by
John W. Elman, OD
The newest category of contact lenses is silicone hydrogels. The main advantage of using silicone in a contact lens is that it transmits much more oxygen than any other element used in contact lens materials. The disadvantage is that silicone is hydrophobic--it does not absorb water at all. A lens made of pure silicone is very uncomfortable. It must be combined with some other material to make it hydrophilic ("water loving") and that is what has been done in such continous wear contact lenses as Ciba's NIGHT & DAY® and Bausch & Lomb's PureVision™. It is also used in Ciba's O2OPTIX™ and Vistakon's ACUVUE® ADVANCE™, ACUVUE® ADVANCE™ for ASTIGMATISM, and ACUVUE® OASYS™ materials. Silicone has been used as a component of gas permeable rigid contact lenses for at least 20 years. Combining the silicone elastomer, or rubber, to the soft lens, or hydrogel material, was not easy for chemists to do. It has been likened to combining oil and water, but the task was accomplished by 1999, by chemists at competing companies using competing techniques.
Once they had developed the new silicone hydrogel materials the companies designed and tested new contact lens designs. Only Ciba's NIGHT & DAY® and Bausch & Lomb's PureVision ™ contact lenses are approved by the FDA for 30 days of continuous wear. That means that people can wear them 24 hours per day for a month without removing them for cleaning and disinfecting, something that needs to be done with all other contact lenses. Not everyone can wear them in this manner. But for those people who can, it provides an alternative to refractive surgery such as LASIK, epecially for those who are poor surgical candidates, such as people whose eyes are changing. A charactertic of silicone hydrogels is that these materials are stiffer than traditional soft contact lenses or hydrogels. This stiffness can be measured in the laboratory as the "modulus of elasticity." An article written October, 2005, and available on the Internet by Eric Papas PhD describes this: Elastic Modulus and Silicone Hydrogel Contact Lens Fitting . Because of this stiffness factor, some people who were wearing traditional soft contact lenses find that the new silicone hydrogel contact lenses are initially not as comfortable as their previous soft contact lenses. Yet others find that the silicone hydrogels are much more comfortable, and most find that they are more comfortable over the long term. Because of the modulus of elastitcity the base curve of silicone hydrogels is more critical than in other soft contact lenses. Unfortunately many silicone hydrogels are available in only one base curve, especially the toric designs, and none are availble in more than two base curves. Of the available silicone hydrogels NIGHT & DAY® has the highest modulus of elasticity (i.e. it is the stiffist) and ACUVUE® ADVANCE™ has least modulus of elasticity (is the softest). Of contact lens brands approved for continous wear we prefer the PureVision ™. Most people report it is more comfortable than the NIGHT & DAY® contact lenses. It has a lower modulus of elasticity than NIGHT & DAY®. We find that often people who find the NIGHT & DAY® very comfortable find the PureVision ™ to be uncomfortable. This might be due to available diameter, base curves, or other factors. The more materials and the more base curves to choose from, the more likely we are to find a lens for a difficult patient. A couple of reasons why the PureVision ™ is often the preferred continuous wear contact lens, besides the comfort factor, it is aspheric , which reduces spherical aberration and tends to provide better clarity than lenses that are not aspheric, and it happens to be the least expensive 30 day continuous wear contact lens. When people are wearing contact lenses in the 24 hour per day continous, or "extended wear" mode, it is important to have the cornea examined regularly with a slit lamp biomicroscope by your eye doctor. Earlier soft contact lenses had been approved by the FDA in the 1980's to be worn overnight, some for 30 days. A certain percentage of people developed keratitis and corneal ulcers with these earlier lenses and people were cautioned not to wear contact lenses overnight (although many still did, without problem). The reason for the corneal ulcers was thought to be from hypoxia, a lack of oxygen getting to the corneal cells when the contact lenses were worn. The new silicone hydrogel materials transmit several times more oxygen than the earlier extended wear contact lenses. Rigorous testing studies have been done in various locations around the world, using both the PureVision ™ and NIGHT & DAY® contact lenses, having subjects wear the contact lenses continously for a month at a time. The results showed that adverse events were minimal, and the FDA has approved both of these contact lenses for 30 days of continous wear. Ciba's patent infringement lawsuit against B & L prevented B & L from making or selling PureVision ™ in the United States until the summer of 2005. B & L has added a toric lens version of the lens PureVision Toric™, to correct astigmatim, which became available in December, 2005 and multifocal version, called PureVision Multifocal™ to act correct presbyopia. ACUVUE® OASYS™, Ciba's O2OPTIX™, as well as Ciba's Air Optix for Astigmatism are approved as 6 day extended wear contact lenses.
The internet site for silicone hydrogels is www.siliconehydrogels.org . A 2002 referenced essay of The Physical and Clinical Characteristics of Silicone Hydrogel Lenses: How They Work?, which includes a description of the chemical properties of silicone hydrogel contact lenses and the differences between PureVision™ and Night and Day® contact lenses written by Kathryn Dumbleton, Senior Researcher, Centre for Contact Lens Research (CCLR), University of Waterloo, Ontario, Canada, can be found on that website.
