Which statement best describes silicone hydrogel lens Dk for high water content?

• A. Increases
• B. Decreases
• C. Remains constant
• D. Varies

Answer: (B)

Oxygen permeability (Dk) in silicone hydrogel lenses is dominated by the silicone component, which provides a high pathway for oxygen. When water content is increased, the lens becomes more water-rich and the fraction of silicone decreases. Although some oxygen can diffuse through water, it is far less efficient than through silicone. So diluting with water reduces the overall permeability to oxygen, lowering Dk. That’s why the best description at high water content is that Dk decreases.