How IOL platform designs are achieving quality of vision

Reviewed by Albert Augustin

Take-home: Patient satisfaction is high following implant of the Symfony IOL, which is designed with an elongated focal zone in an attempt to minimise visual compromises and avoid visual dysphotopsias.

A primary unintended consequence of lens platforms offering vision correction in multiple vision zones is the incidence of visual dysphotopsias. Glare and haloes after implantation of a multifocal IOL are highly variable. In controlled clinical trials with the Tecnis IOLs (Abbott), the incidence of moderate haloes was 6% and severe halo occurred in 5%.¹ In studies of the AcrySof multifocal lens (Alcon), 2.7% reported severe glare and 11% severe haloes.²

Fundamentally, design considerations may explain these differences. However, additional factors such as residual refractive error, wavefront anomalies, IOL decentring, posterior capsular opacification and dry eye disease may also induce postoperative photic phenomena.³ Thus, although lens design is integral to reducing the potential for postoperative visual disturbances, the surgeon must still pay careful attention to optimising the ocular surface and performing meticulous surgery.

It is hoped that a new-design lens will lower incidence of glare and halo via elongation of the focal zone. The TECNIS Symfony IOL (Abbott) may also provide improvements in chromatic aberration, providing patients with sharper vision. A recently completed patient satisfaction survey demonstrated that these design considerations translate to improved performance.

The role of competing vision zones in visual disturbances

The role of competing vision zones in visual disturbances

Traditional multifocal lenses are constructed using diffractive optics, which split light into multiple zones for vision at near, intermediate and distance. The lens options on the market employ different design considerations to provide vision at different distances without compromising quality of vision.

The AcrySof IOL utilises apodised optics to spread light distribution across the surface of the IOL based on the size of the pupil and vision demand. That is, more light is theoretically available in the near-distance zone when the focus is near. These lenses are available in two models, +2.5 and +3.0, with the former intended to provide greater distance vision and the latter intended to provide equal vision across all three distances. However, an inescapable fact of such a design is that light diffracted through multiple vision zones at the optical surface results in competing images on the retina, which is the basis for visual phenomena such as halo and glare.

The Tecnis family of IOLs employ a slightly different mechanism, with a wavefront-designed aspheric surface and a posterior diffractive surface. Three different lens options are offered to permit customisation of the lens to a patient’s individual vision needs: the +4.0 model has a theoretical reading distance of 33 cm, the +3.25 lens a theoretical reading distance of 42 cm and the +2.75 model a theoretical reading distance of 50 cm. The aspheric surface is intended to reduce contrast sensitivity, and may affect the path of light transmitted through the posterior diffractive zone. However, again, a design fundamental of the Tecnis family is that the diffractive optics split light to different foci, which has the potential to cause dysphotopsias.

Another lens option corrects vision across the multiple viewing ranges via pseudoaccommodation (Crystalens; Bausch & Lomb). Postmarketing evidence suggests that, although these lenses provide good distance vision, patients may require glasses for near demands, and so the lens may not supply the full range of vision of other IOL technologies.

A new design in IOLs: extended range of vision

A new design in IOLs: extended range of vision

The Tecnis Symfony IOL is designed with a diffractive echelete that creates a novel diffraction pattern to elongate the depth of focus (Figure 1). Even though light comes from distinct points, the elongated focus brings the light to a similar point on the retina. Correspondingly, this increased depth of focus relative to monofocal and other multifocal platforms creates a mechanism to ensure less defocus in the visual plane – in other words, less light that is out of focus. Because haloes and glare result from defocused images in the viewing plane along the z-axis, the Symfony’s elongated depth of focus yields the ideal solution to reduce dysphotopsias.

Such an effect is demonstrated in defocus curves demonstrating 20/20 or better mean visual acuity from distance to 1.5 D of defocus, 20/40 or better mean visual acuity from distance to 2.5 D of defocus and a 1.0 D increase in depth of focus throughout the defocus curve (Figure 2).⁴

Furthermore, the Symfony is constructed with achromatic technology intended to reduce wavelength-dependent defocus. Chromatic aberration results from certain wavelengths of light stopping short of the point of focus (the macula), resulting in blur and reductions in contrast. Studies suggest that the average eye has around 2.0 D of chromatic aberration between 400 and 700 nm and 0.8 D between 500 and 640 nm.⁵ The achromatic technology in the Symfony platform yields sharper focus, in effect bringing the focal point of each wavelength closer to the macula, thereby resulting in sharper vision less given to chromatic variance. Because it is coupled with correction of spherical aberration, the resulting vision does not sacrifice depth of focus (Figure 3).^6,7

Patient satisfaction survey

Patient satisfaction survey

I recently completed a patient satisfaction survey in my clinic, assessing visual outcomes after implantation with the Tecnis Symfony IOL in a population of 160 eyes of 80 patients undergoing refractive lens exchange. This was a retrospective review with the primary objectives of measuring uncorrected distance visual acuity (UCDVA), uncorrected near visual acuity (UCNVA), patient satisfaction and complications.

One month after surgery, UCDVA was 20/32 or better in 93% of patients while UCNVA was 20/50 in 88%. In the questionnaire, 83% of patients reported being satisfied or very satisfied and 90% of patients said they would recommend the procedure. Severe difficulty with glare, haloes and ghosting was reported by only 7%, 6% and 1% of patients, respectively. Little or no difficulty with driving at night was reported by 72% of patients reported. There were no reported intra- or postoperative complications (Figure 4).

Conclusion

Conclusion

Many variables affect presbyopic patients’ ability to see at multiple distances after implantation of a phakic IOL. Thus, even with new designs on the market, such as the Symfony, it remains critically important to understand the individual vision demands and desires of the patient, as well as what he or she is willing to compromise to achieve the desired refractive outcome. The level of compromise the patient is willing to accept will direct the lens choice. If the patient wants good reading, perhaps a higher add multifocal lens would be appropriate, assuming there is an understanding that there may be some issues with quality of vision. In contrast, a patient who drives a lot might be happier with a low add multifocal, with the caveat that some distance acuity may be lost.

Ultimately, then, what the Symfony technology offers by way of an extended range of focus is another lens option to match to patients’ specific needs. There is less chance of a visual compromise at any given distance, and an improved chance that visual dysphotopsias can be avoided. Lastly, studies demonstrate that the proprietary features of the Symfony allow it to transmit light with achromatic correction in order to deliver the best overall quality of vision. Studies demonstrating that Symfony offers optimal modular translation function are not simply data emanating from the laboratory: the conclusions are reinforced by surveys demonstrating that patients are able to perceive the differences in quality of vision at all distances, and, in turn, are extremely satisfied with the vision the lens provides.

References

1. Tecnis Multifocal 1-Piece IOL, Model ZMB00 [package insert]. Santa Ana, CA: Abbott Medical Optics, Inc.; 2009.

2. AcrySof IQ Restor [package insert]. Fort Worth, TX: Alcon Laboratories, Inc.; 2008.

3. N.E. De Vries et al.,J. Cataract. Refract. Surg. 2011; 37: 859-865.

4. 166 Data on File_Extended Range of Vision IOL 3-Month Study Results (NZ).

5. L.N. Thibos et al.,Appl. Opt. 1992; 31(19): 3594-3600.

6. H.A. Weeber and P.A. Piers. J. Refr. Surg. 2012; 28(1): 48-52.

7. P. Artal et al.,Opt. Express 2010; 18(2): 1637-1648.

Albert Augustin

E: albertjaugustin@googlemail.com