Extended keratometer and keratometry procedure

KERATOMETRY
Presented by – Rahul Deb Bera (M.Optm 2nd yr)
VIDYASAGAR COLLEGE OF OPTOMETRY & VISION
SCIENCE
RD 1

CONTENTS
• Definition
• Principle
• Parts of Keratometer
• Procedure of keratometry
• Interpretation of findings
• Clinical uses
• Limitations
• Sources of error
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KERATOMETRY
• Kerato = Cornea
• Metry = Measurement
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DEFINITION
• 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.
• Keratometer also called as Ophthalmometer.
AK Khurana,Theory and practice of optics and refraction 2nd edition.2008;160.
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PRINCIPLE
• Keratometry is based on the fact that the anterior surface of the cornea acts as a convex
mirror and the size of the image formed varies with its curvature.
Greater the curvature of cornea, Lesser is the image size.
• From image size formed by anterior surface of cornea( 1st Purkinje image) – radius of
curvature of cornea can be calculated.
AK Khurana,Theory and practice of optics and refraction 2nd edition.2008;160
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Therefore,
B’P/A’B’ = BP/AB B’P = v
F/i = u/o A’B’ = i
r/2i = u/o ( since F = r/2) BP = u
r = 2ui/o AB = o
U (BP) is constant for any instrument
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Relationship between radius of curvature and dioptric power of cornea -
D = n-1/r
D = 0.3375/r
D – Dioptric power of cornea
n – Refractive index of cornea
r – Radius of curvature of cornea (m)
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 Doubling Principle :
 Because of involuntary eye movement image formed on cornea would be constantly
moving.
To overcome this Doubling technique is used.
Two prism ( Base Up prism in left diaphragmatic aperture & Base Out prism in
right diaphragmatic aperture) is introduced into the optical system so that 2 images
are formed.
The prism is moved until the images touch each other.
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• Keratometer is based on 2 concepts :
Variable Doubling Fixed Doubling
Eg. Bausch and Lomb keratometer Eg. Javal-Schiotz
Ha
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PARTS OF BAUSCH AND LOMB
KERATOMETER
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OPTICAL SYSTEM OF KERATOMETER
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• Total area of upper & lower apertures = Area of each the other two apertures
• Upper and lower apertures also act as Scheiner’s disc doubling the central image,
whenever the instrument is not focused precisely on central mire image.
• Thus image – doubling mechanism is unique in Bausch and lomb keratometer, in that
double images are produced side by side as well as 90 degree from each other.
• This allows the instrument of the power of cornea in two meridian , without rotating the
instrument.
“it is called one position keratometer”
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PROCEDURE
1. Instrument calibration and eye piece focusing
2. Patient Adjustment :
• Seated in front of the instrument.
• Chin on chin rest and head against head rest.
• Eye not being examined is covered with occlude.
• Chin raised or lowered till patient’s pupil and projective knob are at same level.
AK Khurana,Theory and practice of optics and refraction 2nd edition.2008;163-164.
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• Instruct the patient to :
• Keep eyes open wide and blink normally.
• Try not to move the head nor speak.
• Look at the reflection of own eye in the keratometer barrel.
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3. Focusing of mires :
• The central image is doubled, indicating that
Instrument is not correctly focused on the
Corneal image of the mire.
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To measure curvature in horizontal meridian,
plus sign of central and left images
are superimposed using horizontal measuring control.
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To measure curvature in vertical meridian,
minus signs of central and upper
imaged are coincided with
the help of vertical measuring control.
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In presence of oblique astigmatism,
two plus signs will not be aligned entire
instrument rotated till they are aligned.
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INTERPRETATION OF FINDINGS
1. SPHERICAL CORNEA
• No difference in power between 2 principle meridian.
• Mires seen as perfect sphere.
2. ASTIGMATISM
• Difference in power between 2 principle meridian.
• Horizontally oval mires in with the rule astigmatism. Eg. 45.00D @ 90/ 43.25D @180
• Vertically oval mires in against the rule astigmatism. Eg. 42.00D @ 90/44.50D @180
• Oblique astigmatism -> principal meridian b/w 300 – 600 & 1200 – 1500 .
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3. IRREGULAR ANTERIOR CORNEAL SURFACE
• Irregular mires
• Doubling of mires
4. KERATOCONUS
• Pulsating mires ( inclination & jumping of mires on attempt to adjust the mires)
• Minification of mires in advanced cases ( k > 52D ) due to increased amount of myopia.
• Oval mires due to large astigmatism.
• Irregular, wavy & distorted mires in advanced keratoconus.
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BAUSCH & LOMB KERATOMETER
• Range - 36.00 to 52.00 D
• Normal Values – 44.00 to 45.00 D
• Extended Keratometer -
Place +1.25 D lens in front of aperture
to extend range to 61 D ADD 9 D
Place -1.00 D lens in front of aperture
to extend range to 30 D SUBSTRACT 6 D
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CLINICAL USES OF KERATOMETERS
• It helps to measure corneal astigmatic error.
• Helps to estimate radius of curvature of the anterior surface of cornea.
• IOL power calculation.
• To monitor pre & post surgical astigmatism.
• It is used to monitor the shape of cornea in keratoconus and keratoglobus.
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LIMITATIONS OF KERATOMETRY
• Measurements of keratometer based on false assumption that the cornea is a act
like a convex mirror, whereas the cornea in reality is aspheric.
• Measure refractive status of small central cornea ( 3-4 mm)
• Loses accuracy when measuring very flat or very steep cornea.
• One position instruments assume regular astigmatism.
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SOURCES OF ERRORS IN KERATOMETRY
• Improper calibration
• Faulty positioning of the patient.
• Excessive tearing.
• Abnormal lid position.
• Improper focusing of central image.
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Extended keratometer and keratometry procedure

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