46 Diffractive Multifocal Intraocular Lenses The ultimate goal of visual rehabilitation in cataract surgery has always been to restore the best level of visual acuity, across a natural range of focal distances, much like the normal pre-presbyopic human eye. Utilizing the optical property of diffraction and interference to separate light rays into two distinct focal lengths enables an individual to perceive both near and distant images simultaneously, depending on the target of regard. Newer aspheric diffractive multifocal intraocular lenses (IOLs) provide high-quality images at both distance and near foci, and improved near vision quality compared with refractive multifocal lenses.1 There are two types of multifocal diffractive IOLs currently available in the US in different models and iterations. First, the aspheric Restor (Alcon, Fort Worth, TX) is a biconvex apodized diffractive acrylic lens with an embedded blue-filtering cromophore. This lens is available in a single-piece IOL design in 3 different add powers, 4.0 D, 3.0 D, and 2.5 D (models SN6AD3, SN6AD1, and SV25T0). It is also available in a 3 piece design suitable for sulcus fixation with either a 4.0 D or 3.0 D add power (MA60 D3, MA60D1). All of the Restor lenses have a central anterior surface diffractive, ringed, multifocal optic encompassing the central portion of the IOL. The central diffractive optic measures 3.6 mm in the Restor 4.0 and 3.0 add, while the 2.5 D lens model has a central diffractive optic measuring 3.4 mm. The peripheral portion of all of the Restor lenses is devoted to distance focus. This lens design is intended to reduce glare and halo under mesopic and scotopic conditions when the pupil is potentially larger. At small pupil sizes, the lens provides an equal distribution of light rays for both near and distant foci. However, at a 5mm pupil size, 84% of light rays entering the Restor lens are distributed to distance focus, due to the peripheral refractive zone of the lens. The 2.5 D model has other modifications to enhance distance vision quality, including a larger central ring which is 100% distance-dominant (0.94 mm vs 0.84 mm), as opposed to 60% distance focus in the Restor 3.0 D lens. The 4.0 D and 3.0 D Restor lenses have −0.10 of negative asphericity, while the 2.5 D version and −0.20 um of negative asphericity. (All of this information is Alcon, data on file) Second, the Tecnis multifocal (Abbott Medical Optics, Abbott Park, IL) is also available in 3 different add powers, a 4.0 D, 3.25 D, and 2.75D single-piece IOL design (ZMBOO, ZLBOO, ZKBOO). It too is available in a 3-piece design suitable for sulcus fixation, with a 4.0 D add power (ZMAOO). All of the Tecnis multifocal lenses add −0.27 um of negative asphericity to the optical system of the eye. The Tecnis multifocal lens is designed to be pupil-independent, with a 50/50 light split at all pupil sizes, due to the full-optic diffractive design. Three multifocal diffractive IOL types are available in the United States. First, the Tecnis Multifocal (model ZMBOO, Abbott Medical Optics, Abbott Park, IL) is a one-piece acrylic, pupil-independent full optic diffractive lens with a +4.00 add power (Fig. 46.1). Second, the aspheric Restor (Alcon, Fort Worth, TX) multifocal lens is a biconvex one-piece acrylic, apodized diffractive multifocal lens with an embedded blue-filtering chromophore (Fig. 46.2). This lens is available in the United States in two add powers, 3.0 D and 4.0 D (models SN6AD1 and SN6AD3). In other parts of the world a 2.5 D add power is available as well. The lens has a central anterior surface diffractive, ringed multifocal optic encompassing the central 3.6 mm of the lens, whereas the peripheral 2.4 mm of the remaining optic is a full distance refractive portion. This design is intended to reduce glare and halo under mesopic and scotopic conditions when the pupil is dilated. The aspheric Restor lens corrects −0.10 µm of spherical aberration. Third, the Restor lens is also available in a three-piece format (models MA60D1 and MA60D3) that may be sulcus fixated. Before discussing surgical options with the patient, the patient exam may be used as a time-saving screening tool to eliminate lens choice options based on patient anatomy and comorbidities. Performing a corneal topography and macula optical coherence tomography (OCT) in the cataract evaluation helps to effectively tailor the discussion of IOL options and reduce the need to discuss options for which the patient is not a candidate. A multifocal lens should not be implanted if there is evidence of significant ocular pathology. The splitting of light rays into near and distant foci in a diffractive multifocal optic causes 18% of light rays to be lost to high orders. The inherent reduction of contrast sensitivity associated with a multifocal lens may further compromise any preexisting contrast loss related to macular dysfunction.2 This may include epiretinal membrane and any other macular degenerative process. Optic nerve disease may also produce impaired contrast sensitivity. Therefore, in any patient with documented optic nerve dysfunction or visual field loss, caution with multifocal lens implantation is warranted.3 Corneal topography is helpful in identifying preexisting corneal astigmatism. If a corneal topography is available at the time of surgical evaluation, discussion may then be further tailored to include the need for concomitant astigmatic correction, or to eliminate a multifocal IOL option if there is corneal pathology. Significant corneal irregular astigmatism is a contraindication to the use of a multifocal IOL, as poor vision quality may result from preexisting high-order aberrations (HOAs) and the combined impact of reduced contrast sensitivity.4 Treatable forms of irregular astigmatism such as pterygium or anterior basement membrane dystrophy should be identified and addressed prior to obtaining biometric measurements. If a patient elects to undergo pterygium excision or superficial keratectomy due to preexisting corneal disease, then several months of corneal stabilization should be allowed prior to obtaining corneal measurements for cataract surgery. If repeat corneal topography or wavefront aberrometry shows an acceptable improvement in corneal regularity, a multifocal IOL may be considered. Ideal candidates for multifocal IOLs should have minimal amounts of corneal irregularity or HOA. There are few studies regarding acceptable parameters for irregular astigmatism or HOAs to guide candidacy for multifocal IOLs. However, patients with coma measuring greater than 0.32 µm implanted with multifocal IOLs have been demonstrated to have increased dysphotopsia.5 Fig. 46.1 Tecnis multifocal lens, model ZMBOO. The lens has a prolate anterior surface with −0.27 µm of asphericity and a back-surface diffractive ringed multifocal surface. This lens provides a 50/50 split for distant and near foci for pupil sizes greater than 1.00 mm. The central 1.00 mm of the lens is distance focus only. This lens is also available in a three-piece platform that may be used for sulcus placement. (Courtesy of Abbott Medical Optics.) Corneal topography may also help identify patients with underlying aqueous tear deficiency that may impact corneal regularity and visual quality.6 If significant dry eye is identified, a regimen of artificial tears or punctal occlusion should be considered. Use of topical cyclosporine or lifetegrast may be considered as well. If corneal irregularity is noted due to dry eye, it is often necessary to treat the dry eye aggressively for 1 to 2 weeks prior to obtaining biometric measurements. Fig. 46.2 Alcon model SN6AD1 Restor apodized diffractive multifocal intraocular lens (IOL). (Courtesy of Alcon.) Pupil size may be an important variable to consider in terms of candidacy for a multifocal lens. The Tecnis multifocal lens is pupil independent, providing a 50/50 split of light rays focused for distance and near across all pupil sizes from 1.0 mm and larger. This lens provides excellent near and distance acuity even in patients with large or irregular pupils. The Restor lens is pupil dependent, due to its central diffractive optic being 3.6 mm in diameter. Therefore, in patients with a mesopic pupil measuring significantly larger than 3.6 mm, near function of this lens may be compromised. Conversely, the shift in function from a diffractive to a refractive optic when the pupil is larger than 3.6 mm may reduce photic phenomena with nighttime driving.7 Careful measurement of mesopic pupil size as a part of preoperative testing may help identify which multifocal lens is appropriate for each patient’s unique anatomy and functional needs. Certain medication, such as antidepressants, may cause pupillary dilation due to anticholinergic properties. This should be a consideration in preoperative planning, as large pupil size may contribute to glare and dysphotopsia symptoms.8 Conversely, in patients with poorly dilating small pupils, where mechanical pupil enlargement or a Malyugin ring may be required, a non–pupil-dependent lens may be favorable to achieve effective near vision, as pupil size and function may be permanently altered. Advances in corneal imaging now make identification of abnormal corneas much simpler. Parameters such as HOA may be evaluated as well. Wavefront analyzers such as the iTrace (Tracey Technologies, Houston, TX) and the optical path difference (OPD)-Scan III (Marco, Jacksonville, FL) enable the evaluation of other information such as angle kappa, spherical aberration, and HOA. These indices may be predictors of patient satisfaction, and also help to guide the choice of appropriate IOL9 (Fig. 46.3). Patients with prior refractive surgery may be suboptimal candidates due to corneal irregularity and HOA. However, some studies have demonstrated that diffractive multifocal IOLs may be implanted successfully in post-refractive patients.10 The regularity of the post-refractive cornea and measured HOA should be a strong consideration in these cases, and additional discussion with the patient regarding vision quality is advisable. Use of intraoperative aberrometry to refine IOL selection may be helpful in prior myopic refractive surgery patients.11 Fig. 46.3 Nidek optical path difference (OPD) III. Images show high angle kappa and large mesopic pupil size. Large angle kappa may be a relative contraindication to multifocal IOL usage. (Courtesy of Mitchell Jackson, MD.) Patients with prior myopic refractive surgery typically have high positive spherical aberration (oblate cornea), whereas those with prior hyperopic refractive surgery more typically have negative spherical aberration (prolate cornea). Spherical aberration matching with the most appropriate lens based on preexisting spherical aberration may help to normalize postoperative spherical aberration. Highly aberrant corneas typically have some degree of induced multifocality and reduction in contrast sensitivity. Placement of a multifocal lens in this clinical scenario is more likely to adversely impact visual quality. Utilizing presbyopia-correcting lenses enables us to offer patients more options for visual rehabilitation after cataract surgery. Every lens option or vision system such as monovision has some inherent compromise: accommodating lenses may not provide a uniformly satisfactory degree of near acuity, whereas multifocal lenses reduce contrast sensitivity and are associated with increased risk of photic phenomena. Therefore, it is incumbent upon the surgeon to educate the patient so that an informed decision can be reached, while considering all options. There are numerous video education platforms available, including Eyemaginations™ (Baltimore, MD) and even iPad based apps for patient education including SightSelector™ (Patient Education Concepts, Houston, TX). It is helpful to have patients watch one of these educational videos prior to meeting with the surgeon or surgical counselor so that the concepts of accommodation, lens removal, and multifocality can be clearly grasped. If patients do not fully understand the absolute loss of accommodation typically experienced with a monofocal lens, they are more likely to be disappointed with their lack of near vision postoperatively. Also, patients are correspondingly less likely to consider a presbyopia-correcting lens if the loss of accommodation with standard cataract surgery is not clearly understood. Specific examples of tasks that could require glasses post-operatively may be helpful to explain the anticipated range of visual function with a monofocal lens, such as needing glasses for seeing the dashboard instrument panel in a car, or needing glasses for computer use. Most cataract patients anticipate the need for reading glasses, but are less aware of the need for glasses for intermediate tasks after cataract surgery. Explaining that even seeing the food on one’s dinner plate may be less clear than one would anticipate, due to the profound accommodation loss with a monofocal lens, may be helpful. During preoperative counseling, it is also important to determine patient lifestyle, hobbies, reading needs, and personality type. To successfully implant multifocal lenses and reduce patient dissatisfaction and risk of lens exchange, a full and thorough discussion of the potential for aberrant light phenomena (glare and haloes) is warranted. Occupational night drivers may not be ideal candidates. The process of neuroadaptation, and allowing 3 to 6 months for reduction in glare and halo phenomena, should be explained. It is also extremely important to demonstrate and discuss the typical focal distance of the lens being considered. For a Tecnis Multifocal or Restor with +4 add, the optimal reading distance is 33 cm, with a fairly rigid depth of focus in front or behind this zone. For the Restor +3 lens, the optimal near focus is 43.5 cm, with a larger depth of focus. With lower add power, intermediate acuity is logically improved, as illustrated by the binocular defocus curve of the lower add power lens, measuring 20/25 compared with 20/40 with the +4 add12 (Fig. 46.4). An additional consideration is the amount of intermediate-range work that the patient does on a regular basis. Multifocal lenses with lower powers are designed to better optimize intermediate-range vision. The highest add power (+4) multifocal lenses lead to more patient complaints in this regard, although over time the intermediate vision can improve with neuroadaptation. Even so, +1.00 reading glasses are often required to optimize intermediate vision in patients implanted with a +4 add multifocal. Lower power multifocal lenses such as the ZKBOO or Restor +2.5 add lens may provide increased intermediate acuity. However, these low add multifocal lenses do not provide the amount of near correction to reliably provide consistent unaided near acuity for most reading tasks. If a low-add multifocal lens is implanted in the first eye, sometimes it may be desirable to implant a higher power MFIOL in the second eye to optimize patient needs. Other products available outside the United States such as the FineVision trifocal IOL (PhysIOL, Liège, Belgium) seek to address this shortcoming of multifocal IOLs with more than two optimal focal distances.
Multifocal Intraocular Lenses
Patient Selection
Pupil Size
Other Preoperative Considerations
Preoperative Counseling
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