The Guide to Intraocular Lenses (IOLs) and Customized Cataract Surgery

By Dr Errol Chan

Biometry of your eye is absolutely necessary and uses sophisticated technology to quantify the length of your eyeball, your corneal curvature, and other important eye parameters, such as corneal width and the depth of the anterior chamber of your eye.

Mathematical calculations are then conducted based on these measurements to allow your surgeon to choose the intraocular lens (IOL) power most appropriate for your visual requirements.

Your eye surgeon may also require other tests to be done prior to cataract surgery, for example:

An intraocular lens (IOL) is a clear plastic disc-like implant that is placed into the capsular bag of the eye to take on the optical function of the natural crystalline lens.

The IOL selected for your eye is of a specific prescription and customized to your eye to allow you the most optimal vision and visual quality, based on your profile of vision-related activities.

A wide array of intraocular lenses are available for implantation, although the specific type of IOL that your surgeon may recommend will depend on certain parameters of your eye structure, your lifestyle needs, hobbies, driving habits, and desire for spectacle-freedom over a range of viewing distances.

Broadly speaking, viewing distances are classified into 3 main categories: distance (at 100 cm and beyond), intermediate (about 66 cm to 80 cm), and near (about 33 cm)

There are 3 main groups of intraocular lens options available for patients:

1. Monofocal Aspheric IOLs

   Monofocal IOLs, as the name suggests, have a single focus point. In the majority of individuals who have monofocal IOLs placed, the specific prescription of the IOL is customized to a distance focal point, and so allow good distance vision.

   

   What your vision might look like after cataract surgery with monofocal IOL placement

   These IOLs offer excellent quality of distance vision with very minimal to no glare* or halos, compared with multifocal, trifocal, or extended-depth of focus (EDOF) IOLs. If you are happy with using glasses for near or intermediate distance activities, these lenses provide very good quality distance vision, e.g. for driving in the day and night.

   However, you would very likely still need spectacle correction for near or intermediate distance activities, e.g. using glasses for reading, using the computer, or watching television.

   

   *Glare refers to the indistinct edge of fuzzy light, observed when one looks at a point source of light, e.g. a street lamp at night. Halos are additional rings of light seen when one looks at a point light source. Starbursts appear as radially scattered light.

   Monofocal aspheric IOLs may also be implanted and customized to a near focus, instead of a distance focus. This would allow near objects to be in focus, whilst distance objects would be out of focus. Such refractive targeting is most commonly performed to achieve monovision correction in a patient.

   What is monovision correction?

   Some individuals are able to use one eye for distance viewing, and another for near, in a state known as “monovision”. This strategy is sometimes employed in appropriate individuals who undergo cataract surgery with monofocal IOLs.

   In monovision correction, the prescription of the monofocal IOLs are optimized such that the IOL in one eye allows for good distance vision, while the IOL in the other eye is optimized for good near vision. Usually, the dominant eye is corrected for distance vision, and the non-dominant eye is corrected for near vision.

   This may feel awkward at first for individuals who are not already using monovision with their current spectacle or contact lens. Therefore, for individuals who are keen to consider monovision correction, suitability must first be tested with a trial using contact lenses to simulate what this feels like for the individual.

   The downside to monovision is that it reduces one’s appreciation of space and depth, and so individuals may observe that objects appear to be less 3-dimensional than what they really are.

2. Multifocal, Trifocal & Extended Depth of Focus (EDOF) IOLs

   Multifocal IOLs (which are really “bifocal” IOLs) aim to allow the individual to have both good distance vision and near vision without spectacles.

   Trifocal IOLs, as the name suggests, target distance, intermediate and near vision.

   Extended depth-of-focus (EDOF) IOLs address some of the fall-off in vision within specific viewing ranges with the bifocal or trifocal IOLs. For instance, the reduced vision between distance and intermediate distances may be an issue for some individuals. EDOF IOLs provide a more gradual transition in vision between these distances.

   

   What your vision might look like with a trifocal IOL

   While the ability to see well without spectacles at specific distances, or over a range of distances, are certainly advantages of such IOLs, it must be stressed that patients with these IOLS implanted MUST trade off accepting some degree of reduced contrast (i.e. images appear dimmer or less vibrant than expected), as well as a much higher likelihood of appearance of glare and halos around lights at night, than with monofocal IOLs.

   These symptoms usually get better with time as individuals tend to adapt to the new visual quality, but nevertheless these visual phenomena will still be apparent from time to time. Within the group of such IOLs, EDOF IOLs may cause less glare and haloes than trifocals, and may thus be more suited for individuals desiring spectacle-freedom who wish to drive at night.

   Such IOLs can be implanted in the majority of patients with normal eyes, including patients who have had previous laser refractive surgery (i.e. LASIK or PRK).

   However, patients with underlying retinal diseases involving the macula, or glaucoma, are not suitable candidates for such IOLs because these eye diseases compromise the optimal optical functioning of such IOLs.

   Lastly, it must also be mentioned that healthy tear production is a very important factor that also affects the optimal visual performance of such IOLs. Patients with dry eyes can still be considered for these IOLs. However, adequately moisturizing dry eyes both before and after the cataract surgery procedure will be very important in this group of patients.

3. Toric IOLs

   These lenses are used to correct corneal astigmatism (“san guang”). Astigmatism is a specific type of refractive error that arises from the cornea being more curved along one plane, than in the rest of the cornea. Just as there is a great variety of heights and weights amongst different people in the population, corneal curvature likewise varies quite a bit amongst different people.

   In order to customize the IOL option specific to your eye, an IOL neutralizing the corneal astigmatism is required. Usually, if the corneal astigmatism is less than 0.5 diopters, you may not need a toric IOL.

   Monofocal, multifocal, trifocal and EDOF IOLs come with toric lens options as well.

   Your ophthalmologist would be able to advise you on the need for a toric lens option to correct your corneal astigmatism.

Those with significant astigmatism (usually more than -0.50 D) would usually require toric IOLs. Monofocal, trifocal and EDOF IOLs all come with toric options. For those who have had trifocal and EDOF IOLs, reading is generally encouraged under good lighting conditions, e.g. using a reading lamp.