Small description on keratometry

1. KERATOMETRY
Dr. Litty K S
Junior Resident
Department of Ophthalmology
Government Medical College ,Kottayam.

2. KERATOMETRY(OPHTHALMOMETRY)
Kerato = Cornea
Metry = Measurement

3. Definition
Keratometry→ Measurement of curvature of the anterior surface of cornea across a
fixed chord length of 2-3 mm which lies within the optical spherical zone of
cornea.

4. (4 mm)
(4-8 mm)
(8-11 mm)
(11-12mm)

5. PRINCIPLE
Anterior surface of cornea → CONVEX MIRROR
Greater the curvature of cornea, lesser is the image size.
From the size of image formed by the ant.surface of cornea( 1st Purkinje image),
Radius of curvature of cornea can be calculated.

6. PURKINJE IMAGES

7. u v
i
o
For a convex mirror,
M = i/o = v/u
i→ image size
o→object size
v→distance of image from mirror
u→distance of object from mirror
When object is at infinity,
i/o = f/u
f→focal length
(Radius of curvature/2)
ie, i/o = R/2u
So,
R = 2ui/o
u is a constant for any instrument. Ie, for known object size,
measurements of image size will help to determine the r = radius of curvature

8. Relationship between Radius of curvature and Diopteric
Power of Cornea
D - diopteric power of cornea
n - Refractive index of cornea (1.3375)
r - Radius of cornea in metres
D = n-1/r
r= 1.3375-1/D mm

9. Miniature
involuntary eye
movements during
fixation of eye
Image formed by
ant.surface of cornea also
moves(impossible to
measure image size)
DOUBLING
PRINCIPLE

10. ● Biprisms introduced into the optical system so that 2 images are formed
● Lower edge of one image coincides with upper edge of the other
● If eye moves - both moves
● The prism is moved until the images touch each other.
Depending upon the position of prism - if distance decrease , doubling increases

11. KERATOMETER IS BASED ON TWO CONCEPTS:
Fixed object size with
variable image size
(Variable doubling)
Eg. Bausch & Lomb keratometer
Fixed image size with
variable object size
(Fixed doubling)
Eg. Javal-Schiotz keratometer

12. HELMHOLTZ KERATOMETER

13. BAUSCH AND LOMB KERATOMETER
Principle:
Constant object size & variable image size.

16. Total area of upper & lower aperture = Area of each of the other 2 apertures
Therefore, brightness of images is equal
★ Upper & lower apertures also act as Scheiner’s disc doubling the central
image,whenever the instrument is not focussed precisely on central image.
★ Thus, image doubling mechanism is unique in Bausch & Lomb keratometer, in
that double images are produced side by side as well as at 90° from each other.

17. This allows the measurements of the power of cornea in two meridia, without
rotating the instrument.
Therefore it is also known as as ‘one-position keratometer’.
The eyepiece lens enables examiner to observe the magnified view of doubled image.

18. Procedure of keratometry.
Instrument adjustment:
● Instrument is calibrated before use
● White paper is held in front of objective piece & a black line is focused sharply
on it.
● Keratometer then calibrated with steel balls.
● Steel ball of known radius of curvature is placed before keratometer & its value
is set on the scale or dial.

19. ● Mires are focused by clockwise & anticlockwise movement of eyepiece through
trial & error.
● When mires are on focus calibration is complete.

20. Patient adjustment:
1. Seated in front of the instrument.
2. Chin on chin rest & head against head rest.
3. Eye not being examined is covered with occluder.
4. Chin raised or lowered till patient’s pupil and projective knob are at the same
level.

21. Instruct the patient to:
● Keep eyes open wide & blink normally.
● Try not to move head nor speak.
● Look at the reflection of own eye in the keratometer barrel.

22. Focusing of mires:
Mire is focused in the centre of cornea.
Patient’s view of mire. The central image is doubled, indicating that
………….. instrument is not correctly focused on
the corneal image of the mire

24. Interpretation of findings
Spherical cornea :
● No difference in power between two principal meridians
● The mires seen as perfect sphere.
Astigmatism:
● Difference in power between two principal meridians.
● Horizontally oval mires seen in with-the-rule astigmatism
● Vertically oval mires are seen in against-the-rule astigmatism.
● In oblique astigmatism, the principal meridians are between 30-60° & 120-
150°

25. Irregular anterior corneal surface is characterized by the following:
● Irregular mires
● Doubling of mires.

26. Keratoconus is characterized by:
● Inclination & jumping of mires seen while attempting to adjust the mires.(When
an attempt is done to superimpose the plus mires,they will jump above & below
each other(pulsating mires)
● Minification of mires → advanced keratoconus(K>52D) due to increased
myopia
● Oval mires → large astigmatism
● Irregular,wavy & distorted mires – advanced keratoconus.

27. Automated keratometer
● Focuses the reflected corneal image to an electronic photosensitive device,which
instantly records the size & compute the radius of curvature.
● Target mires are illuminated with infrared light & an infrared photodetector is
used.
ADVANTAGES
- Compact device
- Less time consuming
- Easy to operate

28. ● 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

29. USES
● Measurement of corneal astigmatic error
● Radius of curvature of cornea→ contact lens fitting
● Shape of cornea -keratoconus , keratoglobus
● Asses refractive error in case of hazy media
● IOL power calculation
● Pre & post surgical astigmatism
● Differentiate axial v/s curvatural anisometropia

30. Limitations
● Measurement of keratometer based on false assumption that cornea is a
symmetrical spherical/spherocylindrical structure with 2 principal meridia
separated from each other by 90°
● Measure refractive status of very small central area of cornea (3-4 mm),
ignoring the peripheral corneal zones
● Loses accuracy while measuring very flat(<40D) or very steep(>50D) cornea.
● Small corneal irregularities preclude use of keratometer due to irregular
astigmatism

31. ● One-position instruments assume regular astigmatism
● Distance to focal point is approximated by distance to image