Rigid gas permeable lenses
RGPs - the hard but breathable solution...
RGPs - the hard but breathable solution...
Rigid gas permeable (RGP) contact lenses were first made available in 1979 as an alternative to the soft lens. The Polycon lens was launched by Syntex in 1980.
The rationale for producing gas-permeable materials was provided by an American chemist resident in London, Irving Fatt. He carried out some ground-breaking studies into oxygen tension and permeability with Jennifer Chaston, FCOptom at St Thomas' Hospital and Judith Morris, FCOptom at Moorfields.
The photo to the right is of the original polaragraphic cell (c.1976) used by Irving Fatt and Judith Morris for the early measurement of oxygen permeabiliites of materials (DK values).
The image to the left shows a fitting set of ten 'Excel O2' hard gas permeable corneal contact lenses dating from the early 1980s by G. Nissel & Co.
The Excel O2 was an early second generation GP lens noted within the profession for its excellent wettability (making it, therefore, very comfortable to wear). Its standardised parameters made it suitable as a stock lens in busy practices.
G. Nissel & Co
G. Nissel & Co was founded by the brother-in-law to Josef Dallos in 1946 and started making contact lenses in PMMA as soon as that material became available after the war. George Nissel, born in Transylvania in 1913, nearly went to America in 1942 as the evidence from correspondence held in the museum shows, but he elected instead to become a British citizen, opening factories in London (1946-1972) and Hemel Hempstead (from 1958).
In 1963 Nissel was probably the first company to produce soft lenses outside of Czechoslovakia. Nissel used lathing and was asked by Otto Wichterle to find a suitable way of polishing hydrophilic lenses. These attempts were discontinued in 1968 however, because of inconsistencies in the materials then available.
The 'Hartflex' gas permeable contact lens fitting set from Wöhlk-Contact-Linsen (1977) was the first type of hard gas permeable lens manufactured by compression moulding from cellulose acetate butyrate (CAB) and launched in the UK during 1977. It was based on the Parabolar and Parabolett designs which had been produced in PMMA (Polymethylmethacrylate - this was not a purpose-designed lens material and thus had had the disadvantage of negligible oxygen transmission and a relatively hydrophobic surface).
CAB - cellulose acetate butyrate. CAB had superior wetting characteristics, was permeable to oxygen and less brittle, with greater thermal conductivity. Its thermoplastic nature made it suitable for either moulding or lathe cutting. CAB materials were first made available for contact lens use in 1973. The first CAB lens was the Persecon (1977). Frank Dickinson found that problem patients could wear CAB lenses all day and wrote an article in 1977 entitled 'There's a lot of mileage in hard lenses yet'.
Tinted RGP lenses were made available in 1983 and Extended wear RGP lenses in 1986.
Diffrax (1987) was the first diffractive RGP bifocal contact lens, designed by Dr Michael Freeman of London (author of later editions of Fincham's Optics): Unlike conventional bifocal designs they provided distance and near powers created by a centrally located diffractive zone on the back surface of the lens. Since the diffractive design involved two focal powers giving two focal points, independent of pupil size, lighting conditions would not affect the image. The lens was available in Polycon II or Fluorocon 60 material. It could be fitted in a similar way to a single vision gas permeable lens.
From about the mid 1980s the oxygen permeability of contact lenses was increased by introducing larger amounts of silcone to the lens material. A consequence was that 'rigid' lenses developed a degree of flexibility. Unfortunately it was possible for this to go too far and high minus lenses in particular had a tendency to flatten in use after six months or so. 1990s fluoro-silicates overcame this problem.
Comfort O2 lens, 2009
This is the packaging from one of the new generation rigid gas permeable lenses made from silicone hydrogel, to Ledaperm design, with a blue handling tint. Made by David Thomas (Contact Lenses) Ltd of Northampton, it is one of that company's range of custom-made speciality GP lenses for the individual eye (i.e. they were normally supplied singly). It was made of the only silicone hydrogel RGP material in the UK, available exclusively via them.
As RGPs have become ever softer, the tendency in optometry schools at the present moment is to discourage the use of the term 'rigid' gas permeables in favour of 'gas permeables'. Even some of the traditional scleral (i.e. hard ) lenses are made from GP materials... so the new challenge for historians is not to be too 'rigid' in our classification of lenses!