6. SHAPE OF THE NORMAL CORNEA
Cornea is not a perfect sphere ,assumed to have a conic section.
X 2 + Y 2 + (1 + Q)Z 2 – 2RZ = 0
Z axis - axis of revolution of the conic
R is the radius at the corneal apex
Q -asphericity, used to specify type of conicoid.
8. Anterior corneal configuration tends to be prolate, i.e., the cornea progressively
flattens out towards periphery by 2–4 diopters .
The asphericity of the normal cornea, depending on different studies, ranges
from –0.26 to –0.11.
This tendency to flatten towards periphery can be detected in the topographic map.
Toward the periphery, dioptric power appears to decline, and the nasal area flattens more
than the temporal area .
The topographic patterns of the two corneas of the same individual often show
mirror-image symmetry.
SHAPE OF THE NORMAL CORNEA
13. Corneal Topographic Patterns:
• Depending on corneal curvature
• Rabinowitz et al in 1996 described 10 different patterns:
• REGULAR PATTERNS :
– Round
– Oval
– Steepening : Superior or Inferior
• ASTIGMATIC PATTERNS:
– Symmetrical & Orthogonal : (Bow-Tie Effect)
• With or without skewed axis
– Asymmetrical & Orthogonal:
• With superior steepening
• With inferior steepening
• Bow-tie with skewed radial axis
– Irregular : no pattern and non-orthogonal
15. FUNDAMENTALS AND TECHNOLOGICAL APPROACHES TO CORNEAL TOPOGRAPHY
PLACIDO DISC SYSTEM-
Based on specular reflection technoque
16.
17. Placido Disc: the Original Corneal
Topographer
Placido Disc: observer
views the pattern of
concentric white rings
(mires) reflected from the
patient’s cornea
through a central +2 D
lens.
Very “qualitative”
18. Images formed by Placido Disc
• Based on the overlay of concentric mires on the cornea.
– The closer the mires, the steeper the axis.
– The wider the rings, the flatter the axis.
21. Limitations of Placido Disc System
• It misses data on the central cornea
• It is only able to acquire limited data points
• It measures slope not height
• It is difficult to focus and align
• In most topographers, the patient is exposed to
high light
22. • Computerized VideoKeratoscopy
• Capturing the keratoscopic details onto a
video and displaying data analysed with
mutiple algorithms
• Measures larger area with more points
• Produce permanent reproducible records
• One of the most important developments in
diagnostic instrumentation
23. The Real Need –
Analysing each &
every point over
cornea
Video -keratoscopy
25. Key Points
•Avoid all eye drops, particularly local
anaesthetics as they decrease TBUT
• Explain the patient & make comfortable
• Ask patient to blink normally
• Other contact procedures on cornea
(tonometry, A-scan) should be done
after topography
27. READING OF TOPOGRAPHICAL DATA
• Check the name of the patient, date of exam and examined eye.
• Type of measurement (height in microns, curvature in mm, power in D)
• Check the scale & step interval
• Study the map (type of map, form of abnormalities)
• Evaluate statistical information
• Compare with topography of the other eye
• Compare with the previous maps
28.
29. HOW TO INTERPRET A CORNEAL TOPOGRAPHY MAP
Photokeratoscope raw image
32. • Most widely used
• Most useful
• Quick interpretation possible
• User-friendly
COLOUR-CODED TOPOGRAPHIC MAPS
Louisiana State University Color-Coded Map
1987 by Stephen Klyce
33. Interpretation of a colour map:
1. Colour Codes:
– Hot colours: red-orange
– steep portions
– Cool colours: blue-purple
– flat portions
1. The Scale used:
– Absolute Scale: routine practice / screening
• 35-50D : each color = 1.5D interval
• <35D or >50D : each color > 5D interval
– Normalized Scale: more minute details
• 11 equal colours spanning ‘that’ eyes’ dioptric power
34. Normalized scale (variable scale)
Uses a given colour for different curvatures or elevations on each cornea analyzed, depending
on the range for that particular cornea, determined by its flattest and steepest values.
Difficult to interpret and can lead to an incorrect diagnosis
Magnify subtle changes in corneal surface if the scale is too narrow, or minimize large
distortions if the scale is too wide.
Color recognition is lost with a variable scale, since it uses different colors for different eyes.
35. ii. Absolute scale (fixed scale)
uses the same color for the same curvature or elevation no matter which eye is examined.
However, there are many different absolute scales since the examiner can choose different
variables such as range or step size (intervals in color changes)
36. Topographic Displays: Corneal Maps
Axial Map (Sagittal Map)
Most commonly used map, good
approximation for the paracentral cornea
Measures the radius of curvature for a
comparable sphere (with the same tangent
as the point in question) with a center of
rotation on the axis of the
videokeratoscope.
Localized changes in curvature and
peripheral data are poorly represented
37. Displays tangential radius of
curvature or tangential power, which
is calculated by referring to the
neighboring points and not to the
axis of the videokeratoscope .
Reflect local changes and peripheral
data better than axial maps.
Very useful in detecting local
irregularities, corneal ectactic
diseases, or surgically induced
changes.
Local Tangential Curvature Map
(Instantaneous Map)
39. Refractive Map
displays the refractive power of the cornea,
which is calculated based on Snell’s law of
refraction, assuming optical infinity .
Correlates corneal shape to vision, and is
useful in understanding the
effects of surgery.
42. Distortion of the Placido rings
because of tear film breakup
Topographic irregularities and
patches of the map without
analysis because of a tear film
with large instability
43. Loss of information of certain areas of the cornea due to eyelids not opened enough (A), and
due to nose (B)
46. CLINICAL USES OF CORNEAL TOPOGRAPHY
Keratoconus
1. An area of increased corneal power surrounded by concentric areas of decreasing power.
2. A inferior-superior power asymmetry.
3. A skewing of the steepest radial axes above and below the horizontal meridian.
47. PELLUCID MARGINAL DEGENERATION
Inferiorcorneal thinning between 4 and 8 o’clock positions above a narrow band of clear thinned
corneal stroma. The ectasia is extremely peripheral and it presents a crescent-shaped
morphology. This pattern has a classical “butterfly” appearance that results in a flattening of the
vertical meridian and a marked against-the-rule irregular astigmatism
48.
49. Keratoglobus
Rare bilateral disorder
Entire cornea is thinned out most markedly near the corneal limbus
Reliable topographic examinations show an arc of peripheral increase
in corneal power (steepening) and a very asymmetrical bow-tie
configuration
50.
51. Terrien´s marginal degeneration
Flattening over the areas of peripheral thinning.
When thinning is restricted to the superior and/or inferior areas of
the peripheral cornea, there is a relative steepening of the corneal
surface approximately 90 degrees away from the midpoint of the
thinned area.
High against-the-rule or oblique astigmatism is a common feature,
as this disorder involves more frequently the superior and/or
inferior peripheral cornea.
If the area of thinning is small or if the disorder extends around the
entire circumference of the cornea, central cornea may remain
relatively spared with a spherical configuration
52. Patient with Terrien marginal degeneration displaying superior
stromal thinning with intact epithelium, overlying pannus, and lipid
at the leading edge.
53.
54. Pterygium
Triangular encroachment of the conjunctiva onto the cornea usually near the medial canthus
When the lesion continues to grow out onto the cornea, it could lead to a high degree of
astigmatism.
When the growth of pterygium is about 2 mm or more, a flattening of the cornea at the axis
of the lesion occurs . This produces a marked with the-rule astigmatism, even of more than 4 D.
The evolution of the pathology and the surgical outcome could be monitored by changes in
corneal topography.
57. Topographic pattern after penetrating
keratoplasty
Superior corneal steepening caused
by a tight suture
58. Topographic patterns of LASIK
decenterd ablations
after myopic treatment
after hyperopic treatment
59. Topographic analysis in a post-LASIK
cornea with an epithelial in-growth at
the inferonasal area:
Placido rings image
axial map
60. CONTACT LENS INDUCED CORNEAL WARPAGE
Characterised by topograhic changes in cornea following contact
lens wear as a result of mechanical pressure exerted by lens
Usually 4 different form which occur alone or with one another
i.Peripheral steepening
ii. Central flattening
iii. Furrow depression
iv. Central moulding
62. Other Uses of Corneal Topography
1. To guide removal of tight sutures after corneal surgery (keratoplasty, cataract
surgery, etc.) that are causing steepening of the cornea.
2. To help in the designing the astigmatic keratotomy.
3. To guide contact lens fitting: election of the probe lens and design of the lens.
4. To calculate the keratometry values for the calculation of the required power of
an intraocular lens for implantation.
5. To evaluate the effect and evolution of a keratorefractive procedure.
