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IOLs: Choosing the best implant for cataract surgery

Intraocular lenses (IOLs) are medical devices that are implanted inside the eye to replace the eye's natural lens when it is removed during cataract surgery. IOLs are also used for a type of vision correction surgery called refractive lens exchange.

Before the use of intraocular lenses, if you had cataracts removed, you had to wear very thick glasses or special contact lenses in order to see clearly after cataract surgery, since no device was implanted in the eye to replace the focusing power of the natural lens.

Today there is a wide variety of premium IOLs to choose from. The best intraocular lens for you depends on many factors, including your lifestyle and your specific visual needs.

The following is an overview of premium IOLs currently available. With the NHS, you will usually be offered monofocal lenses, which have a single point of focus. This means the lens will be fixed for either near or distance vision, but not both.

If you go private, you should be able to choose from a wider selection of IOLs and during your preoperative exam and consultation, your cataract surgeon can help you choose the best IOL for your needs, as well as additional cataract surgery costs involved if you choose one of the following premium lens implants.

Aspheric IOLs

Traditional intraocular lenses have a spherical optical design, meaning the front surface is uniformly curved from the centre of the lens to its periphery. Though a spherical IOL is relatively easy to manufacture, this design does not mimic the shape of the natural lens inside the eye, which varies in curvature from center to periphery. In other words, the eye's natural lens is aspheric ("not spherical").

Why is this important?

A spherical intraocular lens can induce minor optical imperfections called higher-order aberrations (HOAs), which can affect quality of vision, particularly in low-light conditions such as driving at night.

Premium aspheric IOLs, on the other hand, match more closely the shape and optical quality of the eye's natural lens, and thereby can provide sharper vision — especially in low light conditions and for people with large pupils.

Toric IOLs

Toric IOLs are premium intraocular lenses that correct astigmatism as well as near sightedness or long sightedness.

Like toric soft contact lenses, toric IOLs can correct astigmatism because they have different powers in different meridians of the lens. They also have alignment markings on the peripheral part of the lens that enable the surgeon to adjust the orientation of the IOL inside the eye for optimal astigmatism correction.

Just prior to cataract surgery, the surgeon places temporary markings on the patient's cornea that identify the location of the most curved meridian of the front of the eye. Then, when the toric IOL is implanted during the cataract procedure, the surgeon rotates the IOL so the markings on the IOL are aligned with the markings on the cornea to ensure proper astigmatism correction.

Prior to the development of toric IOLs, cataract surgeons had to perform a procedure call limbal relaxing incisions (LRI) to correct astigmatism during or after cataract surgery.

In LRI, small incisions are made at opposite ends of the cornea, very near the junction between the cornea and the surrounding white sclera. When these incisions heal, the cornea becomes more spherical in shape, reducing or eliminating astigmatism.

In some cases — even when a toric IOL is used — limbal relaxing incisions may be needed after cataract surgery to fully correct astigmatism. But typically in such cases, the amount of astigmatism remaining after implantation of a toric IOL is far less, making a better LRI outcome more likely.

LASIK and PRK can also be performed after cataract surgery to correct residual astigmatism, but toric IOLs decrease the likelihood of needing these additional surgical procedures.

Accommodating IOLs

Conventional spherical IOLs are monofocal lenses, meaning they are designed to provide clear vision at a single focal point (usually far away for good driving vision, for example). With conventional IOLs, typically you must wear eyeglasses or contact lenses in order to use a computer, read or perform other close-up tasks within arm's length.

Accommodating IOLs are premium intraocular lenses that expand the range of clear vision with both an aspheric design and flexible "haptics" — the supporting legs that hold the IOL in place inside the eye.

These flexible legs allow the accommodating IOL to move forward slightly when you look at near objects, which increases the focusing power of the eye enough to provide better near vision than a conventional monofocal lens.

Accommodating IOLs may not provide the same level of magnification for near vision that a multifocal IOL does. But many people find these premium IOLs greatly reduce their need for reading glasses or computer glasses after cataract surgery, while providing exceptionally clear distance vision similar to that offered by a monofocal IOL.

Multifocal IOLs

Multifocal IOLs are another category of presbyopia-correcting IOLs that can decrease your need for reading glasses or computer glasses after cataract surgery.

Like multifocal contact lenses, these premium IOLs contain added magnification in different parts of the lens to expand your range of vision so you can see objects clearly at all distances without glasses or contact lenses.

Some studies have shown multifocal IOLs tend to provide better near vision than accommodating IOLs, but they are also more likely to cause glare or mildly blurred distance vision as a trade off.

Your cataract surgeon can help you decide if you are a good candidate for multifocal IOLs at your preoperative exam and consultation.

Laser cataract surgery is often recommended if you are interested in multifocal IOLs, because precise alignment of these lenses is very important to give you the best visual outcome at all distances.

Monovision

An alternative to accommodating and multifocal IOLs for correcting presbyopia is monovision.

There is no such thing as a "monovision IOL." Monovision is the technique of fully correcting the refractive error of one eye and intentionally making the other eye mildly near sighted.

In this scenario, the fully corrected eye sees distant objects clearly (but cannot see very well up close without glasses), and the mildly near sighted eye sees very well up close without glasses (but not so clearly far away).

Monovision may sound odd the first time you hear about it, but this technique has been used very successfully with contact lenses for many years. And it is now being used frequently with cataract surgery to decrease a person's dependency on reading glasses and computer glasses after surgery.

Any combination of premium IOLs can be used for monovision cataract surgery.

When accommodating or multifocal IOLs are used for monovision, the term "modified monovision" is often used, since these lenses offer an expanded range of vision by nature of their design in addition to a prescribed monovision effect.

A different type of IOL for each eye

Sometimes, the best visual outcome after cataract surgery is achieved by using a different type of premium IOL in each eye.

For example, you may have more astigmatism in one eye than the other. If this is the case, your cataract surgeon may recommend a toric IOL in that eye, and perhaps an accommodating IOL in the other eye to also decrease your need for computer glasses.

Another scenario is for your cataract surgeon to recommend one brand of multifocal lens for one eye and a different brand for the other. This is because one brand may provide better computer vision and the other may provide sharper vision at a closer distance, for reading and other close-up tasks.

Your cataract surgeon can fully evaluate your specific needs during your pre-op exam and consultation, and help you choose the best combination of premium IOLs for a successful visual outcome.