Modulus, Water Content and Dk in Silicone Hydrogels,
LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
In the first
two parts of this series, I compared physical properties of all of the known silicone
hydrogel contact lens materials. We found that modulus decreases linearly with increasing
water content. Dk generally
increases as water content decreases, but some unexpected combinations exist highlighting
the fact that we can't consider silicone hydrogel lenses homogeneous within this
class. Rather, members may be more like distant cousins within the silicone hydrogel
family. For example, CooperVision's new comfilcon A material has unusually
high Dk for a given water content as a consequence of its unique macromer structure.
Figure 1. Dk and modulus values for the six
current silicone hydrogel materials.
Modulus vs. Dk
In this last column of the series, I explore the relationship
between silicone hydrogel lens modulus and oxygen permeability (Dk). When these
relationships are mapped out, it becomes clear that distinctions exist so that we
can't predict lens modulus from Dk.
You might think that the highest Dk materials must be the stiffest
as a consequence of greater silicone content. The first generation materials, balafilcon
A (PureVision, Bausch & Lomb) and lotrafilcon A (Night & Day, CIBA Vision)
were relatively stiff, likely as a consequence of their low water content, and indeed
lotrafilcon A has both the highest Dk and the greatest modulus. The softest material,
Vistakon's galyfilcon A (Acuvue Advance) material, has a very high water content
(47 percent) and, as expected, the Dk value drops to the lowest of any lens in this
class (60). However, the comfilcon A material now has the highest water content
of any silicone hydrogel lens product (48 percent) and the Dk remains the second
highest in this class (128), yet the modulus remains relatively low.
Figure 1 displays the relationships between Dk and lens modulus
for the six known silicone hydrogel contact lens materials. What becomes evident
is that there may be placeholders on the ends of the Dk-modulus graph, but these
physical properties can in fact be dissociated. We cannot consider silicone hydrogel
contact lenses homogeneous within their class, and if you fit them you must be keenly
aware of the properties inherent to each material so you can make the best silicone
hydrogel lens selection for appropriate patients.
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.
Contact Lens Spectrum, Issue: December 2006