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Previous investigators have shown that thin polymethyl methacrylate (PMMA) contact lenses (less than or equal to 0.12 mm) flex on toric corneas, thereby altering the residual astigmatism. The present study was designed to evaluate the flexure and residual astigmatism of gas permeable Polycon contact lenses on toric corneas and compare the results with those obtained with PMMA lenses on the same corneas. Eight subjects (16 eyes) with corneal toricities ranging from 1.25 to 4.25 D wore four pairs of identical Polycon and PMMA lenses. The only lens parameter that varied was center thickness, which ranged from 0.07 to 0.16 mm. Flexure and residual astigmatism were measured on all corneas with all lenses in a double-masked fashion. Our results indicate that Polycon lenses undergo significantly more flexure and alter residual astigmatism more on toric corneas than PMMA lenses at all center thicknesses evaluated. For both materials, flexure and residual astigmatism increased as center thickness decreased. The clinical implications of these findings are discussed.

Two techniques for fitting long-term polymethyl methylcrylate (PMMA) lens wearers with a gas-permeable hard lens are compared. These are: complete cessation of all lens wear for the period necessary to achieve stability in the patient's refraction and corneal curvature, and immediate refitting with a Polycon lens. Eleven patients who had worn hard contact lenses for at least 3 years and had wearing times of at least 8 hr per day were refitted with each technique. Refitting with Polycon lenses slowed but did not otherwise inhibit the recuperative changes taking place in the cornea after PMMA lens removal; vision was better and more stable with this group.

The artificial materials currently used in ophthalmology are reviewed. Those include poly(methyl methacrylate) in contact lenses, keratoprostheses, and intraocular lenses; cellulose acetate butyrate and the siloxane-containing polymethacrylates in contact lenses; the silicones in contact lenses, scleral buckling materials, and drainage implants in glaucoma; the hydrogels for contact lenses and retinal surgery implants; and the cyanoacrylate adhesives for corneal perforations and ulcers. The properties of the materials and their relationship to ocular tissues, as well as the advantages and disadvantages of their use in the eye are discussed. Probable future advances of biomaterials in ophthalmology are also discussed.

The minimum corneal oxygen need varies between 2 and 5% depending on individual characteristics. The tear pump alone is not sufficient to provide the corneal oxygen needs of many PMMA wearers. Hard permeable contact lenses may provide oxygen to the cornea by the tear pump and/or direct transmission through the lens. The relationship between the various oxygen permeable hard materials and the oxygen provided at the corneal surface is shown in this paper.