Corneal Topography ( Types, classification, different instrumentation and their interpretation)

1. CORNEAL TOPOGRAPHY
Presented by – Florina Deka & Dhanjit Borah
B.Optom 3rd year, RCOOJ

2. CONTENT
• Introduction
• Topography vs tomography
• Techniques
• Display of data
• Corneal topographic patterns in normal corneas
• Formats for display of data on colour maps

3. INTRODUCTION
• Corneal topography refers to the study of shape of the
corneal surface
• It implies a computerized, video assisted technique that
provides detailed information about the shape of the corneal
surface
• Present day technologies allow 3D evaluation with cross
sectional images and are thus referred to as corneal
tomography system

4. TOPOGRAPHY VS TOMOGRAPHY
• The term topography refers
to a detailed representation
or description of the surface
characteristics of a structure
• Limiting to the anterior
surface
• Tomography is the process
of generating a two
dimensional cross sectional
image of a slice through a
three dimensional object
• Topography + Pachymetry

5. TECHNIQUES
Topography Techniques
Reflection based Projection based Computer
techniques techniques topography system
Keratometry Keratoscopy Rastersterography Interferometry
Placido disc Photokeratoscopy Videokeratoscopy

6. REFLECTION BASED TECHNIQUE
• This technique is based on the fact that the anterior surface of
the cornea act as a convex mirror and that the size of the
image formed varies with its curvature
• These calculate the slope of corneal surface, then the
curvature and power

7. A. KERATOMETRY
• Keratometry is the process of measuring radius of curvature
of anterior corneal surface from within central 3mm of cornea

8. TYPES
BAUSCH & LOMB
• Constant object size and
variable image size
• One position keratometer
JAVAL SCHIOTZ
• Constant image size variable
object size
• Two position keratometer

9. CLINICAL USES
• Helps in measurement of corneal astigmatic error
• Helps to estimate the radius of curvature of the anterior
surface of the cornea
• Measure the power of IOL
• Help in the contact lens fitting
• To monitor the shaped of cornea in keratoconus

10. B. KERATOSCOPY
• For information about corneal shape outside of the central
3mm, keratoscopy rather than keratometer was needed
• It refers to the evaluation of the corneal surface by direct
observation of images of mires reflected from the surface of
cornea

11. 1. PLACIDO DISC KERATOSCOPE
• In 1880, Antonia Placido developed the first keratoscope,
known as placido disc
• It consist of equally spaced alternating black and white rings
with a hole in the centre to observe the patients cornea.
• Distortions in corneal shape
appear as deviations from
evenly spaced concentric
circles.

12. Disadvantage
• Small degrees of abnormalities of corneal shape are not easily
identifiable
• Cannot be used in corneas with epithelial defects or stromal
ulcers, etc. because of non reflection of the target by the
cornea

13. 2. PHOTOKERATOSCOPY
• When a photographic film camera is attached to a
keratoscope, the instrument is called as photokeratoscope
• In this technique, keratoscopic image is photographed and the
size of images on the photographic film can be varied to
change the size of the corneal image
• The corneal curvature is then measured by utilizing the
distance of the keratoscopic rings from the cornea, the
magnification of the virtual image formed by the anterior
surface of the cornea and the focal length of the objective of
the camera

14. • The image of photokeratoscope covers the paracentral area,
overlapping into the central and peripheral zones but leaving
the optically important central 2-3 mm as well as peripheral
cornea

15. 3. VIDEOKERATOSCOPY
• When a television camera is attached to a keratoscope, it is
called as videokeratoscope
• Covers approx 95% of the corneal surface
• With the advent of computers, the videokeratoscopy has been
computerized
• Two types – large cone placido
small cone placido

16. PROJECTION BASED TECHNIQUES
• It project and image on the corneal surface and measure the
true shape of the cornea by calculating the height or
elevation, above a reference plane
• This data can be used to calculate slope, curvature and power.
• Includes – Rasterstereography and - interferometry

17. A. RASTERSTEREOGRAPHY
• This device projected a calibrated grid (a grid pattern of
horizontal and vertical lines spaced 0.2mm apart is used) on to
the flourescein stained tear film
• It makes images of projected grid onto the cornea
• The photograph is captured by video camera and uses computer
algorithms to analyse the pictures
• Modified operating room microscope or slit lamp is used
• The accuracy of system is 0.3D
for a diameter of 7mm
• The advantage of this system
is that it includes information
across the whole of cornea
and even includes part of the
sclera

18. B. INTERFEROMETRY
• It uses the technique of light wave interference
• The interference fringes can cover the entire anterior ocular
surface not just cornea
• It comprises a reference sphere that is compared to tested
surface
• Interference fringes are produced
as a result of difference in 2 shapes
• This difference is then interpreteted
as a contour map of surface elevation
• This method is not in widespread
clinical use

19. COMPUTER TOPOGRAPHY SYSTEM
COMPUTER TOPOGRAPHY SYSTEM
Placido disc Slit scan Scheimpflug image OCT
based based based based
Eyesys Orbscan Pentacam IOL master
I & II Galilei 700

20. PLACIDO DISC BASED
• Originally, they were limited to evaluation of the anterior
corneal surface.
• Presently, it includes imaging of posterior surface and direct
evaluation of elevational changes of both anterior and
posterior corneal surfaces
• Corneal Topography system can now be called corneal
tomography, as they allow 3D evaluation of corneal tissue

21. PRINCIPLE
• Reflection principle
• Anterior surface of cornea act like a convex mirror and hence
the size of image formed by it is determined by its curvature
• A steeply curved cornea will produce a smaller image, while a
flatter cornea will produce a larger image of the same object
situated at the same distance from the cornea
• Different projection devices use lighted circular rings of
varying sizes and numbers, these rings are reflected by convex
cornea and through an opening in the centre of target, images
are obtained by an acquisition camera

22. BASIC UNIT OF CORNEAL
TOPOGRAPHY SYSTEM
The basic unit primarily consist of the following :
• A projection device
• Acquisition device (video camera)
• Analytic device (computer)

23. COMMERCIALLY AVAILABLE PLACIDO
DISC CTS
• Eyesys desktop
• TMS-4 topographic modelling system
• Astramax
• Atlas 9000 corneal topography system
• OPD Scan III
• Cassini TCA

24. i. EYESYS DESKTOP
• It is a 25 ring videokeratoscopic device with USB 2 connectivity
with fast image processing time of 3 sec
• Analyses 9000 data points
• Use stiles crawford effect and allows
display of relative brightness of light
entering the pupil
Eyevista is a portable available model with similar functions,
useful for patients which are confined to a bed or wheelchair

25. ii. TMS-4 topographic modelling system
• Utilizes 31 projected rings providing 7000 data
points
• Corneal coverage is 0.02-11.00 with an accuracy of 0.10D
• Has a patented laser allignment
system for accurate allignment
and an exclusive refractive
surgery planning programme

26. iii. Astramax
• It is a three camera imaging system that uses stereo raytracing
for high precession, patient specific corneal measurement
• Patented polar grid yields critical
measurement to measure complex
corneal shapes
• High definition graphics provide eye
simulation and 3D surface modelling

27. iV. Atlas 9000 CTS
• Uses ray tracing technology to display higher order corneal
aberration
• It has non visible placido ring
illumination which is comfortable
for even the most light sensitive
patient
• The 22 ring placido disc is optimized
to avoid ring crossover which allows reliable results for a wide
range of patients
• It is a diagnostic instrument for CL
fitting, pathology detection and
management and selection of
aspherical IOLs

28. V. OPD Scan iii
• Provides intuitive maps and numerical data for the corneal
surface and provides neural networks assisted detection of
corneal pathology such as keratoconus and pellucid marginal
degeneration

29. Vi. Cassini TCA
• Cassini total corneal astigmatism (TCA) uses multicoloured
LED point to point ray tracing to provide a GPS like analysis of
cornea along with high resolution images utilised for surgical
guidelines
• There is a total of 679 LEDs; 224 red, 224 green, 224 yellow
and 7 white
• The unique measuring principle enables highly accurate and
repeatable measurement of the TCA
• It measures the posterior cornea using second purkinje
reflections and provides a TCA measurement

30. • The multicoloured LED coverage is equal across the entire
cornea, leaving no space for central scotoma
• The accurate axis and magnitude of astigmatism play a vital
part in the correct selection and positioning of a toric IOL

31. SLIT SCAN BASED
• Uses scanning slits that step over the corneal surface to
acquire topographic information
• Two slits are used, positioned at 45 degree angles to the right
and left of the instrument axis
• 20 slit images are captured from each direction with overlap
in a 7mm diameter central area
• Total corneal coverage – 10mm
• All images are captured within 1.5 sec
• It is also capable of measuring posterior surface thus, corneal
diameter can also be measured
• Eg. Orbscan

32. ORBSCAN
• In 1995, orbscan I was introduced
• In 1999, orbscan II
• In 2014, orbscan III
• It combines the advantages of slit scanning technology with
an advanced placido disc system
• Uses principle of projection
• 40 scanning slit beams (20 from right and 20 from left with up
to 240 data points per slit) are used to scan the cornea from
limbus to limbus
• Acquires over 23000 data points

33. • The images captured are then used to construct the anterior
corneal surface, posterior corneal surface, anterior irir and
anterior lens surfaces
• Data regarding corneal pachymetry and AC depth are also
displayed

34. BEST FIT SPHERE (BFS)
• The computer calculates a hypothetical sphere that matches
as long as possible to the actual corneal shaped being
measured, this is called the BFS
• It then compares the real surface to the hypothetical sphere,
showing areas ‘above’ the surface of the sphere in warm
colours and areas ‘below’ the surface in cool colours
• Green is ‘sea level’
• Warmer colours are above the ‘sea level’ and cooler colours
are below the ‘sea level’

35. FUNCTION
• Anterior and posterior elevation
• Anterior and posterior Curvature
• Full corneal pachymetry
• White to white diameter
• Pre and postop difference map

36. QUAD MAP
Quad map refers to the typical orbscan map which comprises
following 4 different maps:
• Anterior elevation map (anterior float)
• Posterior elevation map (posterior float)
• Curvature map (axial keratomretry map)
• Pachymetry map

37. Anterior and posterior elevation map
The meshwork effect indicates how the cornea would appear,
if it were entirely spherical and is referred to as the reference
sphere. This elevation data can be interpreted usefully in a
number of ways

38. Curvature map (axial keratometric map)
• Provides detailed keratometric information across the
diameter of the cornea
• K readings are between certain values that the cornea is
neither too steep nor too flat
• K readings more than 48D are an indication of potential
keratoconus

39. Pachymetry map
• Orbscan provides pachymetry readings from the precorneal
tear film to the endothelium thus, thicker than that obtained
in ultrasound
• Provide thickness information across cornea from limbus to
limbus, not just in single points as with ultrasound
• Detect areas of weakness, thinning and scarring

40. SCHIEMPFLUG IMAGING BASED
• Based on geometric rule that describes the orientation of the
plane of focus of an optical system when the lens plane is not
parallel to the image plane .
• In this scenerio, an oblique tangent can be drawn from the
image, object and lens planes and the point of intersection is
scheimpflug point, where image is in best focus
• Eg. – Pentacam, Galilei and Sirius

42. PENTACAM
• Can obtain 50 images less than 2 sec
• Each image has 500 true elevation points for a total of 25000
true elevation points for surface of cornea
• Able to image both anterior and posterior surface of cornea
• Has 2 cameras
• One is for detection and measurements of pupil and helps in
fixation and orientation
• Second is used for visualization of anterior segment
• Pentacam AXL is the latest version with axial length
measurement

43. TECHNICAL DATA
Technically, pentacam system uses the following data:
• Blue light (UV free), 475nm, to illuminate the eye
• Rotating scheimpflug camera for taking images
• Processor – an ultrafast DSP
(digital signal processor)
with 400 million operations
per second

44. CLINICAL APPLICATIONS
• Measurement of corneal shape
• Measurement of corneal thickness
• Measurement of corneal power
• Corneal elevation maps
• Corneal curvature maps
• Keratoconus screening
• Preoperative screening before refractive surgery
• Corneal wavefront analysis
• Densitometry of the lens
• Improved IOL calculation in post LASIK patients
• Measurement of axial length

45. Measurement of corneal shape
• Topography includes making a map that describes elevation
and depression on the surface of cornea
• Topographic analysis of front and back surfaces is based on
the true elevation measurement from one side of the cornea
to the other (limbus to limbus)
• Pentacam provides significantly more accurate elevation
measurements than other devices

46. Measurement of corneal thickness
• Describe the true thickness of cornea across its entire breadth
and width
• Corneal thickness is calculated from the top of the epithelium
to the anterior surface of the endothelium, excluding the tear
film
• It is displayed as a colour image over its entire area from
limbus to limbus
• Importance parameters like thickness in the centre of pupil,
apical corneal thickness and the thinnest location are
provided
• The distance and position of the thinnest point relative to the
apex of cornea are also available which are useful for early
detection of keratoconus

47. Measurement of corneal power
• It is also used to measure corneal power
• This application is especially useful in patients following
excimer keratorefractive surgery, in which relationship
between anterior and posterior surfaces is altered yielding
inaccurate keratometry readings required for IOL calculation

48. Keratoconus screening
• Pentacam is the only technology which gives the direct
measurement of elevation data and hence detection of
keratoconus
• There is an inbuilt keratoconus screening software which also
helps in grading of keratoconus

49. Preoperative screening before
refractive surgery
• Pentacam has facility for preoperative screening before
refractive surgery to exclude ectasia and formefruste
• Diagnosting criteria for formefruste based on magnitude of
elevation maps put forth by Michel W. Belin is as follows:
 Normal values for anterior elevation are differences less than
+12µm
 Between +12 and +15 µm are suspicious
 Greater than +15µm indicates keratoconus
 Normal values for posterior elevation are approximately 5µm
higher than those for anterior elevation

50. ADVANTAGES OF PENTACM
• Higher resolution central cornea
• Measure surface irregularities – keratoconus
• Calculate pachymetry from limbus to limbus
• Wavefront analysis to detect higher order aberration

51. DISPLAY OF THE DATA
1. Numerical power plots
2. Keratometry view
3. Photokeratoscopic view
4. Profile view
5. Colour coded topographic maps

52. 1. NUMERICAL POWER PLOTS
• Corneal curvature of specific areas is shown in dioptric
values
• Data are displayed in 10 concentric circular zones with
1mm interval between each
• Numerical values are displayed in
colour, which are in agreement
with the colour scale being used
• It also shows the average dioptric
value of each of the 10 concentric
circular zones individually along
with the average overall corneal
curvature

53. 2. KERATOMETRIC VIEW
• It depicts the keratometric reading in two principle meridians
in three different zones simultaneously
• The three zones measured are central 3mm zone,
intermediate 3-5mm zone and peripheral 5-7mm zone
• It is an important map for assessing
the skewing of semimeridians
• The more the keratometric readings
in principle meridians deviate from
being perpendicular to each other,
the more irregular or nonorthogonal
corneal astigmatisn exist

54. 3. PHOTOKERATOSCOPIC VIEW
• It depicts the actual black and white photograph of the
placido ring captured by the video camera
• This view helps in confirming the proper patient fixation and
in identifying the eye captured
• The reflected rings on cornea
are situated more towards
the limbus on one side than
the other, and on the nasal
side of the distance
between the rings is
comparatively narrower

55. 4. PAR CTS profile view
• Shows the graphical plotting along the XY axis of the steepest
and flattest meridians of the cornea and the difference
between the two in dioptres
• The display button shows the astigmatic difference between
the flat and steep meridians
• A gray zone in this difference plot denoted the pupillary area
• In the symmetrical eye, tracing across this grey band is a
straight line
• In presence of astigmatism, an apparent slag is seen
• The more this slag increases, the more asymmetrical cornea
is.

56. 5. COLOUR CODED TOPOGRAPHIC MAP
• This are the most useful and commonly used display formation
• For interpretation following parameters should be considered :
 Colour codes
 Scale used
 Quantitative indices

57.  COLOUR CODES
• Hot colour = red and its various hues indicates steep portions
of cornea
• Cool colour – blue and its various hues indicates flat portion
• The colours Red, orange, yellow, green, purple & blue denotes
progressively refractive power

58.  SCALE USED
Absolute scale
• Each colour represents
1.50D interval between
35.00 & 50.00 D, whereas
above and below this range
colours represent 5.00D
interval
• Disadvantage – it doesn’t
show subtle changes of
curvature
Normalised scale
• Here, the cornea is divided
into 11 equal colours
• Advantage – shows more
detailed description of the
surface
• Disadvantage – colours of
two different maps cannot
be compared

59.  QUANTITATIVE INDICES
This include the following :
• Predicted VA based on cornea shaped
• Simulated k reading
• Mean k reading
• Surface regularity index
• Surface asymmetry index
• Point spread function

60. CORNEAL TOPOGRAPHIC PATTERNS IN
NORMAL CORNEAS
• 10 different topographic patterns are seen in normal eyes
which was described by Rabinowitz in 1996, depending upon
the corneal curvature
• These can be grouped as follows :-
1. Regular pattern
 Round (23%)
 Oval (21%)
 Steepening
- Superior steepening
- Inferior steepening

61. 2. Astigmatic pattern
 Symmetrical and orthogonal i.e. bow tie effect (18%)
- Symmetrical bow tie with nonskewed axis
- Symmetrical bow tie with skewed axis
 Asymmetrical and orthogonal (31%)
- Asymmetrical bow tie with superior steepening
- Asymmetrical bow tie with inferior steepening
- Asymmetrical bow tie with skewed radial axis
 Irregular – no pattern and no orthogonal (7%)

62. Corneal topographic pattern seen on colour coded maps in
absolute scale

63. FORMATS FOR DISPLAY OF DATA ON
COLOUR MAPS
1. Corneal power map (Sagittal or axial map)
2. Tangential map
3. Elevation map
4. Refractive power map
5. Irregularity map
6. Trend and time display
7. Difference display map
8. Rt/Lt eye compare map

64. 1. SAGITTAL OR AXIAL MAP
• The corneal power map is a 24 colour representation of
dioptric power at various points on the cornea
• The radius of curvature is measured 360 times for each
placido ring image from
centre to vertex
• The sagittal algorithm
averages the data points
from first to the next ring
and so on

65. 2. TANGENTIAL MAP
• Tangent are projected outwards from the centre vertex 360
degree
• Ring curvature is measured along the tangent, this is also
known as instantaneous curvature map
• This is the best indicator of corneal shape but is a poor
indicator of corneal power. Therefore tangential reading must
never be used for calculating K values
• This map is very useful tool for the diagnosis of corneal ectatic
condition like keratoconus

67. 3. ELEVATION MAP
• Elevation of a point on the corneal surface displays the height
of the point on the corneal surface
• The elevation maps helps in distinguishing localized elevations
(steep because of projection) from otherwise steep corneal
area
• The interpretetion – Red is raised (steep),and blue is below
(flat)
• The hotter colours shows area that are elevated above the
reference sphere and cooler colours represent the areas that
are depressed under the reference sphere

69. 4. REFRACTIVE POWER MAP
• It illustrates how the corneal curvature refracts light in true
dioptres of power and not curvature
• It uses ray tracing and snellen law of optics to perform
calculations
• In it the spherical cornea has cooler colours the centre with
increasing hotter colours extending out to the periphery
• Thus, it is also called as asphericity map of the cornea
• Useful for determining the optical zone for RGP lenses and
also in performing refractive corneal surgery

71. 5.IRREGULARITY MAP
• Shows area on cornea that are hot in colours
• It displays the distortion of the cornea using previous
elevation maps
• Hotter colours represent
the higher values of
distortion measured in
units of wavefront errors
• If there is a significant
hot colours within the
pupil zone, the VA will be
compromised

72. 6. TREND AND TIME DISPLAY
• Here changes occuring in topography in time can be displayed
in chronological order

73. 7. DIFFERENCE DISPLAY MAP
• It exhibits the comparative difference in two given
topographic maps

74. 8. RIGHT EYE/LEFT EYE COMPARE MAP
• It allows comparison of both eyes simultaneously

77. THANK YOU