What do K readings measure?

K readings measure the curvature of the central anterior corneal surface. They express the corneal power in diopters and the radius of curvature in millimeters for the two principal meridians (steepest and flattest). The difference between the two readings indicates the amount of corneal astigmatism.

How are K readings used for contact lens fitting?

K readings help select the initial base curve for contact lenses, especially rigid gas permeable (RGP) lenses. The base curve is chosen relative to the flattest K reading. The K values also indicate whether the cornea has significant astigmatism that may require a toric lens design. Soft lens fitting relies less on K values but still uses them for reference.

What is the difference between corneal and total astigmatism?

Corneal astigmatism is the astigmatism caused by the corneal surface curvature, measured by the keratometer. Total astigmatism, measured during refraction, includes both corneal astigmatism and internal (lenticular) astigmatism from the crystalline lens. The two values may differ, which is why a refraction is needed in addition to keratometry.

What do distorted mires on a keratometer indicate?

Distorted mires indicate an irregular corneal surface. Common causes include dry eye (tear film irregularity), keratoconus (corneal steepening and thinning), corneal scarring, contact lens warpage, and other corneal pathology. When mires are distorted, the K readings may be unreliable, and further evaluation such as corneal topography may be needed.

Keratometer: Measuring Corneal Curvature for Contact Lens Fitting and Astigmatism Detection

What Is a Keratometer?

A keratometer (also called an ophthalmometer) measures the curvature of the central anterior corneal surface. It provides measurements in both diopters of corneal power and millimeters of radius of curvature. These measurements, commonly called K readings, are essential for contact lens fitting, astigmatism assessment, and monitoring corneal conditions.

How the Keratometer Works

The keratometer projects illuminated targets (mires) onto the cornea and uses the size of the reflected images to calculate the corneal curvature. A steeper cornea (smaller radius) produces a smaller reflected image, while a flatter cornea (larger radius) produces a larger image.

The instrument measures the two principal meridians of the cornea, which are the steepest and flattest meridians, oriented 90 degrees apart. The power difference between these meridians indicates the amount of corneal astigmatism.

K Readings Explained

K readings are expressed as:

Power in diopters: The refractive power of the corneal surface (e.g., 43.50 D @ 180 / 44.75 D @ 090)
Radius of curvature in millimeters: The physical measurement of the curve (e.g., 7.76 mm @ 180 / 7.53 mm @ 090)

K Reading	Classification	Approximate Radius
Below 41.00 D	Flat cornea	Above 8.23 mm
41.00 to 45.00 D	Average cornea	7.50 to 8.23 mm
Above 45.00 D	Steep cornea	Below 7.50 mm

The keratometer measures only the central 3 to 4 mm of the cornea. It does not provide information about peripheral corneal curvature. For detailed corneal mapping including the periphery, corneal topography is required.

Clinical Applications

Contact Lens Fitting

K readings are fundamental for selecting the initial base curve of a contact lens, especially rigid gas permeable (RGP) lenses. The base curve of an RGP lens is typically selected in relation to the flattest K reading:

On-K fit: Base curve matches the flattest K
Flatter than K: Base curve is flatter (larger radius) than the flattest K
Steeper than K: Base curve is steeper (smaller radius) than the flattest K

Soft contact lens fitting relies less heavily on K readings since soft lenses drape over the cornea, but K values still help identify patients with unusual corneal curvature.

Corneal Astigmatism Assessment

The difference between the two K readings indicates the amount of corneal astigmatism. For example, K readings of 43.00 @ 180 and 45.00 @ 090 show 2.00 D of with-the-rule corneal astigmatism (the steeper meridian is vertical).

Corneal Monitoring

Serial K readings track corneal changes over time, useful for:

Monitoring keratoconus progression (increasing corneal steepening and astigmatism)
Post-surgical follow-up after corneal procedures
Detecting corneal warpage from contact lens wear

When K readings show irregular or distorted mires, this suggests corneal surface irregularity. Causes include dry eye, corneal scarring, keratoconus, or contact lens warpage. Note the mire quality alongside the K values when recording results.

Types of Keratometers

Manual keratometers: The operator aligns the mires visually and reads the measurements from calibrated scales. The Bausch & Lomb and Javal-Schiotz designs are the two classic types.
Automated keratometers: Measure corneal curvature electronically with minimal operator input. Often integrated into auto-refractor/keratometer combination instruments.

Confusing corneal astigmatism (measured by the keratometer) with total astigmatism (measured during refraction). Total astigmatism includes both corneal and internal (lenticular) astigmatism. The corneal component alone does not tell you the full astigmatic correction needed in the spectacle prescription.

Mire Quality

Beyond the numerical readings, the quality of the reflected mires provides important clinical information:

Clear, sharp mires: Indicate a smooth, regular corneal surface
Distorted mires: Suggest corneal irregularity (keratoconus, scarring, dry eye)
Doubled or split mires: May indicate significant corneal irregularity

Clinical Relevance

The keratometer provides essential data for contact lens fitting, particularly for RGP lenses where the base curve must closely match the corneal curvature. It also helps differentiate between corneal and lenticular astigmatism, which affects the choice between toric and spherical contact lenses. Understanding K readings is a core competency for opticians involved in contact lens dispensing.

Key Takeaways

The keratometer measures central corneal curvature in diopters and millimeters
K readings provide the two principal meridians and the corneal astigmatism
K values guide RGP contact lens base curve selection
The instrument measures only the central 3 to 4 mm of the cornea
Mire quality indicates corneal surface regularity
Corneal astigmatism is only one component of total refractive astigmatism