7.1 Keratometry Overview

7.1 Keratometry Overview

Keratometry is the measurement of the curvature of the central anterior cornea using a keratometer (also called an ophthalmometer). It samples only the central paracentral zone, roughly the central 3 mm, and assumes the cornea is a spherocylinder. Results are called K readings and are reported either as a radius of curvature in millimeters or as a dioptric power in diopters (D). The COA candidate must be able to take K readings, interpret the two principal meridians, recognize astigmatism, and know what the numbers mean for contact lenses and surgery.

Why keratometry matters

K readings feed three clinical jobs: (1) contact-lens fitting, where base curve is selected relative to flat K; (2) intraocular lens (IOL) power calculation before cataract surgery, where K combines with axial length; and (3) detection of corneal astigmatism and irregularity, such as keratoconus. An inaccurate K reading propagates directly into a wrong IOL power or a poorly fitting lens, so technique discipline is the testable point.

The keratometric index and the core formula

The keratometer does not measure power directly; it measures radius from the size of a reflected image (the mires) on the corneal tear film, then converts radius to power using a fixed keratometric index of 1.3375. This index is a convention, not the true corneal index, chosen so the anterior radius estimates the total corneal power and accounts for the back surface.

$D = \frac{(n - 1)}{r} = \frac{0.3375}{r\,(m)} = \frac{337.5}{r\,(mm)}$

Worked example: A radius of 7.50 mm gives 337.5 / 7.50 = 45.00 D (a steep cornea). A radius of 8.04 mm gives 337.5 / 8.04 = 41.98 D (a flat cornea). Remember the inverse rule: a smaller radius is a steeper, higher-power cornea, and a larger radius is a flatter, lower-power cornea. Mixing this up is the single most common keratometry error.

Two principal meridians

A keratometer measures two principal meridians roughly 90 degrees apart. If both read the same (e.g., 43.50 @ 180 and 43.50 @ 90), the cornea is spherical. If they differ, the difference is the corneal astigmatism. For example, 43.00 @ 180 and 45.00 @ 090 means 2.00 D of astigmatism, with the steep meridian vertical. The COA records both readings and their axes.

Exam logistics anchor

The COA is administered by IJCAHPO as a 200-question multiple-choice exam with a 180-minute time limit, delivered at Pearson VUE centers or via OnVUE remote proctoring. Pass/fail uses a scaled, criterion-referenced cut score rather than a fixed percentage. Keratometry is a small clinical-skills domain, but its questions are formulaic: know the index, the formula, the normal range, and the steep-or-flat direction, and you can answer most of them quickly.

Manual versus automated keratometry

The COA should know both instrument families. The manual keratometer (Bausch & Lomb single-position or Javal-Schiotz two-position) requires the technician to focus the eyepiece, align mires by hand, and rotate the barrel to find the two meridians. The single-position type doubles the image with a fixed prism and reports power directly on calibrated drums; the Javal-Schiotz type moves two illuminated mires until they just touch. Automated keratometers and combined autorefractor-keratometers capture the rings electronically in under a second and print the K values and axes.

Automated units are faster and reduce examiner-induced error, but they still depend on a good tear film and proper patient fixation, and they can be fooled by a blink or a poorly centered eye. On the exam, manual technique is tested more heavily because it exposes the underlying optics.

What keratometry does not measure

Keratometry samples only the central paracentral ring and assumes the cornea is a smooth, symmetric spherocylinder. It does not map the corneal periphery, the posterior surface, or focal irregularities. It also assumes the two principal meridians are exactly 90 degrees apart, which is untrue in irregular corneas. For full-surface analysis the practice uses corneal topography (Placido-disc) or tomography (Scheimpflug), which generate thousands of data points and color maps.

Understanding this limitation is exactly what separates a confident keratometry reading from an inappropriate one, and the exam tests whether you know when a keratometer is the wrong tool.

Units and recording conventions

Most clinics record K readings in diopters because IOL formulas and contact-lens base-curve tables use diopters and millimeters interchangeably. A standard chart entry looks like "K: 43.50 @ 180 / 44.75 @ 090," listing the flat meridian first. Always include the axis; a power without an axis is incomplete and useless for astigmatism management.

Quick reference numbers to memorize

• Average corneal power: about 43.50 D; normal range roughly 42-46 D.
• Average anterior radius: about 7.7-7.8 mm.
• Conversion constant: 337.5 (from index 1.3375 times 1000).
• Manual keratometer measuring range: about 36 to 52 D.
• The cornea supplies roughly two-thirds of the eye's total refractive power (about 43 D of the eye's ~60 D), which is why K accuracy matters so much for IOL planning.

Knowing these anchors lets you sanity-check any reading instantly: a value far outside 42-46 D should prompt a second look at technique, the tear film, or the instrument before you accept it as the patient's true cornea.