Toric lenses are the standard method for correcting astigmatism, a common vision issue resulting from an irregularity in the eye’s curvature. This specialized lens design is used in both contact lenses and intraocular lenses. Toric lenses properly focus light onto the retina, providing clear vision.
Understanding Astigmatism
Astigmatism is a type of refractive error where the eye is not perfectly spherical, causing light to bend unevenly as it enters the eye. Instead of being shaped like a basketball, an eye with astigmatism is often shaped more like an American football or the back of a spoon. This irregular curvature prevents incoming light from focusing to a single point on the retina.
The result is blurred or distorted vision that affects both near and far objects. Astigmatism can stem from an irregularly shaped cornea, which is called corneal astigmatism and is the most common form. It can also be caused by an irregular curvature of the lens located inside the eye, a condition known as lenticular astigmatism.
What Toric Lenses Are and How They Correct Vision
Toric lenses are specifically designed to counteract the uneven curvature of the astigmatic eye. Unlike standard spherical lenses, which have the same curvature and power across the entire surface, a toric lens is shaped like a slice of a torus, a geometric shape similar to a doughnut. This unique design incorporates two different optical powers across two distinct meridians, which are perpendicular to each other.
This dual-power system ensures that the lens can provide the necessary correction for both the flatter and steeper curves of the astigmatic eye. The lens provides a cylindrical power that precisely compensates for the eye’s irregular shape, making sure that light rays converge correctly onto the retina. Because the correctional power is only applied along a specific axis, the lens must maintain a precise orientation on the eye to function effectively.
Where Toric Lenses Are Used
Toric lens technology is primarily used in two major forms of vision correction: contact lenses and intraocular lenses (IOLs). Toric contact lenses are used for daily or extended wear to correct astigmatism. They are available in both soft and rigid gas permeable materials, offering a comfortable and non-permanent way to achieve clear vision.
Toric intraocular lenses (IOLs) offer a permanent solution, as they are implanted into the eye during cataract surgery or refractive lens exchange. Implanting a toric IOL allows surgeons to correct the patient’s pre-existing astigmatism along with the cataract, often reducing the patient’s dependence on glasses for distance vision.
The Fitting and Stabilization Process
Prescribing a toric lens requires a detailed measurement of the eye to determine the precise location and degree of the astigmatism. This involves measuring the axis of astigmatism, which is specified in degrees ranging from 0 to 180. Even a slight misalignment can significantly reduce the lens’s effectiveness, as a rotation of just 10 degrees can lead to a considerable loss in visual clarity.
Toric contact lenses are manufactured with specialized stabilization features to keep them from rotating during blinks and eye movements. For toric IOLs, the surgeon carefully rotates the implanted lens during surgery to align its corrective axis perfectly with the eye’s astigmatism. This precise alignment is necessary because for every three degrees of misalignment, the lens loses about 10% of its astigmatism-correcting power.