Cataract Surgery Feature Story

SEPTEMBER 2009 I CATARACT & REFRACTIVE SURGERY TODAY EUROPE I 31 O ur visual system integrates the refractive surfaces of the cornea and the crystalline lens; the positive spherical aberration (SA) of the cornea is partially cancelled out by the negative SA of the crystalline lens.1,2 As the eye ages, the cornea remains fairly stable, but the crystalline lens thickens at the periphery, inducing positive SA.1-3 The nucleus hardens4 and changes occur in the internal refractive index gradient,5 equivalent refractive index,6 and lens shape.6 As these changes occur, the compensation for aberration gradually declines, leading to increased total ocular aberrations and loss of optical7 and visual8 quality. Contrast sensitivity is reduced, and glare and halos become more common.9 Traditionally, cataract patients have been implanted with standard spherical IOLs. This strategy produces good visual acuity;10 however, total optical aberrations increase, affecting visual quality. Aspheric IOLs were introduced to eliminate or reduce SA, with the aim of improving functional vision.11 The optical advantages of aspheric IOLs have now been largely accepted.12 Two types of aspheric IOL are in common use: (1) negative SA and (2) aberration-free. IOLs with a small amount of negative SA, such as the Tecnis Z9000 (Abbott Medical Optics Inc., Santa Ana, California) and the AcrySof IQ (Alcon Laboratories, Inc., Fort Worth, Texas), are designed to cancel out the positive corneal SA. Aberrationfree IOLs aim to leave the eye with a small amount of positive SA. Examples of this include the SofPort AO (Bausch & Lomb, Rochester, New York) and the At.Smart 46LC (Carl Zeiss Meditec, Jena, Germany). The rationale for leaving a small amount of positive SA in the eye is that this may have beneficial effects on contrast sensitivity.13 The At.Smart 46LC has been found to produce significantly fewer higher-order aberrations over a 6-mm optical zone than conventional spherical IOLs, suggesting that it could be particularly helpful in improving visual quality in mesopic conditions.14 However, little has been published to date about the At.Smart 46LC IOL. We recently conducted a study exploring the performance of this IOL and comparing it with its sister lens, the spherical At.Smart 46S in each patient’s contralateral eye. The only difference between the two IOLs is their sphericity. The At.Smart family of IOLs are foldable single-piece acrylic lenses (25% water content) with a four-haptic design. After hydration, the refractive index is 1.46. The 46S (power range, 16.00–27.00 D) has a conventional biconvex spherical optic and the 46LC (power range, 0.00–32.00 D) is equiconvex and aspheric aberration-free and designed for convergent rays entering the IOL. Comparison of the At.Smart Aspheric and Spherical IOLs