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.
Contact Lens Spectrum, Issue: July 2005