Toric Intraocular Lenses for Correction of Astigmatism in Primary Cataract Surgery

Astigmatism is an optical aberration caused by the toricity of an optical system. If two meridians of an optical system with the least and highest refractive power are positioned perpendicular to each other, the resulting aberration is defined as regular astigmatism. Instead of one focal point, regular astigmatism results in two perpendicular focal lines and a circle of least confusion (Fig. 5.1). Regular astigmatism is defined as ‘with-the-rule’ (WTR) when the steepest meridian of the optical system is positioned between 75 and 105° (Fig. 5.2b) and as ‘against-the-rule’ (ATR) when the steepest meridian is positioned between 165 and 195° (Fig. 5.2c). Astigmatism in other directions (15–75° and 105–165°) is defined as oblique. In contrast to regular appearances of astigmatism, irregular astigmatism is characterized by, e.g., three or more main meridians of the optical system (generating higher order aberrations like trefoil, quadrafoil, etc.). Corneal dystrophies like kera-toconus also result in irregular astigmatism. This astig matism does not follow a typical kind of regular pattern. Figure 5.2d shows an irregular post laser in situ keratomi-leusis (LASIK) astigmatism. As technical-optical systems can be designed with regular toricity, they are able to cor rect astigmatic aberrations of another toric optical system, just like spherical optics correct defocus aberrations of defocused optical systems (Fig. 5.2a). This is also the basic principle of toric intraocular lenses (TIOL) (Fig. 5.3).