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Contact lenses manufactured from silicone hydrogel represent the most important advance in soft contact lens technology in recent years. Wearers would obviouly prefer lenses they can sleep in with total safety, so it was only a matter of time before the industry developed a safe product for continuous wear. After years of ongoing development and testing, such lenses are now readilly available for everone to wear at an affordable price. This article looks at what's so special about silicone hydrogel lenses and why they might be just the right choice for you.

YEARS IN DEVELOPMENT

The concept of lenses made from silicone hydrogel was first proposed over 20 years ago. The potential benefits were clear but the technological challenge was as difficult as needing to combine oil and water to produce an optically clear product. Researcher organizations have to date invested considerable academic and financial resources to achieve the high performance soft lenses of today. They have done this quite simply because they see this material as the way forward and the ultimate standard of the future.

At the time of writing, silicone hydrogel lenses are available in the USA from four manufacturers: "Night & Day" and "O2OPTIX" from CIBA Vision, "Pure Vision" from Bausch & Lomb, and "Acuvue OASYS" and "Advance" from J&J Vision Care.

OXYGEN TRANSPORT

So what's so special about silicone hydrogel as a lens material? Essentially, the great benefit of this material is it's ability to transport oxygen to the eye and thus maintain proper eye health. Modern contact lens technology is all to do with oxygen permability, which is measured using the "Dk" index. And for optimal oxygen transmission, this must be as high as possible.

The Dk of traditional, non-silicone based hydrogel lenses is directly related to the amount of water that the material can hold, since oxygen dissolves into the water-component of the material and thus diffuses through the lens. The Dk in fact increases logarithmically with increasing water content. Water itself has a Dk of only 80, thus placing a ceiling on the Dk of convential materials, and the original soft contact lens material, polyHEMA, possesses a Dk of only around 10 (with a water content of about 38%).

However, in silicone hydrogels, the relationship between Dk and water content is the opposite. Here higher water content implies lower Dk. With these lenses Dk increases significantly as the silicone content - not water content - of the lens increases. The Dk values of silicone hydrogel contact lenses are much higher than those of older soft lens materials.

SLEEPING IN CONTACT LENSES

According to market data from CIBA Vision, 84% of soft contact lens wearers occasionally sleep with their lenses in, and one-third admit to doing this regularly. 25% of wearers of one to two-week disposable lenses with low oxygen transmissibility occasionally or routinely sleep in their lenses overnight. And 72% of soft contact lens wearers said they would prefer to be allowed to wear their lenses overnight provided it was safe.

Thus the demand for lenses with the capablities of silicone hydrogels is evidently very high with almost three-quarters of soft lens wearers wanting to be able to sleep in their lenses.

LENS HANDLING

Silicone hydrogel lenses are significantly more rigid than their conventional counterparts, due to the presence of silicone. This increased rigidity does possess some advantages, making the lenses markedly easier to handle and thus the ideal choice for wearers with a less delicate touch. There is a downside to this however in that the rigidity makes the lenses less prone to adopting the shape of the eye so readily, resulting in the possibility for some of reduced comfort.

REDUCED DEPOSITIONS

Historically, the deposition of proteins and lipids has been a problem with contact lenses. The amount of depositions is essentially linked to the duration of wear, the lens material and the tear composition of each individual wearer. With traditional lenses, a frequent replacement schedule greatly reduces the buildup of harmful deposits.

Silicone hydrogel contact lenses have been proven to offer major benefits in the reduction of protein and lysozyme deposition. Indeed, combined with the correct replacement schedule and appropriate care system, problems associated with protein deposition have become almost a thing of the past.

DRY EYES

A problem frequently encountered by wearers of traditional soft contact lenses is one of a feeling of discomfort and "grittiness" in the eyes towards the end of the day. It has been reported that after one week wearing silicone hydrogel lenses, the quantity of regular soft lens wearers in a particular study group who previously suffered significantly from end-of-day dryness, was reduced dramatically to only 13% of the sample.

LIFESTYLE BENEFITS

Silicone hydrogel contact lenses also provide some benefits that conventional lenses don't. Continuous wear contact lenses can be particularly beneficial for use by members of certain professions, such as medical workers, police, the armed forces and firefighters who frequently work unpredictable and long hours.

Furthermore, an increasing numbers of contact lens wearers can now enjoy active outdoor lifestyles, enjoying pursuits such as camping and hiking where it is often difficult to handle and disinfect the lenses with the appropriate level of hygiene.

THE FUTURE FOR CONTACT LENSES

There is much evidence to suggest that, owing to their significant benefits over conventional soft lenses, silicone hydrogel lenses will ultimately, and sooner rather than later, displace other types of contact lens as the prefferred choice for both daily and overnight wear for the majority of contact lens wearers. Silicone hydrogel lenses provide a much improved physical performance, excellent handling properties and greater comfort. They might be considered as the ideal lens for general purpose use that exists today and are likely to become the new everyday standard.