Intraocular Lens Characteristics

Modern posterior chamber IOLs typically have the following characteristics (Table 9-1):

• are foldable and injectable

• are made from either silicone or acrylic materials

• have a biconvex aspheric optic with a square posterior edge

• are either single-piece or 3-piece

Foldable IOLs allow for a smaller incision size, which minimizes surgically induced corneal astigmatism and decreases postoperative wound complications. Injectable IOLs (either manually loaded into the injector cartridge or preloaded by the manufacturer) reduce exposure of the IOL to possible ocular surface contamination. Both silicone (which is hydrophobic) and acrylic (which can be either hydrophobic or hydrophilic) IOLs are suitable for the majority of patients.

Adherence of silicone oil to the surface of a silicone IOL can occur (Fig 9-1). Use of an IOL material other than silicone may be preferable in patients who will likely later require vitrectomy with silicone oil injection (eg, presence of proliferative diabetic retinopathy, retinal detachment in the fellow eye). Additionally, postoperative optic calcification of hydrophilic acrylic IOLs has been associated with exposure to air or gas. In patients who will be undergoing future intraocular surgeries that require the intraoperative use of gas (eg, endothelial or lamellar keratoplasty, vitrectomy), it may be advisable to consider a different material. Nd:YAG laser capsulotomy is not effective in treating this opacification, which may require IOL explantation.

Table 9-1 Modern IOL Characteristics

Figure 9-1 A large silicone oil droplet is adherent to the posterior surface of this 3-piece silicone intraocular lens (IOL).

(Courtesy of Christopher Kirkpatrick, MD and Anna Kitzmann, MD, Photographer: Toni Venckus. CRA. Available from https://webeye.ophth.uiowa.edu/eyeforum/atlas/pages/silicone-oil-drop-IOL.htm)

IOL optic geometry has evolved from earlier plano-convex models to the newer biconvex design (see BCSC Section 3, Clinical Optics). The addition of a square posterior optic edge design has reduced posterior capsular opacification (PCO) by blocking cell migration behind the optic (Fig 9-2). For more discussion of IOL design and PCO, including photos and illustrations, please see Chapter 6 in BCSC Section 3, Clinical Optics.

Most corneas have some degree of positive spherical aberration. The designs of older types of IOLs were spherical, adding positive spherical aberration to the optical system of the eye, thereby decreasing contrast sensitivity. Newer IOLs are aspheric, with varying degrees of zero or negative spherical aberration (ranging from 0 to −0.27 μm) to offset any positive spherical aberration of the cornea and thus improve contrast sensitivity. It should be noted that corneas with prior hyperopic laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) treatments often have negative spherical aberration, which can affect IOL selection. A decentered IOL that has any amount of positive or negative spherical aberration will induce coma.

Almost all modern IOLs also incorporate ultraviolet (UV)-absorbing chromophores into the material of the IOL that protects the retina from UV radiation. Some IOLs also incorporate blue-light filtering to attenuate blue-wavelength light (the optic of these IOLs therefore appears yellow to the surgeon). Proponents of these “blue-blocking” IOLs contend that they protect the macula from blue-light exposure; however, opponents claim that there is no evidence of benefit from blue-blocking IOLs, and they are concerned that these lenses might create problems with circadian rhythms or with mesopic or scotopic vision.

IOL technology has advanced, enabling it to address presbyopia and astigmatism, thereby reducing dependence on spectacles. The specialized IOL designs include accommodating, multifocal, extended depth of focus (EDOF), toric, and phakic IOLs. Toric IOLs are discussed later in this chapter in the section Modification of Preexisting Astigmatism. Phakic IOLs are discussed in BCSC Section 13, Refractive Surgery.

Figure 9-2 The actual geometry of the “square” posterior optic edge depends on the configuration of the optic and the IOL power. A, Scanning electron micrograph (SEM) shows the IOL at 25× magnification, with the posterior edge on the left. B, SEM shows the junction of the posterior surface with the lateral edge of the optic at 1000× magnification. Increasing the power of the IOL on the posterior surface increases the curvature of that surface and increases the obtuse angle of the intersecting surfaces.

(Courtesy of Werner L, Müller M, Tetz M. Evaluating and defining the sharpness of intraocular lenses: microedge structure of commercially available square-edged hydrophobic lenses. J Cataract Refract Surg. 2008;34(2):310–317.)

• Mainster MA, Turner PL. Blue-blocking IOLs vs. short-wavelength visible light: hypothesisbased vs. evidence-based medical practice. Ophthalmology. 2011;118(1):1–2.

• Srinivasan S. Intraocular lens opacification: What have we learned so far. J Cataract Refract Surg. 2018;44(11):1301–1302.

Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.