IOLs of the future will combine best features of current designs
Roibeard î'hÉineacháin
in Munich
THE IOLs of the future will be wavefront-customised aspheric accommodating lenses with large optic diameters and will be composed of new materials with the elasticity of silicone and the high refractive index of acrylic, predicts Richard L. Lindstrom MD.
The optics of future IOLs will probably be mainly biconvex although meniscus lenses and they may also be included in future IOL designs
Speaking at the XXI Congress of the ESCRS, Dr Lindstrom said that as IOLs have evolved over the past couple of decades new design ideas have emerged, each of which have their own particular advantages. As experience grows with the different lenses ophthalmologists are reaching a consensus as to the true value of each design feature. This will lead in time to new designs that incorporate the best features of several lens types.
In addition, what is at one time seen as an advantage in an IOL may later become a disadvantage that must be overcome with new IOL designs and materials.
So, for example, while a small optic offers the advantage of allowing the lens to be inserted through a small incision, the dysphotopsia they can cause under scotopic conditions is stimulating the development of new large-optic lenses that may be inserted through equally small if not smaller incisions.
At present the two types of material used in foldable lenses, silicone and acrylic both have special properties with specific advantages. For a foldable and injectable IOL to have a large 9.0 mm optic it would have to be composed of a material that combined the special properties of acrylic and silicone. "The acrylic lenses have higher refractive indexes but the silicone lenses are elastic, which allows them to go through a smaller incision. If we want a large optic that we can implant through a small incision we need to create new polymers. I believe the future will see the development of new hybrid polymers, elastic Acrylics with desirable features of both the higher refractive index of acrylics and the elasticity of silicone."
To reduce the incidence of PCO, the optics of the IOLs of the future will probably have squared posterior edges, a design feature of several currently available lenses that numerous studies have shown to be effective in the prevention of secondary cataract. Alternatively, IOLs may fill the capsular bag and prevent PCO through contact inhibition, he said.
Future IOLs will also have better filters for protecting the macula. One possibility is that they will have photochromic filters, that is, they will filter out different wavelengths depending on the intensity of the incoming light. "The filters of current IOLs are fixed at 380 - 450 nm. What we'd really like to have is a photochromic lens. At night I don't want to wear 450 nm absorbing lenses but I would like them during the day to protect my lens and macula from ultraviolet light."
To have IOLs with large optics that can be implanted through micro- incisions, new polymers will be necessary which combine the higher refractive index of acrylics and the elasticity of silcone
The optics of future IOLs will probably be mainly biconvex in shape, similar to the natural shape of the crystalline lens. However laboratory research suggests that good results may also be obtained with meniscus lenses and they may also be included in future IOL designs.
In addition, the curvature of the optics of future IOLS will be aspheric with a wavefront-enhanced design, like the current Tecnis IOL, to minimise aberrations. They will also be individually customised for each patient's eyes.
"Customised wavefront-adjusted IOLs will give patients the highest quality of vision just as we provide our patients with our wavefront-driven customised LASIK and LASEK ablations."
To achieve the best possible optical outcome the IOLs should be made of a material that is adjustable not only after surgery, like Calhoun Vision's Light adjustable IOL, but at any subsequent time during a patient's life.
"It appears that it is technically possible to make a lens that may be repeatedly adjustable after implantation. So if your wavefront changes with age or there is some induced astigmatism you could go back and have it adjusted again. I think that lenses like this will be available within the next ten years. The Calhoun lens may be the pioneer but it's only the beginning."
To meet the raised expectations of cataract patients and the increasing demands of presbyopic baby-boomers, future IOLs will also have to restore accommodation. Current multifocals can provide satisfactory results in most cataract patients and ametropic presbyopes, but they are far from ideal. The currently available accommodating IOLs also fall short of the visual needs of patients who wish to have spectacle-free vision after surgery for cataracts and presbyopia.
"The only way to achieve 20/20 at distance and J1 at near without loss of vision is through an accommodating optic. I believe these accommodating optics will not simply move forward like the CrystaLens and the 1CU but also change shape. There are lenses coming down the pipeline which apart from axial movement can also change shape like the natural lens and we only have to change that shape by 0.2 mm to 0.3 mm to achieve an accommodative amplitude of 3.0 D."
With all the lens design possibilities their most important property will be that they are easy to implant in the eye with the maximum of safety. That may mean that the lenses are pre-packed in their injection cartridges during their manufacture. The lenses may also be impregnated with agents that prevent PCO and intraocular infection.
Dr Lindstrom noted that lenses that fulfil all of the criteria for optimum safety, efficacy and multifocality might actually one day serve to replace corneal refractive surgery to a large extent.
"When such lenses become available probably refractive lens exchange will be come the procedure of choice."
Richard L. Lindstrom, M.D.
University of Minnesota
Minneapolis, Minnesota, US
rllindstrom@mneye.com
