Eyeworld

EW INTERNATIONAL 76 January 2018 with a Q of –0.45; in both cases the cornea is prolate, but the prolaticity of the second case is greater, which means that the surface has a radius of peripheral curvature more flat than the first, and this means that it will have greater negative spher- ical aberration. A normal aspheric cornea has a Q factor between –0.20 and –0.45; a Q of zero would correspond to a completely spherical cornea, and a Q greater than zero corresponds to an oblate cornea, i.e., more powerful at the periphery than in the center, inducing positive spherical aberration. A hyperprolate cornea is considered to have a Q factor >0.6. Any corneal topographer nowadays will give us the Q value (Figure 2). Intraocular lenses can induce positive spherical aberration, be neutral or induce negative spherical aberration. Lenses that induce positive spherical aberration Spherical intraocular lenses increase spherical positive aberration in the eye, reducing the quality of the retinal image in most patients. There are few cases in which these lens- es would be prescribed, such as in patients with hyperprolate corneas (operated with hypermetropic exci- mer laser). Among the lenses with positive spherical aberration are the MA60AT (Alcon, Fort Worth, Texas) with a spherical aberration of +0.14 +/–0.09 μm, the CT Spheris 204 (Carl Zeiss Meditec, Jena, Germany), and the Sensar lens (Johnson & Johnson Vision, Santa Ana, California). Lenses that do not modify spherical aberration The neutral lenses currently avail- able on the market with prolate anterior and posterior surfaces are the Akreos, SofPort, L161 (Bausch +Lomb, Bridgewater, New Jersey), and the CT Asphina 409M (Carl Zeiss Meditec). These do not modify corneal spherical aberration, are less sensitive to tilt than aspherical lens- es, and provide better image quality than spherical IOLs. Lenses that induce negative spherical aberration Some aspherical lenses have been designed with a prolate anterior surface (Tecnis, Johnson & Johnson Vision), a prolate posterior surface (AcrySof IQ, Alcon), or with both prolate surfaces (FineVision, PhysI- OL, Liège, Belgium, and CT Asphina Understanding continued from page 75 Figure 3. Oculus Pentacam tomographer. Q –0.19. The Q factor varies depending on the diameter at which the measurement is made, generally measured at 6 mm (Figure 2) or at 8 mm (Figure 3), but smaller or larger diameters may be measured if necessary. Figure 4. Positive spherical aberration of +0.34 μm of a virgin cornea; same case as Figures 2 and 3, coefficient Q of –0.22 (prolate cornea); ideal case to use an aspheric lens that induces a negative spherical aberration, for example the Tecnis with negative spherical aberration of –0.27 μm, resulting in a final aberration close to +0.07 μm Figure 2. Carl Zeiss Meditec topographer. Q –0.22 509M). These give better contrast sensitivity when correcting positive spherical aberration of the cornea but generate less depth of focus than spherical lenses (Figure 4). In addi- tion, they require a better centering, since a decentration of the optics of the lens induces other aberrations like coma. The performance of the lens depends on the pupil, and its function deteriorates in mesopic conditions. There are lenses with aspherical profiles that tolerate a greater lens offset, such as the CT LUCIA lens (Carl Zeiss Meditec). The "aspheric Zeiss optics" has a different design known as bi-sign, 4 in which the IOL combines the advantages of neutral and correcting aspherical IOLs, mak- ing this lens a good choice for most patients and ideal for cases with an alpha angle greater than 0.5 mm or cases with the risk of the lens off center (Figure 5). The alpha angle, unlike the kappa angle (best known for its im- portance in refractive surgery), is the distance between the center of the limbus and the visual axis. Under normal conditions an intraocular lens will be centered within the capsular bag, and the center of the bag will be very close to the center of the limbus. The alpha angle can be measured with the Pentacam (Oc- ulus, Wetzlar, Germany) or iTrace (Tracey Technologies, Houston) (Figure 6). Some authors do not rec- ommend implanting multifocal intraocular lenses in patients with alpha angles greater than 0.5 mm. 5 The iTrace device classifies with col- ors the amount of alpha angle; an angle less than or equal to 0.3 mm is green, 0.3 to 0.5 mm is yellow, and greater than 0.5 mm is red. A green alpha angle provides greater confidence that the patient will be looking through the center of the optic zone, as intended. One may consider implanting some multifo- cal lenses when the alpha angle is in the yellow zone. When there is an alpha angle in the red zone (>0.5 mm), the optical axis (center of the capsular bag) may not coincide with the visual axis of the patient, which may lead to a refractive surprise or an unsatisfied patient. This same principle used for multifocal lenses could be used for aspheric lenses. Some colleagues have pro- posed selecting the intraocular lens platform depending on the corneal asphericity to correct; others prefer to leave a residual positive spherical

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