Eyeworld

4 Aspheric profiles: What is the real impact? T o understand the value of asphericity for an intraocu- lar lens, we must first appreci- ate the fact that the human visual system is, in essence, a contrast sensitivity detection system. For most optical systems, image quality is measured by an ability to faithfully transmit contrast from the object being viewed to the image being formed, and maintaining contrast is a fundamental as- pect of image quality. One need look no further than the image quality differences between television technology from 20 years ago and what is readily available today for the importance of maintaining contrast to become immediately apparent. More than 60 years ago, a Dutch physicist name Fritz Zernike described a method for quantifying how image con- trast may be influenced in the presence of different types of aberrations, referred to as the Zernike polynomials. The spherical and astigmatic corrections in a typical pair of glasses, referred to as defocus and regular astigmatism, are known as second order aberrations. A significant increase in any aberration results in a decrease in image contrast. When we describe some- thing as being "in focus" what is actually meant is that it has high contrast at high spatial frequencies. In the visual world in which we live, maintaining contrast plays an important role and the presence of aberrations work against this. For traditional spherical IOLs, the image quality degrades toward the optic's periphery. 1 This is because the marginal rays are brought into focus in front of the central rays. It's well known that the human cornea is also an imperfect lens and shares this property re- sulting in an elevated fourth order aberra- tion known as positive spherical aberration. In the pseudophakic state, combining a traditional spherical IOL with the naturally occurring positive Including an additional correction for any regular corneal astigmatism further in- creases contrast and improves visual qual- ity. This is one reason why our AcrySof toric IOL patients with corneal astigma- tism are often very happy with the quality of their vision. Here, three separate aberra- tions are being addressed: defocus, regular astigmatism, and spherical aberration. The AcrySof IQ adds a physiologic amount of negative spherical aberration to help offset the naturally occurring amount of positive spherical aberration of the cornea. Such a strategy has been developed to maintain contrast and is less likely to result in an over cor- rection. Maintaining contrast to maintain visual quality is the name of the game. References 1. Kohnen T, Kiaproth OK, Buren J. Effect of intraocular lens asphericity on quality of vision after cataract removal: an intraindividual comparison. Ophthalmology. 2009; 116: 1697–1706. 2. Wang L, Santaella RM, Booth M, Koch DD. Higher-order aberrations from the internal optics of the eye. J Cataract Refract Surg. 2005;31(8):1512–9. 3. He JC, Gwiazda J, Thorn F, Held R. Wave-front aberrations in the anterior corneal surface and the whole eye. J Opt Soc Am A Opt Image Sci Vis. 2003;20(7):1155–63. 4. He JC, Burns SA, Marcos S. Monochromatic aberrations in the ac- commodated human eye. Vision Res. 2000;40(1):41–8. 5. Artal P. Optics of the eye and its impact in vision: a tutorial. Ad- vances in Optics and Photonics. 2014;6(3):340. 6. Beiko GH. Personalized correction of spherical aberration in cataract surgery. J Cataract Refract Surg. 2007;33(8):1455–60. 7. Wang L, Koch DD. Ocular higher-order aberrations in individuals screened for refractive surgery. J Cataract Refract Surg. 2003;29(10):1896–903. 8. Awwad ST, Warmerdam D, Bowman RW, et al. Contrast sensitivity and higher order aberrations in eyes implanted with AcrySof IQ SN60WF and AcrySof SN60AT intraocular lenses. J Refract Surg. 2008;24(6):619–25. 9. Tzelikis PF, Akaishi L, Trindade FC, Boteon JE. Ocular aberrations and contrast sensitivity after cataract surgery with AcrySof IQ intraocular lens implantation clinical comparative study. J Cataract Refract Surg. 2007;33(11):1918–24. 10. Pandita D, Raj SM, Vasavada VA, et al. Contrast sensitivity and glare disability after implantation of AcrySof IQ Natural aspherical intraocular lens: prospective randomized masked clinical trial. J Cataract Refract Surg. 2007;33(4):603–10. 11. Legras R, Chateau N, Charman WN. Assessment of just-notice- able differences for refractive errors and spherical aberration using visual simulation. Optom Vis Sci. 2004;81(9):718–28. Dr. Hill is in private practice at East Valley Ophthalmology in Mesa, Ariz. He can be contacted at 480-981-6130 or hill@doctor-hill.com. Dr. Hill is a paid consultant and investigator for Alcon. by Warren E. Hill, MD spherical aberration of the cornea will re- sult in a significant increase in the total amount of spherical aberration. Above a certain level, this leads to an increasing loss of contrast with increasing pupil size. One of several important goals of cataract surgery should be to maintain image qual- ity—said another way, to maintain con- trast. The crystalline lens in the youthful eye, without a cataract, has somewhere be- tween –0.138 µm and –0.24 µm of nega- tive spherical aberration, which means it typically has more power in the center than it does toward the periphery. 2–5 The mean value for positive spherical aberration in the human cornea is about 0.274 µm. 6 The net result is that, on average, a small amount of naturally occurring positive spherical aberration is commonly present for those at the peak of visual function in early adulthood. It is interesting to note that this amount is often symmetrical. 7 The AcrySof difference In the presence of a small pupil, the AcrySof IQ (Alcon, Fort Worth, Texas), Tec- nis (Abbott Medical Optics, Abbott Park, Ill.), and any aberration neutral IOL, such as the SofPort AO (Bausch + Lomb, Bridge- water, N.J.), perform about the same in terms of contrast sensitivity. With a pupil size of 4 mm or larger, however, helping to neutralize the naturally occurring positive spherical aberration of the cornea begins to make a difference in terms of both image quality and visual performance as shown by contrast sensitivity testing. 8–10 Beiko found that targeting a residual positive spherical aberration of 0.10 µm following cataract surgery re- sulted in superior visual performance at a 6-mm pupil size. 6 And Legras et al found that for a 6-mm pupil the optimal value for spherical aberration was not 0.0 µm but actually 0.08 µm. 11 It's this research that led Alcon to target a value for spheri- cal aberration with the AcrySof IQ lens of about 0.10 µm for the average pseudopha- kic eye. The strategy behind the AcrySof IQ lens is to offset this naturally occurring positive spherical aberration of the cornea with a slightly lower negative spherical aberration value such that the final resid- ual amount of spherical aberration approx- imates that of the youthful eye. Clinical update on blue light filtering and aspheric technologies

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