When comparing BCR to flatter Ks, which approach should be used to select tear layer?

• A. Use flatter Ks
• B. Use steeper Ks
• C. Ignore Ks and rely on BCR
• D. Use average Ks

Answer: (A)

The tear layer between a GP lens and the cornea is governed most by the flatter corneal curvature when you’re comparing the lens back surface to keratometry readings. Using the flatter keratometry value as the reference to select the tear layer ensures central clearance remains consistent and avoids bearing or excessive thinning that could occur if you relied on a steeper curvature. It also accounts for the fact that the flatter portion of the cornea largely dominates the lens’ central alignment, so picking the flatter Ks helps achieve a stable, comfortable fit. Using the steeper curvature could create too thin a tear layer centrally on the flatter areas, while averaging or ignoring Ks ignores important corneal geometry that guides proper tear film thickness.