Corneal topography provides a non-invasive method to map the surface curvature of the cornea. Various techniques have been developed over time including keratometry, keratoscopy, rasterstereography, and interferometry. A normal topography map will show the cornea progressively flattening towards the periphery. Corneal topography is useful for diagnosing conditions like keratoconus by detecting irregular steepening, and for guiding contact lens or refractive surgery. Interpretation of topography maps involves analyzing features like color scales, axial maps, and topographic indices to evaluate the cornea's shape and detect any abnormalities.

1. Corneal Topography
Sana Azam
M Phil. (VS) 3rd semester
PICO HMC Peshawar

2. Contents
• Introduction
• Development of different systems for corneal measurements
• Different systems/techniques used for corneal topography
• Normal topographic map
• Indications for corneal topography
• Interpretations of corneal topography maps
• Corneal aberrometry
• Corneal topography in different clinical conditions

3. Introduction
• Corneal topography is a non-invasive imaging
technique used to map the surface curvature of
cornea

4. Overview of corneal dimensions
• Cornea comprises 2/3rd of optical power of eye
• Refractive index 1.376
• Vertical diameter is 10.6mm
• Horizontal diameter is 11.7mm

5. Overview of corneal dimensions (cont.)
 Radius of curvature
Anterior surface:7.7mm
Posterior surface:6.9mm

6. Overview of corneal dimensions (cont.)
 Zones of cornea
• 3-4 mm………. Apical zone
• 4-8 mm………. Para-central
• 8-11 mm……….. peripheral
• 11-12 mm………..limbal

7. Overview of corneal dimensions (cont.)
 Corneal thickness
• Central 0.52 - 0.57 mm
• Peripheral 0.66 - 0.76mm
• Limbus 1.2mm

8. Principle of Corneal topography
• Cornea acts as a convex mirror
• Image formed is inversely related to curvature of
mirror

9. Development of different systems for
corneal measurements
1. Scheiner’s method
2. Keratometer
3. Keratoscopy
4. Rasterstereography
5. Interferometry

10. Development of different systems for
corneal measurements
1. Scheiner in 1619 analyzed corneal curvature by
matching image of window frame reflected onto
subject’s cornea with the image produced by one of his
calibrated spheres

11. Development of different systems for
corneal measurements (cont.)
2. Keratometer described by Helmholtz in 1854
measures the radius of curvature of anterior corneal
surface from 4 reflected points approximately 3mm
apart

12. Development of different systems for
corneal measurements (cont.)
3. Keratoscopy involves evaluation of topographic
abnormalities of corneal surface by direct observation of
images of mires
 Advances in Keratoscopy are:
a. Placido disc Keratoscopy
b. Photokeratoscopy
c. Videokeratoscopy

13. Development of different systems for
corneal measurements (cont.)
3. Keratoscopy
a. Placido disc Keratoscopy was introduced by
Antonio Placido in 1880
• Disc consists of black and white mires with a central
hole
• Distortion in corneal shape will be observed as
deviation from evenly spaced concentric mires

15. Development of different systems for
corneal measurements (cont.)
3. Keratoscopy
b. Photokeratoscopy
• The technique was given by Gullstrand in 1896
• The system consists of a photographic film camera
attached to Keratoscope
• There are 9-15 rings covering 55-75% of corneal area
• Closer the rings, steeper will be the cornea

17. Development of different systems for
corneal measurements (cont.)
3. Keratoscopy
c. Videokeratoscopy consists of a television camera
attached to it
• Its mires cover about 95% of corneal area

19. Development of different systems for
corneal measurements (cont.)
4. Rasterstereography uses calibrated grid which is
projected on fluorescein stained tear-film

20. Development of different systems for
corneal measurements (cont.)
5. Interferometry 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 interpreted as contour map of
surface elevation

21. Techniques used for corneal
measurements
1. Specular reflection technique
• Placido disc system
• Interferometry based systems
2. Diffuse reflection technique
• Rasterstereography
3. Scattered light slit based system
• ORBSCAN II

22. Topography of normal cornea

23. Characteristics of corneal shape
 Corneal surface is not a perfect sphere and can be
studied in terms of;
• Eccentricity e
• Shape factor P
• Asphericity parameter Q

24. Characteristics of corneal shape
• Eccentricity is the degree of peripheral flattening of
cornea (e =0.41-0.58)
• Shape factor/ p-value mathematically defines
corneal asphericity
• p = 1 – e2
• e = √1-p
• e2 =1-p
 e2 is sometimes taken as asphericity parameter Q

25. Eccentricity

26. Characteristics of corneal shape
 Cornea progressively flattens out towards periphery
by 2-4 D
• Nasal area flattens more than temporal

27. Topographic patterns in normal
cornea
 Regular patterns
• Round 23%
• Oval 21%
• Steepening
o Superior
o Inferior
 These patterns are considered normal w.r.t to keratoconic cornea however
for their presence astigmatism is necessary to present

28. Topographic patterns in normal cornea
(cont.)
 Astigmatic patterns
• Symmetric bow-tie 18%
o Symmetric bow-tie with skewed axis
o Symmetric bow-tie without skewed axis
• Asymmetric bow-tie 32%
o Asymmetric bow-tie with superior steepening
o Asymmetric bow-tie with inferior steepening
o Asymmetric bow-tie with skewed radial axis
• Irregular astigmatism 7%

30. Indications for corneal topography
• To diagnose corneal diseases i.e keratoconus
• To guide CL fitting
• To evaluate the effect of keratorefractive procedures
• To guide the removal of tight sutures after corneal
surgery
• To determine keratometry values in order to calculate
required IOL power

31. Corneal topography examination &
interpretations

32. Pre-requisites for good topographic
examination
• Patient should be seated facing the bowl and
adjustments should be done for proper alignment
• Exactly center and focus the mires on cornea
• Observe if there are tear film irregularities
• Observe if there are any artifacts induced by nose or
eyelids

33. Interpretations of corneal topography
maps
• Interpretations of maps are based on analysis of:
1. Raw Photokeratoscope image
2. Color coded scales
3. Topographic displays
4. Basic topographic index

34. Interpretations of corneal topography maps
1. Raw photokeratoscope image
• This image is based on unprocessed data and verifies
the reliability of resulting topographic map

35. Interpretations of corneal topography maps
2. Color-coded Scales
• Hot colors = steepness
• Cool colors = flatness

36. 2. Color-coded Scales (cont.)
Absolute Scale Normalized Scale
Each color represents 1.5D
interval between 35-50D
and above this range colors
represent 5D intervals
In this scale cornea is
divided into 11 colors
spanning the eye's total
dioptric power
Doesn't show subtle
changes of curvature
Identifies minute
topographic changes
Easier to read Can magnify subtle
changes

38. Interpretations of corneal topography maps
3. Topographic displays
• Axial maps
• Instantaneous map
• Refractive map
• Elevation map
• Difference map
• Relative map
• Irregularity map

39. 3.Topographic displays
3.1 Axial map
• Axial or corneal power map is a 24 color
representation of dioptric power of cornea at various
points on cornea
• Radius of curvature is measured 360 times for each
Placido ring image from center to vertex
• Sagittal algorithm averages the data points from first
to next ring and so on

41. 3. Topographic displays
3.2 Tangential Map
• In this scale ring curvature is measured along the
tangents which are projected from center vertex 360
degrees
• This map is best indicator of corneal shape but poor
indicator of corneal power

43. 3. Topographic displays
3.3 Elevation Map
• Elevation of a point on corneal surface displays the
height of point on corneal surface relative to spherical
reference surface
• Distinguishes localized elevation from otherwise
steep cornea
• Helpful in determining ablation depth after refractive
surgeries

44. ss

45. 3. Topographic displays
3.4 Refractive Map / Asphericity Map
• Displays refractive power of cornea
• Relates corneal shape to vision by considering the
effects of spherical aberrations
• Spherical cornea has cooler colors in center with
increasing hotter colors towards periphery

46. 3. Topographic displays-Refractive Map
• Useful in understanding the effects of refractive
surgery and determining the optical zone for RGP
lenses

48. 3. Topographic displays
3.5 Irregularity Maps
• Same as elevation map but reference surface is a
toric surface
• Difference between corneal surface and reference
toric surface represents the part of cornea that cannot
be optically corrected

49. Topographic displays – Irregularity Maps
• Hotter colors represent higher value of distortion in
units of wave-front error which can be translated into
diopters of distorted power

50. 3. Topographic displays
3.6 Difference Map
• Displays the changes in certain values between two
maps
• Use to monitor any type of change i.e. recovery from
contact lens-induced warpage

52. 3. Topographic displays
3.7 Relative Map
• Displays some values by comparing them to an
arbitrary standard i.e. sphere, asphere, normal cornea
3.8 OD/OS comparison map
• It allows simultaneous comparison between both eyes

54. 4- Basic topographic indices
 Basic topographic indexes include:
• Sim-K reading
• Min-K reading
• Corneal eccentricity index
• Average corneal Power
• Surface regularity index
• Surface asymmetry index

55. 4. Basic topographic indices
4.1 Sim-K reading
• It provides the power and axis of steepest and flattest
corneal curvatures
• Common uses are contact-lens fitting, refractive
surgery, assessing irregular corneal shape

56. 4.Basic topographic indices
4.2 Min-K reading
• Minimum Meridional power from rings 7, 8, and 9
• The average power and axis are displayed

57. 4.Basic topographic indices
4.3 Corneal eccentricity index
• Estimates eccentricity of central cornea and is
calculated by fitting an ellipse to corneal elevation
data
• Positive value is for prolate surface
• Negative value is for oblate surface
• Zero value is for perfect sphere
 This value is used for contact lens fitting

58. 4. Basic topographic indices
4.4 Average corneal power
• This is the area-corrected average corneal power in
front of pupil
• Helpful in determining central corneal curvature
when calculating appropriate intraocular lens power

59. 4. Basic topographic indices
4.5 Surface regularity index (SRI)
• It measures the regularity of corneal surface that
correlates with best spectacle corrected visual acuity
assuming the cornea to be only limiting factor
• SRI index increases with increase in irregularity in
corneal surface
• It measures optical quality

60. 4. Basic topographic indices
4.6 Surface asymmetry index
• The index of asphericity indicates how much the
curvature changes upon movement from center to
periphery of cornea
• When cornea becomes less symmetric, the index
differs more from zero

61. 5. Corneal aberrometry & wave-front
maps
• Corneal aberrometry is measure of aberrations that
occur during refraction through cornea
• Wavefront aberration is deviation of resulting
wavefront from ideal wavefront
• Greater the difference between resulting and ideal
wavefront, more worse will be the image quality

62. 5. Corneal aberrometry & wave-front maps
• Zernike polynomials are used to define and quantify
monochromatic aberrations
• Zernike terms are defined using double index
notation
a. Radial order (n)
b. Angular frequency (m)

63. Corneal aberrometry terminologies (cont.)
 Low order aberrations
• 1st order is not related to wavefront & is corrected by prisms
• 2nd order = spherical / astigmatism defocus = corrected by
spectacles , CL
 High order aberrations (not corrected by spectacles/CL)
• 3rd order = coma/ trefoil defects
• 4th order = spherical aberrations

64. 5. Corneal aberrometry & wave-front maps
(cont.)
• Wavefront maps measure the pathway difference
between measured wavefront and reference
wavefront in microns at pupil entrance
• Each color represents a specific degree of wavefront
error in microns

65. 5. Wavefront Maps (cont.)
• In order to evaluate the impact of aberrations on
vision quality following quantitative parameters are
defined;
i. Peal to valley error (PV error)
ii. Root mean square error (RMS)
iii. Strehl ratio
iv. Point spread function (PSF)
v. Modulation transfer, phase transfer, optical transfer function

66. 5. Wavefront Maps (cont.)
5.1 PV error is measure of distance from lowest to
highest point in wavefront
• It doesn't represent the extent of defect so is not a best
measure of optical quality

67. PV error

68. 5. Wavefront Maps (cont.)
5.2 RMS Error is statistical measure of deviation of
corneal wavefront from the ideal wavefront
• It describes the overall aberration and indicates how
bad aberrations are

69. 5. Wavefront Maps (cont.)
5.3 PSF measures how well a point object is imaged on
output plane (retina) through optical system
• In small pupils (1mm) PSF is affected by diffraction
• In large pupil aberrations are source of image
degradation

70. 5. Wavefront Maps (cont.)
5.4 Strehl ratio represents the ratio of maximum
intensity of actual image to maximum intensity of fully
diffracted image both being normalized to same
integrated flux

71. 5. Wavefront Maps (cont.)
5.5 Modulation transfer, phase transfer, optical
transfer function defines the ability of optical system
to affect the image of grating by reducing the contrast
and is known as modulation transfer function (MTF)
or
• By shifting the image sideways called phase transfer
function (PTF)
 Optical transfer function is made up of MTF and
PTF

72. 6. Corneal topography in different
clinical conditions
6.01 Keratoconus
• Keratoconus appears as an area of increased corneal
power surrounded by concentric area of decreasing
power
• Inferior-superior power asymmetry
• Skewed radial axis

74. 6. Corneal topography in different
clinical conditions
6.02 Pellucid Marginal Degeneration (PMD)
• Inferior corneal thinning between 4 & 8 o’ clock
position
• Flattening of vertical meridian
• Against the-rule astigmatism

76. 6. Corneal topography in different
clinical conditions
6.03 Keratoglobus is a bilateral disorder in which
entire cornea is thinned out most markedly near limbus
 Corneal topography shows;
• Peripheral steepening
• Asymmetrical bow-tie configuration

77. s

78. 6. Corneal topography in different
clinical conditions
6.04 Terrien’s Marginal Degeneration
• Flattening over the area of peripheral thinning
• Relative steepening of corneal surface 90 degrees
away from midpoint of thinned area
• Against the-rule or oblique astigmatism

80. 6. Corneal topography in different
clinical conditions
6.05 Pterygium
• Flattening of cornea at axis of lesion
• Marked WTR astigmatism even more than 4D

82. 6. Corneal topography in different
clinical conditions
6.06 Post-radial keratotomy (post-RK)
• RK corrects myopia by placing series of radial
incisions leaving a clear optical zone
• Typical topographic pattern is polygonal shape
• Incisions cause flattening of central cornea
surrounded by bulging ring of steepening

84. 6. Corneal topography in different
clinical conditions
6.07 Post-astigmatic Keratotomy
• Incisions are placed on steepest meridian to cause
flattening of that particular meridian
• 90 degree apart meridian becomes steeper

86. 6. Corneal topography in different
clinical conditions
6.08(a) Post-photorefractive keratotomy in Myopia
• Laser beam flattens central cornea to correct myopia
thus giving the cornea an oblate shape

88. 6. Corneal topography in different
clinical conditions
6.08(b) Post –PRK in Hypermetropia
• Laser beam flattens mid-peripheral cornea in
hyperopia giving it a prolate profile
6.08(c) Post-PRK in Astigmatism
• The zone on which laser beam is directed will appear
oval

89. sss

90. 6. Corneal topography in different
clinical conditions
6.09 Post LASIK
• In LASIK laser beam ablates the tissue under
superficial corneal flap
• Topographic pattern for myopia is oblate
• Topographic pattern for hyperopia is prolate

91. 6. Corneal topography in different
clinical conditions
6.10 Post-laser thermal keratoplasty
• Holmium laser heats the corneal stromal collagen in a
ring around the outside of pupil causing localized
flattening
• The surrounding zone will become steeper
• Typical topography pattern is central corneal
steepening and ring of flattening around it

92. 6. Corneal topography in different
clinical conditions
6.11 Post INTACS
• INTACS are inserted into periphery of cornea to
correct small degrees of myopia or hyperopia by
changing orientation of collagen lamellae

94. 6. Corneal topography in different
clinical conditions
6.12 Post-keratoplasty
• Sutures usually induce a central bulge in corneal graft
• Prolate configuration is common

95. 6. Corneal topography in different
clinical conditions
6.13 CL induced corneal warpage
• Corneal warpage is topographic change in cornea
caused by mechanical pressure exerted by CL use
 Common topographic patterns are:
• Peripheral steepening
• Central flattening
• Furrow depression
• Central molding/irregularity
• Pseudokeratoconus