Progress in the accuracy of intraocular lens (IOL) biometry, new IOL designs, and the concomitant rise in patients’ refractive expectations has brought emphasis to the optics of intraocular lenses. This chapter focuses on the optical considerations relevant to IOLs. For information on the history and development of IOLs, and more surgical information with respect to IOLs, see BCSC Section 11, Lens and Cataract.
In the 1970s, surgeons implanting IOLs included those who used intracapsular cataract extraction (ICCE) and those who used small-incision phacoemulsification (phaco). The IOL optic was made from polymethylmethacrylate (PMMA), with supporting haptics of metal, polypropylene, or PMMA. The rigidity of these materials required that a small phaco incision be enlarged for IOL insertion. However, following the introduction of a foldable optic (made from silicone) in the late 1980s, enlargement was no longer required, and the combination of phaco and IOL implantation was widely adopted.
The basic lens designs currently in use are differentiated by the plane in which the lens is placed (posterior chamber or anterior chamber) and by the tissue supporting the lens (capsule/ciliary sulcus or chamber angle). Figure 6-1 illustrates the major types of IOLs and optics.
The effect of lens material on factors such as posterior capsular opacification (PCO) has been investigated. Earlier studies suggested that IOLs made from acrylic are associated with lower rates of PCO than are those made from silicone or PMMA. However, more recent studies suggest that lens edge design is a more important factor in PCO than is lens material, as Hoffer proposed in 1979 in the lens edge barrier theory. An IOL with an annular, ridged edge or a square, truncated edge creates a barrier effect at the optic edge, exerting a 69% increase in pressure of the IOL edge against the posterior capsule. This reduces cell migration behind the optic and thus reduces PCO (Figs 6-2, 6-3, 6-4). The ridge concept led to the development of partial-ridge and meniscus IOLs, which were used for a time, and the sharp-edge designs now in use.
Plano and even negative-power IOLs are available for patients with very high myopia. The presence of even a plano IOL helps maintain the structural integrity of the anterior segment and decreases the incidence of posterior capsular opacity over simply leaving the eye aphakic. Moreover, yttrium-aluminum-garnet (YAG) capsulotomy in aphakic eyes is associated with a higher incidence of retinal detachment than in eyes with IOLs.
“Piggyback” lenses (ie, 2 IOLs in 1 eye; biphakia), implanted either simultaneously or sequentially, may be used when the postoperative IOL power is incorrect or when the needed IOL power is higher than what is commercially available. Minus-power IOLs can be used to correct extreme myopia and (as piggybacks) to correct IOL power errors. Intralenticular fibrosis may occur after placement of a piggyback lens arrangement. To the extent that this fibrosis may displace or tilt the piggyback IOL, it will change the refractive effect of the IOL and result in ametropia.
Figure 6-1 The major types of intraocular lenses (IOLs) and optics. A, Anterior chamber lens. B, Posterior chamber lens in the capsular bag. C, Posterior chamber lens in the ciliary sulcus. D, Biconvex optic. E, Planoconvex optic. F, Meniscus optic.
(Redrawn by C. H. Wooley.)
Figure 6-2 Schematic illustrating the concept of a 69% increase in pressure (green arrow) at the edge of an intraocular lens.
(Courtesy of Kenneth J. Hoffer, MD.)
Current IOLs are foldable, injectable, aspheric, sharp edged, and single piece (or 3 piece), and they have higher refractive indices; together, these features allow for implantation through smaller incisions than used for the earlier designs.
Figure 6-3 Square edge intraocular lens (IOL) design. A, TECNIS 1-piece IOL with the square edge design extending to the haptics. B, Detail of Rayner Superflex aspheric IOL demonstrating a square edge design.
(Courtesy of Baye Vision.)
Figure 6-4 Increasing the pressure at the edge of an intraocular lens leads to a blockage of cells to the central posterior capsule (A, B).
(Courtesy of Kenneth J. Hoffer, MD.)
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Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.