This document provides an overview of corneal topography. It begins by defining corneal topography as the study of the shape of the corneal surface. It then describes several techniques for evaluating corneal topography including keratometry, keratoscopy using Placido discs and photokeratoscopy, rasterstereography, and interferometry. Computerized topography systems that provide detailed maps of the corneal surface are also discussed. The document outlines clinical applications of corneal topography and variations in topographic patterns seen in normal and diseased corneas.

1. CORNEAL TOPOGRAPHY
Dr Saurabh Kushwaha
Resident Ophthalmology

2. SCOPE
 Introduction
 Topography techniques
 Keratoscopy
 Computer topography systems
 Colour coded topographic maps
 Clinical Applications
 Variations in corneal topographic patterns

3. INTRODUCTION
 topos : place
 graphō : write
 Planetary science : study of surface, shape and
features of earth
 Corneal topography refers to study of the shape
of corneal surface

4. TOPOGRAPHY TECHNIQUES
Topography techniques
Keratometry
Keratoscopy
1. Placido disc
2. Photokeratoscopy
3. Videokeratoscopy
Rastersterography
Interferometry
Reflection Based
techniques
Projection Based
techniques

5. KERATOSCOPY
 It is the evaluation of topographic abnormalities
of the corneal surface by direct observation of
images of mires reflected from the surface of
cornea
 Techniques
1. Placido-disc keratoscope
2. Photokeratoscopy
3. Videokeratoscopy

6. PLACIDO-DISC KERATOSCOPE
 Consists of equally spaced alternating black and white
rings with a hole in the centre to observe patients cornea
 Observer views the pattern of concentric white rings
(mires) reflected from the patient’s cornea through a central
+2 D lens
 The closer the mires, the steeper the cornea
 The wider the rings, the flatter the cornea

7. PLACIDO-DISC KERATOSCOPE
 Disadvantages
• Small degree of abnormality of corneal shape are not
easily identifiable
• Not used in corneas with epithelial defects & stromal
ulcers because of non reflection of image by the cornea
• Anatomy of nose and orbit may limit field size and
restrict the corneal area that can be examined

8. PHOTOKERATOSCOPY
 When a photographic film camera is attached to a
keratoscope, it functions as a photokeratoscope
 In the technique, the keratoscopic image is
photographed wherein the size of the images on the
photographic film can be changed to change the size
of the corneal image
 The image of most photokeratoscope rings covers
the paracentral zone, overlapping into the central and
peripheral zones but leaving the optically important
central 2-3 mm as well as the peripheral cornea

9. PHOTOKERATOSCOPY
 Nidek PKS 1,000 is the current model, has 9-15
rings which cover 55 -75% of the corneal surface

10. VIDEOKERATOSCOPY
 When a television camera is attached to a
keratoscope, it functions as a videokeratoscope
 Covers approx 95% of the corneal surface
 With the advent of computers, the
videokeratoscopy has been computerised

11. RASTEREROGRAPHY
 A calibrated grid pattern of horizontal and
vertical lines (spacing of 0.2 mm) is projected onto
fluroscein stained tear film, photographs are taken
and computer assisted algorithms are used to
analyze the pictures
 Can measure epithelial defects

12. INTERFEROMETRY
 Uses the technique of lightwave interfernce
 The interference fringes can cover the entire
ocular surface and not just cornea
 Applies 3-dimensional imaging
 Not in widespread clinical use

13. COMPUTER TOPOGRAPHY
SYSTEMS
 Term corneal topography system (CTS), or
videokeratography, implies computerised, videoassisted
technique that provides detailed information about the
shape of the corneal surface
 Most corneal topographers evaluate 8,000 - 10,000
specific points across the entire corneal surface
 Excellent accuracy & repeatability

14. COMPUTER TOPOGRAPHY
SYSTEMS
 Basic unit of CTS primarily consists of
• A projection device
• Video Camera
• Digital computer attached to a slit-lamp chin rest

15. COMPUTER TOPOGRAPHY
SYSTEMS
Topography systems
EyeSys Orbscan
I & II
IOL master 700
Scheimpflug
image based
OCT
based
Placido-disc
based
Slit scan
based
Pentacam
Galilei

16. COLOUR CODED
TOPOGRAPHIC MAP
 Most useful and most commonly used display
formation
 Parameters for interpretation are as follows:
• Colour codes
• Scale used
• Quantitative indices

17. COLOUR CODES
 Hot colour – red, indicates steep portions of
cornea
 Cool colour – blue, indicates flat portions
 Red - orange – yellow – green – purple - blue
denotes progressively lessening refractive power

18. SCALES
 Absolute scale: each
colour represents 1.5
D interval between 35
& 50D, where as
above and below this
range colours
represent 5D interval
 Disadvantage: it
does not show subtle
changes of curvature
 Normalised scale: in
it the cornea is divided
into 11 equal colours
 Advantage: it shows
more detailed
description of the
surface
 Disadvantage:
colours of two different
maps cannot be
compared.

19. QUANTITATIVE INDICES

20. ORBSCAN
 Slit scanning topography system
 40 scanning slit beams (20 from left and 20
from right) to scan the cornea
 Each of the 40 slit images triangulates one slice
of ocular surface
 Total duration of examination 1.5 sec

21. ORBSCAN II
 Placido disc added in Orbscan I

22. INTERPRETATION OF MAPS

23. CURVATURE MAPS

24. ELEVATION MAPS
 Direct data obtained by slit scanning
 Represent the true corneal shape : topography
 Elevation data compared against a refernce
surface : usually spherical

25. PACHYMETRY MAPS

26. THE QUAD MAP

28. PENTACAM
 Pentacam (Oculus Inc) obtains images of
anterior segment by rotating Schiempflug camera
which is a digital charged coupling device
 Comprehensive anterior aegment analyser
 Perform following 5 functions
1. Scheimpflug image of anterior segment
2. Three dimensional anterior chamber
analyser
3. Pachymetry
4. Corneal topography
5. Cataract analyser

29.  Advantages of pentacam
• Takes 50 meridonial sections through centre of
cornea which allows the system to realign the
central thinnest point of each section before it
reconstructs the corneal image. Thus it
eliminates any eye movement occurring during
the examination
• Enables measurement of cornea with severe
irregularities such as keratoconus that may not
be possible with placido disc imaging
• Enables calculation of pachymetry from limbus
to limbus

30. REFRACTIVE MAP

36. CLINICAL APPLICATIONS
 Preoperative and postoperative assessment of the
refractive patient
 Preoperative and postoperative assessment of
penetrating keratoplasty
 Irregular astigmatism
 Corneal distrophies, bullous keratopathy
 Keratoconus (diagnostic and follow-up)
 Follow-up of corneal ulceration or abscess
 Post-traumatic corneal scarring
 Contact lens fitting
 IOL power calculation

37. CORNEAL TOPOGAPHIC PATTERNS
IN NORMAL CORNEAS
 The normal cornea flattens progressively from the
centre to the periphery by 2-4 D, with the nasal area
flattening more than the temporal area
 Two corneas of an individual show mirror-image
asymmetry
 Variations are common in nature

38. VARIATIONS IN CORNEAL
TOPOGRAPHIC PATTERNS

39. Round: 22.6%
Symmetric bow-tie: 17.5%Asymmetric bow-tie: 32.1%
Irregular: 7.1%

40. KERATOCONUS
 Keratoconus is almost always bilateral, and one
cornea is more involved than the other (here OD)
 Here OS, at first impression looks fairly normal, the
‘lazy eight’ astigmatic pattern is a common
characteristic of keratoconus

41. PELLUCID MARGINAL
DEGENERATION
 Topographical findings include high against-the-rule
corneal astigmatism and inferior mid-peripheral steepening at
4 and 8 o’clock. This pattern in creates a “kissing pigeon” or
“butterfly wing-like” or “crab claw” pattern diagnostic of PMD

42. TERRIENS DEGENERATION
 In Terriens degeneration where the thinning is
restricted to the superior or inferior area of the
peripheral cornea, there is relative steepening
approximately 90º away.
 There will be ATR Astigmatism with vertical
flattening

43. PTERYGIUM
 Asymmetrical flattening of the cornea with
pterygium

44. THANK YOU