Advances in small incision surgery, improved clinical outcomes, and the recent availability of bifocal intraocular lenses (IOLs) have made lens surgery as a refractive modality a reality. By employing bifocal IOLs in refractive procedures to correct for both distance and near vision, it has become possible for ophthalmic surgeons to help cataract patients achieve independence from spectacle wear. However, the variety of bifocal IOL options available coupled with issues of cost can make this a confusing and difficult decision for patients. Consequently, as more ophthalmic surgeons offer bifocal IOLs to their patients, it is critical to understand the strengths and weaknesses of this new generation of lenses, because not every one of the current bifocal IOLs is ideal for every patient.
Three Bifocal IOL Options
Bifocal IOLs may be divided into 2 classes: accommodative IOLs, which move within the eye and, as a consequence of this movement, change power; and multifocal IOLs, which contain multiple zones that refract light differently, giving rise to 2 or more simultaneous images. At the present time, 1 accommodative IOL is available (Crystalens from Eyeonics, Inc.), and 2 multifocal IOLs are available (ReZoom from Advanced Medical Optics, Inc. and AcrySof ReSTOR from Alcon, Inc.).
The Crystalens is the only accommodating IOL approved by the Food and Drug Administration (FDA) in the United States. This modified plate-haptic, silicone IOL with polyimide haptics offers excellent distance vision, no loss of contrast sensitivity, minimal night vision disturbances, and good intermediate visual acuity. Unlike the ReZoom lens and the AcrySof ReSTOR lens, the crystalens is unaffected by pupil size, but near vision is weak and variable compared to the 2 multifocal alternatives (J Cataract Refract Surg. 2003;29:677-685). Many crystalens patients have also found it necessary to wear supplemental reading glasses, and in the FDA clinical trial only 26% of patients were totally independent of spectacles at all times.
Some surgeons are concerned about the small optic size (4.5 mm in diameter) of the crystalens and its inability to block ultraviolet (UV) rays up to 400 nm. Also of concern is Z-syndrome, in which the crystalens dislocates due to the compressive forces generated by the capsular bag as it contracts. Yittrium-aluminum-garnet (YAG) capsulotomy can, however, be curative, and the problem may be avoidable with appropriately sized capsulorhexi and meticulous cortical cleanup. In sum, the crystalens is a good bifocal option for those patients who want good intermediate and distance vision but who do not mind wearing glasses occasionally for near vision tasks.
The ReZoom bifocal lens is a zonal aspheric, refractive multifocal IOL, which was modified from an earlier design (i.e., multifocal, silicone optic Array IOL). The ReZoom lens is a hydrophobic, acrylic 3-piece lens featuring technology that uses zones proportioned to provide good visual function across a range of focal distances in varying light conditions. It has a near power +3.50 diopters (D) add in the near portion that translates to about a +2.6 D add at the spectacle plane.
The 5 zones (rings) enable available light that travels through the optic to provide distance, intermediate, and near visual acuity. As such, pupil size is important when considering the use of this lens. The number of zones exposed by the pupil will have a significant effect on what the patient sees. For example, a patient with a pupil size of about 2.5 mm or less will have difficulty with near vision, because the central zone is for distance only and is 2.3 mm in diameter. The 5 zones also contribute to the potential for night vision difficulties such as glare and haloes. As the pupil dilates in dark and dim conditions, more rings of this IOL are exposed, giving rise to light spread at the inner and outer boundaries of the rings, which is perceived by patients as haloes. Both clinical investigators and the manufacturer (Advanced Medical Optics) have said that redesigning the Array IOL to create the ReZoom lens has decreased this halo effect. The ReZoom lens optic also features a triple-edge design that minimizes edge glare and reduces posterior capsular opacification (PCO).
Data from Europe indicated total spectacle independence of about 31%. Spectacle independence data from the U.S. were unavailable at the time of this writing. The ReZoom bifocal IOL appears to be a good choice for those patients who want good distance and intermediate vision with occasional spectacle use for near vision. Prospective patients should optimally have a pupil size larger than 3 mm under accommodative conditions and be willing to accept possible night vision symptoms and a slight loss of contrast sensitivity.
The AcrySof ReSTOR bifocal lens is a refractive-diffractive, apodized single-piece, truncated hydrophobic acrylic IOL. The lens has a central 3.6-mm apodized diffractive optic region with 12 concentric, gradually decreasing step height diffractive zones on the anterior surface, which divide incoming light into 2 diffraction orders to create 2 lens powers. This central 3.6-mm zone is surrounded by a region with no diffractive structure over the remainder of the 6-mm diameter lens. Near correction is +4.0 D at the lens plane, resulting in a 3.2 D correction at the spectacle plane. The near point is slightly closer than that of the ReZoom bifocal IOL; but combined with the apodized diffractive design, this construction creates a distinct transition between the 2 foci (distant and near image) and reduces potentially disturbing optical phenomena such as glare and haloes.
Like the ReZoom IOL, the AcrySof ReSTOR IOL provides good distance vision with some mild compromise in contrast sensitivity. Pupil size concerns are less critical with the AcrySof ReSTOR lens than with the ReZoom lens. The AcrySof ReSTOR optic was designed to be distant dominant with increasing pupil size. This not only improves night distance vision, but it also reduces glare and haloes at night. With decreasing pupil size (as in accommodation), equal energy is devoted to distance and near vision, allowing for excellent near vision.
As with any multifocal lens, however, the AcrySof ReSTOR lens may clinically compromise vision. Because the incoming light is divided mostly between distance and near, very little of it is focused directly for intermediate vision. For those patients who have significant intermediate vision demands, spectacles may be necessary. Nonetheless, the FDA clinical trial demonstrated that 80% of patients who received bilateral AcrySof ReSTOR IOLs had total spectacle independence. The AcrySof ReSTOR bifocal IOL may be recommended for those patients who desire good distance and near vision but who understand that they may require spectacles for specific intermediate tasks, and they may have significant (5% in the FDA clinical trial) difficulties with glare and haloes at night.
Educating patients will be the key to successful bifocal IOL implementation in clinical practice. Ophthalmic surgeons should exercise restraint in “selling” this exciting technology and concentrate instead on understanding the individual lifestyle and expectations of patients in order to help them select the best IOL option. Emphasis should be placed on decreased spectacle dependence and not spectacle independence. Significantly more time should also be spent discussing bifocal IOL options with patients in order to help them understand why bifocal IOL options carry a premium price. Finally, since patients are paying for a service that includes the correction of astigmatism by incisional techniques or laser, the ophthalmic surgeon should have mastered the surgical techniques involved to be able to intelligently discuss the various IOL options with patients and to maximize their clinical outcomes.
Each bifocal IOL has strengths and weaknesses, and while none of the lenses is perfect, surgeons have an obligation to be knowledgeable and frank about the options available to prospective cataract and refractive patients. There is a potential disadvantage with any choice, and not all patients are suitable candidates for a bifocal IOL. Ophthalmic surgeons have the ability to provide patients with something very special, and for those patients who are interested and well-suited, bifocal IOLs can be a true miracle.
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The author discloses a financial interest as a consultant and medical monitor of ophthalmic devices for Alcon Laboratories, Inc. He has no proprietary interest in any of the products discussed in this article.