contact lens materials
A Silicone Hydrogel Material Gets an Upgrade
BY LYNDON JONES, PHD, FCOPTOM, FAAO, & KATHY DUMBLETON, MSC, MCOPTOM, FAAO
The long-awaited commercialization of silicone hydrogels became a clinical reality in 1999 with the release of Bausch & Lomb's PureVision and CIBA Vision's Night & Day lenses. Despite recent releases of almost half a dozen other silicone hydrogels, both "original" lenses have stood the test of time and enjoy significant sales. Recently, the PureVision lens "relaunched" with some significant upgrades to improve comfort, fit and vision quality.
Some silicone hydrogel lens materials are significantly "stiffer" than conventional hydrogel materials due, in part, to the incorporation of silicone. The modulus of both PureVision and Night & Day is significantly higher than low-rigidity conventional lens materials such as etafilcon A, used in the Acuvue 2 lens (Vistakon).
Early silicone hydrogel studies demonstrated that certain patients had some initial comfort and mechanical issues, such as contact lens-associated papillary conjunctivitis and superior epithelial arcuate lesions. Some issues were related to the availability of only one relatively flat base curve, less than optimal front or back surface design and higher modulus of the materials.
Updated Manufacturing Process
The entire PureVision family of contact lenses, including the toric and multifocal, has undergone lens material improvements that have decreased the modulus by percent from the original. That upgrade, in combination with a redesigned front surface, has improved the comfort of the PureVision lens from the original design.
The new PureVision lens has two base curve options with the addition of the 8.3mm to expand its fitting potential. (Table 1) In addition, its anterior surface aspheric optics result in improved night vision, in conjunction with reduced spherical aberration.
Continuous Wear Option
PureVision is one of only two lenses approved for up to 30 days of continuous wear, and clinical trials show the lens to be effective on this schedule. It's also one of only three silicone hydrogel contact lenses approved for use as a therapeutic bandage lens, and several studies have demonstrated its value when used in this manner. CLS
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Dr. Jones is the associate director of the Centre for Contact Lens Research and a professor at the School of Optometry at the University of Waterloo, Ontario, Canada. Dr. Dumbleton is a senior clinical scientist at the Centre for Contact Lens Research.