Until recently, the main goal in the treatment of astigmatism was complete refractive correction of the defect. The commonly adopted methods followed two main strategies: ablation on one meridian only, or split on to the two meridians of astigmatism, axis and power. Treating all astigmatism on one meridian leads to marked corneal asymmetry, similar to the previous situation of astigmatism: asymmetry is merely moved from the center of the cornea towards the periphery. It is important to note that the shape of the cornea resulting from this correction is frequently oblate, very different from a physiological, normal, anastigmatic cornea (Fig. 1). When the pupil dilates even slightly, i.e., to 5 mm, eyes that received this type of treatment show an increase in aberrations, including spherical aberration, quadrafoil, and astigmatism in the periphery.
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