Keratometry
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Direct answer
Keratometry reads only the central three millimetres of the anterior cornea, measuring the radius of curvature of its two principal meridians from the size of the reflected mire image, then converting radius to power by the working formula D = 337.5 divided by radius in millimetres. Two different readings in the two meridians are corneal astigmatism; which meridian is steeper names it — steeper vertical is with-the-rule, steeper horizontal is against-the-rule. The numbers feed three clinics at once: contact lens base-curve selection, intraocular lens power calculation, and keratoconus suspicion when the mires go irregular.
What you must remember
- Principle: the cornea acts as a convex mirror; the keratometer measures the image size of a target of known size and distance (first Purkinje image) and computes the radius.
- Conversion: power = 337.5 / r(mm), built on the keratometric index 1.3375 — a simplification treating the cornea as a single refracting surface, blind to the posterior surface.
- Normal values: roughly 40–46 D (average about 43–44 D; radius about 7.5–7.8 mm); the vertical meridian is typically marginally steeper, creating physiological with-the-rule astigmatism of 0.25–0.50 D.
- Steep vertical meridian = with-the-rule astigmatism (plus cylinder axis 90); steep horizontal = against-the-rule.
- Distorted or irregular mires suggest keratoconus, scarring or tear-film disruption — an indication for corneal topography.
- One-position instruments (Javal–Schiøtz type) versus two-position (Bausch + Lomb type with rotatable mires); calibration against steel test balls keeps either honest.
- Uses: rigid and soft lens fitting (base curve from the flat K), IOL power calculation (the K of SRK-family formulas), keratoconus monitoring, and over-refraction over diagnostic lenses.
- After corneal refractive surgery, standard K values mislead IOL formulas — historical or double-K methods are required, a favourite viva nuance.
- Fixation on the central mire and a focused, aligned image are preconditions; the tear film distorts mires before the cornea does, so blink first, image immediately.
From a printout to three clinical decisions
A young man referred for contact lens fitting: K readings right eye 43.00 D at 180, 44.50 D at 90. First decision — astigmatism: 1.50 D corneal, steeper vertical, therefore with-the-rule. Second — the contact lens: the rigid base curve is chosen on the flat K; converting 43.00 D back to radius (337.5/43.00 ≈ 7.85 mm), a diagnostic lens of about 7.85–7.90 mm is selected for an alignment fit, with the fluorescein pattern judging any adjustment. Third — the optics: if spectacle refraction shows more cylinder than the corneal 1.50 D, the residue is lenticular; the keratometer has just separated corneal from lenticular astigmatism.
Now the trap case: a 22-year-old with one-line acuity loss whose readings climb visit by visit, mires smudged inferiorly, radius drifting below 7.0 mm (power above 48 D). That is the keratometric signature of keratoconus progression, and the referral is for topography — the keratometer measures only the central 3 mm and misses paracentral cones, which is why it screens but never excludes. Reverse the scenario to the cataract clinic: the same 337.5-based readings feed the SRK formula family; had this patient once had LASIK, the flattened central K would under-power the implanted IOL unless a post-refractive method is applied. One instrument, three consultations, one formula holding them together.
Where students slip
Three errors recur. Quoting the conversion constant wrongly — 337.5, derived from the keratometric index (1.3375 − 1 = 0.3375) times 1000. Mixing up the astigmatism naming: with-the-rule means the steep meridian is vertical, which in minus-cylinder prescription language is axis 180 — candidates blend axis and meridian into one confusion. And believing the keratometer measures "the cornea": it measures the anterior central 3 mm, assumes an index, ignores the posterior surface and averages away irregularity — exactly why topography exists. The post-LASIK IOL trap and the paracentral-cone blind spot are the two viva answers that separate the trained refractionist from the operator.
Frequently asked questions
What converts corneal radius to dioptric power?
D = 337.5 / r, with radius in millimetres; the constant derives from the keratometric index 1.3375 treating the cornea as one refracting surface.
What is the keratometric index and why is it a simplification?
1.3375 — a fictitious index compensating for the ignored posterior cornea; it makes anterior-surface measurement approximate total corneal power.
What defines with-the-rule astigmatism on keratometry?
A steeper vertical meridian (higher power at 90), equivalent to plus cylinder at axis 90 or minus cylinder at axis 180 — the physiological pattern in younger eyes.
What do irregular mires suggest?
An irregular surface — keratoconus, corneal scar or unstable tear film — and an indication for topography, which maps far more than the central 3 mm.
Which reading guides rigid lens base-curve selection?
The flat K: a diagnostic rigid lens is chosen near alignment with it, then refined by fluorescein pattern and movement.
Why do post-refractive-surgery K readings mislead IOL calculation?
LASIK flattens the central cornea the keratometer reads, so standard K values overestimate corneal power and under-power the IOL; historical or double-K formulas correct for it.