Implants
Patients have refractive errors, such as myopia (short sightedness) or hyperopia (long sightedness), because the curve of the cornea does not match the length of the eye. Focusing of light, therefore, takes place other than on the retina of the eye. Most of the refraction, or bending of the light rays, occurs at the corneal but some refraction, especially those concerned with fine-tuning, also occurs within the eye at the natural lens.
It is not possible to change the length of the eye, but it is possible for the Excimer laser to change the curve of the cornea, this is made flatter if you are short sighted or steeper (more curved) when you are long sighted. However only a certain amount of corneal curvature change is possible and with high degrees of myopia or hyperopia excessive change of cornea curvature results in loss of quality of vision.
Also, if the Excimer laser removes too much corneal tissue the cornea can become too thin and a danger of the cornea beginning to become weak arises. With these high refractive errors it is better to leave the cornea alone or perhaps use it for fine tuning later. To correct high refractive errors we need to look at the role that the natural lens plays within the eye to focus.
Phakic Implants
With a phakic implant we supplement the strength of the natural lens by placing an extra lens within the eye on top of the natural lens. The phakic implant is inserted into the eye as a minor surgical procedure and is done under local or general anaesthesia.
The phakic implant is rolled into a tight tube and placed through a minute incision into the eye, once inside it unrolls itself into the correct position. By supplementing the strength of the natural lens the phakic implant allows the patient to focus normally.
Visual recovery is rapid and usually within a day, although slight improvement occurs over the following two or three weeks. Any residual astigmatism or minor refractive error can, of course, be corrected with subsequent Excimer laser surgery of the cornea.
Because the natural lens is left untouched the patient is still able to focus naturally for near. In those patients over the age of 45 who are already having problems focussing for near, spectacles will be required for reading. Phakic implants are relatively new but have been used extensively in patients with high refractive errors during the past five years.
It is known that a small percentage of patients will develop cataracts in the natural lens, but these are usually older patients. For these patients we normally recommend a different procedure where the natural lens is replaced by a special implant as opposed to the phakic implant that is inserted in addition to the natural lens.
If a patient does develop a cataract this is easily remedied but does require a further operation. All these complications will be discussed with you prior to surgery.
Lens Implants
One of the most successful operations performed by eye surgeons is the cataract operation. This is extremely common and over one and a quarter million people in the USA have cataract operations every year. It is therefore a safe and very predictable operation. It is performed when the natural lens becomes opaque or not able to transmit light anymore.
In the standard cataract operation the natural lens is removed and replaced by an implant (man made lens). It is possible to calculate what implant is required so that the patient does not require glasses for distance vision after the operation.
In patients who have high refractive errors and where the focusing of the eye is abnormal it is also possible to remove the lens and replace it with an implant of the correct power. The operation is exactly the same as a standard cataract operation although the indications are different. By assessing the length of the eye with ultra sound and measuring the curvature of cornea it is possible to accurately predict what strength implant is required to allow patients to see well in the distance. If there is any residual astigmatism or other refractive error, fine-tuning on the corneal surface is, of course, possible later.
The operation is very simple and is done under local anaesthesia. The whole procedure takes approximately 15 minutes and visual recovery is very quick and most patients see well the next day. Because it is a minor operation it is done within an operating theatre but as it is done under local anaesthesia patients are not required to stay over night and are discharged from hospital usually within an hour of the operation.
Unlike the young natural lens, the implant has a fixed focus. This is usually made for distance vision and the patient then requires reading glasses. The ideal patient for a lens implant is one who has a high refractive error and who already requires an extra prescription for near vision. These are usually patients over the age of 45 years.
In patients who are having both eyes operated, it is possible for one eye to be left slightly short sighted allowing the patient to have a greater range of vision and to see relatively well close up with one eye. This situation, were one eye sees better for distance and the other for near, is known as mono vision and we will discuss this with you prior to the operation.