Astigmatism is an optical aberration that displaces the eyes vertical focal plane with respect to its horizontal focal plane. This displacement in focal planes, which may be caused by an irregularly shaped cornea and/or crystalline lens, causes images to appear blurry. Astigmatism can be corrected by implanting an optic, such as section of a spherical lens, whose projection onto a plane perpendicular to the optical axis is noncircular (e.g., rectangular or elliptical). Because the optic is noncircular, it provides more optical power along one axis than along another axis. As a result, it introduces an astigmatism that can be used to offset or compensate the eyes corneal and/or lenticular astigmatism when aligned properly with respect to the principal meridians of the cornea and/or crystalline lens.
