Keratometry Analysis Tool — Complete User Guide
K1/K2 conversion, simulated keratometry readings, astigmatism axis analysis, detailed with-the-rule/against-the-rule/oblique astigmatism criteria.
keratometryK valueK1/K2astigmatism axiswith-the-ruleagainst-the-ruleSim K
Corneal Curvature Basics
Corneal curvature (K value) is one of the most important refractive media of the eye, accounting for about 2/3 of total refractive power (43D/59D). Accurate corneal curvature measurement is essential for IOL power calculation, corneal refractive surgery planning, and contact lens fitting.
Measurement Parameters
- K1 (Flat curvature): Direction of maximum corneal curvature radius, usually horizontal
- K2 (Steep curvature): Direction of minimum corneal curvature radius, perpendicular to K1
- Corneal astigmatism: K2 - K1
- Sim K: Simulated keratometry value from corneal topography, using 6mm optical zone data
Curvature to Refraction Conversion
Based on Gullstrand model (n=1.3375): D = (1.3375 - 1) × 1000 / r(mm)
- 43.0D corresponds to curvature radius ~7.80mm
- 45.0D corresponds to ~7.04mm
With-The-Rule vs Against-The-Rule Astigmatism
| Type | Steep Axis | Refraction Manifestation |
|---|---|---|
| With-the-rule (WTR) | 90°±20° | Negative cylinder axis at 180°±20° |
| Against-the-rule (ATR) | 180°±20° | Negative cylinder axis at 90°±20° |
| Oblique | 30°-60° or 120°-150° | Axis in these ranges |
With-the-rule astigmatism has better postoperative predictability; against-the-rule astigmatism has slower visual recovery and requires thorough preoperative communication.
Precautions
- Take average of multiple measurements (at least 3 times) for corneal curvature
- IOL Master and corneal topography data usually have 0.1-0.3D difference
- Irregular astigmatism or suspected keratoconus should be referred to cornea specialist
References
- American Academy of Ophthalmology. Keratometry. Basic and Clinical Science Course, 2023.
- Grosvenor T. Primary Care Optometry. 5th ed. Butterworth-Heinemann, 2007.