3. CORNEAL TOPOGRAPHY
• Corneal topography refers to study of the shape of corneal surface
• provides us with a detailed description of various curvature and shape
characteristics of the cornea.
• This information is very helpful for
• the illustration of corneal astigmatism
• detection of corneal pathologies
• perfection of contact lens fitting
• cataract and keratorefractive surgeries
6. KERATOMETRY
• Devised by Helmholtz, who originally used the term ophthalmometer
• Calculations are based on the geometry of a spherical reflecting surface
• Based on fact that anterior surface of cornea acts as convex mirror and size of image
varies with curvature
Object of known size and distance is reflected off the corneal surface to determine the
size of the reflected image with a measuring telescope
• Helps to measure the radius of curvature of anterior corneal surface from 4 reflected
points within central 3mm of cornea
1. Helmholtz keratometer
2. Bausch and lomb keratometer
3. Javal schiotz keratometer
7. BAUSCH & LOMB
Object size constant
Amount of doubling varied to produce
the image of fixed size
JAVAL SCHIOTZ
Amount of image doubling is constant
Measures the object size needed to
produce an image of fixed size
8.
9. RANGE OF KERATOMETRIC READING
• Dioptric Power : 36D to 52D
• Radius of Curvature : 6.5mm to 9.38mm
10. APPLICATIONS
1. Objective method for determining
curvature of the cornea.
2. To estimate the amount and direction of
corneal astigmatism
3. The ocular biometery for the IOL power
calculation
4. To monitor pre and post surgical
astigmatism.
5. Differential diagnosis of axial versus
refractive anisometropia.
6. To diagnose and monitor keratoconus and
other corneal diseases.
7. For contact lens fitting by base curve
selection
LIMITATIONS
1. Assumes cornea to be spherical
2. Details of only central 3mm ignoring
peripheral corneal zones
3. Loses accuracy when measuring
very steep or flat corneas
4. Small corneal irregularities preclude
the use due to irregular astigmatism.
11. KERATOSCOPY
• Refers to the evaluation of topographic abnormalities of the corneal surface
by direct observation of images of mires reflected from the surface of cornea.
• Keratoscope: instrument that projects multiple concentric rings (mires) on
the cornea
• Keratoscopy: direct visualization of the rings
1. Placido-disc keratoscope
2. Photokeratoscope: when a still camera is added to photograph the mires
3. Videokeratoscope: when a video camera is added
12. HAND-HELD PLACIDO
DISC KERATOSCOPE
• Equally spaced
alternating black and
white rings with a hole
in the centre to observe
patients cornea.
• Utilizes corneal
reflections (purkinje
image) of bright rings
• The steeper the cornea
the closer reflected
rings of placido disc lie
PHOTOKERATOSCOPY
• photographic film
camera attached to a
keratoscope.
• Current
photokeratoscopes (eg
Nidek PKS 1000 or
keracorneoscope) have
9-15 rings
• cover 55-75% of corneal
surface.
• Closer the line steeper
is the corneal surface
• Corneal cylinders of
upto 3D can escape
detection by use of
photokeratoscopy.
VIDEOKERATOSCOPY
• Television camera is
attached to a
keratoscope.
• Computer assisted
videokeratoscopy has
become synonymous
with the corneal
topography.
• It covers approximately
95% of the corneal
surface.
13.
14.
15.
16. RASTERSTEREOGRAPHY
• Uses a direct image on the corneal surface.
• It projects a calibrated grid onto the fluorescein stained tear film.
• The advantage is that it includes information across the whole cornea.
• The projected nature of the test does not allow interference due to corneal surface
or stromal defects.
17. INTERFEROMETRY
• Uses the technique of light wave interference.
• This includes both holography and moire fringe techniques.
• This method is not in widespread clinical use.
18. MODERN TOPOGRAPHY
PLACIDO DISC SYSTEMS
• project a series of concentric rings of
light on the anterior corneal surface.
• The corneal shape or curvature is
directly measured in diopters of
curvature along thousands of points on
the rings.
• do not actually measure elevation
• they derive anterior corneal elevation
data by reconstructing actual anterior
curvature measurements via
sophisticated algorithms
SLIT-SCANNING DEVICES
• directly measure the elevation of both
the anterior and posterior cornea via time
domain or light-based analysis.
• These devices process elevation data
along several points on the anterior and
posterior corneal surfaces.
• This data is then converted into anterior
and posterior curvature in diopters as well
as corneal thickness or pachymetry in
microns
19. PLACIDO DISC SYSTEMS
Small-cone Placido disc topographers-
project more rings on the cornea and have a
shorter working distance
Medmont E300 (Medmont)
Scout and Keratron (EyeQuip)
Magellan Mapper (Nidek)
Large-cone Placido disc systems use a
longer working distance and project fewer rings
onto the cornea than small-cone topographers
and are more forgiving when measuring
patients with very deep set eyes.
ATLAS 995 and 9000 (Carl Zeiss
Meditec)
ReSeeVit (Veatch Ophthalmic
Instruments)
SLIT-SCANNING DEVICES
1. Orbscan (Bausch & Lomb)
2. Visante OCT (Carl Zeiss Meditec).
Scheimpflug imaging-
•uses a rotating camera to photograph
corneal cross-sections illuminated by slit
beams at different angles .
•This method corrects for the non-planar
shape of the cornea and, thus, allows
greater accuracy and resolution in
creating a 3-D map of the cornea
•Pentacam (Oculus)
22. Scheimpflug-based Devices
• There are four devices adopting the Scheimpflug principle and using the
Scheimpflug camera. These devices are
1. TMS-5 (Tomey, Nagoya, Japan),
2. Pentacam® HR (OCULUS, Wetzlar, Germany),
3. Sirius® (CSO Florence, Italy),
4. Galilei® (Ziemer, Port, Switzerland).
25. Comprehensive anterior aegment analyser.
Perform following 5 functions
1. Scheimpflug image of anterior segment
2. Three dimensional anterior chamber analyser
3. Pachymetry
4. Corneal topography
5. Cataract analyser
26. THE SCHEIMPFLUG PRINCIPLE
The Scheimpflug principle, first introduced by Theodor Scheimpflug, a cartographer
of the Austrian navy, describesan optical imaging condition, which allows
documentation of an obliquely tilted object with the maximally possible depth of focus
and minimal image distortion under given conditions.
It is a geometric rule that describes the orientation of the plane of focus of an optical
system (such as a camera) when the lens plane is not parallel to the image plane
27. Normally, the lens and image (film or sensor)
planes of a camera are parallel and the plane of
focus (PoF) is parallel to the lens and image
planes . If a planar subject is also parallel to the
image plane, it can coincide with the PoF, and the
entire subject can be rendered sharply.
If the subject plane is not parallel to the image
plane, it will be in focus only along a line where it
intersects the PoF, as illustrated in Figure 1B.
When an oblique tangent is extended from the
image plane, and another is extended from the
lens plane, they meet at a line through which the
PoF also passes, as illustrated in Figure 1C
(referred as Scheimpflug line). With this condition,
a planar subject that is not parallel to the image
plane can be completely in focus.
The Scheimpflug principle has been applied in
ophthalmology to obtain optical sections of the
entire anterior segment of the eye, from the
anterior surface of the cornea to the posterior
surface of the lens
28.
29. A nonrotating Scheimpflug camera was first produced by the Oxford group, later on
marketed as Oxford CASE 2000
The Zeiss SLC was the first rotating video Scheimpflug system. The EAS 1000 (Eye
analysis system) was the first electronic rotating Scheimpflug camera, which recorded
a retroillumination image as well.
The latest development among ophthalmic camera systems based on Scheimpflug’s
principle is the Pentacam (Oculus,
Wetzlar, Germany)
The GALILEI™ Dual Scheimpflug Analyzer (Zeimer Ophthalmic systems) is another
high precision optical system for corneal topography and three dimensional analysis of
the anterior eye segment, also based on a Revolving Dual Channel Scheimpflug
Camera and a Placido Disk
30. THE DEVICE
•The Pentacam obtains images of the anterior segment
by a rotating Scheimpflug camera (digital CCD camera
with synchronous pixel sampling) measurement.
•The light source - UV-free blue LED’s (wavelength=475
nm).
2 cameras
• 1st -located in the center for the purposes of
detection of the size and orientation of the pupil and
to control fixation.
• 2nd- mounted on rotating wheel to capture images
from the anterior segment. This rotating process
supplies pictures in three dimensions and also
allows the center of the cornea to be measured
precisely.
31. • The slit images are photographed on an angle form 0 to 180
degrees to avoid shadows from nose.
• Every picture is a complete image through the cornea at a
specific angle, combination of such slit images creates a real
360 degrees image of the anterior segment.
• The software utilizes a ray tracing algorithm to construct
and calculate the anterior segment. It acquires a total of 50
images in approximately two seconds, extracting 2,760 true
elevation points from these images which in turn generates
138,000 true elevation points for both the corneal front and
back surfaces, from limbus to limbus, including the center of
the cornea, a major advantage over keratometers and
Placido-based corneal topographers.
• The measurement process lasts less than 2 seconds and
minute eye movements are captured and corrected
simultaneously.
32. CLINICAL APPLICATION
1. Pachymetry
2. Scheimpflug Imaging in LASIK
3. Topography and contact lenses.
4. Topography in RK.
5. Post keratoplasty astigmatism.
6. Keraoconus screening
7. Corneal Pathologies
8. Anterior Chamber implantation of phakic IOLs
9. Assessment of Lens Density + anterior and posterior subcapsular cataracts
10. Capsular bag distension syndrome (CBDS)
11. Posterior Capsule Opacification (PCO)
33. 12. Improved IOL Calculations
13. identifying intralenticular foreign bodies
14. Glaucoma Screening
a) Effect of pilocarpine on anterior chamber depth and anterior chamber volume
in eyes with narrow angle and open angles
b) Anterior chamber Volume has been found to be a good screening tool for
diagnosing eyes with narrow angles
c) dynamics of the anterior chamber including the ACV can be studied following
procedures like laser peripheral iridotomy (PLI).
34. PACHYMETRIC MAPS
The remaining map on Pentacam four-map view is called topometric map, or
pachymetric map.
•determines central or paracentral corneal thickness
+
•describes distribution of corneal thickness throughout entire corneal diameter
Pachymetric data is useful in screening refractive surgery candidates, as it assists in
the estimation of residual stromal bed thickness.
Provides invaluable data when ruling out subclinical keratoconus (FFKC), as it
distinguishes whether thinnest point corresponds with corneal apex.
35.
36. CORNEAL TOMOGRAPHY
• corneal tomography consisting of
two parts:
• corneal parameters on the left side,
• 4-view refractive composite map on
the right
40. CORNEAL PARAMETERS
Qs: Quality specification
• It specifies the quality of the
tomographic capture
• Should be “OK”, otherwise there
is some missed information which
was virtually reproduced
(extrapolated) by the computer; in
this case, the capture should
preferably be repeated.
Q-val: Q Value
• Represents the asphericity of the anterior surface
of the cornea.
• The ideal value is measured within the 6-mm
central zone as shown between two brackets.
• Normal value is (–1 to 0).
• Plus Q (>0) oblate corneas (e.g. after > –3 D
myopic photoablation and after radial keratotomy).
• Over minus Q (<–1) hyperprolate corneas (e.g.
after > +3 D hyperopic photoablation and in
keratoconus).
• Both oblate and hyperprolate corneas produce
spherical aberrations.
41.
42. K1: (Kf):
• Curvature power of the flat meridian of
the anterior surface of the cornea
measured within the 3-mm central zone
(Sim-K) and expressed in diopters (D).
• Normal K1 = > 34 D.
• It should be considered in myopic
correction; each –1 D correction reduces
flat K by 0.75 D to 0.8 D.
• Final flat K should be > 34 D, otherwise
positive spherical aberrations will be
induced.
K2: (Ks):
• Curvature power of the steep meridian
of the anterior surface of the cornea
measured within the 3- mm central zone
(Sim-K) and expressed in diopters (D).
• Normal K2 is < 49 D.
• It should be considered in hyperopic
correction; each +1 D correction will add
1.2 D to steep K.
• Final steep K should be < 49 D
43. Km: (K-avg):
• Mean curvature power of the anterior
surface of the cornea within the 3-mm
central zone (Sim-K) and expressed in
diopters (D).
• It should be considered to avoid flap
complications.
• Km <40 D free-flap complication
may occur
• Km > 46 D button-hole complication.
K-max:
• Maximum curvature power of the whole
anterior surface of the cornea expressed
in diopters (D).
• Normal K-max < 49 D
• Normal difference in Kmax between the
two eyes is < 2 D
• Normal (Kmax - K2) difference in the
same eye is < 1 D.
• Whenever the difference is ≥ 1D, K-max
should be used instead of K2 into the
calculations for hyperopic correction to
avoid postphotorefractive irregularities
44.
45. Astig:
Amount of corneal (topographic)
astigmatism (TA) on the anterior surface
of the cornea, i.e. the difference
between the two curvature radii (K2 –
K1) within the 3-mm central zone (Sim-
K). TA should be compared with the
manifest astigmatism (MA).
• Axis: The axis of anterior corneal
astigmatism within the 3-mm central
zone. It should be compared with the
axis of MA.
•
Pachy Apex:
It represents thickness at the apex of the cornea.
The computer considers the apex as the origin of the coordinates,
where X and Y are horizontal and vertical meridians respectively.
Zero is displayed in both squares of pachy apex coordinates.
The direction of axis X is from the patient’s right to his/her left when
the patient is seated opposite to the physician.
The direction of axis Y is from the bottom up. Example: a point
“e” in the left cornea is located at “+0.2,–0.4” position, i.e. this
point is located 0.2 mm temporal to and 0.4 mm inferior to
corneal apex.
46. Pupil Center:
• Corneal thickness corresponding to
pupil center location and its coordinates.
• Pupil center coordinates are necessary
for the decentration technique when
treating hyperopia, astigmatism or
corneal irregularities.
• They are also important to evaluate
angle kappa
• normal x-coordinate—in absolute
value—is ≤ 200 μm (or ≤ 5°).
Pupil diameter:
• It is the diameter of pupil in the
circumstance of capture (photopic,
mesopic or scotopic according to the
amount of illumination).
• It is necessary for adjusting optical
zone (OZ) diameter, which should be at
least 0.5 mm larger than the scotopic
pupil size.
47.
48. •THINNEST LOCATION (TL):
•Thickness and location of the thinnest point of the cornea.
•The new definition of thin cornea is a cornea below 470 μm with normal tomography,
or a cornea below 500 μm with abnormal tomography.
•The normal difference in thickness at the TL between the two eyes is < 30 μm.
•The difference in thickness between TL and pachy apex is normally ≤ 10 μm.
•Y-coordinate is most often normal, suspected or abnormal when it is < 0.500 mm,
0.500 mm to 1.000 mm, or >1.000 mm respectively; the important algebraic sign is
the minus indicating inferior displacement of the TL.
49. ANTERIOR CHAMBER VOLUME (ACV), ANGLE (ACA) AND
DEPTH (ACD):
•Anterior Chambers with
•ACV < 100 mm3
•ACA < 24°
•ACD < 2.1 mm may have the risk to develop angle closure glaucoma.
•On the other hand, safe parameters for phakic IOL (PIOL) implantation are-
•ACD ≥ 3.0 mm
•ACA > 30°
•ACV ≥ 100 mm3.
52. • The four most important tomographic maps are the anterior curvature sagittal map,
the anterior and posterior elevation maps, and the pachymetry map
• In each map, both shape and parameters should be studied.
• It is necessary sometimes to study the anterior curvature tangential map.
53. ANTERIOR SAGITTAL MAP
•Steep areas hot colours (red and
orange), while flat areas cold colours
(green and blue).
•The cross point of this segmentation
represents apex (anatomical center) of the
cornea.
•Parameters studied particularly on the
steep axis at the 5-mm central circle.
•The normal pattern is the symmetric
bowtie (SB) .
54. • The two segments (a) and (b) are equal in
size, and their axes are aligned.
• The two opposing points (S and I) on the 5-
mm central circle on the steep axis.
• Normally, the inferior (I) point has a higher
value than the superior (S) one, and the I-S
difference should be < 1.5 D.
• The superior point may rarely have a higher
value than the inferior one the S-I difference
should be < 2.5D
• The SB pattern represents regular
astigmatism, which can be with-the-rule (WTR),
against-the-rule (ATR) or oblique according to
the orientation of the SB.
55. • WTR astigmatism SB is on or within
―15° of the vertical meridian of the cornea
.
• ATR astigmatism SB is on or within
―15° of the Horizontal meridian of the
cornea .
• Oblique astigmatism SB is neither
vertical nor horizontal .
• The SB pattern can be encountered in KC
when K readings are abnormally high .
56. Abnormal patterns
• They include the following:
• 1. Round (R) .
• 2. Oval (O) .
• 3. Superior Steep (SS) .
• 4. Inferior Steep (IS) .
• 5. Irregular (Irr) .
57.
58.
59. ANTERIOR TANGENTIAL MAP
•This map helps in describing corneal irregularities.
•It is also useful for determining morphologic patterns of the cone in ectatic corneal
disorders.
•Depending on this map, there are three patterns of the cone: nipple, oval and
globus.
61. REFERENCE BODY
•The computer adjusts the reference surface with the measured surface.
•The computer considers all points above reference surface as elevations, being
displayed as positive values, and considers all points below the reference surface as
depressions, being displayed as negative values ,all values are in microns.
•The coincidence points between reference surface and measured surfaceare
displayed as zeros, i.e. exactly like the sea level .
62. 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 reference surface 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 .
63.
64.
65. REFERENCE BODY
• The computer of the camera proposes a
reference body for each corneal surface
being captured .
The reference body of the front surface
may differ from that of the back surface,
although both surfaces are of the same
cornea
66. REFERENCE BODY TYPES
Toric Ellipsoid Body
• It is an aspherical shape which is
rotationally symmetric according to two axes,
major and minor.
But it has a coronal elliptical cross-section ,
i.e. there are two perpendicular axes, one is
steeper than the other.
advantage very good approach to the
real course of, e.g. astigmatic corneal
surface.
•
67. Spherical Body
• It is better than the previous bodies in highlighting cornealirregularities since the
normal cornea has a toric ellipsoid shape.
It is well known that to recognize something, it should be matched with other different
things.
Therefore, if we want to show the details of an abnormal cornea, we should relate it
to a spherical reference body.
68. FLOAT MODE
•• The reference body can be adjusted with examined surface of the cornea in various
locations .
•Accordingly, details of central part might appear (or disappear).
•If the reference body is adjusted in contact with apex of the cornea, it is called “no
float mode” .
•when the reference body is represented to be optimized with respect to the cornea, it
is called “float mode” , i.e. the distance between the two bodies (corneal surface and
reference body) should be equal in sum and minimum.
• The float mode is most commonly used as a standard to compare examinations
carried out by various topographic systems.
•• very early stages of keratoconus (KC) are difficult to recognize on the float shape
due to distance optimized adjustment.
69.
70. • In general, we have to use both the Best Fit Sphere (BFS) and the Best Fit Toric
Ellipsoid (BFTE).
• The BFS is important for three reasons:
(1) To see the shape of the cornea
(2) To search for an important risk factor, that is the isolated island or the
tongue like extension
(3) To locate the cone in KC .
• the BFTE is important for two reasons:
(1) To evaluate the details of corneal surface
(2) To evaluate the severity of the cone in KC .
71. THE PACHYMETRY MAP
• The pachymetry map has three main landmarks :
cornea apex (orange arrow)
Thinnest location TL (red arrow)
two opposing points on the vertical meridian at the central 5-mm circle (white
dotted arrows).
The normal difference between the superior (S) and inferior (I) points is ≤ 30 μm.
Shape: The normal pachymetry map has a concentric shape .
72.
73. ABNORMAL SHAPES include
• a. Horizontal displacement of the thinnest
Location .
• b. Dome shape. The Thinnest Location is
vertically displaced .
• c. Bell shape. There is a thin band in the
inferior part of the cornea . It is a hallmark
for Pellucid Marginal Degeneration (PMD).
• d. Keratoglobus. A generalized thinning
reaching the limbus .
74. THICKNESS PROFILES
• These profiles are only displayed in the Pentacam.
There are two pachymetry profiles:
1. Corneal Thickness Spatial Profile (CTSP) describes the average
progression of thickness starting from the Thinnest Location to corneal periphery
in relation to zones concentric with the Thinnest Location
1. Percentage Thickness Increase (PTI) describes the percentage of
progression of the same.
75. Normal profile curved line plotted
in red, following (but not necessarily
within) the course of the normative
black dotted curves, with an average of
0.8–1.1 .
• When there is a fast transition of
thickness between the Thinnest
location and corneal periphery, the
average will be high, and vice versa
e.g. in an oedematous cornea, the
average will be low and the curve will
be flat.
76. Abnormal profiles include:
• a. Quick Slope . The red curve leaves its course before
6-mm zone. It is encountered in FFKC & ectatic
disorders. The average is usually high > 1.1 .
• b. S-shape . The red curve has a shape of an “S”. It is
encountered in FFKC and ectatic disorders. The
average is usually high > 1.1 .
• c. Flat shape . The red curve takes a straight course. It
is encountered in diseased thickened (oedematous)
corneas such as Fuch’s dystrophy & cornea Guttata.
The average is low < 0.8 .
• d. Inverted . The red curve follows an upward course. It
is encountered in some cases of PMD. The average is
very low < 0.8 and may take a minus value.
