4.4 Keratometry & corneal curvature

What keratometry measures

Keratometry measures the curvature of the central roughly 3 mm of the anterior cornea in two principal meridians and derives corneal power in diopters from the radius of curvature. The cornea supplies about +43 D of the eye's roughly +60 D total power, so its curvature dominates the refraction and is essential for contact-lens fitting and IOL power calculation.

Radius and dioptric power

The keratometer converts radius r in millimeters to power K in diopters using the standardized keratometric index 1.3375:

K (D) = (n - 1) / r = 0.3375 / r(meters) = 337.5 / r(mm)

Worked examples:

• r = 7.50 mm gives K = 337.5 / 7.50 = 45.00 D
• r = 8.00 mm gives K = 337.5 / 8.00 = 42.19 D
• r = 7.80 mm gives K = 337.5 / 7.80 = 43.27 D

A steeper cornea has a shorter radius and a higher dioptric power; a flatter cornea has a longer radius and lower power. Remember the reciprocal relationship: small radius means steep means strong.

Flat K, steep K, and corneal astigmatism

The keratometer reports two readings, one per principal meridian:

• Flat K: the flatter meridian (longer radius, lower power).
• Steep K: the steeper meridian (shorter radius, higher power).

Corneal (keratometric) astigmatism = Steep K minus Flat K, located at the axis of the steep meridian.

Worked example: readings of 43.00 D at 180 and 44.00 D at 090 give astigmatism = 44.00 - 43.00 = 1.00 D. Because the steeper meridian is vertical (near 90), this is with-the-rule (WTR) astigmatism, typical of younger eyes. If the steeper meridian were horizontal (near 180) it would be against-the-rule (ATR), common with age; oblique astigmatism sits near 45 or 135. By convention a minus-cylinder spectacle axis lands near the flat corneal meridian.

Reading the mires

On a manual Bausch and Lomb type keratometer you align and superimpose the plus and minus mires:

• Focus and center the reticle on the cornea.
• Eliminate the doubling by adjusting each drum; the horizontal drum reads one meridian, the vertical drum the other.
• If the mires will not line up (steps or rotation), the axes are oblique or the cornea is irregular (keratoconus, scar, or dry eye), so flag it. A poor tear film is the most common cause of unstable, unrepeatable mires; ask the patient to blink and re-take.

Clinical uses

Contact-lens fitting

A rigid gas-permeable lens base curve is chosen relative to flat K ("fit on K," or steeper or flatter depending on design and corneal shape). Soft-lens fitting relies less on exact K but still uses corneal curvature and diameter (sagittal depth). Very flat or very steep K readings predict fitting difficulty.

IOL power calculation

Cataract IOL formulas (SRK/T, Holladay, Barrett, Hoffer Q) require average K or both Ks, plus axial length and anterior-chamber depth. Erroneous Ks are a leading cause of postoperative refractive surprise, so K readings must be repeatable, and for toric IOLs the steep-meridian axis must be accurate. Modern practice increasingly uses corneal topography or optical biometry (IOLMaster or Lenstar), which map thousands of points versus the keratometer's four, but the underlying radius-to-power math is identical.

Quick reference

Step-by-step keratometry

1. Calibrate the eyepiece and seat the patient with chin and forehead firm against the rests.
2. Have the patient fixate the reflected target and blink to smooth the tear film.
3. Focus the reticle until the central mire is crisp.
4. Rotate the instrument so the horizontal (plus) mires just touch, and read the drum; this is one meridian.
5. Rotate 90 degrees, superimpose the vertical (minus) mires, and read the second meridian.
6. If the two axes are not 90 degrees apart or the mires show steps, record irregular astigmatism and flag it.

Millimeters and diopters

Because K = 337.5 / r, the two units move inversely; a shorter radius always means a steeper, stronger cornea:

An average K (mean of the two meridians) summarizes overall corneal power; for the 43.00 and 44.00 D example the average K is 43.50 D. Some devices also report simulated keratometry (SimK) derived from topography.

Contact-lens base curve math

For a spherical rigid lens, a common starting base curve equals the flat K in millimeters (or about 0.50 D flatter for a lid-attachment fit). Convert as needed: a flat K of 43.00 D corresponds to r = 337.5 / 43.00 = 7.85 mm. As corneal cylinder rises above roughly 2.50 D, a toric or specialty design is usually required, and unusually steep K values may signal keratoconus, warranting topography before fitting.

IOL calculation in brief

Modern optical biometry feeds average K, axial length, and anterior-chamber depth into a formula (SRK/T, Barrett Universal II, Holladay) to output the intraocular lens power that leaves the eye near emmetropia. A 1 mm axial-length error shifts the result by roughly 2.5 to 3 D, and a 1 D K error shifts it by about 1 D at the spectacle plane, so accurate, repeatable K readings are critical, and for toric IOLs the steep-axis alignment must be exact.