Which Dk/t value is associated with Holden-Mertz's extended wear recommendation?

Which Dk/t value is associated with Holden-Mertz's extended wear recommendation?

• A. 87 x 10^-9
• B. 125 x 10^-9
• C. 24 x 10^-9
• D. 65-175

Answer: (B)

The important idea here is oxygen transmissibility through the lens, expressed as Dk/t, and how Holden and Mertz tied a practical minimum to safe extended wear. Dk/t combines the material’s oxygen permeability (Dk) with lens thickness (t) to describe how much oxygen can reach the cornea while the lens is worn. For extended wear, corneal health hinges on enough oxygen getting through the lens during sleep or long wear, so Holden and Mertz established a threshold value that’s high enough to minimize hypoxia and edema. That threshold is about 125 x 10^-9, meaning lenses with a Dk/t at or above this level are considered capable of supporting extended wear with reduced risk. The other numbers either fall short of that established criterion or represent broader ranges rather than a specific target, so they don’t align with the Holden-Mertz extended-wear recommendation.

The important idea here is oxygen transmissibility through the lens, expressed as Dk/t, and how Holden and Mertz tied a practical minimum to safe extended wear. Dk/t combines the material’s oxygen permeability (Dk) with lens thickness (t) to describe how much oxygen can reach the cornea while the lens is worn. For extended wear, corneal health hinges on enough oxygen getting through the lens during sleep or long wear, so Holden and Mertz established a threshold value that’s high enough to minimize hypoxia and edema. That threshold is about 125 x 10^-9, meaning lenses with a Dk/t at or above this level are considered capable of supporting extended wear with reduced risk. The other numbers either fall short of that established criterion or represent broader ranges rather than a specific target, so they don’t align with the Holden-Mertz extended-wear recommendation.