What happens to Dk when center thickness increases?

• A. Increases
• B. Decreases
• C. No change
• D. Depends on material

Answer: (B)

Oxygen transport through a lens is determined by two things: the material’s permeability and the distance the gas must travel through the lens. The Dk value is an intrinsic property of the lens material, so it doesn’t change just by making the lens thicker. What does change with thickness is the effective oxygen delivery, often called Dk/t, where t is the center thickness. Since Dk stays the same but the path length increases, the overall transmissibility declines as thickness increases. So, the quantity that actually decreases with thicker center thickness is the transmissibility (Dk/t), reflecting less oxygen reaching the cornea. If you’re looking at Dk alone, it would remain unchanged.