BLOG: Refractive forgiveness and EDOF lenses
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Extended depth of focus IOLs differ from multifocal IOLs in an important way.
As the name suggests, they broaden the range of continuous vision, rather than apportioning light to specific focal points. Their broad “landing zone” has significant advantages beyond just correcting presbyopia. It also makes EDOF lenses more forgiving of residual refractive error and less likely to require enhancement postoperatively.
In an early study with the Johnson & Johnson Vision Tecnis Symfony IOL (the New Zealand Duet study), for example, patients could tolerate up to 1.5 D of astigmatism without loss of distance visual acuity. The defocus curves for the lens also demonstrate that mean binocular visual acuity of 20/25 or better can be sustained through 1.5 D of defocus. So, the EDOF characteristics appear to apply to not only spherical defocus, but astigmatism as well, and this has been consistent with our clinical experience.
This degree of tolerance is especially useful in post-refractive eyes in which accurate IOL power calculations are more difficult. But even when performing surgery in normal eyes using contemporary third- and fourth-generation formulae, residual refractive error still exists. In an analysis of more than 280,000 cases, Lundström and colleagues reported that a biometry prediction error within 0.5 D was achieved in just under three-quarters of the eyes (73%). A large national registry study this year had similar results, while another found that emmetropia (defined as a spherical equivalent –0.5 to +0.5 and less than 1 D of astigmatism) was achieved in only 55% of eyes targeted for emmetropia. Although there are smaller studies with much tighter predictive accuracy, the refractive outcomes of cataract surgery still fall significantly short of those achieved with LASIK, making a little bit of tolerance to refractive error a very good thing.
Of course, when it is necessary to utilize the defocus tolerance for residual error, there is a consequence: One loses some of the range intended for near to intermediate vision. This may be one of the reasons that many surgeons like the personalized vision approach of combining an EDOF and multifocal IOL. Binocularly, this combination provides excellent near vision for reading, with the refractive tolerance and other benefits of an EDOF lens.
Symfony was the first FDA-approved EDOF lens in the U.S. and has been on the market for several years. Recently, the AcrySof IQ Vivity IOL (Alcon) was introduced, and there are a number of other lenses with EDOF features in the pipeline. It is a positive development for our patients to see this category expanding.
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