Holden-Mertz daily wear Dk/t recommendation?

• A. low, unknown
• B. 65-175
• C. 35 x 10^-9, based on stromal acidosis
• D. 24 x 10^-9, based on edema

Answer: (D)

Oxygen supply to the cornea during lens wear is the key idea. Dk/t combines how easily oxygen passes through the lens material (Dk) with how thick the lens is (t) to give a measure of how much oxygen reaches the cornea. For daily wear, the Holden–Mertz data identify a minimum Dk/t that keeps corneal swelling (edema) at bay during a typical day of wear. The value you should remember is 24 x 10^-9, and it’s specifically tied to preventing edema. That’s why this option is the best answer: it reflects the threshold linked to edema, not other effects. The other numbers either aren’t the standard daily-wear threshold, or they’re attributing a rationale (like stromal acidosis) that isn’t the classic basis used in this recommendation.