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Current status of intraocular lenses.

The past 5 years have seen a reawakening of interest in intraocular lenses after the early disasters of the mainly angle-fixated lenses. The subject has been kept alive owing to the work of Choyce in England and Binkhorst in Holland, and it can now be conservatively estimated that there have been 50 000 American and 10 000 European intraocular implant operations. Implant societies are being formed in many parts of the world. Recently the United Kingdom Intraocular Implant Society was formed, with Mr Neil Dallas as its first president (see p. 000). The well-known advantages of intraocular lens implantation are: Freedom from patient handling, minimum aniseikonia, rapid return of binocularity, and normal peripheral vision. However, the operation is undoubtedly technically difficult, and such complications as corneal dystrophy, implant dislocation, pupil block glaucoma, and cystoid maculopathy have led to a conservative approach by many surgeons. All intraocular lenses should be made ofmonomerfree polymethylmethacrylate (PMMA), as was used by Ridley for his first posterior chamber implant in 1949. PMMA has been shown in 30 years of clinical use to be remarkably free from degradation in vivo. At present there are 4 European manufacturers of lathe-turned lenses, while the 8 American firms generally produce moulded lenses. Lenses are biconvex or plano/convex, 4 to 6 mm in diameter. Most firms supply lens powers from 10 to 25 dioptres in aqueous. The standard-power lens of 19-50 dioptres in aqueous is approximately equivalent to an 11 0-dioptre sphere spectacle lens; this usually produces iseikonia. Individual lens power can be assessed in an aphakic eye for secondary implantation by refraction. However, lens-power measurements for primary implantation necessitate axial length measurement with ultrasonography, keratometry, and the use of standard calculation tables as produced by Binkhorst and others. Generally speaking, the standard 19 50-dioptre lens is used except in children, who require a more powerful 22-50-dioptre lens. Supporting lens loops are either made from polyamide (nylon), which, while being light and easily manufactured, may be a potential source of weakness, as it is hygroscopic (though nylon has been shown not to be attacked by pure water), or polypropylene (Prolene), which is probably more stable, being less hygroscopic and already available. Metal loops of platinum/ iridium, titanium, and gold alloy are said to be