Astigmatism is a condition whereby
the cornea is shaped like a football (elliptical) instead of a basketball
(spherical). This creates distortion by elongating images on the
retina. For example, a square would be seen as a rectangular image
on the retina. Thus an object appears blurred at all distances.
To correct astigmatism, the corneal
shape must be changed from an ellipse to a sphere (football to basketball).
By changing the pattern of the beam, tissue is removed more in one
direction than in the other.
In the normal eye, light traveling through the cornea in
both the horizontal plane (blue) and vertical plane (red)
is refracted (bent) the same, so that the image is properly
focused on the retina in the back of the eye. With astigmatism,
an abnormal curvature of the cornea in one direction causes
light traveling in one plane, the horizontal (blue) plane
in this case, to be focused in front of the retina. The light
striking the retina through the horizontal plane is out of
focus on the retina and vision is unclear.
the cornea will be treated in such a way with the Excimer laser
as to effectively "flatten" the cornea in the steep axis. In
the example above, the steep horizontal axis (blue) is
flattened in the green area shown in the figure on the right.