Eyeworld Supplements

EW DEC 2014 - Supported by an independent educational grant from Abbott Medical Optics

This is a supplement to EyeWorld Magazine.

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IOL material properties: Contributions to visual quality and patient satisfaction surgery with a high-index IOL. 1,2 Although no one has yet shown that this cosmetic issue carries any optical consequences, we know that, at a min- imum, light reflected out is not hitting the retina as intended. It may also contribute to the negative dysphotopsias that some patients complain about. 3 These are always noted in the temporal field of vision, which is processed by the larger, nasal side of the retina (80 degrees compared to 60 degrees on the temporal side). It is possible that light reflected beyond the critical angle of the lens casts a shadow on that side of the retina. Finally, a low index of refraction seems to provide a larger sweet spot in which patients are likely to be satisfied with their vision. In my practice, when I recently switched to a lower index-of- refraction lens, the technicians and I suddenly found that our "hug factor" (the percentage of my patients who are so thankful for their new vision that they hug me) had gone way up. Not everyone will be interested in optical science and the complex interac- tions of index of refraction, reflectance, and chromatic aberration. But at the end of the day, it's that hug factor that defines success in cataract surgery for all of us. Reflectance is also closely related to the index of refraction. Total internal reflection is the optical principle that makes fiber optic cable work so well. Light in one end of the cable keeps bouncing and propagating through the cable, allowing telecommunications sig- nals to travel hundreds of miles without being lost because of the critical angle of the material. This critical angle is based on the change in index of refrac- tion from the optical material to the surrounding media. IOL materials with a high index of refraction will have a shorter critical angle of, for example, 60 degrees. That means that light incident on the lens from an angle greater than 60 degrees gets bounced or reflected out of the eye, just as it would get bounced down the length of the fiber optic cable. To make matters worse, a high-index lens will be thinner (that's the benefit, remember?) and flatter. The flatter the lens, the more it will act like a mirror and allow light to bounce out more eas- ily. A relatively thicker lens will have a shorter radius of curvature, and incom- ing light rays will strike at an angle and be reflected centrally inward toward the retina instead of outward. We see reflectance as an external glint in patients who have had cataract Use the human crystalline lens as your guide in seeking out materials that provide superior optics for your patients T here are many IOL material and design properties that go into providing excellent vision. The most important material quality on my priority list is the index of refraction. For intraocular lenses, the refractive index directly affects not only lens thickness but is also related to optical quality: chromatic aberration; the range of high quality vision; and how light rays reach the retina. The crystalline lens has an index of refraction of 1.41, and I prefer to stay as close to this physiologic index of refraction as possible. The IOLs with sim- ilarly low indices of refraction include the older silicone IOLs (most of which are unavailable or rarely used now), the STAAR (Monrovia, Calif.) Collamer material, and the Abbott Medical Optics (Abbott Park, Ill.) Tecnis acrylic (Figure 1). In recent years, most other manufac- turers have been focused on the mantra of "smaller is better," opting for thin- ner, higher-index materials that can fit through smaller incisions. While there are some advantages of microincisional cataract surgery, I think the emphasis on shaving a few tenths of millimeters off the incision size may have come at the cost of optical quality. A higher index of refraction spreads white light out across its spectrum, creating chromatic aberration. In the world of photography, it is well known that a high-index camera lens causes blue edge blur. Instead of a crisp contrast between the edge of an object and the background of the photograph, there is a slightly blurred border resulting from the out-of-focus blue light. The same type of image distortion can occur with a high-index IOL material. It is interesting that the highest-index lenses on the mar- ket have a blue-blocking chromophore. Without that chromophore, we would surely notice a much stronger optical effect from out-of-focus blue light. Gary N. W├Ârtz, MD Figure 1. The index of refraction of various lens materials, including contemporary hydrophobic acrylic IOLs (in green). 2

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