Rigid gas permeable lenses
RGPs - the hard but breathable solution...
RGPs - the hard but breathable solution...
Rigid gas-permeable (RGP) contact lenses were a development of hard lenses first made available in 1979 as an alternative to the breathable soft lens. As RGPs got smaller and thinner their rigidity came to be seen as an asset rather than a problem.
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. They used a polaragraphic cell for the early measurement of oxygen permeabiliites of materials (DK values).
The Japanese firm Toyo Contact Lens Co., Ltd, launched the world’s smallest (hard) lens in 1970, but as this was not oxygen-permeable they started to investigate RGP materials immediately thereafter and spent almost a decade developing a product. The 'Polycon' lens was launched by an American company, Syntex, in 1980. Toyo originally explored using the same material but with added water-affinity, which led them to develop the Menicon O2, usually hailed as the world’s first RGP lens, sold in Japan from January 1979 and France from May 1979. This proved to be good timing; sales at launch were boosted by adverse publicity for the safety of soft lenses then being developed from silicone rubber. 10 million units a year were sold for the next 20 years.
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 the British manufacturer 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. It is noted above how the company then joined the RGP lens revolution.
Tinted RGP lenses were made available in 1983.
Extended wear RGPs
Even as the Menicon O2 remained available, Toyo developed the improved Menicon EX, which was five times more permeable. It launched in Japan in May 1986 and was approved for American distribution three years later. The Menicon EX lenses could be worn for a full week.
Diffractive Bifocal RGPs
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.
The Menicon O2-32 was launched in September 1987 as the successor to the EX. (Now 3x more permeable than the original O2).
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.
The Menicon Super EX was launched in 1991. This boasted 2x the permeablility of the EX, with no consequent loss of strength. The Menifocal Z was that company's first bifocal RGP, launched in 2000.
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 gas-permeable 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!