Intraocular Lens Implants
and Cataract Surgery


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In 1949, Sir Harold Ridley implanted the first intraocular lens in London. He had observed that despite having bits of plastic in their eyes from shattered canopies, the World War II Royal Air Force pilots did not experience any problems or reaction to them. However, instead of being lauded for his visionary approach, he was initially ostracized for his efforts.



Sir Harold Ridley, the pioneer of lens implantation. While he is most famous for his work with implants, he also had a great interest in onchocerciasis. Some of his important research on onchocerciasis was based on his work in Ghana, where he had been stationed during World War II.


(Image adapted from the internet)



Lens implantation only became widespread in the 1970s when new designs and technologies were introduced. FDA approval in the United States was obtained in 1981. More than 3 million implants are now inserted into the eye worldwide each year.



WHAT IS AN INTRAOCULAR LENS?

An intraocular lens (IOL) is a plastic lens that is inserted into the eye as a direct replacement for the natural eye lens. It is inserted in the same place where the original lens is located. This retains the normal focusing mechanism of the eye. The commonest reason for implantation is during cataract surgery. The removal of the cataract (cloudiness of the natural lens) and its replacement by an implant improves the vision significantly, even without glasses.

Previously before these lenses were introduced, the patients would not have their natural lens replaced and so had to wear very thick eyeglasses or some special type of contact lenses. Nowadays, different lenses have been specifically designed for patients with different vision problems.

The implants are made of inert material to avoid causing a reaction when inside the eye. The 3 main materials used are: polymethylmethacrylate, silicon or acrylic material. Polymethylmethacrylate is the cheapest, but is rigid and cannot be folded. Therefore, it needs a bigger incision wound (which may need to be sutured) before it can be inserted into the eye. Silicon and acrylic lenses are softer and can be folded before being inserted into the eye. Therefore, only a smaller incision size is needed. Some lenses are folded into an injector, which allows smoother ‘injection’ of the lenses into the eye.



WHAT ARE THE DIFFERENT TYPES OF INTRAOCULAR LENS?

There are many different types of implants in the market. It is definitely worth discussing your visual acuity needs with your ophthalmologist so that the most appropriate lens specific to your requirements can be selected. Please bear in mind the cost issue. Most insurance companies will cover the costs of standard monofocal lenses. However, you will likely to need to pay the price difference if you choose any of the more advanced (and therefore premium) intraocular lenses.

The different implant design types. The circular part is called the optic. This is the part that focuses light onto the retina. The 'arms' or 'legs' are the haptics. These are important to maintain the position of the optic in the correction location within the capsular bag. The focusing power of the optic depends on the length of the eyeball and curvature of the cornea (front window of the eye). The process of determining the power of the implant is called biometry.

(Image adapted from the internet)




Monofocal intraocular lens. Monofocal lenses are the traditional and standard lenses. They are able to focus light at the chosen distance only: far, intermediate, or near. Simply put, your vision will be clear only at one distance (focal length). The norm is to choose the lens that focuses for far objects. This means that you will see well for distance, but will need glasses or contact lenses for reading (near vision) or computer work (intermediate vision). Often you will also need glasses to sharpen up your focus for distance This most closely mimics what happens during the aging process. As we grow older, our natural lens loses its ability to focus for near presbyopia and this is why we need reading glasses. For most people, a standard monofocal lens supplemented with reading glasses is sufficient for their day to day activities. However, other newer lenses are now available that allow focusing at different distances.



Aspheric intraocular lens. The standard traditional monofocal lens has a front surface that is uniformly curved or 'spherical'. In contrast, aspheric lenses have a flatter periphery. This design helps to improve contrast sensitivity, reduce optical aberrations, and make the vision crisper. However, the advantages of improved contrast sensitivity are debated by some, especially for the older patients. Nevertheless, most younger patients requiring cataract surgery should benefit from the implantation of aspheric lenses. Examples include Akreos AO Aspheric, Tecnis Z9000 and AcrySof SN60WF.


Blue light--filtering intraocular lens. Some of the newer implants have been manufactured to have a yellow tint to provide blue light protection. This is to mimic the original lens which naturally filters these potentially harmful rays. Blue light ranges from 400 to 500 nanometers, and is present in natural and artificially produced light. Some of the premium lenses already incorporate this feature in their design. Theoretically, minimizing entry of blue light into the eye reduces the risk of damage to the macula, such as in age-related macular degeneration. At present, there is no strong evidence to suggest any significant benefit of blue filter over non-filter lenses. In fact, a study from Austria found that there may be subtle loss of contrast sensitivity in eyes receiving the blue light-filtering implant. This loss of contrast sensitivity may be more pronounced for patients with moderate to severe glaucoma. An example of such a lens is Acrysof Natural.

The blue-filtering implant (far right) has a yellow tint when compared to the other intraocular lenses. Most lens implant types are already designed to block harmful ultraviolet rays, but the yellow tint adds extra protection against blue light.

(Image adapted from the internet)



Toric intraocular lens. Toric lenses were developed to treat astigmatism. Astigmatism is an eye focusing problem where light rays entering the eye become focused at different distances from the retina. To correct astigmatism with glasses, there will be 2 different strengths within the same spectacle lens. Toric lenses apply the same principle to correct astigmatism. Before surgery, your ophthalmologist will mark your eye carefully so that the implant can be placed in the correct position and angle. A potential risk of implanting this lens is distortion and reduced vision if it rotates out of position (which occurs very rarely). If this happens, further surgery may be needed to reposition or replace the implant. These lenses include the Staar Surgical Lens and Acrysof IQ Toric Lens.

(Images adapted from the internet)

Toric implants are able to correct for astigmatism present in the eye. For a successful outcome, the lens has to be rotated to the correct axis. Your ophthalmologist will make marks on your eye before surgery in order to be able to align the marks on your eye with the axis marks on the implant surface.



Multifocal intraocular lens. Multifocal lenses have different zones that allow focusing of light from distant, intermediate and near objects. It is effectively like having trifocal eyeglasses inside the eye. This means that it becomes possible to see well at more than one distance without glasses or contact lenses. However, these lenses may not be suitable for everyone. The main minor complaints from multifocal lenses are: glare, halos, and mild starbursts around light sources at night.

(Image adapted from the internet)

There may also still be a requirement for reading glasses to sharpen up the focus while performing close-up work. Available brands in the market include AcrySof IQ ReSTOR, ReZoom and Tecnis MF.


Accommodating intraocular lens. This lens also allows you to see well at more than one distance without glasses or contact lenses. It is designed with a hinge similar to the mechanics of the eye's natural lens. This hinge allows back and forth movement of the implant, thereby giving natural focusing ability at different distances. Some have suggested that the reading focus for the accommodating intraocular lens may not be as strong as the multifocal intraocular lens. There is also the concern that these lenses are only able to retain the near vision capability over a period of a few months rather than years. A commercially available accommodating intraocular lens is the Crystalens.

(Image adapted from the internet)

Left: The accomodative implant sits in a more backward position when focusing at the distance. Right: The accommodative implant has moved to a more forward position to enable focusing at near objects


Light-Adjustable Lens. This is still undergoing FDA clinical trials. After implantation, the lens is treated with light of a certain wavelength to change its curvature. This adjusts the focusing power to enable even better vision correction tailored to your needs.


“Piggyback” intraocular lens. If the result from your first operation was not satisfactory, your ophthalmologist may recommend inserting an extra lens over the already implanted one (“piggybacking”). This approach is considered to be safer than removing and replacing the initial lens. The piggyback implant may cause problems by rubbing against the iris because of its location in front of the existing lens implant. Patients receiving these lenses will need regular monitoring, at least in the initial period after surgery, to look for potential complications such as uveitis and angle closure glaucoma. However, when implanted in carefully selected and suitable patients, the visual and safety outcomes can be excellent.




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