What Is the Keratometry Index?

Why Does Keratometry Use 1.3375?

Keratometry directly measures only the front corneal curvature, but the posterior cornea also contributes to total corneal power. The 1.3375 keratometry index is a convention that folds the average posterior surface effect into a single simplified value. This lets devices estimate corneal power from the anterior radius without directly measuring posterior curvature. It works reasonably well for many normal corneas, but it remains an approximation.

How Is Radius Converted to Diopters?

With the keratometry index convention, corneal power is calculated from the anterior radius using a constant derived from the index. Clinically, the conversion is often written as D = 337.5 / r, where r is in millimeters. A smaller radius means a steeper cornea and a higher diopter value, while a larger radius means a flatter cornea and a lower diopter value.

• Example 7.50 mm is about 45.00 D
• Example 8.00 mm is about 42.19 D

Where Will You See the Keratometry Index Used?

You will see keratometric values in contact lens fitting, corneal topography reports, and many cataract surgery planning workflows. Simulated K readings from Placido topography are based on keratometric conventions and typically represent the central cornea. Clinicians use these values to estimate corneal astigmatism, select lens parameters, and track changes over time. Some devices also report total corneal power measures that go beyond the keratometry index.

Limitations and Special Cases

The keratometry index can be less accurate when the relationship between the anterior and posterior cornea is altered. This is common after corneal refractive surgery, corneal transplantation, or in some ectasia disorders. In these cases, relying only on keratometric power can misestimate true corneal power and affect intraocular lens calculations. Tomography-based measurements that include posterior curvature can be more informative when available.