Cataract lens replacement: How IOLs work

Like your eye's natural lens, an IOL focuses light that comes into your eye through the cornea and pupil onto the retina, the sensitive tissue at the back of the eye that relays images through the optic nerve to the brain. Most IOLs are made of a flexible, foldable material and are about one-third of the size of a dime.  Like the lenses of prescription eyeglasses, your IOL will contain the appropriate prescription to give you the best vision possible. Read below to learn about how IOL types correct specific vision problems.

Which lens option is right for you?

  • Before surgery your eyes are measured to determine your IOL prescription, and you and your Eye M.D. will compare options to decide which IOL type is best for you, depending in part on how you feel about wearing glasses for reading and near vision.
  • The type of IOL implanted will affect how you see when not wearing eyeglasses. Glasses may still be needed by some people for some activities.
  • If you have astigmatism, your Eye M.D. will discuss toric IOLs and related treatment options with you.
  • In certain cases, cost may be a deciding factor for you if you have the option of selecting special premium lOLs that may reduce your need for glasses.

Intraocular lens (IOL) types

Monofocal lens
This common IOL type has been used for several decades.

  • Monofocals are set to provide best corrected vision at near, intermediate or far distances.
  • Most people who choose monofocals have their IOLs set for distance vision and use reading glasses for near activities. On the other hand, a person whose IOLs were set to correct near vision would need glasses to see distant objects clearly.
  • Some who choose monofocals decide to have the IOL for one eye set for distance vision, and the other set for near vision, a strategy called "monovision." The brain adapts and synthesizes the information from both eyes to provide vision at intermediate distances. Often this reduces the need for reading glasses. People who regularly use computers, PDAs or other digital devices may find this especially useful. Individuals considering monovision may be able to try this technique with contact lenses first to see how well they can adapt to monovision. Those who require crisp, detailed vision may decide monovision is not for them. People with appropriate vision prescriptions may find that monovision allows them see well at most distances with little or no need for eyeglasses.
  • Presbyopia is a condition that affects everyone at some point after age 40, when the eye's lens becomes less flexible and makes near vision more difficult, especially in low light. Since presbyopia makes it difficult to see near objects clearly, even people without cataracts need reading glasses or an equivalent form of vision correction.

Multifocal or accommodative lenses
These newer IOL types reduce or eliminate the need for glasses or contact lenses.

  • In the multifocal type, a series of focal zones or rings is designed into the IOL. Depending on where incoming light focuses through the zones, the person may be able to see both near and distant objects clearly.
  • The design of the accommodative lens allows certain eye muscles to move the IOL forward and backward, changing the focus much as it would with a natural lens, allowing near and distance vision.
  • The ability to read and perform other tasks without glasses varies from person to person but is generally best when multifocal or accommodative IOLs are placed in both eyes.
  • It usually takes 6 to 12 weeks after surgery on the second eye for the brain to adapt and vision improvement to be complete with either of these IOL types.

Considerations with multifocal or accommodative IOLs

  • For many people, these IOL types reduce but do not eliminate the need for glasses or contact lenses. For example, a person can read without glasses, but the words appear less clear than with glasses.
  • Each person's success with these IOLs may depend on the size of his/her pupils and other eye health factors. People with astigmatism can ask their Eye M.D. about toric IOLs and related treatments.
  • Side effects such as glare or halos around lights, or decreased sharpness of vision (contrast sensitivity) may occur, especially at night or in dim light. Most people adapt to and are not bothered by these effects, but those who frequently drive at night or need to focus on close-up work may be more satisfied with monofocal IOLs.

Toric IOL for astigmatism
This is a monofocal IOL with astigmatism correction built into the lens.

  • Astigmatism: This eye condition distorts or blurs the ability to see both near and distant objects. With astigmatism the cornea (the clear front window of the eye) is not round and smooth (like a basketball), but instead is curved like a football. People with significant degrees of astigmatism are usually most satisfied with toric IOLs.
  • People who want to reduce (or possibly eliminate) the need for eyeglasses may opt for an additional treatment called limbal relaxing incisions, which may be done at the same time as cataract surgery or separately. These small incisions allow the cornea's shape to be rounder or more symmetrical.


Protective IOL filters
IOLs include filters to protect the eye's retina from exposure to UV and other potentially damaging light radiation. The Eye M.D. selects the filters that will provide appropriate protection for the patient's specific needs.

Other important cataract lens replacement considerations

  • In some cases, after healing completely from the cataract lens surgery, some people may need further correction to achieve the best vision possible. Their ophthalmologist may recommend additional surgery to exchange an IOL for another type, implant an additional IOL, or make limbal relaxing incisions in the cornea. Other laser refractive surgery may be recommended in some cases.
  • People who have had refractive surgery such as LASIK need to be carefully evaluated before getting IOLs because the ability to calculate the correct IOL prescription (PDF 650K) may be affected by the previous refractive surgery.

Additional cataract information resources

If you're interested in learning more about cataract from the ophthalmologist's perspective, follow the link below to read American Academy of Ophthalmology practice guidelines for clinicians.

American Academy of Ophthalmology Preferred Practice Pattern: Cataract in the Adult Eye 

Written by
Reviewed by Dr. Elena Jiménez on Mar. 1, 2014

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