The AcrySof? ReSTOR? lens is the first apodized diffractive intraocular lens (IOL) to become available, and clinical results with this lens seem to be different from those seen with older technology. We're familiar with previous lens technology, either zonal refractive or full aperture diffractive designs. I will discuss the advantages and disadvantages of these older designs and explain how the new AcrySof? ReSTOR? lens represents a breakthrough technology.
David Allen, FRCS, FRCOphth
The AcrySof® ReSTOR® lens is the first apodized diffractive intraocular lens (IOL) to become available, and clinical results with this lens seem to be different from those seen with older technology. We're familiar with previous lens technology, either zonal refractive or full aperture diffractive designs. I will discuss the advantages and disadvantages of these older designs and explain how the new AcrySof® ReSTOR® lens represents a breakthrough technology.
Refractive Multifocal Lenses
Pupil restrictions with this lens arise because of the design of the lens. The central 2 mm or so of the lens is purely for distance vision. It is only when the pupil opens up to about 3 mm that you actually get effective contribution to the near image from the more peripheral regions of the lens. Normally, when people want to read, they switch on the light, and their pupils constrict due to both light and convergence. This lens doesn't perform terribly well for near in those lighting or activity conditions. Clearly, this lens design requires excellent centration.
There are various reasons why the image is actually not as good as that with other designs. The zones do not actually yield clear and crisp images because of issues of light scatter caused by the edge of each zone. This significantly degrades the quality of the image formed on the retina. In addition, the light that goes to the near focus-in front of the retina-forms a ring of unfocused light around the center of the distant image. This is the source of the halo effect experienced at night. If a significant proportion of the light goes to this effect, it is more troublesome for the patient.
Diffractive Multifocal Lenses
Consider now the full optic diffractive lenses, such as the previous 3M lens and the Pharmacia lens, the original polymethylmethacrylate IOL, and now the Tecnis® foldable lens. This design has the significant advantage of providing multifocal or bifocal vision, but with no consideration for pupil size. The un-aided reading vision with this type of lens can actually be very good.1
But that advantage is also a disadvantage. This lens equally distributes the light to both the distance and near foci, regardless of pupil size. Every part of the lens is simultaneously producing a distance and a near focus, and the distribution of light to these 2 images is the same, regardless of pupil size. Thus, there is always 41% of light going to the distance focus and 41% of light going to the near focus (and the rest is lost to higher order images and aberration).
So the full optic diffractive lens will perform much better in small pupil conditions and provide good reading vision. But in large pupil conditions-certainly greater than 4 mm-there are 2 distinct foci. Still you don't get the very precise annular rings of scattered light from the near focus; you get a much more diffused image. In low light/large pupil conditions, light is going to a focal point that is not being utilized. Thus, the distance image is not going to be as good. Essentially, light is being wasted with this sort of lens design. As the light energy is balanced between the 2 foci, this design also has image quality issues.
The AcrySof® ReSTOR® Apodized Diffractive Lens