Refractive Multifocal IOLs
Refractive multifocal IOLs have a lens optic that has different optical powers in different parts of the lens. The original model was called the AMO Array® and I used this type of multifocal IOL from 1999 to 2004. The AMO Array® was the first successful multifocal IOL and was the first to be approved by the US FDA. It was replaced in 2004 by the AMO ReZoom®.While these IOLs give good distance vision and good intermediate range vision the reading performance is variable. In my experience the night time halos and glare are worse than the newer diffractive multifocal IOLs. I stopped using refractive multifocal IOLs in 2004 because of these two problems. However, I still have many patients who have done well with these IOLs and are happy with their result.
Diffractive Multifocal IOLs
There are two different models of diffractive multifocal IOL that I use:
- Alcon AcrySof® aspheric ReSTOR® (SN6AD1) with +3 add
- AMO Tecnis® multifocal aspheric acrylic optic (ZMA00) with +4 add
Both these diffractive multifocal IOLs are designed to:
- Allow the eye to see both in the distance and to read without the aid of spectacles.
- Reduce the severity of night time halos compared to the refractive multifocal IOLs.
Alcon Laboratories is the world’s largest supplier of devices, instruments and eye drops for eye surgery and is based in Fort Worth, Texas. AMO Surgical is the second largest eye surgery supplier. For more information you can Google the Alcon AcrySof ReSTOR or the AMO Tecnis multifocal acrylic IOL.
How a diffractive multifocal IOL works
On the front surface of the ReSTOR lens are a series of concentric rings that form what is called a diffraction grating. Diffraction gratings have the property of bending light rays to focus light at two places simultaneously. This allows the lens to have two separate focal points, one for distance and the other for near vision. With the ReSTOR IOL there are 16 rings on the front surface of the lens over a diameter of 3.6 mm. The total diameter of the optical part of the lens implant is 6.0 mm.
The Tecnis multifocal works in the same way but has the diffraction grating rings on the entire back surface of the IOL. The Tecnis will theoretically give better reading performance in low light conditions such as in restaurant but probably has slightly worse halos at night time.
The optical properties of the ReSTOR IOL change with changes in your pupil diameter.
On the ReSTOR IOL the height of the steps in the rings is highest in the more central rings and decreases gradually in the more peripheral rings. This property of the lens is called “apodized”.
Apodization means that in bright light when your pupil is small the amount of light that goes to the distance focus is roughly equal to the amount of light going to the near focal point.
This means that as your pupil enlarges in dim light the amount of light going to the distance focus increases and the amount of light going to the near focus decreases. This helps to reduce night time halos around lights.
There is no change in the balance between the amount of light going to the near or distance focal points with the Tecnis multifocal. In theory this means slightly more disturbance of night time vision but better reading ability when the pupil is larger in dim light.
Reading properties of the Tecnis multifocal or ReSTOR lens
The reading distance of the Tecnis multifocal IOL is much closer than most people are used to. It is about 25cm from your eye. The newest version of the ReSTOR IOL has moved this out to about 33 cm which is a more normal reading distance. The ability to read at this close distance is typically very good and most people can read much smaller print than with either the older refractive multifocal IOLs or with the accommodating IOL.
Disadvantages of the Alcon ReSTOR and AMO Tecnis multifocal IOLs
- The reading distance is very precise and is a little closer than most people are used to at 33cm with the ReSTOR and 25-30cm with the Tecnis. This was done to separate the two focal points of the lens so as to reduce night time halos.
- There is a “dead spot” at about 1 metre away that is between the two focal points of the IOL. You may need +1.00 dioptre glasses to bring the distance focal point in to see a computer screen or read music to play a musical instrument.
- Critical observers may complain that the distance vision is not quite as “sharp” as it was with their glasses. There is always some trade off to achieve near vision without glasses. If you are particularly fussy about the quality of your vision I do not recommend diffractive multifocal IOLs.
- There is a small but measurable reduction in contrast sensitivity.
- The Alcon ReSTOR or AMO Tecnis multifocal IOL are both more expensive than a non-multifocal IOL. I charge an additional fee of $1200 including GST for diffractive multifocal IOLs.
- Multifocal IOLs are very sensitive to any opacity of the posterior capsule. About 50% will require a YAG laser capsulotomy at some time in the first 5 years after surgery. There is a charge for this but it can be claimed for on medical insurance.
Post-Operative Problems with Diffractive Multifocal IOL Implantation
- Halos around lights at night time are common but rarely prevent night driving.
- Ghost images – often due to astigmatism.
- Reading vision may not be good after only one eye has been done. Most people find reading much easier after both eyes have been operated on
- The clarity and sharpness of distance vision depends on correction of pre-existing astigmatism. I perform limbal relaxing incisions for astigmatism correction at the time of surgery without any additional charge. I do not guarantee their effectiveness as they may under or over correct astigmatism. If there is residual uncorrected astigmatism that affects your vision then I will offer to correct it with excimer laser surgery at cost price (half the normal price).
- Post-operative refractive error, whether that be hyperopia (far sightedness), myopia (short sightedness) or astigmatism can affect the clarity of distance and near vision after any IOL implantation. We do everything possible to minimise this by using the most modern technology (IOLMaster) and intraocular lens formulae to calculate the correct optical power of the IOL. Excimer laser surgery may be required to achieve a perfect refractive result and I charge cost price (half the normal price) for PRK or LASIK after IOL surgery.
- Dr Kent is now offering the crystalens “accommodating” intraocular lens. Unlike other intraocular lenses used to replace the cataractous lens after removal the crystalens accommodating intraocular lens restores your vision to the level you had in your 40’s, enabling you to see in the distance, mid range & for most near tasks without the need for spectacles. Dr Kent was the first eye surgeon in New Zealand to implant this remarkable revolutionary lens.