The current trend toward small incision cataract surgery has resulted in the use of elastomers as intraocular lens materials. Little has been published on appropriate methods of evaluating biomaterials intended for implantation in the eye. We present a methodology for such a study and report the results for two elastomeric silicone intraocular lens materials. The chemical, optical, and mechanical properties of the two materials were evaluated, as was their stability to hydrolytic and ultraviolet degradation. Qualitative correlations between these properties and clinical requirements are discussed. Both silicone materials possessed the necessary properties for use as small incision intraocular lenses.
A transparent poly (ether urethane) (PEU) was considered for use as a foldable intraocular lens material. The PEU was found to possess excellent mechanical, optical, and surface characteristics for this application. In vitro hydrolytic and ultraviolet aging studies suggested the PEU to be tolerant to conditions simulating 3-10 years of normal intraocular exposure. Different behavior was obtained, however, from intraocular and subcutaneous implantation of the PEU. After 6 months of intraocular exposure in the feline model, prototype PEU lenses had lost most or all of their optical resolving power. SEM analysis demonstrated scattered pitting and cracking on the lens surfaces. Degradation was found to be more extreme after as little as 30 days of subcutaneous exposure in rabbits. Severe pitting over the entire surface of implanted flat PEU specimens was observed by SEM. Macroscopic examination showed the samples to be frosty in appearance. It was postulated that the subcutaneous implant environment provides an accelerated in vivo model for materials intended for intraocular use. A minimum acceleration of 6-10x was estimated on a preliminary basis. The PEU studied here was found to be unsuitable for use as a foldable intraocular lens material.
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