Modulus, Water Content and Dk in Silicone Hydrogels, Part 3
BY 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.