Multifocal Intraocular Lenses
Conventional IOLs are monofocal and correct the refractive ametropia associated with removal of the crystalline lens. Because a standard IOL has no accommodative power, it provides a clear focus for visual targets at a single distance only. However, the improved visual acuity resulting from IOL implantation may allow a patient to see with acceptable clarity over a range of distances. If the patient is left with a residual refractive error of simple myopic astigmatism, the ability to see with acceptable clarity over a range of distances may be further augmented. In this situation, one endpoint of the astigmatic conoid of Sturm corresponds to the distance focus and the other endpoint represents myopia and, thus, a near focus; satisfactory clarity of vision may be possible if the object in view is focused between these 2 endpoints. In bilateral, asymmetric, and oblique myopic astigmatism, the blurred axis images are ignored and the clearest axis images are chosen to form one clear image for distance vision; the opposite images are selected for near vision. Thus, even standard IOLs may provide some degree of depth of focus and “bifocal” capabilities.
An alternate approach to this problem is to correct one eye for distance and the other for near vision; this approach is called monovision—this approach is also used with contact lenses. Nevertheless, most patients who receive IOLs are corrected for distance vision and wear reading glasses as needed.
Multifocal IOLs are designed to improve both near and distance vision to decrease patients’ dependence on glasses. With a multifocal IOL, the correcting lens is placed in a fixed location within the eye, and the patient cannot voluntarily change the focus. Because object rays encounter both the distance and near portions of the optic, both near and far images are presented to the eye at the same time. The brain then processes the clearest image, ignoring the other(s). Most patients, but not all, can adapt to the use of multifocal IOLs.
The performance of certain types of IOLs is greatly impaired by decentration if the visual axis does not pass through the center of the IOL. On the one hand, the use of modern surgical techniques generally results in adequate lens centration. Pupil size, on the other hand, is an active variable, but it can be employed in some situations to improve multifocal function. Other disadvantages of multifocal IOLs are image degradation, “ghost” images (or monocular diplopia), decreased contrast sensitivity, and reduced performance in lower light (eg, decreased night vision). (See Videos 6-1 and 6-2.) These potential problems make multifocal IOLs less desirable for use in eyes with impending macular disease.
Accuracy of IOL power calculation is very important for multifocal IOLs because their purpose is to reduce the patient’s dependence on glasses. Postoperative astigmatism should be low, given that visual acuity and contrast sensitivity are degraded with greater degrees of residual astigmatism.
Courtesy of Mark Packer, MD, FACS, CPI.
Access all Section 3 videos at www.aao.org/bcscvideo_section03.
The optical cost of multifocal IOLs.
Animation developed by Joshua A. Young, MD.
Types of Multifocal Intraocular Lenses
Bifocal intraocular lenses
Of the various IOL designs, the bifocal IOL is conceptually the simplest. The bifocal concept is based on the idea that when there are 2 superimposed images on the retina, the brain always selects the clearer image and suppresses the blurred one. The first bifocal IOL implanted in a human was invented by Hoffer in 1982. The split bifocal was implanted in a patient in Santa Monica, Callifornia, in 1990. In this simple design, which was independent of pupil size, half the optic was focused for distance vision and the other half for near vision (Fig 6-12A). This design was reintroduced in 2010 as the Lentis Mplus (Oculentis, Berlin, Germany) and is now showing encouraging results in Europe.
Figure 6-12 Multifocal intraocular lenses (IOLs). A, Hoffer split bifocal IOL (left) and photograph of a lens implanted in a patient in 1984 (right). B, Three-zone multifocal design. C, Multifocal IOL with several annular zones. D, Diffractive multifocal IOL; the cross section of the central portion is magnified (the depth of the grooves is exaggerated).
(Photograph courtesy of Kenneth J. Hoffer, MD; all illustrations redrawn by C. H. Wooley.)
The additional power needed for near vision is not affected by the AL or by corneal power, but it is affected by the ELP. A posterior chamber IOL requires more near-addition power than does an anterior chamber IOL for the same focal distance. Approximately 3.75 D of added power is required to provide the necessary 2.75 D of myopia for a 14-inch (35 cm) reading distance.
Multiple-zone refractive intraocular lenses
These lenses use concentric refractive rings of different optical powers. In 3-zone bifocal lenses (Fig 6-12B) the central and outer zones are for distance vision; the inner annulus is for near vision. The diameters were selected to provide near correction for moderately small pupils and distance correction for both large and small pupils.
Another design uses several annular zones (Fig 6-12C), each of which varies continuously in power over a range of 3.50 D. The intention is that whatever the size, shape, or location of the pupil, all the focal distances are represented on the macula.
Diffractive multifocal intraocular lenses
Diffractive multifocal IOL designs (Fig 6-12D) use diffraction optics to achieve a multifocal effect. The overall spherical shape of the surfaces produces an image for distance vision. The posterior surface has a stepped structure, and the diffraction from these multiple rings produces a second image, with an effective add power. At a particular point along the axis, waves diffracted by the various zones add in phase, providing a focus for that wavelength. Approximately 20% of the light entering the pupil is dissipated in this process and does not reach the retina. Therefore, a variable decrease in contrast sensitivity should be expected when implanting these lenses. Diffractive IOLs are somewhat tolerant to decentration, but the optical aberrations that could develop due to IOL malposition can be troublesome for some patients
Second-generation diffractive multifocal intraocular lenses
The industry provides numerous refractive IOL designs, such as improvement in the rays taken to the intermediate focus providing some level of intermediate vision (trifocal IOLs), total eye asphericity control by adding negative asphericity to the IOL profile, and optical achromatization. These designs aim to decrease contrast sensitivity loss, extending at the same time the depth of focus to improve polyfocality.
Some of these IOLs incorporate an apodized diffractive lens. Apodization refers to the gradual tapering of the diffractive steps from the center to the outside edge of a lens to create a smooth transition of light between the distance, intermediate, and near vision focal points.
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.