The document discusses keratometry, the measurement of the curvature of the anterior surface of the cornea, covering its principles, methods, and clinical applications. It describes both manual and automated keratometers, their advantages, and limitations, including common sources of errors in measurement. Key applications include assessing corneal astigmatism, guiding contact lens fitting, and monitoring corneal shape and post-surgical changes.

1. KERATOMETRY
Dr Saurabh Kushwaha
Resident Ophthalmology

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

3. 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

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

8. 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

9. DOUBLING PRINCIPLE

10. 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

11. METHODS OF KERATOMETERY

12. BAUSCH & LOMB KERATOMETER

13. ILLUMINATION SYSTEM

14. OBSERVATION SYSTEM

15. MIRES
 Circular
 Forms an image on the cornea

16. 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

17. RECORDING OF THE CORNEAL
CURVATURE

18. 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

19. 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

20. 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

21. JAVAL SCHIOTZ KERATOMETER

22. MIRES

24. OPTICAL SYSTEM

25. FOCUSING OF MIRES

28. KERATOMETERS
 Ziess Oberkochen
Keratometer
 Humphrey Keratometer

29. 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

30. 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

31. 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

32. 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

33. 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

34. THANK YOU