LENSES VS. IOLS

Multifocal Lenses Versus Multifocal IOLs

IOLs and contact lenses provide vision correction for presbyopes. Here's a closer look at the options.

By Aaron B. Zimmerman, OD, MS, FAAO, & Andrew Emch, OD, MS, FAAO

	Dr. Zimmerman is an assistant clinical professor of optometry at The Ohio University College of Optometry and associate chief of the contact lens clinic. He can be reached at azimmerman@optometry.osu.edu
	Dr. Emch is a clinical assistant professor at The Ohio State University College of Optometry and also serves part-time as an optometrist at the Columbus Laser and Cataract Center in Westerville, Ohio.

After age 40, humans subjectively notice a reduced ability to focus on near targets. Presbyopia is estimated to affect more than 1 billion individuals worldwide and nearly one-third of the U. S. population (Holden et al, 2008). Presbyopia is a gradual reduction of accommodative ability due to loss of flexibility of the crystalline lens. To address the near defocus, patients seek correction with multifocal spectacles, over-the-counter (OTC) readers, and soft and GP lenses. As patients continue to age, the next major visual obstacle encountered is often decreased vision due to cataract formation.

Following cataract extraction, pseudophakic patients experience absolute presbyopia and often prefer the same corrective options used prior to surgery. Unlike phakic presbyopes, pseudophakes have access to multifocal intraocular lenses (IOLs) as an additional presbyopia-correcting option.

This article will discuss corrective options for phakic and pseudophakic presbyopia, highlighting specific designs and the current landscape of soft and GP multifocals. Additionally, we'll discuss designs and visual performance of various multifocal IOLs. When possible, we'll compare IOL and contact lens designs.

Forms of Presbyopia Correction

Presbyopic contact lens correction falls into three categories: single vision distance and reading spectacles; monovision; and multifocal lenses in soft, GP, hybrid, and scleral options. Tables 1 and 2 summarize current soft and GP contact lens designs.

To contrast the contact lens experience, a patient undergoing cataract extraction—either via clear lens exchange or as a necessity due to a visually impairing cataract—will undergo an immediate loss of all accommodation. Yet many ametropes gladly embrace absolute presbyopia in this fashion as an appropriately selected IOL may offer them spectacle-free distance vision for the first time in years. Binocular monofocal distance correction yields 2.5D of blur at near, which may be managed with OTC readers or by selecting corrective powers to induce monovision. Just as with contact lenses, unique multifocal IOLs exist, but their optical designs differ from those of the most common multifocal contact lenses.

Monovision

The 2011 International Contact Lens Prescribing report (Morgan et al, 2012) indicates that approximately 12 percent of contact lens-wearing presbyopic patients are corrected with monovision, which presents a reasonable option, particularly for early-to-mid presbyopia (< +1.50D add) (Bennett, 2008; Nichols, 2012). It is often a preferred method for patients who have moderate astigmatism as alternative presbyopic options are limited. Uncomplicated monofocal optics allow for straightforward lens selection; a more plus-powered sphere or toric contact lens would simply be fit on one eye. Unfortunately, nearly 25 percent of patients fail to adapt to monovision, and it is associated with reduced stereopsis (Richdale et al, 2006) and binocular contrast sensitivity (Durrie, 2006). It may be associated with increased risk of falling (Vale et al, 2008), and has been implicated in an aviation accident (Nakagawara and Veronneau, 2000).

Surgeons may elect to recreate a monovision system using monofocal IOLs in a previously welladapted monovision lens wearer. Depending on cataract severity, contact lenses may be trialed to demonstrate monovision's benefits and drawbacks. Other fitting philosophies incorporate modified monovision, in which the dominant eye is corrected for single vision distance while the nondominant eye is corrected with a multifocal IOL or contact lens. Regardless, visual compromise resulting from monovision via contact lenses or IOLs will often improve with over-spectacle correction at distance or near, and should be considered as needed.

Zonal Multifocal Designs

The first multifocal IOL approved for use in the United States was AMO Array (Advanced Medical Optics, now Abbott Medical Optics) in 1997. Similar to concentric contact lens designs, this lens consisted of a center-distance zone surrounded by four alternating near and far zones (Steinert et al, 1999). During the two years following its approval, a study compared Array to a monofocal IOL. No difference was found for distance visual acuity; however, near acuity was nearly two lines better (Steinert et al, 1999).

Although spectacle dependency was reduced with Array, subjects noted glare and halos as well as reduced low-contrast visual acuity (Steinert et al, 1999). In 2005, the FDA approved AMO ReZoom, which again used a center-distance zone with four alternating near and far zones; however, this design featured aspheric transitions between zones. Both the Array and ReZoom IOLs are pupil size dependent, with smaller pupil diameters directing the majority of light to the distance focal point. At a pupil diameter of 5mm, two-thirds of the light is dedicated to distance (Lane et al, 2006). Although ReZoom was an upgrade of the Array, reports of glare and halos continued.

A zonal multifocal contact lens, the Acuvue Bifocal (Vistakon) was approved in 2001, and its design is similar to the Array multifocal IOL—a center distance zone, surrounded by four alternating near and far zones. Similar to the Array IOL, the Acuvue Bifocal design is associated with ghosting and halos, particularly at higher add powers (Ardaya et al, 2004). Currently these designs are not as visually favorable as other lenses, likely due to the center-distance optics and associated dysphotopsias.

A more recent design is Balanced Progressive Technology developed by CooperVision. It uses a zonal progressive design that invokes ocular dominance to help satisfy visual needs at all distances, incorporating a monovision element. The dominant lens design consists of a 2.3mm spherical center-distance lens surrounded by an aspheric intermediatenear optical zone, while the nondominant design consists of 1.7mm spherical center-near design with an intermediate-distance aspheric surrounding zone. Add powers range from +1.00D to +2.50D. This system has been shown to have minimal detrimental effects on stereopsis and visual acuity as compared to single vision lenses with readers (Ferrer-Blasco et al, 2011). This design is available in Biofinity (comfilcon A), Proclear (omafilcon A), and Frequency 55 (methafilcon A), which allows for material versatility while maintaining consistent optics. The Proclear multifocal also is available in a toric design.

Aspheric Contact Lenses

Use of aspheric surfaces is one of the more prominent and effective methods of providing add in GP and soft multifocal designs. Asphericity can be applied to anterior, posterior, or both (bi-aspheric) surfaces. Altering the eccentricity value on either surface generates more or less power as desired.

Aspheric surfaces can be applied for a variety of optical phenomena. For example, an aspheric single vision soft lens, such as PureVision 2 (Bausch + Lomb [B+L]), is designed with relatively low eccentricity to incorporate a small amount of negative spherical aberration to correct the inherent positive spherical aberration of the cornea (Wang et al, 2003) . The eccentricity values of multifocal designs, however, are much larger than those of monofocal aspheric designs. This generates positive spherical aberration, which increases depth of focus and enhances intermediate and near vision.

Many soft lens aspheric designs are center-near, with the asphericity on the anterior surface. The center-near design is purposely employed for soft lenses for two reasons. First, the limited movement associated with soft lenses (ideal movement is 0.30mm to 0.80mm) ensures that near optics are in place over the visual axis. Also, pupil diameter decreases with age (Winn et al, 1994), and more importantly, pupillary miosis occurs with the near triad. Current soft lenses incorporating a center-near anterior aspheric design are the SofLens Multi-Focal, PureVision Multi-Focal (both B+L) and the Air Optix Aqua Multifocal (Alcon).

In addition to widely distributed lenses, custom designs such as the C-Vue multifocal (Unilens) and the Intelliwave multifocal (Art Optical) have been reinvigorated by the Definitive (Contamac) latheable silicone hydrogel material, efrofilcon A. While these lenses are also center-near with an anterior aspheric surface, they are customizable for high spherical and cylindrical powers.

GP aspheric lenses are often center-distance in design, and the asphericity can be imparted to one or both sides of the lens. Unlike soft lenses, these will effectively translate on the eye. This design yields progressively more positive power toward the periphery. As the lens translates, the midperipheral lens approaches the pupil and focuses the divergent light from the near target. Accordingly, these lenses are most effective for pupil sizes less than 5mm. The ever-present caveat with aspheric GPs is that high posterior surface eccentricity values coupled with lenses designed to fit steep may potentially mold the cornea (Wang et al, 2002). Regardless, they remain the multifocal GP design of choice for many.

Translating Contact Lenses

Translating contact lenses have the potential to provide excellent distance, intermediate, and near vision. Distinct zones exist within a lens dedicated to monofocal vision at a given distance. Superior distance and inferior near zones are typically separated by a visible line that ideally settles in the lower third of the pupil following the blink. Depending on the contact lens, a horizontal intermediate segment may exist between the upper (distance) and lower (near) zones. Prism ballasting and truncation are used to maintain proper orientation of the segment following the blink. While single vision optics offer potentially excellent vision, these contact lenses may require substantial modification to troubleshoot structural anatomical obstacles and blink dynamics, especially considering the visual demands of patients opting for this design.

Accommodating IOLs

As IOLs are placed securely in the capsular bag or ciliary sulcus, there are no designs that correlate directly with vertically translating GP contact lenses. Nevertheless, some IOL designs do function by translating in the axial dimension.

While the crystalline lens becomes inflexible with age, the ciliary muscle maintains its ability to contract (Sheppard and Davies, 2011). IOLs have been developed to harness ciliary muscle contraction and produce axial movement, a mechanism obviously inaccessible to contact lenses. In 2004, Crystalens (B+L) (Figure 1) was introduced. This design features a 5.0mm optic attached via a hinge to two 2.75mm plate haptics on each side of the lens. During ciliary muscle contraction, the pressure in the vitreous cavity increases, which theoretically moves the optic anteriorly via the hinge mechanism. This anterior lens displacement induces temporary myopia, which improves near vision.

Figure 1. The Crystalens IOL.

Crystalens is available in two options: Aspheric Optics (AO) and High Definition (HD). The AO has a bi-aspheric design and induces no higher-order aberrations. As previously mentioned, the cornea tends to be associated with positive spherical aberration while the crystalline lens is associated with some negative spherical aberration. Because AO does not induce negative spherical aberration, there is inevitably net positive spherical aberration in the system. Fortunately, this has been associated with an increased depth of focus (Marco et al, 2005).

Crystalens HD uses a 1.5mm central bispheric portion of the 5mm optic and is associated with inherent positive spherical aberration. The spherical aberration from the implant combines with the positive corneal spherical aberration to reduce distance image quality, but this also expands depth of focus as compared to AO (Pepose et al, 2011).

Diffractive IOLs

According to Market Scope data, presbyopia-correcting IOLs comprise about 7 percent of the U.S. market. Of the multifocals already described, AcrySof IQ ReStor (Alcon) is currently the leading lens (Krader, 2011). One gaining market share is Tecnis (AMO) at 25 percent. Both AcrySof IQ ReStor and Tecnis use a diffraction grating that directs light for both distance and near optics. The Tecnis IOL uses a prolate aspheric anterior surface and a posterior that features a 1mm central zone surrounded by 32 concentric rings, each ring with the same height. The aspheric surface induces −0.27mm of spherical aberration, which offsets the average positive corneal spherical aberration. The structure of the diffractive grating is independent of pupil diameter, which means an equal 41 percent of light energy is dedicated for distance and near correction, while the remaining 18 percent is lost due to destructive interference (Pepose et al, 2011). The overall lens is designed to incorporate a +3.2D add at the spectacle plane.

AcrySof IQ ReStor is available in both a +4.0D and +3.0D add (+3.2D and +2.4D spectacle plane, respectively). This design differs from Tecnis as the diffraction grating is on the anterior lens surface and is apodized, which indicates that the step height of each concentric ring is lower than that of the previous, more central step. Peripheral to the 3.6mm diameter diffractive portion of the lens is a 2.4mm distance dedicated refractive zone. Of note, ReStor +4.0 consists of 12 concentric rings while ReStor +3.0 (Figure 2) consists of nine rings. The posterior IOL surface is convex aspheric, which creates −0.10mm of spherical aberration, again to offset the positive corneal spherical aberration. Due to the apodization, ReStor is pupil size dependent. With a pupil diameter of 2mm, light energy is evenly divided between distance and near; however, as the pupil diameter increases, the distance optics become far more dominant.

Figure 2. The Restor +3 IOL.

Visual Performance of Multifocal IOLs

When Array was introduced its only competitor was a monofocal lens. Steinert (1999) found that subjects with bilateral Array lenses had more comfortable vision at near without spectacles compared to a monofocal/multifocal combination. This result was unfortunately accompanied by a moderate loss in low contrast visual acuity and complaints of halos and glare (Sen et al, 2004). ReZoom was an enhanced IOL design, and although one study reported that less than 10 percent of patients who had this lens experienced glare and halo problems (Forte and Ursoleo, 2009), other studies reported these symptoms in nearly 30 percent of patients (Chiam et al, 2007).

ReStor +4.0 IOL was approved in 2005 and was a departure from the previous model of the zonal progressive. Initial visual outcomes were impressive as more than 90 percent of patients read 20/25 or better at distance and 20/32 or better at near (Reeves, 2009). Less than 10 percent of patients corrected with ReStor reported severe halos or glare (Vingolo et al, 2007; Cochener et al, 2010). Complaints did arise regarding reduced intermediate vision with this lens, which prompted development of ReStor +3.0. When directly comparing visual performance of ReStor +4.0 to ReStor +3.0, the +3.0 model resulted in better intermediate visual acuity, a more realistic near working distance, less of a detrimental effect on distance visual acuity, and lower higher-order aberrations (de Vries et al, 2010).

Tecnis is the most recent addition to the diffractive IOL class (Figure 3). When compared to the refractive multifocal ReZoom, it provides more patient satisfaction, better near vision, and fewer photic complaints (Cillino et al, 2008). When comparing binocular implantation of ReStor, ReZoom, and Tecnis, all three performed similarly, but ReStor and Tecnis provided more spectacle independence compared to ReZoom (Gierek-Ciaciura et al, 2010).

Figure 3. The Tecnis IOL.

Per the previously discussed market data, diffractive multifocal IOLs are the preferred implant for patients desiring spectacle-free vision. While Crystalens products produce better corrected distance vision, better uncorrected intermediate vision, better contrast sensitivity, and less glare as compared to ReStor or ReZoom, they do not perform as well as ReStor at near (Pepose et al, 2007). Interestingly, when one eye is corrected with Crystalens and the fellow eye with ReStor, near vision improves and contrast sensitivity testing is similar to bilateral Crystalens correction (Pepose et al, 2007). Studies have shown that 67 percent of patients with bilateral Crystalens implants required less than 1.25D of near add, with additional studies showing at least 1.75D of accommodation (Cumming et al, 2006; Alio et al, 2004; Macsai et al, 2006). Others have shown far less (Stachs et al, 2005; Stachs et al, 2006). While this is an improvement from monofocal distance correction, reduced near vision continues to inhibit spectacle-free vision with this design. Also, complications secondary to the hinge mechanism, such as Z-syndrome, may occur (Yuen et al, 2008).

Conclusion—Looking to the Future

Direct comparison of presbyopia-correcting contact lenses and IOLs is a challenge as presbyopia and cataractogenesis are very different, non-parallel processes. Presbyopia limits focus, while cataracts impact color, brightness, and best-corrected vision. One population requires removal of the crystalline lens and a static implanted optic, while the other population has clear optical media and requires a dynamic optical correction. Ocular surface complications may be more significant for contact lens wearers, and contact lenses' dynamic nature can be beneficial or detrimental to visual performance.

Just as contact lenses are regularly introduced, there is buzz around new IOLs. To contrast the current popularity of diffractive IOLs, most new technology is focusing on accommodative designs as they offer the best potential visual acuity and fewest photic effects. Foreign markets continue to introduce IOL designs that claim stronger, more accurate accommodative mechanisms. Some focus on optimizing lens axial translation (Tetraflex, Lenstec), others on channeling fluid (Fluidvision, Powervision), dual optic systems (Synchrony, Visiogen), or altering IOL shape to aid near focus (Dynacurve, NuLens Ltd.) Expect to hear more about them as they progress through the FDA approval process. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #201.