Extended Depth-of-Focus IOLs

All current options for presbyopia-correcting IOLs involve some form of optical compromise. Multifocal IOLs split light into two foci and also scatter some light energy, resulting in a loss of contrast sensitivity. Accommodating IOLs cause no loss of contrast sensitivity, but current designs do not provide sufficient near acuity for sustained, high-grade reading performance. A small degree of monovision is typically used to very effectively remedy this situation. Because presbyopia-correcting implants involve a significant financial contribution from patients; therefore, asking them to settle for an optical compromise can be tricky.

US PHYSICIANs’ USE OF PRESBYOPIA-CORRECTING IOLs

Although the IOLs available today in the United States provide very good results, according to a physician survey conducted at the 2014 American Society of Cataract and Refractive Surgeons Annual Meeting, these lenses account for only about 7.2% of cataract implants.1 This relatively low penetration level has remained stable for several years. Monovision with standard IOLs is three times more common than presbyopia-correcting IOLs, according to the same survey. Why? Is it a lack of access to advanced multifocal or accommodating IOL designs currently available only outside the United States?

When both US and non-US physicians were surveyed at the 2014 ASCRS meeting regarding the incidence of night vision aberrations in their multifocal IOL patients, both groups indicated that about 6% of their patients had significant night vision complaints. Although some newer multifocal implants are touted as having beaten the dysphotopsia problem, when carefully studied, many of these IOLs actually have dysphotopsia incidences that are similar to those seen in the original FDA trials of US-approved designs.2-6

A TOTAL NEAR VISION SOLUTION?

If a near vision solution could be developed that matched the distance performance of a monofocal IOL, it would disrupt the industry. One possible candidate is a new category of IOLs known as extended depth-of-focus IOLs (EDOF-IOLs). An example of this type of lens is the Symfony from Abbott Medical Optics, which is currently available outside the United States. This lens uses three principal optical strategies to achieve its near performance: spherical aberration control, a diffractive optic to expand a single focal zone as opposed to creating multiple foci, and chromatic aberration control.

Spherical aberration control has been extensively described previously, and it simply amounts to placing negative spherical aberration in an IOL to counteract the naturally occurring positive spherical aberration in the normal cornea. This is the strategy used by a variety of currently available IOLs, including the monofocal Tecnis (Abbott Medical Optics) and AcrySof (Alcon) IOLs.

DIFFRACTIVE OPTICS

Diffractive optics have historically been used in IOLs to create two distinct foci; however, through a modified diffractive ridge design, the Symfony IOL has a single elongated zone of focus. Although the Symfony physically resembles a multifocal IOL, it is, in fact, not a multifocal (Figure 1). It is important to realize that the loss of contrast sensitivity seen in diffractive multifocal IOLs relates principally to the multifocality as opposed to the diffractive optic. Diffractive lenses are quite compatible with very high-performance optics. Diffractive lenses are commonly used in the professional telephoto lens designs available from the Canon camera company (Figure 2).7

Professional photographers like diffractive optic designs for a variety of reasons, but an important one is chromatic aberration control. All IOLs create chromatic aberration when light gets dispersed, owing to the prismatic effect of the IOL. Peripheral light rays are bent differently based upon their wavelength, and short-wavelength blue light is bent more than long-wavelength red light. With a diffractive optic, however, the dispersion of light according to wavelength is exactly the opposite, with red light bent more and blue light bent less. When diffractive and refractive optics are combined, chromatic aberration is significantly reduced (Figure 3).

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 (Figure 4). In essence, this means that a patient could be refracted to his or her best distance visual acuity and then overminused by an additional 1.50 D, but still maintain a visual acuity of 20/20 or better. As would be expected with this range of focus, intermediate vision was excellent. At 3 months postoperatively, Symfony patients in the study achieved a mean intermediate visual acuity of better than 20/20 through their distance correction (Figure 5). Although mean distance-corrected near visual acuity was only 20/30, the real surprise was that these Symfony patients experienced no difference in glare or halos compared with the monofocal controls.8,9

CONCLUSION

The near acuity of the Symfony EDOF-IOL does not match that of a multifocal but excellent intermediate vision with quality of vision matching that with a monofocal is a very compelling proposition. This new category of EDOF-IOLs has the potential to change the way surgeons approach presbyopia. If ophthalmologists can truly deliver functional near vision without optical compromise, I predict the presbyopia-correcting IOL category will grow tremendously.n

Steven J. Dell, MD
• director of refractive and corneal surgery for Texan Eye in Austin
• chief medical editor of CRST
• (512) 327-7000
• consultant to Abbott Medical Optics and Alcon