contact lens materials
Introducing the Latest Silicone Hydrogel Lens
Another lens has joined the ranks in the silicone hydrogel market. The Acuvue Oasys (Vistakon) now adds to a class of lens materials that many believe will replace traditional hydrogel products in the not-so-distant future.
Acuvue Oasys is made of senofilcon A, a 38 percent water content lens that has more silicone, less water and more Hydraclear than Vistakon's Acuvue Advance galyfilcon A material. Hydraclear is an internal wetting agent that eliminates the need for a surface treatment, as other lenses in this class require. The company claims that the Hydraclear material is more compressed in the senofilcon A product, creating a smoother, wettable surface that approaches the smoothness and wetness of the cornea.
The Dk value of Acuvue Oasys is 103, which provides a Dk/t value of 147 at a center thickness of 0.07mm for minus powers. Currently, the FDA has cleared Vistakon's 510(k) application for daily wear of senofilcon A.
Comparing Apples to Apples
Table 1 shows how the available silicone hydrogel lenses stack up to one another. Remember that industry reported Dk/t values simply provide the transmissibility through the thinnest portion of a –3.00D lens. The Dk/t will vary across any lens, with significantly diminished values in the periphery of a high-minus lens or in the center of a plus lens. Note that manufacturers have expanded many of the silicone hydrogel parameters since their initial launch, with available sphere powers now from +8.00D to –12.00D.
Silicone Hydrogel Advantages
Compared to traditional hydrogels, silicone hydrogel materials have lower water contents, higher bound water and greater oxygen transmissibility, providing more than 95 percent oxygen availability to the cornea in the open eye state, according to Brennan.
Not only are silicone hydrogels the lenses of choice for overnight use, but they may soon be positioned for patients who suffer from situational dryness and exposure to constant or occasional adverse environments. These include computer use (because of low blink rate and larger palpebral aperture),
TABLE 1 - Currently Available Silicone Hydrogel Lenses
|NIGHT & DAY||ACUVUE OASYS||O2OPTIX||PUREVISION||ACUVUE ADVANCE|
|Material||Lotrafilcon A||Senofilcon A||Lotrafilcon B||Balafilcon A||Galyfilcon A|
|Powers||+6.00 to –10.00||–0.50 to –6.00||–1.00 to –6.00||+6.00 to –12.00||+8.00 to –12.00|
|Base Curves||8.4, 8.6||8.4||8.6||8.6||8.3, 8.7|
Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens Service at University Hospitals of Cleveland.