Astigmatism is a very common yet frequently misunderstood vision problem. A person with astigmatism has blurred vision at all distances, which can range from barely noticeable to severe.
Astigmatism occurs when the cornea has different degrees of curvature in different meridians.
Think of corneal meridians like lines connecting opposing numbers on the face of a clock or watch. For example, a line connecting the 12 and the 6 on the clock dial is one meridian. A line connecting the 3 and the 9 is another meridian. If all corneal meridians are curved equally, there is no astigmatism (though nearsightedness or farsightedness may be present). But in an astigmatic eye, different corneal meridians have different amounts of curve and therefore images cannot come to a clear focus on the retina.
An analogy that is commonly used to explain astigmatism is to compare the shape of your cornea to a football or a baseball. If you have astigmatism, your cornea is shaped more like a football (or the back of a spoon) than a baseball. Like a football (or a spoon), an astigmatic eye has unequal curvature in different meridians. In contrast, a baseball is a perfect sphere with equal curves in all meridians.
How toric soft contacts correct astigmatism
Toric soft lenses are a special type of soft contact lens that have different curves in different meridians to correct astigmatism. Special weighting and/or thin zones in toric lenses keeps the proper lens meridian in front of the corresponding meridian of the eye and prevents the lens from rotating during wear.
Because they have a more sophisticated design, toric soft lenses are more expensive to manufacture and cost more than regular soft lenses.
Regular soft contact lenses have the same curve in all meridians and cannot correct astigmatism.
How rigid gas permeable (RGP) contacts correct astigmatism
As their name implies, rigid gas permeable (RGP) lenses are hard lenses. They maintain their shape on the eye and don’t conform to the unequal contours of an astigmatic eye like soft lenses do.
When an RGP contact lens is worn on an eye that has mild to moderate astigmatism, the space between the (spherical) back surface of the contact lens and the (astigmatic) front surface of the cornea is filled with tears. This layer of tears is shaped in such a way that it becomes, in effect, a lens that fully corrects the astigmatism of the eye.
This “tear lens” behind the RGP contact lens is unaffected by lens rotation. Therefore the only lens power required in the contact lens is the power needed to correct any nearsightedness or farsightedness that exists along with the astigmatism.
In cases of moderate to severe astigmatism, a standard rigid gas permeable contact lens may be uncomfortable or unstable on the eye because the shape of the cornea is so irregular that the contact lens doesn’t center properly on the eye and moves (or “rocks”) too much during blinks. In these cases, a toric RGP contact lens may be used.
A toric RGP lens has a spoon-shaped back surface that more closely matches the front surface of an astigmatic eye for a more stable and comfortable fit. This closer fitting relationship also keeps the lens from rotating on the eye, allowing different powers (curves) to be ground on different meridians of a toric RGP lens. The variable powers on the lens surface correct the astigmatism without having to rely solely on the power of the “tear lens” between the cornea and the back surface of the lens. The front surface of a toric RGP lens may be spherical or toric, depending on the required prescription. (If both the front and back surfaces are toric, the lens is called a bitoric lens.)
Because toric rigid gas permeable lenses have a more sophisticated design, they are more expensive to manufacture and therefore cost more than regular RGP contact lenses.
If you have astigmatism, your eye doctor will help you determine the type of contact lens that is best suited for your particular needs.