Chapter 20

The IOLs manufactured today are biconvex, convexplano, planoconvex, or meniscus.1 As mentioned above there are two effects of changing the media in the posterior compartment of the eye. The first effect is a result of the change in the effective power of the posterior surface of the IOL due to a refractive medium other than the normal vitreous. The second effect is due to the light traveling through the posterior compartment at a speed different from that of the vitreous. These two effects must be considered independently because they are uniquely different for each lens shape and material.

BSS

The most common replacement material for the vitreous is BSS. Because it has a refractive index similar to vitreous (this is due to the high water content of the vitreous), index of refraction adjustments to power are unnecessary. Therefore, only the location of the lens will dictate IOL power. If the IOL is to be placed in the sulcus, the power should be decreased by approximately 0.5 diopter (D) for a standard-power IOL to maintain the desired refractive outcome. The sulcus is approximately 0.25 mm anterior to the in-the-bag plane, so the effective anterior chamber depth (ACD) is about 0.25 mm less. For a 20-D lens it is a 0.50-D difference, but for a 40-D lens it is a 1.0-D difference, and for a 10-D lens it is only a 0.25-D difference. If the IOL is placed in the bag or the optic is captured beneath the anterior capsule, no power change is necessary.

LENS MATERIAL AND POSITION

Foldable three-piece IOLs have haptics, which, whether extruded polymethylmethacrylate (PMMA) or polypropylene, have almost no memory. They therefore remain at the same angulation in the ciliary sulcus. In some patients with crowded anterior segments, this may be the source of iris chafe leading to iris depigmentation and pigmentary glaucoma. Plate silicone IOLs are contraindicated for sulcus fixation. Polypropylene haptics, when placed in the ciliary sulcus, may erode into the sulcus, and the material has been implicated as a cause of low-grade inflammation. Therefore, the best choice for small-incision IOL implantation would be a foldable optic material, with the optic captured within the anterior or posterior capsule to prevent iris chafe, combined with extruded PMMA haptics for sulcus compatibility. If optic capture is impossible, the incision should be enlarged and a one-piece PMMA IOL with at least 2.5-degree angulation or a 0.5-mm step vault should be implanted. This will prevent iris chafe. The haptic should have a large radius of curvature for broad sulcus contact and stability. In addition, an optic of 6.0 mm should be considered in an effort to minimize the effects of possible IOL decentration resulting in edge glare.

An excellent study on the ideal shape of an IOL when silicone oil is used demonstrated that a meniscus lens can actually neutralize both the shape and path length effect of silicone, such that when the silicone oil is removed there is no optical effect.2 Meniscus lenses, however, have many other optical disadvantages such as spherical aberration. More importantly, they have the highest degree of induced astigmatism when tilted.^3,4

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