If the spectacle cylinder is equal to 1.5 times the corneal cylinder, which GP lens type is appropriate?

If the spectacle cylinder is equal to 1.5 times the corneal cylinder, which GP lens type is appropriate?

• A. Spherical
• B. Back surface toric
• C. Front surface toric
• D. Bitoric

Answer: (B)

When shaping a GP lens for astigmatism, place toricity where it most effectively neutralizes the corneal shape. If the spectacle cylinder is larger than the corneal cylinder—about 1.5 times in this case—the corneal astigmatism isn’t fully accounted for by a spherical front surface. A back surface toric design corrects the corneal cylinder on the posterior surface while keeping the front surface spherical, which tends to give stable optics and less rotation. Front surface torics can be more prone to rotation and instability due to tear film and lid movement, and bitoric lenses are typically used when both surfaces require substantial toricity. So, with this mismatch in cylinder magnitudes, the appropriate choice is a back surface toric GP lens.

When shaping a GP lens for astigmatism, place toricity where it most effectively neutralizes the corneal shape. If the spectacle cylinder is larger than the corneal cylinder—about 1.5 times in this case—the corneal astigmatism isn’t fully accounted for by a spherical front surface. A back surface toric design corrects the corneal cylinder on the posterior surface while keeping the front surface spherical, which tends to give stable optics and less rotation. Front surface torics can be more prone to rotation and instability due to tear film and lid movement, and bitoric lenses are typically used when both surfaces require substantial toricity. So, with this mismatch in cylinder magnitudes, the appropriate choice is a back surface toric GP lens.