What are the different types of lens for Cataract?
An intraocular lens (IOL) is a small artificial lens that replaces the eye's natural lens during cataract surgery. The IOL provides similar focusing power to the natural lens; without it very thick glasses would be required after cataract surgery.
IOLs can be made with different focusing powers, like prescription spectacles or contact lenses. Here in Melbourne, before cataract surgery careful measurements will be made of the dimensions of your eye, known as biometry, (see preceding topic) to help determine the right power IOL for your eye.
Figure 1. Intraocular lens. It is coloured yellow to filter harmful UV light
What are IOLs made of?
As synthetic implants, most IOLs are made of acrylic, silicone or other plastics. These materials are inert and do not react with your eye or cause harm to the eye’s natural structures. IOLs are coated with a special material that protects your eyes from harmful ultraviolet (UV) light.
What are the different types of IOL?
Fortunately today we have a wide variety of IOLs to choose from. The best IOL for you depends on several factors, including your lifestyle and specific visual needs.
1. Monofocal IOLs
The most common type of lens used with cataract surgery is a monofocal IOL. It is called monofocal because it has one focusing distance. It is set to focus for close work, medium range or distance vision – chosen depending on your visual needs. Like most people you may wish to have it set for clear distance vision; this is best for driving, walking and seeing people at a distance. Generally eyeglasses are needed for reading or close work.
Monofocal IOLs are very successful and may well be the best choice lens for you. They are unlikely to be suitable if you have astigmatism – which may be detected on biometry measurements.
It is important that monofocal IOLs are aspheric (meaning the curvature varies from centre to periphery). Aspheric IOLs mimic the curvature of the eye’s natural lens; this is required for great vision. Without this aspheric curvature there will be minor optical imperfections (known as higher-order aberrations) that reduce the quality of vision, particularly in low-light settings. Premium aspheric IOLs, in comparison, provide sharper vision as they closely follow the shape and optical quality of the natural lens — especially in low light conditions (like driving at night).
I only use premium aspheric IOLs when I perform cataract surgery in Melbourne.
2. Toric IOLs
Toric IOLs are premium lenses that correct astigmatism as well as short-sightedness or long-sightedness. Astigmatism is when the eye’s focusing power differs in different directions, for instance is greater horizontally, and lesser vertically. Toric IOLs can correct astigmatism because, like the astigmatic eye, they also have different powers in different directions that balances the eye’s astigmatism. Toric IOLs need to be aligned to the correct orientation to fully correct the astigmatism. Computer Assisted Cataract surgery, available in Melbourne, involves eye-recognition software that guides the Ophthalmologist through the operating microscope to orientate the lens correctly for best visual outcomes.
Figure 2. Astigmatism. The focusing power of the eye differs for vertical and horizontal light. A toric shaped lens corrects for this.
3. Multifocal IOLs
A multifocal IOL is a premium IOL that provides both distance and near focus at the same time. The IOL has different zones set at different focusing powers. It may allow you to see clearly in the distance without glasses as well as read or use a computer, also without using glasses (although you still may need glasses for some circumstances eg reading small newspaper print).
The different zones of the IOL focus light differently, splitting the light between seeing near and far, which means unlike a monofocal IOL, there is never 100% of the light being used for distance viewing or close work. Not everyone can tolerate this - because of the special optics of a multifocal IOL, it is only suitable for 5-10% of people considering cataract surgery.
Figure 3. Multifocal IOL
Are you suited a multifocal IOL?
You may be suited for a multifocal IOL if before cataract surgery you don’t require glasses for distance vision or are a little long-sighted with minimal astigmatism.
Who shouldn’t have multifocal IOLs?
If you are very short-sighted or have large amounts of astigmatism, the multifocal IOL may not be right for you. Likewise it should be avoided if you have other eye disease (such as glaucoma, macular degeneration, diabetic retinopathy) or are at risk of developing such eye diseases. (As a glaucoma specialist I take great care to diagnose and treat glaucoma, as well as cataracts). If you are the sort of person that notices every detail, you may not enjoy these lenses due to the bothersome optical side effects (aberrations, like halos) that are often noted by people who have multifocal IOLs implanted.
What are the problems of a multifocal IOL?
These need to be carefully understood before surgery. Due to the special optics of the IOL, after implantation you may see halos or sparkles around lights, particularly when driving at night. These may not bother you greatly, but some people are so bothered by these optical effects they may need further surgery to replace the lens, which has an increased risk of complications.
Monovision is where one eye is implanted with a monofocal IOL aiming for distance vision, and the other (non-dominant) eye has a monofocal IOL aiming for near vision. One eye sees for distance, and the other eye sees for near.
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 and successfully with cataract surgery to decrease a person's dependency on reading glasses and computer glasses after surgery. However reading glasses still may occasionally be required (eg for fine newspaper print).
Monovision does not work for everyone and some people (approximately 20%) are unable to adapt to using one eye for distance viewing and one for near/intermediate viewing. This is why, if you are considering monovision, we recommend you trial monovision with contact lenses prior to surgery. This is to simulate and establish, firstly, whether you can tolerate monovision and secondly, the level of monovision you prefer or tolerate (near or intermediate).