2. It is a non invasive imaging technique for mapping the
corneal surface.
It allows three dimensional evaluation of corneal tissue.
It provide both quantitative and qualitative evaluation of
cornea.
It is also known as Photokeratoscopy or
Videokeratoscopy
3. Indication
Pre and post refractive surgery.
Diagnosis of corneal disorder.
Early screening of keratoconus.
Confirmation of post-operative astigmatism.
Contact lens fitting
ICL fitting
Orthokeratology
C3R
Pre-Operative IOL selection
Post keratoplasty astigmatism evaluation.
4. Principle of topography
Placido disc based system
Non placido disc based system
1.Scanning slit
2.Scheimpflug photography
5. Instrument Design
1.Instrument with small target and short working distance
It is a small instrument
Provide maximum corneal coverage
In this focusing and alignment error is more
2.Instrument with larger target and larger working
distance
This minimise focussing and alignment error
But provide lesser corneal coverage
6. Parts of Topography
Projection device:
To create image of cornea by projecting placido disc.
Video Camera:
Capture reflected image of plasido disc.
Computer
Analyse reflected image ,Calculate topographic data
and display .
7. Keratograph Algorithm
It is the process of building a topographic map of cornea
from keratoscopic data.
1. Capture image of video keratoscopic ring.
2. Measurement of angular size of point
3. Reconstruction of corneal surface
4. Calculate the power and other data
5. Present measured data as a topogram
8. Procedure
Patient is seated comfortably in front of the instrument.
Enter the patient data.
Focus the instrument by aligning crosshairs in the centre
of pupil.
Project alternative black and white concentric ring on to
the cornea.
Camera take the pictures of the reflection.
Measure angular points on the ring.
Computer analyse the reflected images and display result
as colour coded maps.
9. Corneal Topographic Maps
Topogram
It is the method of displaying data from corneal topography
system
Different topographic system generate different types of maps.
Display option of topographic data
1. Numerical power plot
2. Simulated Keratometric view
3. Profile view
4. Photokeratoscopic view
5. Surface elevation map
6. Colour coded maps
12. Colour Coded maps
Colour code Interpretation
Green and yellow Normal cornea
Hot/warm
colours(red and its
hues)
Steeper cornea
Cool colours(blue
and its hues)
Flatter cornea
13. Interpretation
Round or oval - Normal cornea
Bow-tie pattern -Astigmatism
Vertical bow-tie -With the rule astigmatism
Horizontal bow-tie -Against the rule astigmatism
Asymmetrical bow-tie -Keratoconus
Kissing pigeon pattern -Pellucid marginal degenaration
and advanced keratoconus
16. Diagnosis if keratoconus
K value - the average keratometric value of central 2 to
4 rim.
I-S value - average of dioptric power of five points
measured along inferior and superior cornea.
K value I-S value
Normal <47.2D <1.40
Keratoconus
suspect
47.2 to 48.7 1.4 to 1.9
Keratoconus >48.7 >1.9
18. CMS
It is the fist fully automatic corneal topography system
Consist of 32 alternatively black and white ring placido disc for
analysing cornea
Also useful for measuring corneal thickness
TMS
It consist of 31 alternatively black and white ring placido disc
system
For analysing 7000 data points on cornea
19. Orbscan
it is a newer technology which allow mapping of anterior and
posterior surface of cornea.
It use a hybrid technique by combining advantages of placido disc
system and slit scanning system
Pentacam
It is the gold standard anterior segment topography device .
It use rotating scheimpflug technology to evalute anterior and
posterior corneal surface.
