Long-term stability of intraocular lenses: literature review, assessment, and testing protocol

A critical review of the literature confirms the generally excellent, very long-term clinical stability and performance of polymethylmethacrylate (PMMA) in intraocular lenses. In contrast, only short-term, largely nonsystematic data are available on the performance of other, newer materials, such as soft silicones based on polydimethylsiloxane and copolymers of dimethyl- and diphenylsiloxane, hydrogels based on poly(2-hydroxyethylmethacrylate), and various "temporary" and "permanent" hydrophilic coatings. Some conflicting reports have been published on the long-term stability of isotactic polypropylene loop materials. However, the medical consensus seems to be that these are essentially stable to ultraviolet light energy reaching intraocular lenses during the expected service life in excess of 20 years. Uncertainty still surrounds the use of various ultraviolet-absorbing chromophores in the optics and/or haptics of intraocular lenses. Published reports indicate varying transmittance and effectiveness of ultraviolet-absorbing intraocular lenses. Furthermore, there is concern about the fate of the ultraviolet-absorbing chromophores during clinical conditions, especially with regard to degradation and leaching. Although a number of papers has been published on the effect of ultraviolet light energy on intraocular lenses, the experimental approaches differed substantively, so that intercomparative deductions must be made guardedly. A few papers have been published on the long-term biostability (hydrolytic, oxidative, and enzymatic) of intraocular lens materials. However, much of what has appeared in the literature is often illustrative of simplistic assumptions that largely ignore the environment of the eye. Clearly, the eventual success of newer materials must be based on performance characteristics that exceed those of the time-tested polymethylmethacrylate and the refined manufacturing technology designed to transform this polymer into intraocular lenses. The unquestioned clinical success of intraocular lenses rests on long historical data with PMMA that are unavailable with proposed newer materials. Thus, the entry of such new materials into the commercial market, meriting the acceptance of clinicians (and patients), must follow rather than precede thorough in vitro accelerated testing procedures under conditions that permit intercomparative conclusions and recommendations.