Keratometry
 The objective measurement of the curvature of the anterior surface of the cornea
o Cornea acts as a highly convex spherical mirror
o Uses reflective properties of the cornea in order to measure its radius of
curvature
o Each manufacturer uses an assumed corneal index of refraction to convert radii
to diopters
 B&L uses 1.3375
o One position keratometer: variable doubling (B&L)
 Both principle meridians are perpendicular to each other
o Two position keratometer
 Rotate the instrument for the secondary meridian
 Allows measurement if the two principal meridians are not at right angles
to each other
 Clinical uses:
o Objective method for determining curvature of the cornea, amount and direction
of corneal astigmatism, quality and stability of the corneal refracting surface
 Examples: progressive myopia, keratoconus, degenerative anomalies,
and pre-surgical workup for cataract surgery (helps determine power of
IOL)
o Contact lens fitting
 Base curve selection in RGP and hydrogel lens fitting
 “Over keratometry” can help detect CL surface irregularities or poor
wetting qualities
 Monitor corneal changes produced by wear
o Establish baseline data
 Should be performed on all new patients
 Patient may later want CLs or may develop an injured/diseased cornea
o Detection of irregular astigmatism
 Keratoconus: a corneal dystrophy where the center of the cornea thins
and bulges
 Results in steep curves, high astigmatism, and distorted mires
 Pterygium
 Corneal scarring
o Refractive estimate in unresponsive patients, children, amblyopes, aphakia or
high ametropia, patients with media opacities and/or poor quality retinoscopic
reflex and difficult refractions/unreliable subjective data
o Determine the nature of ametropia
 Axial
 Example: high myopia with 40D “K” reading
 Refractive
 Example: high myopia with 49D “K” reading
o Post-surgical management of keratoplasty or cataract surgery
 Limitations
o Area measured is a 0.1mm annular ring with a 3.0mm diameter
o One position keratometers assume that meridians of least and most cylinder are
orthogonal (perpendicular)
o Assumed corneal index may cause problems when comparing K’s from different
instruments
o Autokeratometers do not evaluate the quality of the cornea
 Procedure
o Turn instrument on and clean forehead and chin rests
o Focus the eyepiece
 Turn fully counterclockwise to blur reticule (toward plus)
 Relaxes accommodation
 Turn eyepiece clockwise until reticule is FIRST seen in sharp focus
 Note: Incorrect focusing can produce significant error (up to 0.2mm
radius)
o Position patient comfortably and align canthus marker using instrument height
control
o Place the reflection of the mires in the center of the pupil
 Account for all 3 axes (x, y, and z)
 X: instrument pivot
 Y: instrument height adjustment
 Z: instrument focus and headrest tilt control
o Instruct patient to “keep both eyes open, look at the reflection of your eye in the
instrument, and it is okay to blink”
o Occlude left eye
o Align the crosshairs in the center of the lower right-hand circle
 Means the optical axis and the visual axis of the eye are the same
o Focus the lower right-hand circle to obtain superimposed image and lock
instrument
o Locate the principal meridians
 Make the “plus” signs parallel by rotating the instrument and then overlap
the “plus” signs with the horizontal axis wheel
 Align the “minus” signs with the vertical axis wheel
 KEEP ONE HAND ON THE FOCUSING KNOB
o Record both horizontal and vertical measurements (in eighths of a diopters)
 Record flatter/steeper @ steeper meridian
 Record clarity of mires: clear or distorted
 ALWAYS record findings to 2 decimals (total of 4 digits)
o Problems/Solutions
 Unable to locate keratometric mires instrument and/or patient not
aligned properly
 Mire clarity is transient measure quickly after allowing the patient to
blink
 Mire focus is transient ensure that patient’s forehead is secure against
headrest
 Patient gaze is unsteady ensure that fellow eye is occluded
 H&V mires cannot be measured concurrently patient may have
irregular astigmatism
Only 1 minus sign is visible patient’s eyelid is drooping (have them
open wide)
 Only 1 plus sign is visible occluder is in the way
 Range of the B&L keratometer is 36.00 to 52.00
o Normal values are around 44.00 to 45.00
o To increase the range:
 Place +1.25D lens in front of aperture to extend range to 61D
 ADD 9 D
 Place -1.00D lens in front of aperture to extend range to 30D
 SUBTRACT 6 D
 Astigmatism
o Irregular: principal meridians are not perpendicular to each other
 Produce distorted mires
o Regular: principal meridians are perpendicular
 With-the-rule: more power in the vertical meridian (greatest curvature)
and horizontal meridian is flatter
 Example: 45.00@090/43.25@180
 Against-the-rule: more power in the horizontal meridian and vertical
meridian is flatter
 Example: 42.50@115/44.87@025
 Oblique: principal meridians lie between 20° and 70° and 110° and 160°
o Corneal astigmatism is measured by the keratometer
o Refractive (total) astigmatism is measured by retinoscopy and/or subjective
refraction
 Corneal and refractive do not coincide due to:
 Physiologic lenticular astigmatism (usually ATR and varies with
age)
 Effectivity changes
o ~25% increase in astigmatism going from the corneal plane
to the spectacle plane
 Corneal posterior surface curvature
o Δ K (corneal cylinder)
 Translate K’s into minus cylinder format
 Use the flatter meridian as the minus cylinder axis and the difference in
power between the two meridians as the cylinder power
 Use Javal’s (modified) Rule to predict the correcting cylinder
 Only should be used with </=2.00DC ATR and </=3.00DC WTR
 Convert corneal astigmatism to refractive astigmatism
o WTR: refractive= corneal + (+0.50 x 180)
 WTR gets better by 0.50
o ATR: refractive= corneal + (-0.50 x 090)
 ATR gets worse by 0.50
 Corneal topography is used to provide a color map of the contour of the entire corneal
surface
o Used for corneal surgery (refractive and penetrating keratoplasty), degenerative
corneal conditions (keratoconus), and contact lens fitting (orthokeratology)
