Radius of Curvature and Diopter Values
Radius-Power Relationship
Corneal and lens curvature are inversely related to dioptric power: a shorter (steeper) radius yields higher diopters, a longer (flatter) radius yields lower diopters. Correct conversion anchors base curve choice and interpretation of keratometry or topography.
Core Formula and Quick Examples
Use the keratometric constant: D = 337.5 ÷ r (r in millimeters). Inverse: r = 337.5 ÷ D. Common pairs: 7.8 mm ≈ 43.3 D, 8.0 mm ≈ 42.2 D, 8.4 mm ≈ 40.2 D, 8.6 mm ≈ 39.3 D. Apply per meridian to quantify toricity and select an initial base curve.
Clinical Impact of Accurate Conversion
Precision influences alignment, tear exchange, centration, and comfort. Misestimation can cause excess movement, decentration, or mechanical staining. Numeric results are a starting point refined by slit lamp assessment and patient feedback.
Practical Fitting Tips
- Compare Meridians: Magnitude of difference guides spherical vs toric designs.
- Refine Clinically: Use fluorescein patterns for GP lenses and movement/coverage for soft lenses.
- Document Baselines: Track curvature changes for ectasia, ortho-k, or post-surgical monitoring.