2. • Cornea is the most powerful refractive
element of the eye contributing 43D(70%) of
refractive power of eye.
• Since the shape of corneal surface determines
its refractive power , even a minor
modification of its surface can lead to
significant alteration of image formed on
retina.
3.
4. • Prolate shape
• Anterior surface of cornea: Elliptical
• Horizontal diameter: 11.5mm
• Vertical diameter:10.6mm
• Posterior surface of cornea is circular with average
diameter of 11.5mm
• Thickness of cornea:
• 0.52mm at centre
• 0.8mm at periphery
• 1mm at limbus
• Anterior radius of curvature:7.8mm
• Posterior radius of curvature:6.5mm
5.
6.
7. • CENTRAL ZONE:
Approximately 4mm diameter.
Also called the apical zone
• PARACENTRAL ZONE:
4-8mm diameter. Flatter than the central zone.
• Central + Paracentral zone= optical zone.
• PERIPHERAL ZONE:
8-11 mm.
It is the zone where the normal cornea flattens the most and
becomes ashperic.
• LIMBAL ZONE
8.
9. CORNEAL TOPOGRAPHY
• Study of shape 0f corneal surface.
• Configuration or description of surface
• Aspheric – elliptical shape.
11. PENTACAM
• The Pentacam(Oculus Inc) obtains images of
anterior segment by rotating Schiempflug
camera which is a digital charged coupling
device.
12.
13.
14.
15.
16. KERATOGRAPH ALGORITHMS
Process of building a topographic map of cornea from
keratoscopic data .
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
21. QUANTITATIVE INDICES
Simulated keratometry (Sim K )
Surface Regularity Index (SRI)
Surface Asymmetric Index (SAI)
Irregular astigmatic index (IAI )
Differential sector index(DSI)
Opposite sector index(OSI)
Centre/surround index (CSI )
Analyzed area (AA)
22. SIMULATED KERATOMETRY(Sim K)
• Measure differences in corneal power &
compares asymmetry.
• Provides the power and location of the
steepest and flattest meridians
• Equivalent to Conventional
Keratometry Reading
23. • Surface Regularity Index (SRI)
-Local fluctuation in central corneal power
-Local surface irregularities
• Surface Asymmetric Index (SAI)
-Measures differences in corneal power between
corresponding points at each ring & compares
symmetry.
- Used to monitor changes caused by contact Lens
warpage or keratoplasty
24. Formats for display of datas on colour
maps
1. Corneal power map( saggital&axial)
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
25. The ANTERIOR SAGITTAL map
• Steep areas are displayed in hot colours (red and
orange), while flat areas are displayed in cold colours
(green and blue).
On the other hand, red segments are displayed on
steep areas, while blue segments are displayed on flat
areas.
The cross point of this segmentation represents apex
(anatomical center) of the cornea.
Beside the shape of the map, parameters should be
studied particularly on the steep axis at the 5-mm
central circle. The normal pattern is the symmetric
bowtie (SB) .
26. The ANTERIOR SAGITTAL map
• a. In WTR astigmatism, the SB is on or within
―15° of the vertical meridian of the cornea .
• b. In ATR astigmatism, the SB is on or within
―15° of the Horizontal meridian of the cornea
• c. In oblique astigmatism, the SB is neither
vertical nor horizontal .
• The SB pattern can be encountered in KC
when K readings are abnormally high .
27.
28. • Corneal power map 24 colour representation
of diopteric power at various points on the
cornea
29.
30. Tangential map
• Better geographical representation of cornea
• Best indicator of corneal shape than power
31.
32. Refractive power map
• It illustrates how the corneal curvature
refracts light in true dioptres of power.
• Useful for determining the optical zone for
rigid gas permeable lenses and in performing
refractive corneal surgery.
33.
34. The Elevation Maps
• An elevation map describes the height details of the
measured corneal surface by matching it with a
reference surface (RS).
Points above the RS are considered elevations and
expressed in plus values, and those below the RS are
considered depressions and expressed in minus values
In corneal astigmatism, one meridian is steeper than
the other and is located under the RS taking minus
values, contrary to the flatter meridian which takes
plus values .
35.
36. Irregularity map
• It shows areas in cornea that are hot in
colour.
• It displays the distorsion of cornea using
previous elevation map results with toric
reference instead of sphere reference.
37.
38. Trend and time display
• In this changes ocurring in topography with
time can be displayed in chronological order
42. KERATOCONNUS
• Keratoconus is a noninflammatory pathologic condition
characterized by progressive thinning and protrusion of the
cornea.
• The thinning process occurs in one particular area so that
the surrounding area remains disproportionately thicker.
•
• Physiologically, the normal cornea is thinner in its center
and thicker in the periphery.
• The gradual increase of the corneal thickness from the
center toward the periphery in healthy eyes falls within a
normal range and that this characteristic could lead to a
criterion for identifying pathology such as ectasia.
43.
44.
45. 4MAP ELEVATION AND
KERATOCONUS
• Use best-fit sphere and float and a scale that is ±75μm
• we suggest front elevation first and Look at back
elevation and pachymetry next and at curvature last
• Normal values for front elevation are less than +12μm.
• Differences greater than +15μm typically indicate
keratoconus
• Between +12 and +15μm are suspicious.
• Normal values for posterior elevation are
approximately 5μm higher than those for front
elevation,
46.
47. Pellucid Marginal Degeneration
• Hallmarked by a
thinning of the inferior
peripheral cornea
• The corneal thinning
begins approximately
1.0 to 2.0 mm above
the inferior limbus.
48. • High against the rule
astigmatism.
• Inferior mid-peripheral
steepening at 4 & 8
o’clock position.
• Kissing pigeon pattern
(diagnostic of PMD)
49.
50. CORNEAL TOPOGRPHY IN PTERIGYUM
• Typical with-the-rule
astigmatism is induced.
• Bow-tie pattern oriented
vertically.
51. TOPOGRAPHY IN TRAUMATIC CASES
Depends upon
• Location
• Severity(extent & depth)
• Type of trauma
Flattening along the meridian of
laceration & steepening along 90*
away
52. CLINICAL USES
Photorefractive Surgery
• Preoperative screening
• Surgical planning
• Assessment of surgical outcomes
• Detection & management of complications
• Refinement & development of surgical outcome
Expected shape of cornea after refractive
surgery
55. Determination Of
• Refraction esp. for poor quality retinoscopic
reflex.
• Curvature of cornea
• Amount and direction of corneal astigmatism
• Quality of corneal refracting surface
• Stability of corneal refracting surface.
56. • In cases of trauma
• Progressive Myopia
• Aphakia or high ametropia
• To calculate the power of Intraocular Lens
58. LIMITATIONS
1) Algorithms for power calculations are based
on spherical optical system , while the normal
cornea is aspheric.
2) The formulae employed for power calculation
are centered on the corneal apex and not on
the more relevant line of sight.
3) The keratometric index of refraction
(1.3375)employed underestimates the changes
in corneal power after procedures like PRK as
the actual refractive index of cornea is 1.376
