prescribing
for astigmatism
Finally
Breathing Easier with Soft Toric Lenses
BY
PETER D. BERGENSKE, OD, FAAO
With all the talk over the last several decades regarding permeability of contact lens materials, we've had to accept minimal
oxygen transmission in soft toric lenses. To make toric lenses stable
and to incorporate their more complex optics, the lenses are necessarily larger
and thicker than their spherical counterparts. Combining this greater mass with
low Dk has resulted in compromise for many toric lens wearers. Neovascularization,
myopic creep and corneal distortion have all posed greater problems with soft toric
lenses, even in daily wear. Although a number of soft toric contact lenses have
approval for overnight wear, that wearing schedule isn't a prudent idea for the
vast majority of patients.
A New Era
Fortunately, we've now entered a new era with the introduction
of silicone hydrogel toric lenses. Though in somewhat limited distribution at this
time, expect expansion of parameters and availability to improve steadily.
The first entry in this category is Vistakon's Acuvue Advance
for Astigmatism. This lens isn't a silicone hydrogel version of the Acuvue Toric
— the Advance incorporates an entirely new design. Although the manufacturer
claims that it offers "accelerated stabilization," practitioners can think of this
lens design as a variant of a double thin zone design. It has no prism power, and
the lens may orient in either of two positions.
Sphere powers range from +6.00D to –9.00D, with three cylinder
powers up to –6.00D and two cylinder powers above that. Cylinder axes are
available at every 10 degrees for plano to –6.00D sphere powers and at every
20 degrees for the rest. The single base curve is 8.6mm and the diameter is 14.5mm.
The material is the same as that of the Acuvue Advance lens, with a Dk of 60 and
Class I UVA and UVB blocking. It has approval for daily wear use only.
Bausch & Lomb is in the process of rolling out the PureVision
Toric in somewhat limited parameters and availability at this time. Based on the
successful Soflens 66 Toric design, the PureVision Toric incorporates an aspheric
front surface to improve visual performance. Its current sphere power range is from
–0.25D to –6.00D with three cylinder powers at every 10 degrees. The
PureVision Toric has approval for up to 30 nights continuous wear.
More to Come
On the horizon, CIBA Vision has promised the roll out of its O2Optix
for Astigmatism lens, though little specific information is available at this time.
Word has it that we'll also eventually see a toric in the Night and Day (CIBA) line,
and the high oxygen permeability of that lens material should offer an exceptional
overnight wear option for toric lens patients.
The expansion of silicone hydrogel technology to astigmatic correction
is a needed and welcome development. It remains to be seen whether this generation
of lenses will solve most of the presumed physiological issues with toric lenses
and still allow patients to happily wear them. My guess is that we'll continue to
need and see improvements with time, but in the meantime, these new toric lens entries
are as welcome as a breath of fresh air.
Dr. Bergenske, a past chair
of the American Academy of Optometry's Section on Cornea and Contact Lenses, has
practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University
College of Optometry. E-mail him at: berg1101@pacificu.edu.
Contact Lens Spectrum, Issue: October 2005