Astigmatism and toric intraocular lenses.

The article reviews some of the basic optics of astigmatism and the correction of astigmatism with cylindric lenses. A simple model of the conoid of Sturm is demonstrated, and an ideal position for the conoid is postulated. The orientation of the conoid shows that leaving patients with some simple myopic "against-the-rule" astigmatism is beneficial to near work, whereas "with-the-rule" astigmatism is beneficial for distant viewing. Surgeons should be less aggressive with patients with with-the-rule and against-the-rule astigmatism and more aggressive with oblique astigmatism. The toric intraocular lens (IOL) should be positioned on axis or, if slightly off axis, err on the side away from the vertical or horizontal meridian so that the resultant cylinder is more vertical or horizontal. Clinically significant rotation of the toric IOL occurs in a few cases, but these can be easily rerotated. Rerotation should be done between the first and second weeks after primary implantation.