Astigmatism is a common vision problem resulting from an asymmetrical curve in the cornea or the lens inside the eye. This condition causes light entering the eye to focus on multiple points instead of a single, precise location on the retina. Standard contact lenses have a uniform, spherical shape and cannot effectively correct this uneven focusing, leading to persistent visual blur. A specialized contact lens is necessary to precisely counteract this refractive error and restore clear vision.
Understanding Astigmatism
Astigmatism is a refractive error where the eye is shaped more like a football than a perfectly round baseball. This uneven curve in either the cornea (corneal astigmatism) or the lens (lenticular astigmatism) means the eye bends light rays differently along two perpendicular axes. Instead of a single, sharp focal point, the light is spread out, causing vision to be blurry or distorted at all distances. This irregular shape can make fine details appear wavy or stretched, often leading to symptoms like eyestrain and headaches. Astigmatism is frequently present at birth and can occur alongside nearsightedness or farsightedness.
The Specialized Lens for Astigmatism
The specific lens designed to correct this refractive error is known as the toric contact lens. The term “toric” describes a shape derived from a torus, resembling a slice cut from the side of a donut. This design relates directly to the asymmetrical shape of the astigmatic eye. Unlike standard spherical lenses, a toric lens possesses two different corrective powers oriented along two perpendicular axes. One power corrects the spherical component of the prescription, while the other—the cylindrical power—specifically addresses the astigmatism.
How Toric Lenses Correct Vision
Toric lenses incorporate a cylindrical power that aligns with the specific axis of the astigmatism in the eye. Because of this precise alignment requirement, the lens must remain in a stable, specific orientation on the eye. Even a rotation of just 10 degrees can significantly reduce the clarity of vision.
To prevent rotation, manufacturers use various stabilization mechanisms. These features ensure that the corrective cylinder power remains aligned with the unique axis of the patient’s astigmatism, providing consistently clear vision. Tiny, often invisible, orientation markings are included on the lens surface to help the eye care professional verify this alignment during an examination.
Stabilization Mechanisms
One common method is prism ballast, which makes the bottom edge of the lens slightly thicker and heavier, using gravity to encourage it to settle in the correct inferior position. Another approach is dynamic stabilization, or thin zones, where the lens is thinned at the top and bottom edges, causing the eyelids to center the lens upon blinking. Some designs also incorporate truncation, which involves cutting a small, flat edge on the bottom of the lens to interact with the lower eyelid margin for stability.
Getting Fitted and Wearing Toric Lenses
The fitting process for toric contact lenses is more involved than for standard spherical lenses due to the requirement of precise orientation. An eye care professional must first determine the exact axis and degree of the astigmatism, which is then translated into the lens’s cylindrical power and axis. The optometrist will insert a trial lens and allow it to settle for several minutes, often around five to ten, to assess its fit and stability.
The lens’s orientation is checked using the faint markings on the lens surface, and the eye doctor may use a technique like the LARS (Left Add, Right Subtract) method to compensate for any observed rotation. Because the lens must settle perfectly, multiple trial lenses may be necessary to achieve the best possible clarity and comfort. This specialized requirement means toric lenses are typically more expensive and may take longer to order than standard lenses due to the necessary customization.