Astigmatism and Toric Contacts
Astigmatism means the front of the eye, the cornea and/or the lens within the eye is slightly bent. “Football shaped” is a commonly used analogy but is an exaggeration of the amount of bend. If you were to visualize a balloon which is perfectly spherical and imagine you could squeeze the balloon from its sides it would bend with the steep curve horizontal. This is what astigmatism is. These different curvatures of the eye cause light to focus in two different points in the eye, one behind the other. The greater the amount of bend the farther apart these two focus points are and the blurrier the vision.
The balloon you squeezed could be tilted to a different angle. In fact it could be tilted one hundred and eighty orientations or 180 degrees. Specific orientation is important in the correction of astigmatism. Astigmatism, therefore, has two factors that determine the correction: the amount of bend and its orientation.
A toric lens has two powers: one for the nearsightedness or farsightedness and the other for the astigmatism. Since the astigmatism can be of 180 different orientations, one combination of power could conceivably be made 180 ways! You mathematicians can begin to understand the enormous number of combinations that could be made.
Actually, with disposable lenses, the orientation or as it is called optically the “AXIS,” is made in five or ten degree intervals. Higher powers require more precision and can be made in any axis.
Because toric lens have more power in one direction, they must resist the temptation to rotate on the eye. Toric lenses are therefore ballasted or made slightly thicker at the bottom of the lens to prevent rotation.
Toric lenses today are vastly improved over their predecessors of five to ten years ago. New thinner and more stable designs are introduced frequently. One of the newest innovations is a silicone based lens that has exceptionally high oxygen transmissibility. Another is a toric geared to the dry eye patient that binds water upon it to prevent evaporation.
Astigmatism is extremely common in eyeglass prescriptions. There are, however, far fewer patients wearing their correct astigmatism prescription with soft lenses and experience the world with compromised vision. Torics are an answer to clearer vision with soft contacts. Gas permeable lenses and Synergeyes lenses are other options to correct astigmatism.