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Multifocal Intraocular Lenses

Jay Bansal, MD

Multifocal intraocular lenses (IOLs) have more than one focal point, which enables them to correct near and far vision. They traditionally replace the native lens following cataract or clear lens removal. Multifocal IOLs create 2 or more retinal images, corresponding to focused light rays from objects of different distances. Since the retina is presented with more than one image simultaneously, the patient must concentrate on an object or task at a particular distance.

TYPES OF MULTIFOCAL INTRAOCULAR LENSES

Multifocal IOLs have traditionally been separated into 2 types: diffractive and refractive. These describe the intrinsic way in which a lens design creates the multiple retinal images. However, many newer lens designs include features of both types, making the distinction somewhat less meaningful. Nevertheless, clinicians should understand the similarities and differences between these 2 designs since all existing multifocal IOLs use one or both optic principles.

Refractive Multifocal Intraocular Lenses

Refractive multifocal IOLs possess 2 or more curvatures to form refractive zones. Each refraction zone bends light to form 2 or more retinal images, each with a different dioptric power. It can be thought of functioning in the same way as a bifocal corrective lens. Since each refraction zone also has a different effective aperture, the performance of most refractive multifocal IOLs depends, in part, on pupillary response to light.¹ In other words, as the pupil dilates, the relative proportion of light directed to near and distant focal points changes (Figure 9-1). Image quality can depend on the patient’s natural pupil size. People with small pupils would be expected to have poorer image quality than people with larger mean pupil size.

Diffractive Multifocal Intraocular Lenses

Diffractive multifocal IOLs create more than one retinal image through light diffraction. The surface of a diffractive IOL possesses microscopic concentric rings, or diffractive zones, that slow and bend light rays. Each ring creates a discontinuity of optical density that directs a portion of the light energy to both near and far focal points (Figures 9-2 and 9-3). The result is either 2 or 3 retinal images in the case of bifocal or trifocal diffractive IOLs, respectively. Bifocal diffractive IOLs provide patients with simultaneous near and distant focal points, plus an intermediate focal point in the case of trifocal IOLs.

Diffractive multifocal IOLs may or may not be apodized. A nonapodized diffractive multifocal IOL contains regularly sized concentric rings with a uniform height from center to edge. Thus, for each ring, the optical density is split to the same extent for near and distant focal points. In an apodized lens, the diffractive step heights are gradually reduced in each concentric ring as they extend from the center to the periphery of the lens. Peripheral rings direct more light rays (optical density) to the distant focal point compared to the more central rings. At times in which the pupil is dilated (eg, during night driving), the resolution of far objects should be superior at the expense of near vision resolution.

Purely refractive or diffractive IOLs assume rotational symmetry; however, newer lenses are applying the concept of rotational asymmetry. Rotationally asymmetric lenses are divided into sectors or segments that handle light differently. Thus, instead of creating the same diffraction or refraction across the ring or zone, a segment of the lens provides a different focal point than the rest of the lens.² In other words, the distance section provides a sharp, high-contrast image for far vision, and the near section produces the same for near vision.^3,4

IOLs with aspheric designs are intended to reduce spherical aberrations. Both aspheric and rotationally asymmetric design features should improve contrast sensitivity.^2,5 Lastly, hybrid diffractive-refractive IOLs attempt to achieve the best of both designs and extend the functional visual range of the lens.

In the mid-2000s, attempts were made to advance phakic multifocal IOLs. The goal was to preserve the crystalline lens and place the multifocal IOL in the anterior chamber.^6,7

While visual results were acceptable, anterior placement of the IOL was associated with various, potentially serious complications including endophthalmitis, induced astigmatism, chronic uveitis, pupillary block glaucoma, and cataract. Thus, development was largely abandoned. Efforts to develop posterior chamber phakic IOLs for presbyopia treatment are in the early stages.8

INDICATIONS AND CONSIDERATIONS

Multifocal IOLs were primarily developed as a treatment for aphakia and to correct presbyopia in patients with cataracts.⁹ Indeed, multifocal IOL implantation has become a variation of standard phacoemulsification cataract surgery.¹⁰ The other important indications for multifocal IOLs are in patients with relatively clear crystalline lenses. One patient population is comprised of people who have high refractive error due to abnormal ocular anatomy. The other population is older individuals with presbyopia who have normal axial length and who wish to be free of spectacles. Cataract patients and patients with clear lenses are 2 very different patient populations, however, and the approach and considerations in each group are quite different.

Refractive lens exchange (RLE) in people with relatively clear lenses offers the possibility of treating myopia or hyperopia, astigmatism, and presbyopia simultaneously, while avoiding cataracts in the future. If spectacle independence means treating presbyopia and even astigmatism in addition to myopia or hyperopia, it is a very high bar indeed. The bar is even higher in patients who are naturally emmetropic and seeking a surgical solution for presbyopia that frees them from reading glasses.^11–13

PATIENT SELECTION AND PREOPERATIVE COUNSELING

Even after an optimal surgical result, patients still may be dissatisfied with their optical outcomes following IOL placement. Patient factors and characteristics that may guide the selection of multifocal IOL implantation candidates are shown in Table 9-1.^10,14 When discussing treatment options and obtaining informed consent, patients should be aware of the risks involved when replacing a natural lens with a multifocal IOL.

Prospective patients should be aware of the increased risk of photic phenomena such as glare, halos, monocular diplopia, and starbursts (Figure 9-4). Patients who experienced photic phenomena tend to tolerate them within 6 months of surgery.^15,16 This may be due, in part, to higher order (neural) adaptation.¹⁷ Photic phenomena are less objectionable to patients as uncorrected near visual acuity (UNVA) and contrast sensitivity improves despite stable, residual refractive errors.^16,18,19 Indeed, patients who receive multifocal IOLs generally require a 6-month neuroadaptation period to achieve full benefit from the lenses.²⁰

Importantly, most studies considered presbyopia treatment concurrent with cataract phacoemulsification. A person seeking treatment for cataracts is usually quite familiar with halos and glare, but a person with clear lenses seeking treatment for refraction errors and presbyopia usually is not. This may be at least partially managed through candid presurgical discussions and postsurgical reassurance.

Pupil size is an important consideration in surgical planning. Patients with large pupils are at an increased risk for visual disturbances at night.²¹ As the pupil expands in low light, the edges of the pupil interact with the edge of the treatment zone.22 Various scotopic assessments, such as pupillometry, can be performed before surgical presbyopia treatment to determine the relative risk of photic phenomena at night.²³ Patients with pupils that are likely to impinge on the treatment zone can be counseled before the procedure about their increased risk. On the other hand, patients with small pupils present a technical challenge for surgeons. The surgeon may need to expand the pupil for capsulorrhexis, taking care not to damage the iris sphincter. A decentered capsulorrhexis leads to a decentered IOL, which interferes with IOL function and leads to patient dissatisfaction.²⁴

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