Keratometry

KERATOMETRY
Dr Saurabh Kushwaha
Resident Ophthalmology

SCOPE
 Introduction
 Principle of Keratometry
 Methods of Kertaometery
 Automated Keratometer
 Clinical Applications
 Limitations
 Sources of errors

INTRODUCTION
 Keratometry is measurement of curvature of the
anterior surface of cornea across a fixed chord
length, usually 2-3 mm, which lies within the optical
spherical zone of cornea.
 Kerato : Cornea
 Metry : Measurement
 Expressed in Dioptric power

PRINCIPLE OF KERATOMETRY
 Anterior surface of cornea acts like a convex
mirror and forms 1st Purkinje image

PRINCIPLE OF KERATOMETRY
 The curvature of the anterior surface of cornea
determines the size of the 1st Purkinje image
 Greater the curvature of cornea, lesser is the
image size
 If the object size and distance is known and the
1st Purkinje image is determined, then the
curvature of anterior corneal surface can be
calculated

DOUBLING PRINCIPLE
 Biprisms introduced into the optical system so that two
images are formed
 If eye moves – both moves
 The prism is moved until the images touch each other
such that lower edge of one image coincides with upper
edge of the other

MIRES
 Circular
 Forms an image on the cornea

FOCUSING OF MIRES
 Instrument is correctly focused on corneal image
so that central image is no longer doubled
 Images are superimposed using the horizontal and
vertical measuring control

RECORDING OF THE CORNEAL
CURVATURE

RANGE OF KERATOMETER
 Range - 36.00 to 52.00 D
 Normal values - 44.00 to 45.00 D
 To increase the range - Place +1.25 D lens in
front of the aperture to extend range to 61D
 Place -1.00 D lens in front of the aperture to
extend range to 30 D

ASTIGMATISM
 Difference in power b/w 2 principal meridia
 Horizontally oval mires in WTR Astigmatism
 Vertically oval mires in ATR astigmatism
 Oblique astigmatism - principal meridia b/w 30º- 60º &
120º-150º
 Rotate the head of keratometer to get alignment
 Read out the axis from the amount of rotation

KERATOCONUS
 Inclination & jumpimg of mires on attempt to adjust
the mires - Pulsating mires
 Minification of mires in advanced cases (K >52 D) due
to increased amount of myopia
 Oval mires due to large astigmatism
 Irregular, wavy & distorted
mires in advanced keratoconus

KERATOMETERS
 Ziess Oberkochen
Keratometer
 Humphrey Keratometer

AUTOMATED KERATOMETERS
 Focuses reflected corneal image on to an
electronic photosensitive device, which instantly
records the size & computes the radius of
curvature
 Target mires are illuminated
with infrared light & an infrared
photodetector is used

ADVANTAGES OF AUTOMATED
KERATOMETERS
 Compact device
 Consumes less time
 Comparatively easy to operate
 No doubling device is needed
 Measures angle size in many meridians, so it
computes angle as well as power in many
meridians
 Absence of annoying glare of brightly
illuminated mires

CLINICAL APPLICATIONS OF
KERATOMETERS
 Helps in measurement of corneal astigmatic error
 Helps to estimate radius of curvature of the anterior
surface of cornea - Use in contact lens fitting
 Monitors shape of the cornea - Keratoconus
- Keratoglobus
 To monitor pre- & post-surgical astigmatism
 IOL power calculation
 Used for differential diagnosis of axial versus
curvatural anisometropia

LIMITATIONS OF KERATOMETRY
 Measurements of keratometer based on false assumption
that cornea is a symmetrical spherical or spherocylindrical
structure, with 2 principal meridia separated from each other
by 90°
 Measures refractive status of small central cornea (3-4 mm)
 Loses accuracy when measuring very flat or very steep
cornea
 Small corneal irregularities preclude use of keratometer
due to irregular astigmatism
 It cannot describe corneal asphericity

SOURCES OF ERRORS IN
KERATOMETRY
 Improper calibration
 Faulty positioning of patient
 Improper fixation by patient
 Localized corneal distortion
 Abnormal lid positioning
 Improper focusing of the corneal image

Keratometry

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Keratometry