantages:
- The quality of vision will never be as good as vision obtained with a standard monofocal IOL. This is mostly because the focused image always has an overlay of the unfocused image, as explained above. In the majority of patients, over a period of 3 to 6 months the brain learns to adjust and ignore the unfocused image by only concentrating on the sharp image. A small percentage of people never learn to adapt, which is largely dependent on the person's personality. For instance, someone who is a perfectionist might struggle to adapt.
- The intensity of light is an important factor to consider, since good lighting is essential to obtain good and comfortable reading vision with these lenses. The reason is that, as the total amount of light entering the eye is split into a near and far component, less light per given distance is available to stimulate the retina. So in poorly-lit environments, people with these lenses might experience problems to perceive clear vision. With these lenses it is, therefore, important to operate in good ambient light.
- Under low light conditions, especially when looking at illuminated objects, one may perceive a varying degree of distortion. These distortions mainly take the shape of "halos" or a "starburst" effect. The reason for these distortions is that the lens has two optical elements that function in tandem.
- Two definite focuses are perceived - one for distance and one for near. Depending on the type of lens and its design, glasses may be needed to correct vision for computer work, as a computer screen is often located further away than the normal reading distance.
For the above reasons, certain personality types, as well as people with specific occupational requirements, may be better off using a standard IOL.
Examples of IOLs with their optical elements arranged in a circular fashion are the AcrySof ReSTOR and Technis Multifocal lenses.
An example of a bifocal IOL is the Lentis IOL.
2. Accommodative IOLs
These IOLs provide the patient with limited visual accommodation by interacting with the ciliary muscles, zonules and vitreous inside the eye. They use hinges at both ends to "latch on" and move forward and backward inside the eye, mimicking the eye's accomodative mechanism.
The advantages of accommodating IOLs are:
- Reduction of halos, glare and other visual disturbances, because light comes from, and is focused on, a single focal point.
- Good vision at far, intermediate and near.
- Contrast sensitivity comparable to monofocal IOLs.
- The potential to eliminate or reduce the dependence on glasses after cataract surgery.
The problem with these lenses is that they are theoretically supposed to provide accomodation, but in real life the amount of accomodation claimed by their manufacturers is not that obvious.
Examples of accommodating IOLs are the Crystalens by Bausch & Lomb and the Synchrony lens from the company Visiogen.
3. True accommodative IOLs
A new and novel intra-ocular lens is currently under development, which will enable patients to regain a certain degree of focus-adjusting ability or 'accommodation'. Such an accommodating lens will help patients to see at various distances, adjusting the focus like they were used to when they were younger. This means that patients will be less dependent on glasses for daily tasks such as driving, using a cell phone, looking at price tags, receipts and menus.
At Optimed we are currently conducting research for a company to obtain FDA clearance in the US for such a lens. Our preliminary results are highly encouraging and we anticipate that these lenses will be commercially available in the very near future.