SiHy daily disposables: The future of contact lenses
Lyndon Jones has more than 30 years of experience in optometry and is a Fellow of both the British Contact Lens Association (BCLA) and the American Academy of Optometry (AAO). He is currently a director at the Centre for Ocular Research and Education (CORE) in Waterloo, Canada. Lyndon was part of the team that pioneered the development of Silicone Hydrogel (SiHy) contact lenses in the 1990s. He will be speaking about how SiHy has evolved over the last 20 years at the NCLF event at the end of April.
Give us a bit of background to Silicone Hydrogel (SiHy) in contact lenses
Lyndon Jones: It’s probably best to trace this back to the first regular daily disposable contact lenses - which became widely available in 1971. These were the first “soft” lenses, made from hydrogel, and they were much more comfortable than the hard, rigid reusable lenses that people were used to.
But the biggest problem with soft, hydrogel lenses is that they covered the whole eye, and this meant it was more difficult to get oxygen through to the cornea. Water is not a brilliant transmitter of oxygen - so that limited the amount of oxygen transferred through the lens to the eye. The issue was even worse for patients who had thicker lenses due to a high prescription or those that wore their lenses for long periods of time. The lack of oxygen to the cornea resulted in too many of these patients suffering hypoxic complications.
The challenge for the industry was to produce a material that enabled oxygen to pass through the lens to the cornea without compromising the comfort and simplicity afforded by daily disposables.
Silicone worked well with reusables, and was an obvious choice to use with soft lenses, but the problem was, trying to mix silicone with hydrogel was like mixing oil and water. They just didn’t go together as the chemistry was too complex.
It has taken 25-30 years to solve that dilemma, so the widespread use of SiHy lenses has only really come in this century.
How has the technology of these lenses changed over the years?
LJ: The first generation SiHy lenses contained a large amount of silicone to try to maximise the amount of oxygen transfer, but because silicone is quite resistant to water, that affected the wetness of the lens and caused increased irritation and discomfort.
Later generations were able to use new technologies that reduced the amount of silicone. These lenses were increasingly for the daily wear market and by backing off on the silicone you improve fitting ease and comfort levels.
We found that, despite the reduced levels of silicone, there was still plenty of oxygen transfer happening - more than enough than you might need on a daily basis. So in the modern SiHy lenses, there is now no compromise at all. Wearers are getting enough oxygen, and the mechanical failures of the first generation - rigidity, discomfort, and so on - have been resolved.
The comfort levels of today’s SiHy lenses have been brought up to the levels of regular hydrogels, and they have also been incredibly successful in eliminating hypoxia from contact lens wearers. I train students on hypoxic complications, and it is now merely a history lesson. We simply don’t see patients who wear SiHy lenses suffering from hypoxia at all.
How widely available are SiHy lenses - and what are the cost implications of the new material?
LJ: Over the course of the last 20 years they have become much more widely available. Almost every manufacturer is producing different varieties.
In the early days, there were only two on the market so the fitting and application was limited, but now you have more choice.
There are very few patients who are not fittable with Silicone Hydrogels. It doesn’t matter if you are presbyopic or have astigmatism - there is something on the market that will suit you.
In the reusables category, SiHy is now established as the obvious choice, but because the material is newer to the daily disposables market, there is significant ground to be made. Historically this has been partly due to cost, but, as competition has increased, we have seen big improvements in production capacity and yield - so the unit cost has dropped to much nearer the level of regular hydrogels.
What are some of the challenges that still need to be addressed?
LJ: SiHy lenses might have solved the hypoxia issue, but there are still more general challenges when it comes to contact lens wear.
The rate of infection has not been solved with the new silicone material. We still see inflammatory issues, although these are not related to the silicone itself but are more down to people being irresponsible and not putting them in correctly.
Practitioners still need to use their experience to ensure patients get the best out of their lenses and wear them safely, so there is an important education piece.
But, that said, SiHy is indisputably the safest and healthiest material you can use for contact lenses - and daily disposables remains the best category for comfort and ease of use. In combination, they should be embraced by all practitioners as the best way forward.