Abbott Medical Optics think their new Tecnis Symfony intraocular lens (IOL), approved for use in Europe, can do what multifocal IOLs have not – convince more cataract patients to try presbyopia-correcting lenses.
Currently, presbyopia-correcting IOLs make up just seven per cent of the US market and even less in Europe, says Leonard Borrmann PhD, Abbott Medical Optics’ divisional vice president of research and development. He puts this down to the challenges of multifocal IOLs.
Multifocal lenses do a great job of enabling patients to read, but some multifocal lenses come at the expense of intermediate vision. Splitting incoming light into two or three distinct focal points can also create glare and haloes at night, and reduce image quality and contrast sensitivity.
“Our goal with the Symfony project was to find a solution for a full range of vision without the challenges that may come with today’s options. By significantly reducing the visual disturbances we may be able to expand the use of presbyopia-correcting technology,” Borrmann says.
The Tecnis Symfony IOL uses three optical approaches to maximise quality of vision while providing up to 2.5 dioptres of usable depth of focus, says Patricia Piers PhD, who led development of the lens. First, the Tecnis Symfony IOL uses the proven Tecnis aspheric surface profile, which introduces just enough negative spherical aberration to offset the positive spherical aberration of the typical cornea. We have shown that correcting spherical aberration improves contrast sensitivity and functional vision as measured in driving simulators.
The Tecnis Symfony IOL also applies diffractive principles in two new ways through a diffractive element on the lens’ posterior surface. First, rather than splitting incoming light into two or three separate focal points, it elongates depth of focus, much as a small aperture or adding spherical aberration might.
However, because it is a diffractive effect it does not restrict incoming light as an aperture device does, nor is it affected by pupil diameter, as spherical aberration is. The result is extended depth of focus under all light and pupil conditions. Nonetheless, extending depth of field always comes at the cost of reduced image sharpness, and this diffractive approach is no exception. However, a second diffractive effect, chromatic aberration correction, helps offset this loss, Piers says.
In the natural eye, chromatic aberration results in a spread in focal length of about two dioptres between the shortest and longest visible wavelengths, making the eye myopic for blue and hyperopic for red, Piers notes. The resulting blur is attenuated by the eye being much more sensitive to green, and our brains are used to filtering out chromatic distortion, Piers explains.
Even so chromatic blur remains, degrading image quality. Bench studies show that diffractively correcting chromatic aberration improves modulation transfer function beyond what is achieved with aspheric lenses alone. More important, clinical studies find that it improves contrast sensitivity. Tests of bilateral implants of the Tecnis Symfony lenses compared with standard monofocal aspheric Tecnis lenses find no difference in contrast sensitivity across all frequencies three months after surgery.
Yet the extended focus in the Tecnis Symfony IOL adds about 1.0 dioptre in depth of focus across the entire defocus range. This results in a mean of 20/20 vision or better across 1.5 dioptres defocus, and 20/40 to 2.5 dioptres – enough to allow spectacle independence under most circumstances for most patients.
Glare, haloes and other dysphotopsias are also similar between Tecnis Symfony IOL and monofocal Tecnis lens patients. Nearly 98 per cent reported no night glare or haloes, and those who did reported it as mild to moderate at one month. These figures are comparable to monofocal lenses. Implanting the Tecnis Symfony IOL in a mini-monovision arrangement, with the non-dominant eye -0.75 to -1.0 dioptres, further expands functional vision to about 3.5 dioptres total accommodation.
As reported at Eurotimes Mar 2015