toric contact lenses
ACUVUE® toric contact lens. Image: Johnson & Johnson Vision Care, Inc.
A toric contact lens has two different powers or curvatures so that it can correct for both astigmatism and either myopia (near-sightedness) or hyperopia (far-sightedness). Increasingly, toric contact lenses, which typically combine the effects of a cylindrical lens with that of a spherical lens, are being prescribed for people who are astigmatic (i.e. have corneas that are not perfectly round) and also need help with their far-away or close-up vision. These are people who, in the past, may have been told that they are not suitable for contact lenses.
Sometimes a patient will have astigmatism in only one eye. In such cases, the eye care practitioner may prescribe a toric lens for one eye and an ordinary spherical lens for the other.
Like regular lenses, toric lens may be soft or rigid gas permeable. Unlike regular lenses, which have the same power all around so that it doesn't matter if they rotate on the eye, torics have a definite "up" and "down" because they are not spherically symmetric. This means that a toric lens must keep the same orientation regardless of eye movement, otherwise visual acuity is affected. The usual way of keeping toric lenses in place is to make them slightly thicker, and therefore heavier, at the bottom. This thicker zone is pushed down by the upper eyelid during blinking to allow the lens to rotate into the correct position. Some lenses have ballasting at both top and bottom (see illustration).
fitting and cost
Fitting toric contact lenses takes more time and expertise than in the case of spherical contacts and therefore can be more expensive. (Toric lenses are usually marked with several tiny striations or orientation marks, which invisible to the wearer, to assist their fitting. See illustration) Torics are also usually more expensive to manufacture than non-toric lenses, which is reflected in their additional cost.
soft and RGP torics
Toric lenses are now available in the same wide range of formats as regular contacts. In the past, only rigid gas permeable (hard lenses that allow oxygen through) were prescribed for correcting astigmatism. Today, torics come in soft or RGP types, and are available in extended wear, frequent replacement, disposable, and even daily disposable forms. There is also the option of colored torics to change or enhance natural eye color.
Most people opt for soft lenses over RGP because they are easier to get used to and generally more comfortable to wear. However, some people prefer RGPs because they retain their shape on the cornea better than soft lenses do and tend to provide slightly sharper vision for people with astigmatism.
options for mild astigmatism
People who suffer only a low degree of astigmatism, between about +1.00 or -1.00 diopter, may be able to see well wearing regular spherical rigid gas permeable or even spherical soft contact lenses. In the case of RGPs, the lenses are sufficiently stiff that they make the cornea conform to their shape and thus help mask a need for astigmatic correction. In the cases of soft lenses, a higher powered spherical prescription may achieve the same effect of masking the astigmatism. A qualified eye professional will provide advice on these options.
options for more acute or complex astigmatism
Thanks to multifocal toric contact lenses, people who need vision correction both for reading and for seeing clearly far away as well as astigmatism can have their requirements met with a single contact lens prescription. Although some soft toric multifocal brands are available, more commonly an RGP lens is prescribed in this instance.
B-toric contact lenses are also available for high astigmatic prescriptions. These lenses come in soft, rigid gas permeable, and disposable formats. A bi-toric lens enables a different correction to be made on either side of the lens. Disposable types are available for astigmatisms under 3.00 diopters, whereas custom-made torics can be produced that will handle as high as 12.00 diopters of astigmatism.