More flexible materials will require a higher CT to prevent flexure. Which statement is correct?

• A. Lower CT
• B. Higher CT
• C. Same CT
• D. CT not related to flexure

Answer: (B)

Center thickness determines how stiff a gas-permeable lens is. When a material is more flexible, it deforms more easily under eyelid movement and blinking. Increasing the center thickness makes the lens surface and overall shape more rigid, so it resists that bending and stays aligned on the eye. That’s why a higher CT is needed to prevent flexure with more flexible materials—the added thickness boosts stiffness enough to counteract the forces that cause the lens to bend.

Keep in mind the tradeoff: thicker centers can reduce oxygen transmission through the lens and add a bit more weight, so you balance flexure control with maintaining adequate oxygen supply and comfortable wear. If you used a lower CT, the lens would be more prone to flexure with a flexible material, and keeping the same CT wouldn’t adequately address the added pliability. The idea that CT isn’t related to flexure isn’t accurate.