In GP lenses, the direction of center thickness change associated with astigmatism is:

• A. Lower CT
• B. No change
• C. Higher CT
• D. Depends on design

Answer: (C)

When a gas-permeable lens is used to correct astigmatism, its back surface is made toric, meaning two meridians have different radii of curvature. To achieve that difference in curvature on the posterior surface, the lens must carry more material in the center along the meridian that needs the steeper curvature. That extra material shows up as a thicker center in that direction, so the center thickness is higher. While some designs can vary, the typical outcome for astigmatic correction with GP lenses is a higher center thickness in the relevant meridian.