Why soft contact lenses do not usually provide satisfactory astigmatism correction.
Soft contact lenses have an inherent problem with trying to correct astigmatism.
The problem is that with each blink, contact lenses not only move up and down, they
also rotate either clockwise or counter clockwise. The rotation does not matter if
the correction is only spherical (meaning nearsighted or far sighted corrections).
However with astigmatism, the curvature of the corneal surface and the curvature of
the contact lens each vary in different directions (like the shape of an American
football). When the lens is in perfect position, everything is great, but then we have
to blink and the lens rotates a few degrees, and with the next blink a few more
degrees. Suddenly the soft contact lens is no longer correcting the disparity in
curvature, in fact when it rotates 90 degrees it actually makes the astigmatism twice
as bad as it was before!! In order to compensate for this rotational movement, soft contact lenses for
astigmatism are frequently made with ballasts or prisms so that when an eye blinks
and the lens rotates, the ballasts or prisms cause the lens to rotate back. This helps,
but many patients find this disorienting – especially as the two eyes rotate the
lenses in opposite directions. All contact lenses have to move up and down a little
with each blink to keep tears moving in and out from under the lens. So with each
blink the vision is made a little blurry with contact lenses, even spherical ones. But
with astigmatism we also get rotational disorientation typically different for the two
eyes. This is why many people don’t even have their astigmatism corrected at all
with their soft contact lenses. Lasik is far superior to soft contact lenses in correcting
astigmatism because the correction is applied directly on the surface of the eye.
The correction does not move with each blink; instead, it is stable in any direction of gaze.