Before we can ask "How do toric lens work", we really need to understand what they do, and what they were created for.
Toric
contact lenses can be soft, meaning they are made from soft plastic
that can alter its shape to fit the shape of the eye. This is as opposed
to hard (gas permeable - GP) lenses which are rigid in construction. A
toric lens is special, in that it was designed to help correct
astigmatism.
What Is Astigmatism?
Astigmatism can take two
forms, but it is usually understood to mean an optical malfunction
caused by a misshaping of either the cornea or the optic lens. A normal
cornea is one with is round in shape, just like a ball. In an
astigmatic, the shape is different, and the ends of the cornea have
extended ends. More rugby ball or american football shaped. What happens
then is that the light hits the cornea in two places rather than one.
This is because the shape of the astigmatic eye has a steeper and
flatter curve due to the shape of the cornea. This is called corneal
astigmatism.
Sometimes, astigmatism is the result of an
irregularly shaped optic lens, which is located behind the cornea. This
is called lenticular astigmatism.
The effect to the sufferer is
much the same though. Astigmatism manifests itself as slightly blurred
vision, but can also cause headaches and eye aches.
How Is a Toric Lens Different To Other Contact Lenses?
When
you place a contact lens in your eye, the lens sits on top of a film of
water. If it didn't, your eye would be dry, and it would cause you
discomfort. Because the lens is on a liquid platform, it can rotate, but
in non astigmatics, this is not really a problem. However, in an
astigmatic, the lens needs to stay put in order to correct the
astigmatism. In a toric lens, this is achieved by weighting the lens so
that the bottom is heavier than the top. This of course means that the
bottom of the lens will always point downwards, ensuring that the
prescription of the lens acts correctly.
There is one other
difference of a toric lens over a normal one, in that the lens has two
prescriptions in one. One will correct the astigmatism, and the other
will correct the shortsightedness or long sightedness (depending on what
you have).
