contact lens materials
A Closer Look at Silicone Hydrogel Toric Lens Materials
BY N. REX GHORMLEY, OD, FAAO

By the end of this year, we'll have three silicone hydrogel toric options. Vistakon released its Acuvue Advance for Astigmatism this spring, the PureVision Toric (Bausch & Lomb) will become available this summer and the O₂Optix Toric lens (CIBA Vision) later this fall. Table 1 summarizes the material characteristics of each lens. We should know these characteristics so we can select the best silicone hydrogel toric lens for each patient.

Surface Treatments

Silicone is a hydrophobic lens material. Thus, silicone hydrogel contact lenses must have some modification to enhance the lens wettability. Lotrafilcon B (O₂Optix) has a uniform, dense plasma coating (25nm) that bonds to the lens surface. Balafilcon A (PureVision) has a plasma oxidation surface treatment. Galyfilcon A (Acuvue Advance for Astigmatism) doesn't have a surface treatment, but the polymer incorporates "Hydraclear" to increase surface wetting.

Each lens surface has differences in surface deposition and wettability. A recent article (Ketelson et al, 2005) evaluated the surface wetting of the Night & Day lens (CIBA), O₂Optix, Acuvue Advance and the Acuvue lens upon removal from their blister packs. The results indicated that lotrafilcon A and lotrafilcon B both had a better wetting rating as compared to the Acuvue Advance and the Acuvue lens, presumably because of the plasma surface treatment.

Another article (Jones, 2005) evaluated surface deposition of Night & Day and PureVision as compared to the Acuvue lens. The silicone hydrogel lenses showed decreased protein deposition, but increased lipid deposition as compared to the Acuvue lens. Lipid deposition can pose a problem for some silicone hydrogel patients, who must rub and thoroughly rinse both lens surfaces with multipurpose solution. In addition, an alcohol-based cleaner such as Miraflow (CIBA) can help remove lipids from the lens surface. Instruct these patients to replace their lenses every two weeks and to not "stretch" their lens replacement time.

Oxygen Transmissibility

Silicone hydrogel torics provide greater oxygen transmission compared to low-Dk toric soft lenses. Plus and high-minus lens patients deserve a toric lens that offers a high degree of oxygen transmission. Also, continuous wear patients or noncompliant patients who "occasionally" sleep in their lenses need a lens that provides maximum oxygen supply.

Modulus Musings

Lens modulus can affect soft lens comfort and fit. A softer lens modulus may decrease lens awareness and also drape over a steep cornea to aid in a good lens fit. But, patients may find lenses of higher modulus easier to clean, apply and remove.

Know Your Options

Silicone hydrogel torics will definitely benefit our patients and practices.  But, clinicians must know the characteristics of each material to fully utilize this new technology.

For references, visit www.clspectrum.com/references.asp and click on document #116.

Dr. Ghormley is in private practice in St. Louis, MO. He is a past president of the American Academy of Optometry and is a Diplomate of its Cornea & Contact Lens Section. He is also a consultant for CIBA Vision.