What Is Keratometry?
Keratometry is the measurement of the curvature of the front surface of the cornea. The cornea provides approximately 65-75% of the eye's total refractive power, so its shape and curvature are critically important in understanding refractive error, fitting contact lenses, and planning cataract surgery with accurate IOL power calculations.
The instrument used is called a keratometer (manual) or a topographer/autorefractor with keratometry function (automated). The readings produced are called K readings.
K Readings: Units and Format
K readings describe corneal curvature in two principal meridians, reported in one of two unit systems:
- Diopters (D): Represents the refractive power of the cornea at that curvature. A steeper (more curved) cornea has a higher dioptric power. The average K reading is approximately 43-44 D.
- Millimeters (mm): Represents the radius of curvature of the cornea. Steeper curvatures have smaller radii. The average radius is approximately 7.7-7.8 mm.
Conversion: K (D) = 337.5 / radius (mm). Most instruments display both values.
A keratometry report gives two readings: the flatter meridian (K1, the weaker corneal power) and the steeper meridian (K2, the greater corneal power), along with their respective axes.
Example: K1: 42.50 D at 180 / K2: 44.00 D at 90 -- this indicates a with-the-rule astigmatism pattern (steeper vertically).
Manual Keratometer
The classic manual keratometer (such as the Bausch and Lomb or Javal-Schiotz type) uses the principle of image reflection. It projects a mire (a circular ring or series of marks) onto the corneal surface and measures the reflected image size, which depends on the corneal radius of curvature.
CPOA Role in Manual Keratometry
- Patient preparation: Ask the patient to remove contact lenses at least 2 hours before (soft lenses) or longer (rigid lenses) to allow the corneal surface to recover its natural shape. Contact lens wear distorts K readings.
- Instrument alignment: Position the patient's chin and forehead in the rests. Align the instrument's eyepiece with the patient's eye.
- Instruct fixation: Ask the patient to look at the small light or target inside the instrument and blink once, then hold still.
- Align and read: The examiner (or the CPOA if trained) aligns the mire images in both meridians and reads the K values from the drum scales.
- Record results: Document K1 and K2 (with their axes) for both eyes in the patient record.
💡 Clinical Tip: If the mire images appear distorted, blurry, or irregular during keratometry, this is a clinical sign of corneal irregularity (keratoconus, dry eye, corneal scarring). Alert the supervising doctor to the observation -- do not simply re-measure and ignore irregular images.
Automated Keratometry
Most modern auto-refractor/keratometer combinations measure K readings automatically with the same setup as autorefraction. The CPOA aligns the patient at the instrument, captures readings (typically 3-5 per eye), and records the output. Automated K readings are displayed alongside the refraction and can be printed for the chart.
Clinical Uses of K Readings
| Clinical Application | How K Readings Are Used |
|---|---|
| Contact lens fitting | Base curve of rigid and soft lenses is selected based on K readings |
| Cataract surgery IOL calculation | K readings feed into IOL power formulas |
| Astigmatism assessment | Difference between K1 and K2 reveals corneal astigmatism amount and axis |
| Keratoconus screening | Irregularly high K readings or image distortion signals ectatic disease |
| Post-refractive surgery monitoring | Track corneal flattening or changes after LASIK/PRK |
Normal vs. Abnormal K Values
- Normal K range: approximately 40.00 to 46.00 D (7.32 to 8.44 mm).
- Flat cornea (plano-keratometry): Readings below 40.00 D. May be seen after LASIK (intentional flattening) or after long-term rigid contact lens wear (corneal molding).
- Steep cornea: Readings above 46.00 D. May indicate keratoconus, contact lens-induced corneal steepening, or post-corneal transplant.
- Corneal astigmatism: A difference of more than 0.75-1.00 D between K1 and K2 is clinically significant.
⚠️ Common Mistake: Not asking patients to remove contact lenses before keratometry. Rigid contact lenses can mask significant corneal astigmatism and distort curvature readings for 30 minutes to several hours after removal. Soft lenses typically require at least 15-30 minutes of removal time before accurate K readings can be obtained.
Key Takeaways
- Keratometry measures the curvature of the corneal front surface, reported in diopters (K) or millimeters (radius).
- Normal K readings are approximately 43-44 D (7.7-7.8 mm). Two principal meridians are measured (K1, K2).
- Contact lenses must be removed before keratometry to avoid distorted readings.
- Irregular mire images during keratometry suggest corneal pathology and should be reported to the doctor.
- K readings are used for contact lens fitting, IOL calculation, astigmatism assessment, and keratoconus screening.
- The CPOA positions the patient, ensures proper contact lens removal, captures readings, and records results.