Looking Deep at Extended Depth-of-Focus IOLs
Multifocal IOLs, perhaps the best established solution to the problem of enabling post-cataract patients to see clearly at all distances, work by incorporating discrete refractive optical zones or diffractive echelettes into the lens. Accommodating IOLs, which aim to provide the same benefit, use ciliary muscle power to move the lens and/or change its shape to provide additional focusing power for near and intermediate vision.
Both of these options involve compromise. Multifocal IOL patients experience some degree of glare or halo, although most adjust to it within a matter of weeks. In addition, there is some loss of contrast sensitivity. Typically, though, if the surgery is on target, these patients have good distance vision and don’t need reading glasses for most visual tasks.
The compromise with accommodating lenses is that current designs don’t offer quite enough accommodation to provide both crisp distance vision and perfectly clear reading vision—the ciliary muscle just doesn’t seem to have the necessary strength needed to fully power current lenses. As a result, today’s accommodating lenses typically can’t provide a full range of vision.
These tradeoffs help explain why presbyopia-correcting IOLs have garnered only modest patient interest since becoming available in the US in 1997. According to Market Scope, they accounted for only 2.4% of total IOLs implanted globally in 2014; and use of presbyopia-correcting IOLs has grown only 6% per year since 2010.1 However, a number of new presbyopia-correcting IOLs, including extended depth-of-focus [EDOF] IOLs, may expand the market considerably: Market Scope forecasts nearly 11% growth in presbyopia-correcting IOLs through the end of 2016.1
Elongated Focus
By simply increasing depth of field rather than adding a second or third focal point, EDOF IOLs are designed to provide better near vision than monofocal or accommodating IOLs, and better contrast sensitivity—with less glare and halos—than multifocals or trifocals.
An early entrant to the field, the Tecnis Symfony EDOF, IOL from Abbott Medical Optics received a CE mark in June 2014. This lens uses two strategies to achieve better near vision: spherical aberration control and diffractive optics.
Spherical aberration control is achieved by adding negative spherical aberration to the IOL to counteract the normal cornea’s natural positive spherical aberration, increasing the patient’s depth of field.
The second strategy, diffractive optics, is used in two complementary ways. First, the Symfony makes use of diffractive echelettes to slightly elongate the focus. (This is unlike diffractive multifocals that create two distinct focal distances.)
Typically, the price that’s paid for optics that elongate focus is loss of some degree of image quality. The Symfony compensates for this by sharpening up image quality in other ways. For example, the Symfony makes use of its echelettes to compensate for the normal chromatic aberration inherent in the refractive portion of the optical system.
In a study comparing bilaterally implanted Symfony IOLs in 31 patients with bilateral monofocal Tecnis implants, Symfony patients achieved a mean visual acuity of 20/20 or better throughout 1.50 D of defocus. At 3 months postop, Symfony patients in the study achieved a mean distance corrected intermediate visual acuity of better than 20/20. Mean distance-corrected near visual acuity was only 20/30, which is not the same level of spectacle independence that patients can get with a mutifocal IOL; however, Symfony patients experienced less glare than would be expected with multifocal IOLs.2
The Symfony IOL is being released in Europe, and surgeons who are implanting it are being asked to participate in a Phase 4 trial designed to gather more data on patient outcomes. The FDA Phase 3 study is also currently under way.
Pinhole Approach
A second extended depth-of-focus IOL is the small-aperture IC-8 IOL from AcuFocus, which has received CE Mark approval in Europe. Taking a cue from its Kamra intracorneal inlay, which uses pinhole optics to increase depth of focus for presbyopic patients who don’t need cataract surgery, the IC-8 moves the small aperture optics back to the IOL to give cataract patients a similar extended depth of focus.
Because of the pinhole effect, near vision is improved without too much sacrifice of distance vision or induction of glare. There is a mild decrease in contrast, but not as much as with a multifocal lens.
Results from monocular implantation of a small aperture IOL show improved range of vision after cataract extraction, with a mean of J1 at near, and 20/20 at both intermediate and distance at 12 months. Symptoms reported were similar between the IC-8 eye and the fellow eye implanted with a standard monofocal IOL.3
The company reports that it has designed a tight roll-out of the product in Europe. AcuFocus has not yet applied for a premarket approval in the United States, according to the FDA website.
REFERENCES
1. Freeman W. Expanding PC-IOL Options Expected to Fuel Demand Among Patients. Market Scope. July 17, 2105; 19(7): 1-2.
2. Abbott Medical Optics. Extended range of vision IOL 3-month study results (NZ).http://www.tecnisiol.com/eu/tecnis-symfony-iol.htm. Accessed December 29, 2014.
3. Vukich et al. “Achieving Reliable and Predictable Results with a Small Aperture Inlay.” 2014 AcuFocus white paper.