Which hard lens material had a DK value of 0 and is no longer used today?

Which hard lens material had a DK value of 0 and is no longer used today?

• A. PMMA
• B. SA
• C. FSA
• D. HEMA

Answer: (A)

Oxygen permeability of lens materials is essential for corneal health during wear. PMMA is an old rigid plastic used for early hard lenses that has essentially zero oxygen permeability (Dk ≈ 0). Because the cornea gets most of its oxygen from the tear film, a lens with no O2 transmission reduces oxygen supply to the cornea, risking hypoxia and edema. That’s why PMMA-based lenses were replaced by oxygen-permeable hard materials (like silicone-containing acrylates) and by silicone hydrogel soft lenses. So the material with a Dk value of 0 and that’s no longer used is PMMA. The other materials—silicone-containing acrylates and fluorosilicone acrylates, as well as HEMA-based hydrogels—provide some oxygen passage and remain in use.

Oxygen permeability of lens materials is essential for corneal health during wear. PMMA is an old rigid plastic used for early hard lenses that has essentially zero oxygen permeability (Dk ≈ 0). Because the cornea gets most of its oxygen from the tear film, a lens with no O2 transmission reduces oxygen supply to the cornea, risking hypoxia and edema. That’s why PMMA-based lenses were replaced by oxygen-permeable hard materials (like silicone-containing acrylates) and by silicone hydrogel soft lenses. So the material with a Dk value of 0 and that’s no longer used is PMMA. The other materials—silicone-containing acrylates and fluorosilicone acrylates, as well as HEMA-based hydrogels—provide some oxygen passage and remain in use.