IOL Types — Presbyopia-Correcting

Overview

A standard monofocal IOL has a single refractive power and therefore a single focal point. Vision is clear at one distance, usually far, and glasses cover the rest. Presbyopia-correcting IOLs exist to shrink that spectacle dependence after cataract or refractive lens surgery, and they do it through one of two optical strategies: splitting incoming light into two or more discrete focal points (multifocal and trifocal designs), or stretching a single focal point into a continuous elongated zone (extended depth of focus, EDOF).

Both strategies buy near or intermediate function at some cost to contrast sensitivity, and in the multifocal case, night-vision quality. That trade-off makes patient selection at least as important as the lens design itself.

Diffractive Multifocal IOLs

Multifocal IOLs create multiple simultaneous focal points with a diffractive ring pattern etched into the lens surface: concentric rings that split incoming light into two, or in trifocal designs three, separate images, each focused at a different distance. The brain learns to favor the sharp image at whatever distance is in use and suppress the defocused one, a process called neuroadaptation. Because the light is divided rather than concentrated at a single point, some contrast sensitivity is unavoidably lost compared to a monofocal lens.

Most modern diffractive multifocal IOLs are also apodized: the height of the diffractive steps tapers gradually from the center of the optic toward its periphery. A small, bright-light pupil exposes mostly the central diffractive zone, favoring near vision. A dilated, dim-light pupil exposes more of the peripheral, lower-stepped zone, shifting more light energy toward distance focus. That taper is what keeps night driving usable. A purely uniform, non-apodized diffractive pattern would split light evenly regardless of pupil size and worsen nighttime dysphotopsias.

Extended Depth-of-Focus (EDOF) IOLs

EDOF lenses take the opposite approach. Rather than splitting light into separate images, they elongate a single focal point into a continuous zone, producing one extended range of functional vision instead of two or three discrete ones. There is no second competing image for the brain to suppress, so EDOF lenses generally produce fewer photic disturbances than diffractive multifocal designs. The trade-off is a smaller boost to near vision.

The Tecnis Symfony, a well-known EDOF design, achieves its elongated range with an achromatic diffractive grating: a diffractive pattern engineered to stretch the focal range and correct the eye's inherent chromatic aberration at the same time, rather than the purely refractive, multizone surfaces some other EDOF lenses use. That sets it apart from both purely refractive EDOF designs and the discrete-image splitting of diffractive multifocal lenses.

Trifocal IOLs

Trifocal IOLs extend the same diffractive-splitting principle used by bifocal multifocal lenses one step further, adding a third diffractive zone. A standard bifocal multifocal IOL provides distance and near focal points only. A trifocal design adds a dedicated intermediate focal point, which covers arm's-length tasks like computer work and reading a car dashboard, a range bifocal multifocal lenses tend to leave underserved.

Photic Phenomena

Halos and glare at night are the signature complaint of diffractive multifocal IOLs. The same diffractive rings that split light into multiple focal points also scatter part of it, perceived as halos around light sources and glare. This is most noticeable in low-light, large-pupil conditions such as night driving. EDOF lenses produce one elongated focal zone rather than discrete split images, so they generally cause fewer and milder photic phenomena than true multifocal designs, though not none.

Patient Selection

Multifocal and trifocal IOLs deliberately trade contrast sensitivity for spectacle independence, which makes any coexisting condition that already degrades contrast or image quality a relative or absolute contraindication. An eye with an epiretinal membrane and metamorphopsia is a poor multifocal candidate: the membrane's own image distortion compounds with the lens's inherent contrast loss, and the combination predicts poor patient satisfaction even when visual acuity numbers look acceptable. Irregular corneal astigmatism, significant dry eye, or any other surface or media irregularity that independently degrades retinal image quality argues for a monofocal lens instead.

For a patient who wants reduced spectacle dependence but isn't a good diffractive-lens candidate, or simply declines a premium lens, standard monofocal IOLs can still be used with a monovision strategy: the dominant eye is targeted for distance and the non-dominant eye for mild myopia to provide near vision. This avoids any diffractive optic entirely, at some cost to binocular stereopsis, and is worth trialing with a contact lens before committing to it surgically.

Related Topics

Continue exploring Cataract & Refractive Surgery: