Monofocal Intraocular Lenses

Who are they for?

Monofocal lenses are ideal for patients who require perfect vision for distance. By correcting both eyes for the same distance, they achieve optimum three-dimensional vision and excellent visual acuity.  However, spectacles are still needed for near vision.

Some patients who choose monofocal lenses decide to have one eye set for distance and the other for near vision. This technique is known as “monovision”.  The brain adapts and synthesises the information from both eyes to provide vision at all distances, including intermediate.  In general, this technique reduces the need to use reading glasses. People who use a computer every day may find this solution especially useful.

How do monofocal lenses work?

For decades, monofocal intraocular lenses were the most widely used type of lens.  Nowadays however, they have become the most basic solution for cataracts on the market.   They are designed to correct vision at a specific point in space: distant, intermediate or near.  When the rays of light enter the optic system of a monofocal lens, the light converges on a single point of focus on the retina.

If the calculations are made for distant objects, the intraocular lens is only able to focus on these objects. By contrast, if focus on near objects is required, the power of the lens must be increased.  Light cannot be focused on both distances at the same time with a monofocal lens.  Most patients who choose this type of lens opt to have good vision for distance and rely on reading glasses for near vision.

Diagram of several Monofocal Intraocular lenses
Diagram of several Monofocal Intraocular lenses (https://www.iohoyos.com) 

What are the advantages and disadvantages of monofocal lenses?

One disadvantage in the use of spherical lenses is the appearance of spherical aberration.   Due to the optic of the lens, the rays which pass near the optic axis (called paraxial rays) focus further away than the rays which pass through the periphery of the lens.  The quality of the image from an eye affected by spherical aberration depends on ambient lighting and the pupil size.  When there is little light, the pupil dilates and the quality of vision becomes worse.  By contrast, in the same eye, when there is more light, the pupil contracts and vision improves.  This aberration is therefore especially noticeable in nocturnal conditions: it is difficult to see street lights or car lights and the image may even become blurry.  It is also important to add that the greater the power of the intraocular lens, the greater the total spherical aberration and as a result the poorer the quality of vision. To avoid this visual limitation, aspherical intraocular lenses are used which clearly improve visual results in nocturnal conditions.

monofocales
Diagram of the spherical aberration formed in the eye, when the rays enter in parallel and do not focus on a single point  
Aberrations in a Spherical Monofocal Intraocular Lens
Aberrations in a Spherical Monofocal Intraocular Lens                                      (https://www.alconcirugia.es)

As discussed previously, another disadvantage is that a monofocal lens can only be used to correct refraction at a specific distance.  Therefore, it may not be the best option for patients who hope to do without spectacles.

Patients considering the monovision option can try it out first with contact lenses before having surgery, to see if they are capable of adapting to this type of vision.  The percentage of people who adapt to it is high, but there is a minority who find they are unable to tolerate it.

If you would like to correct distant and near vision at the same time without resorting to monovision, there are other options such as accommodative intraocular lenses or multifocal lenses.

What materials are intraocular lenses made from?

For intraocular lenses to be suitable for insertion into the human eye they must be composed of a material that is biocompatible with the eye’s structures.  Over the years different materials have been used to create intraocular lenses, although the most popular today are those made from Polymethyl methacrylate, silicone and soft acrylic materials.

PMMA is a rigid plastic and has properties similar to those of soft acrylic materials or silicones, but there is a major difference between some lenses and others: PMMA creates hard lenses whilst acrylic or silicone lenses are flexible.  This is a major advantage in cataract surgery because the lenses can be inserted through micro incisions in folded form, to be unfolded directly inside the eye and placed in the correct position.  This technique does not require stitches and results in quicker recovery and fewer complications.  Thanks to the safety and effectiveness of this soft material, in recent years this type of intraocular lenses has become the most commonly used in cataract procedures.

It should be noted that these materials are not only used in monofocal lenses, but in practically all the designs made by a single company: toric, multifocal or accommodative.