Almost every cataract patient needs an intraocular lens, and most surgeons prefer foldable lenses. But should it be made from acrylic or silicone? Are all acrylics alike? Should the lens be single- or multi-piece, and what materials are used? Do the material and design matter as much as the optical properties? And what about blue-blocking and accommodating IOLs? When technology extends options instead of eliminating choices, the result can be confusion. Fortunately, you can rely on some basic principles that are well-established by clinical research. Four basic elements of IOL choice factor into the purchasing decision:
- optics and
The last is probably the most easily understood by surgery center managers, but the first three are at the fore of a surgeon's mind. And as an Outpat-ient Surgery Magazine survey of managers confirmed, surgeon preference was a "very important" factor in IOL choice for a much greater majority (91 percent) than was cost (59 percent, as reported in February 2002). Here we explain the de-signs, materials and optics considerations important to cat-aract surgeons, and the vocabulary used to discuss IOLs.
Design is about haptics
The first IOL implanted in humans in the 1950s was designed to look like the natural human lens: a disk about 11mm in diameter. Most surgeons found it very difficult to place this type of lens in the empty capsule after cataract extraction, so they looked for ways to use a smaller lens that would be easier to implant.
Eventually the designs evolved into a system with a small central optic, to provide focusing power, and some kind of haptic that would hold the optic in the correct position. Today there are two general types of haptics on IOLs.
Loop haptics are relatively thin arms that reach from the optic to touch the capsule and anchor the IOL. Plate haptics, as the name implies, are wide and flat and serve a similar purpose. Both types work well. One difference is that plate-haptic IOLs generally can be implanted only in the lens capsule. If the capsule is torn or its attachments broken during cataract extraction, the plate haptics will create too much stress. Loop-haptic IOLs must be available as a back-up in such cases, because they cause less stress to the capsule and can be implanted above the capsule (in the ciliary sulcus).
Materials: Silicone or acrylic?
The technique and instrumentation of phacoemulsification were developed so that when cataracts appeared, the eye's natural lens could be removed through a small, non-traumatic incision. But for many years, IOLs with hard plastic (polymethylmethacrylate, or PMMA) optics made and loop haptics were the only lenses available to U.S. surgeons. So the small incision was enlarged before the IOL could be implanted.
That changed in the early 1990s, when loop-haptic IOLs made of soft silicone were introduced. STAAR Surgical and Allergan Medical Optics (AMO, now standing for Advanced Medical Optics) marketed the first FDA-approved foldable IOLs. These lenses' optics were silicone, but the loop haptics were made of more rigid materials to provide sufficient support. This makes them three-piece (or multi-piece) IOLs, because two haptic pieces are attached.
After several design and material improvements, the descendants of these original models are still on the market as the STAAR Elastimide and AMO Phacoflex II. Silicone-optic IOLs are also on the U.S. market from Pfizer Ophthalmics (the CeeOn and Tecnis brands) and Bausch & Lomb under many brands under license or developed by companies it acquired. The company's SoFlex IOLs are multipiece and the C11UB is a single-piece silicone model.
STAAR also introduced a plate-haptic silicone lens in the early 1990s. The Elastic, whose design and material have been modified, is made entirely of silicone to avoid the need to have a junction between the haptics and the optic.
Silicone is still a popular material among eye surgeons. According to data from Health Products Research (HPR), silicone IOLs accounted for 38 percent of the lenses implanted after cataract surgery in the United States in 2002. That market share has declined as newer materials become available, with 58 percent of surgeons shifting to acrylic IOL materials, according to HPR.
The strict definition of acrylic is any polymer containing the compound acrylate, which in-cludes PMMA. When surgeons talk about acrylic IOLs, though, they almost always mean soft, foldable acrylic materials. The first foldable acrylic IOL, Alcon Laboratories's AcrySof, was approved by the FDA in 1994.
Better biocompatibility was the impetus for creating a foldable acrylic IOL. However, biocompatibility is vigorously debated in eyecare. Strict FDA standards assure unsuitable IOL materials are barred or quickly removed from the market, so IOL-material choice is not a matter of avoiding bad lenses. While biocompatibility remains controversial, one advantage of the AcrySof emerged in the late 1990s with the work of David Apple, MD, and coworkers at the Center for Research on Ocular Therapeutics and Biodevices in Charleston, S.C.
Through research on thousands of cadaver eyes and IOLs removed post mortem, Dr. Apple found the relatively squared-off edge of the AcrySof optic acted as a barrier to cells that often multiplied and spread behind the IOL to create posterior capsule opacification (PCO). By blocking these cells, the AcrySof reduced the number of patients needing another procedure (Nd:YAG posterior capsulotomy) some time later to give them clear vision.
Other IOL manufacturers soon adopted similar designs. Today, AMO uses a proprietary design called OptiEdge in its multi-piece acrylic IOL, the Sensar, which was approved in 2000. AMO also incorporated the OptiEdge in a silicone IOL, the ClariFlex, and Pfizer did much the same to create the CeeOn Edge silicone IOLs. Whether a squared-off edge is sufficient to reduce PCO is still not settled, but Dr. Apple and coworkers are finding more evidence the bond created between the acrylic material of the AcrySof and the capsule also plays an important role.
The original multi-piece AcrySof IOL's design has been improved several times since its introduction, and it is still widely used worldwide. In 2000, Alcon brought out the AcrySof Single-Piece; its optic and haptics are one seamless piece of acrylic. It has grown quickly in popularity, Alcon says, and modifications of its optical properties continue.
The AcrySof and AMO Sensar are both hydrophobic acrylic: They are relatively resistant to water absorption compared with some other acrylics (hydrophilic). Hydrophilic materials' ability to absorb intraocular fluid made them attractive for use in IOLs because of the potential for greater biocompatibility. The issue is still debated. There are two hydrophilic acrylic IOLs available in the United States.
The Bausch & Lomb Hydroview IOL was approved in 1999. The acrylic optic has about an 18-percent water content; the haptics are PMMA. Because of the relatively high water content, it is often referred to as a hydrogel IOL.
The CV 232 SRE from CIBA Vision is a modification of the unique MemoryLens design, which uses thermoplastic material. This multipiece lens is packaged with the optic rolled, and the surgeon simply removes it from the package and inserts the IOL through the incision. As the IOL warms to the eye's temperature, the optic unrolls, and the haptics move into position. The CV 232 SRE's posterior edge is squared off to help prevent PCO; a rounded anterior edge is designed to give good optical quality.
Pre-rolling means the surgeon or OR staff don't have to fold the lens manually. How IOLS are folded and implanted plays a large role in surgeon preference. Some IOLs have instrumentation designed specifically to fold the lens, then deliver it to the eye. In many cases, the IOL packaging is used in folding, and most IOLs are implanted through an injector that prevents the lens from touching the outside of the eye.
Some early injectors were not easily controlled, but instrument manufacturers have made many refinements. However, some surgeons still prefer to insert foldable IOLs using forceps. Ultimately, the choice of IOL and insertion method/ instrument must be considered a package deal from the manufacturer and for each surgeon.
Options in optics
Until relatively recently, one option for the optical design of an IOL existed: monofocal. This meant the optic functioned at one focal length (its power as measured in diopters), usually chosen to correct distance vision so reading glasses could help near vision. Additionally, IOLs had only been available for spherical correction; they could correct nearsightedness or farsightedness, but not astigmatism. New designs have created options, and those options seem likely to multiply.
The problem of astigmatism after cataract surgery can be addressed for some patients with the STAAR Toric IOL. A toric lens is one in which the optic surface is not a symmetrical sphere but instead is elongated in one direction into a football-like shape. The outer edge of the optic is still round, but the lens is steeper along one direction than the other. This is how eyeglasses and contact lenses correct astigmatism.
The STAAR Toric IOL is a single-piece silicone plate-haptic that corrects from 1.5 to 3.5 diopters of astigmatism. While spherical IOLs can be placed and rotated into any position, a toric IOL must be placed in the correct axis to be effective. Researchers have reported good results implanting the STAAR Toric and maintaining its orientation.
The AMO Array IOL (based on the multipiece silicone Phacoflex II model) was the first FDA-approved multifocal lens. With an optic surface made up of concentric rings with variable curvature rather than a smooth arc, the lens focuses light at distance, intermediate range and near. This is not like the natural lens's ability to change focus, nor like progressive eyeglasses that let the wearer look through different parts of the lens. Instead, incoming light is focused at different distances simultaneously, and the eye/brain visual system learns to concentrate on the image that is in focus.
A different approach to providing multifocal vision is taken with the CrystaLens from eyeonics, inc. This IOL, approved in November 2003, actually changes its focus by using the natural physiological responses of the eye (see "CrystaLens, an $800 IOL That Accommodates for a Range of Focus" on page 46). Several other types of accommodating IOLs under development represent the avant garde of the rapidly evolving IOL marketplace.
But with this basic understanding of the design, materials and optical characteristics of IOLs, you can keep up with the changes and understand why cataract surgeons keep coming back with requests for new and different lenses.