Hydrophilic acrylic intraocular lenses

Small incision cataract extraction with implantation of a foldable intraocular lens (IOL) has evolved significantly over the past 2 decades. In a recent survey of practice styles and preferences of members of the American Society of Cataract and Refractive Surgery (ASCRS),1 respondents indicated that 85% of surgeons prefer implanted IOLs composed of foldable materials. Presently available foldable IOL materials include silicone, hydrophobic acrylic, and recently introduced hydrophilic acrylic or hydrogel materials. Problems associated with various types of hydrophilic acrylic materials have begun to surface. Outbreaks of toxic anterior segment syndrome have been reported with the MemoryLens.2 In addition, possible problems with calcification of the optic in hydrophilic acrylic IOLs have been described.3,4 In this issue, 2 papers discussing calcification of hydrophilic acrylic IOL materials are presented. Werner and coauthors report the results of a clinical pathologic analysis of 9 explanted hydrophilic acrylic IOLs that developed opacification of the optic approximately 24 months after uneventful phacoemulsification with IOL implantation. Clinically, the lenses had a fine granular opacification within the substance of the optic. The analyzed IOLs had multiple fine granular opacities within the substance of the optic; the deposits were parallel to the anterior and posterior curves of the IOL surface, with a distinct clear zone just beneath the surface. Analysis of these granular deposits with special calcium stains as well as scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM/EDS) revealed that these granules were composed, at least in part, of calcium. Buchen et al. present the results of a study analyzing IOL calcification in an animal model. In their rabbit model, slivers or wafers of various foldable IOL materials were placed intramuscularly or subcutaneously. Discs of foldable IOL polymer materials were also implanted within the capsular bag of the rabbit eyes. Silicone, poly(methyl methacrylate), hydrophobic acrylic, as well as hydrophilic acrylic or hydrogel materials were analyzed. Special stains for calcium were done, and SEM/EDS was used for analysis. Calcification was noted on only the experimental acrylic or hydrogel materials. The authors found that the calcification started as a surface phenomenon and progressed into the matrix of the hydrogel material itself. The calcification occurred more quickly in the intramuscularly or subcutaneously implanted material than in the intraocularly implanted lenses. This animal model may allow testing of various materials used in the manufacture of foldable IOLs for their calcification potential. A delayed calcification has now been reported with multiple hydrophilic acrylic IOLs. This includes calcium deposits on the IOL surface as well as within the optic itself. The main question is why this calcification occurs. Is it simple deposition of calcium material on the surface of the IOL in some cases? Is the phenomenon related to a form of diffusion or absorption of substances into the IOL polymer? Are additives within the IOL such as ultraviolet-absorbing materials or other monomers related to this phenomenon? The hydrophilic acrylic material used in some cases of delayed calcification has been reported to have ultraviolet inhibitors within the polymer that were at too high a concentration; this may be related to the calcification (Vista Optics−ASCRS meeting, San Diego, California, USA, April 2001). Is there a potential problem with the packaging or folding system that may incite or contribute to this calcification? All these questions must be addressed. Surgeons as well as manufacturers should closely follow patients implanted with newer hydrophilic acrylic materials. Because many of the problems with calcification appear to be of delayed onset, it is important to be vigilant in the long-term follow-up of these patients. Better ways of analyzing hydrophilic acrylic materials for potential problems such as delayed calcification (eg, animal models and other methods of testing) should be developed and used. The evolution of small incision cataract extraction with foldable IOL implantation has the advantages of faster visual rehabilitation and better patient outcomes. However, newer IOL materials such as hydrophilic acrylics may have potential problems. Continued analysis of these materials, as well as long-term follow-up of patients implanted with hydrophilic acrylic IOLs, is necessary to ensure excellent outcomes in patients having phacoemulsification with foldable IOL implantation.