Figure 5-11 A, Light striking the edge of the IOL may be reflected to another site on the retina, resulting in undesirable dysphotopsias. These problems arise less often with smoother-edged IOLs. B, Light may be internally re-reflected within an IOL, producing an undesirable second image or halo. Such re-reflection may be more likely to occur as the index of refraction of the IOL increases. C, Light may reflect back from the surface of the retina and reach the anterior surface of the IOL. The IOL acts as a concave mirror, reflecting back an undesirable dysphotopsic image. When the anterior surface of the IOL is more curved, the annoying image is displaced relatively far from the fovea. D, When the anterior IOL surface is less steeply curved, the annoying image appears closer to the true image and is likely to be more distracting.(Redrawn b y
C. H. Wooley.)
Davison JA. Positive and negative dysphotopsia in patients with acrylic intraocular lenses. J Cataract Refract Surg. 2000;26(9):1346–1355.
Erie JC, Bandhauer MH. Intraocular lens surfaces and their relationship to postoperative glare. J Cataract Refract Surg. 2003;29(2):336–341.
Franchini A, Gallarati BZ, Vaccari E. Computerized analysis of the effects of intraocular lens edge design on the quality of vision in pseudophakic patients. J Cataract Refract Surg. 2003;29(2):342–347.
Nonspherical Optics
IOLs with more complex optical parameters are now available. It may be possible to offset the positive spherical aberration of the cornea in pseudophakic patients by implanting an IOL with the appropriate negative asphericity on its anterior surface. IOLs with a toric surface may be used to correct astigmatism. Rotational stability may be of greater concern when plate-haptic toric lenses are implanted in the vertical axis than when they are implanted in the horizontal axis. As a toric lens rotates from the optimal desired angular orientation, the benefit of the toric correction diminishes. A properly powered toric IOL that is more than 30° off-axis increases the residual astigmatism of an eye; if it is 90° off-axis, the residual astigmatism doubles. Fortunately, some benefit remains even with lesser degrees of axis error, although the axis of residual cylinder changes. Newer designs are more stable than earlier ones.
Recently, investigators have developed an IOL in which the optical power can be altered by laser after lens implantation. This feature would be useful for correcting both IOL power calculation errors and residual astigmatism.
Mester U, Dillinger P, Anterist N. Impact of a modified optic design on visual function: clinical comparative study. J Cataract Refract Surg. 2003;29(4):652–660.
Multifocal Intraocular Lenses
Conventional IOLs are monofocal and correct the refractive ametropia associated with removal of the crystalline lens. Because a standard plastic IOL has no accommodative power, its focus is essentially for 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. It is difficult to replicate this process clinically. 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. 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. Depending on the type of multifocal IOL and the viewing situation, both near and far images may be 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). 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. Preoperative and postoperative astigmatism should be low, given that visual acuity and contrast sensitivity degrade with against-the-rule astigmatism as low as 1.00 D.
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, CA, 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 5-12A). This design was reintroduced in 2010 as the Lentis Mplus (Oculentis, Berlin, Germany) and is now showing encouraging results in Europe.
Figure 5-12 Multifocal IOLs. A, Hoffer split bifocal IOL (left) and photograph of a lens implanted in a patient in 1984 (right). B, Bullet bifocal IOL. C, Three-zone multifocal design. D, Multifocal IOL with several annular zones. E, 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 b y 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 reading distance.
A later design known as the “bullet” bifocal IOL (Fig 5-12B) had a central zone for near power and an outer zone for distance. When the pupil constricted for near vision, its smaller size reduced or eliminated the contribution from the distance portion of the IOL. When the pupil dilated for distance vision, more of the distance portion of the IOL was exposed and contributed to the final image. Importantly, lens decentration could have a deleterious effect on the IOL’s optical performance. A problem with the design itself was that the pupil size did not always correspond to the desired visual task. For this reason, the bullet bifocal IOL fell into disuse.
Multiple-zone intraocular lenses
To overcome the problems associated with pupil size, ophthalmologists developed a 3-zone bifocal lens (Fig 5-12C). 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 5-12D), each of which varies continuously in power over a range of 3.50 D. The advantage 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 5-12E) use Fresnel 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 absorbed in this process, and optical aberrations with diffractive IOLs can be troublesome.
Second-generation diffractive multifocal intraocular lenses
Currently, 3 second-generation diffractive multifocal IOLs are available. Each increases the patient’s independence from spectacles and decreases the incidence of optical adverse effects.
The first of these IOLs, the AcrySof ReSTOR IOL (Alcon, Fort Worth, TX), is an apodized diffractive lens (Fig 5-13A). 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. This IOL is now available in an aspheric design. The second design, the ReZoom lens (Abbott Medical Optics [AMO], Santa Ana, CA) (Fig 5-13B), has 5 anterior surface zones for distance and near vision; grading between the zones provides intermediate vision. The third IOL, the TECNIS ZM 900 lens (AMO), adds an aspheric surface, whereas the ReZoom lens does not.