The simplest fitting nomogram is based on which keratometry measure?

• A. on K (defined as the flattest keratometry reading)
• B. off K
• C. average K
• D. center K reading

Answer: (A)

The starting point for a simple rigid gas-permeable lens fitting nomogram is the flattest keratometry reading. This “K flat” value is used because it represents the cornea’s least steep curvature and is the most consistent, reproducible measure across measurements. By basing the initial base-curve choice on the flat meridian, you obtain a straightforward, conservative starting point that tends to fit well across the cornea’s meridians without overemphasizing the steep meridian where astigmatism can complicate a quick fit. Using the central or average or off-center keratometry readings adds complexity and can misrepresent the overall corneal shape, making the simple nomogram harder to apply reliably. So, the simplest nomogram anchors on the flattest keratometry measurement, the flat K.