The refractive error of the eye can be reduced by altering the power of the internal lens of the eye. Replacing the natural lens with an artificial intraocular lens has been routinely used in cataract surgery for many decades. A refractive lens exchange is the same as cataract surgery but with the express purpose of diminishing the requirement for spectacles. Unlike cataract surgery this procedure attracts no government and generally no health insurance rebate. The natural lens of the eye serves two functions. Firstly, it provides one third of the optical power of the eye. Secondly, it is flexible up until our mid-forties and therefore allows the eye to adjust its focus from distance to near as required for the visual task at hand. Loss of the flexibility of the natural lens limits this ability, the result of which is the onset of presbyopia. Once the natural lens is removed during refractive lens exchange the ability of the eye to adjust focus from near to far is permanently lost. This is an important limitation and is the reason why refractive lens exchange is confined to patients over the age of 45 years. The major indications for refractive lens exchange are high hyperopia, high myopia and where there exists an absolute contraindication to laser vision correction such as in certain cases of keratoconus.
Once the natural lens is removed an intraocular lens must be used to replace it. This is determined after a detailed pre-operative assessment, in particular after testing the dimensions of the eye with a device such as the IOL Master. The power and style of the intraocular lens to replace the natural lens requires detailed discussion of the needs of the patient. The types of lenses available are:
- Multifocal Toric
- Extended Depth of Focus
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In refractive lens exchange the first priority is good distance vision unaided. Eyes without astigmatism generally have an appropriately powered monofocal lens but when astigmatism exists in the cornea a toric implant is used to correct this refractive error.
When monofocal or toric lenses are set to give good distance vision there is generally limited ability to perform near tasks without reading glasses. For the majority of patients this is a limitation they are happy to live with. If there is also a desire to have more independence from near vision glasses, three strategies can be employed during selection of the intraocular lenses:
- Multifocal Lens;
- Accommodative Lens.
With monovision the non dominant eye is determined and this eye has an implant set for near vision whereas the dominant eye is set for distance. Many people tolerate this well but some people find the disturbance to depth perception and/or the increased blur at night bothersome.
Multifocal lenses use design features within the lens to produce two images, one in the distance and the other near. Multifocal lenses generally provide very detailed near vision but they compromise contrast slightly and can produce glare and halos around lights at night. Because both lenses are the same, binocular vision is preserved. Whilst the side effects generally diminish greatly over the first three months a small percentage of people (2-3%) find that they prefer to have the lenses replaced with monofocal implants. Multifocal lenses also have a tendency to be blurred at intermediate distance, a feature they have in common with monofocal lenses.
Extended Depth of Focus (EDOF) – This lens creates one elongated focal point and has less incidence of glare and halos compared with a multifocal IOL. The EDOF lenses are designed to improve focus at intermediate and near ranges as well as distance. With all correction strategies to enhance near as well as distance vision, there still remains the expectation of wearing near spectacles for some of the time, especially for fine print.
Whenever surgery is performed inside the eye the risk of complications increases in comparison to eye laser surgery correction. The potential complications are the same as for cataract surgery but generally significantly lower due to the age and general health of the patients being better. Nevertheless, there exists the potential for a poor visual outcome in very rare circumstances. Patients with higher myopic refractive errors having refractive lens exchange must carefully consider the increased potential for retinal detachment over their normal risk.
The intraocular lens can be expected to remain clear and in position for life. The lenses are held in place by the lens capsule which surrounds the intraocular lens. If this capsule becomes cloudy there will be a decrease in visual quality until a posterior YAG laser capsulotomy is performed. This laser treatment is very safe, painless and permanently clears the visual path. About 25% of eyes with monofocal implants and over 50% of eyes with multifocal implants require a YAG laser capsulotomy within the first 3 years after refractive lens exchange.