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1. Measurement of corneal
topography:
Videokeratography
Jeewanand Bist

2. References
 Borish’s Clinical Refraction, Benjamin
Franklin.
 A Guide to Videokeratography, Clinical
article; ICLC, vol.23, Nov/Dec, 1996.
 Corneal Disorders; Clinical Management
& Diagnosis; 2nd edition; Howard M.
Leibowitz & Georgo O. WaringIII.
 Internet

3. Videokeratoscopy
 Considerable qualitative data
can be derived from inspection
of the keratoscopic images
when distortions are large.
 For large distortions
quantitative analysis of
keratoscopic images is
necessary.
 A videokeratograph results from
a videokeratoscope working in
conjunction with a computer.

4. History
 In 1980s keratorefractive surgery
provided impetus for development of
better methods for clinicians for
evaluation of corneal topography.
 1987- color topographic maps have
become the standard method for
displaying the output of computerized
videokeratoscopes.

5. History contd…
 First computerized videokeratoscope –
the corneal Modelling System (CMS) by
Computed Anatomy (Gormley et.al
1988).
 EyeSys System 2000 (EyeSys
Technologies) added side cameras that
simultaneously view the cornea in
profile.

6. Principle
 It is based on the concept of
photokeratoscopy. (difference???)
 Video camera substituted for the
photographic camera.
 It has more advantage than
photokeratoscope.
 Videokeratography provides reasonable
accuracy & repeatability in measuring corneal
topography.

7. Principle
 Principle same as keratometer.
 A luminous object (target of rings) is
placed in front of patient’s cornea, and the
image size produced in the corneal
reflection is measured.
 It assumes cornea as the convex mirror &
makes use of the first purkinje image.

8. What is the use when it is
based on the same principle of
keratometer????
 Keratometer measurement is restricted to a
small central corneal area (3-3.5mm).
 It measures corneal curvature at two
positions in each principal meridians. (4
paracentral points).
 Modern viedeokeratoscopes evaluate several
thousands of points from nearly the entire
corneal surface.
 They measure the entire corneal contour.

9. Keratometric measurement of
corneal topography

10. Procedure
 Aligning the subject’s eye in front of the instrument
so that it is centered with respect to instrument.
 Focusing for the corneal image of the target rings.
 Freezing the corneal image by the video camera.
 Image is captured & displayed on a computer screen.
 If the image is unacceptable, it may be discarded
procedure repeated.
 Small areas may be missed.
 In dry eyes lubricating drops should be instilled.

11. Instrument design
 The Tomey
Topographic
Modelling System
(TMS)
 Small target.
 Short working
distance.

12. Instrument design
 The EyeSys 2000
Corneal Analysis
System
 Large target.
 Larger working
distance.

13. Keratograph Algorithms
 Process of building a topographic map of
cornea from keratoscopic data goes through
following general steps:
 Capture video images of the keratoscope rings.
 Measure angular size of points on the rings.
 Reconstruct the corneal surface point by point.
 Assign dioptric or other descriptors for each
surface.
 Present surface descriptors in a color topographic
map.

14. Display Options
 Simulated keratometry
 This display gives values that are meant to
be equivalent of a keratometer reading for
each of the two principal meridians.
 It is produced simply by taking the radius
value from the target ring that corresponds
to the corneal positions where the
reflection takes place from the keratometer
mires.

15. Display Options
 Profile plot
 Shows a plot of radius of curvature (or
power) values with respect to distance
from the center of the corneal map in each
of the two principal meridian.

16. Display Options
 The Corneal Map
 Most common & most
important display.
 It allows the clinician to
visualize the overall
characteristics of the
corneal contour & to
detect various corneal
anomalies.

17. Mapping of Cornea
 Surface Elevation Maps
 Because surface shape is the primary determinant
of corneal optics (Applegat 1994: Applegate &
Howland 1995), a logical way to map the cornea is
to show the relative surface elevation of each
point from a reference surface.
 Surface elevation are mapped relative to a
reference sphere, ellipsoid or other surface that
approximates the corneal shape. (Salmon &
Horner 1995)
 Elevations measured from a plane are nearly
useless as minute elevations are lost.

18. Surface Elevation maps

19.  Dioptric Corneal Maps
 Corneal topography is expressed in terms
of local dioptric values rather than surface
elevations.
 The dioptric maps “speak the language” of
keratometry with which clinicians are
already familiar. (Roberts, 1994a)

20.  Axial Curvatural Maps
 Axial radius also known as the sagittal radius is
the distance along a normal from the point on the
cornea to the optic axis of videokeratograph when
it is aligned with the cornea.
 It is the radius that is measured in keratometry &
was the first radius used in videokeratography.
 It assumes corneal surface as a spherical surface
& this assumption is acceptable for keratometry.
 Introduces major error in videokeratography.

21.  Instantaneous Curvatural Maps
 Instantaneous radius is independent of any axis &
is based on only the local curvature at each
corneal point.
 It is also known as the ‘tangential’, ‘local’, or ‘true’
radius.
 With peripheral corneal flattening, the
instantaneous radius will always be longer than
the axial radius for each peripheral corneal point.

22. Axial Vs Instantaneous
curvatural maps

23. Comparison of different
curvatural maps
 Surface elevation maps show fine
details of corneal surface.
 Particularly useful in the pre-operative &
post-operative management of refractive
surgery patients.
 Monitoring surface anamolies such as
keratoconus.
 Custom-contact lens design.

24. Comparison of different
Curvatural maps…
 Dioptric maps are most familiar & effectively
display changes in corneal contour.
 Useful in monitoring surface shape changes as seen in
keratoconus or in contact lens induced distortion.
 Instantaneous curvature map is more sensitive to
subtle changes than axial curvature maps but is
also more subject to noisy data.
 Axial curvatural maps are used to verify aspheric
contact lens base curve.

25. Interpretation of corneal maps
 Each color corresponds to certain
dioptric power range.
 Cold colors (black, blue, azure)
 Represent flatter surfaces with less
dioptric value.
 Warm colors (orange, red, white)
 Represent steeper surfaces with
greater dioptric value.
 Color belonging to central part of
visible spectrum (green, yellow)
represent surfaces with normal
values.

26. Uses of videokeratography
 To know corneal topography in different
corneal degenerations & dystrophies.
 Early detection of ectatic conditions of cornea
like keratoconus, Pellucid Marginal
Degeneration etc.
 In cases of trauma.
 Evaluating post-op evaluation of refractive
surgeries & penetrating keratoplasty.

27. Topography of Normal Cornea
 Bogan & co-workers
 Round
 Oval pattern represent
corneas with very low
astigmatism.
 Bow- tie patterns indicate
astigmatism

28. Corneal topography in
astigmatism
 Difference in curvature
of two principal corneal
meridians represented
as bow-tie pattern.
 Bow-tie is oriented
along the steeper
meridian.

29. Corneal topography in
astigmatism

30. Corneal topography in
keratoconus
 Keratoconus is a condition
which is characterized by a
non-inflammatory thinning
and steepening of the central
and/or para-central cornea.
 The condition usually
results in a moderate to marked
decrease in visual acuity
secondary irregular astigmatism
and corneal scarring.

31. Bilateral keratoconus

32.  Central & para-central steepening (???)
 Areas beyond central & paracentral area
affects the corneal topography
significantly.

33. Early keratoconus
 Pear shaped infero-
temporal paracentral
steepening.
 Progresses nasally.
 Superior cornea
remains relatively
intact.

34. Early keratoconus
In early keratoconus, there is a characteristic steepening of
the inferior cornea with a subsequent flattening of the

35.  Rotational steepening
occurs at and above the
midline.
 Includes the temporal,
superior-temporal, and
superior cornea.
 The superior-nasal
quadrant of the cornea is
always the last to be
affected.

36.  Modern topographic techniques have
demonstrated that in early keratoconus
there is a characteristic steepening
initially occurring mid-peripherally below
the corneal midline

37. Topographical shapes of
advanced keratoconus
 Nipple
 Oval
 Globus

38. Nipple-Shaped Topography
 The nipple form of
keratoconus
characteristically
consists of a small,
near central ectasia,
less than 5.0 mm in
cord diameter

39. Nipple-shaped keratoconus may also manifest as a small
central ectasia with moderate to high with-the-rule corneal

40. Oval shaped topography
 The most common
corneal shape noted in
advanced keratoconus is
oval topography.
 In oval-form
keratoconus, the corneal
apex is displaced well
below the midline
resulting in varying
degrees of inferior mid-
peripheral steepening.

42. Globus-shaped topography
 The globus form of keratoconus affects
the largest area of the cornea, often
encompassing nearly three quarters of
the corneal surface.

43. Why to talk so much on
keratoconus????
 Keratoconus accounts
for about 15% of all
corneal transplants.
 Early detection is
crucial.
 Progression can be
checked by contact
lenses.
Intra-palpebral, three-point
touch fitting technique for
early keratoconus

44. Pellucid Marginal Degeneration
 Pellucid Marginal
Degeneration (PMD) is
a bilateral corneal
disorder hallmarked by
a thinning of the inferior
peripheral cornea.
 The corneal thinning
begins approximately
1.0 to 2.0 mm above
the inferior limbus.

45. Corneal topography in Pellucid
Marginal Degeneration

46. Corneal topography in Pellucid
Marginal Degeneration
 High against the rule
astigmatism
 Inferior mid-
peripheral
steepening at 4 & 8
o’clock position.
 Kissing pigeon
pattern (diagnostic
of PMD)

47. Typical kissing pigeon
appearance

48. Topography in Terrien’s
Marginal Degeneration
 TMD usually involves the
superior periphery.
 Flattening in the involved
meridian & steepening along
90* away from the ectasia.
 If disease confined to small
corneal arc topography
simulates PMD.
 If involves larger corneal arc,
it simulates keratoconus.

49. Corneal topography in
pterygium
 Pterygium is a triangular
sheet of fibrovascular
tissue which invades
cornea.
 Invades cornea from
nasal or temporal sides.
 Typical with-the-rule
astigmatism is induced.
 Bow-tie pattern oriented
vertically.

50. Topography in Traumatic cases
 Corneal topography in
cases of trauma depends
upon
 Location
 Severity(extent & depth)
 Type of trauma
 Flattening along the
meridian of laceration &
steepening along 90* away.

51. Limitations of
videokeratoscopy
 Measures the contour of peripheral
cornea less accurately than that of the
central.
 Inability to directly measure the optical
performance of complex surface
patterns generated by penetrating
keratoplasty or refractive surgical
procedures.

52. Future Developments
 Rasterstereography
 Tear film is dyed with fluorescein.
 Projects a grid of horizontal & vertical lines onto
the corneal surface & visualizes the image of
transparent cornea.
 Image is captured by video camera & processed
by a computer.
 Analysis of the distance & position of the
projected mires provide data of on the height of
surface at various points rather than on the
curvature.
 Independent of superficial defects or irregularities.

53.  Laser interferometry
 Holography

54. Thanks !!!