The document discusses corneal topography and tomography using the Pentacam device. It provides information on: 1) The Pentacam uses Scheimpflug imaging principles to capture 25,000 elevation points to create a 3D model of the anterior eye segment, allowing for analysis of the anterior and posterior corneal surfaces as well as pachymetry. 2) Compared to Placido disk-based systems, the Pentacam provides direct elevation data rather than deriving it from curvature, allowing for more accurate determination of corneal shape. It can also measure the entire corneal thickness through pachymetric maps. 3) The Pentacam examination involves capturing Scheimpflug images which are then used to generate

1. PENTACAM
TOMOGRAPHY
INDOREDRISHTI.WORDPRESS.COM

2. DR DINESH MITTAL DR SONALEE MITTAL
DRISHTI EYE HOSP VIJAYNAGAR INDORE

3. Corneal topography
• The interpretation of corneal topography has become
an important clinical skill for all eye care professionals
as our ability to surgically alter the cornea has
dramatically improved and patients’ visual
expectations have risen to unprecedented levels.
Despite advancements in automated indices and
reference databases, interpretation of corneal
topography remains an exercise in pattern recognition.
In the case of pre-LASIK evaluation, for example, there
is now a consensus that abnormality in the shape of
the cornea preoperatively is perhaps the most
important risk factor for the development of post-
surgical ectasia. Therefore, understanding the
difference between normal topography and abnormal
topography has become increasingly important.

4. Corneal topography
• Corneal topography initially gained prominence in
the sphere of refractive surgery for the main
purpose of excluding potential candidates for RK or
LASIK due to sub-clinical keratoconus. As
technology developed, corneal topography
developed into a therapeutic corneal tool when the
data was used to drive topography-guided
photorefractive keratectomy (PRK).

5. Corneal topography
• Improving your understanding of corneal
topography/ tomography and its interpretation and
clinical application is an excellent start on the
pathway to making your cataract and refractive
surgeries more successful and your patients more
satisfied with their surgery. Skillful interpretation
of corneal imaging is a key to successful refractive
surgery . However, busy clinicians are challenged
to find the time to keep abreast with the latest
advances in topographic interpretation.

6. Corneal topography
• Modern cataract surgery has evolved into
refractive surgery and now topography
interpretation is an important part of
understanding intraocular lens choice for lens
replacement in cataract and clear lens extraction
surgery. Multifocal IOLs would be contraindicated
in a patient with irregular corneal topography that
may present with normal Ks on keratometry.
Likewise, good decisions can be made in terms of
toric IOL selection using corneal topography.

7. Corneal topography history
• Today’s corneal imaging devices have come a long way
from the 8-ring Placido-disk systems of the early
1990’s, which were limited to topographical maps of
the anterior surface and often contained more artifact
than information. Today’s devices provide a wealth of
information about the cornea, and do so using multiple
imaging sources, often obtained simultaneously.
• They minimize artifact using automated image
capturing and sophisticated validation algorithms, and
even provide diagnostic interpretations, which
incorporate information about the anterior and
posterior cornea, and corneal thickness.

8. Corneal topography
• Corneal images are inherently technical and are
often non-specific. The same map might be normal
or abnormal depending on the clinical setting. For
example, a pachymetry map showing paracentral
corneal thickness of 450 microns might suggest
keratoconus in a 21 year old but be of little concern
in a 65 year old. Central corneal power of 47
Diopters may be expected after hyperopic LASIK
but suggest corneal ectasia after a myopic
ablation.

9. PATTERN RECOGNITION
• The authors use pattern recognition to create
associations between corneal conditions and the
maps they produce, so clinicians learn to recognize
patterns rather than focus on isolated findings.
Pattern recognition is fundamental to everyday life,
from the earliest stages of development through
the most advanced levels of learning.

10. PATTERN RECOGNITION
• Infants use pattern recognition when they differentiate
between familiar and unfamiliar faces. Master chess
players can plan complex strategies after glancing at a
chess board, not by studying the locations of individual
pieces but by seeing patterns and layouts that are
commonly repeated.
• Pattern recognition requires familiarity, which we come
to recognize as expertise. Experienced automobile
drivers come to recognize normal traffic patterns from
potential threats in an instant, where new drivers may
not. Closer to home, ophthalmologists can readily
differentiate an eye that is red from a foreign body from
a one with bacterial conjunctivitis, while the difference
may not be as obvious to an internist.

11. Curvature-based
Devices

12. PLACIDO DISC TOPOGRAPHY
• Placido-disk topography has been the most
commonly used technology since the late 1880s
when the Javal-Placido target was developed.
Placido-disk topography is based on the reflection
of concentric mires (rings) on the cornea. The
closer the mires, the steeper the curvature. The
wider the mires, the flatter the curvature. Clear
surfaces are required for clear reflection of
mires.computerized interpretation of the
reflections generated by these disks became a
reality in the 1970s.

13. Curvature-Based Instruments
• Keratometry
• Keratoscopy or Photokeratoscopy
• Computerized Videokeratoscopy

14. Curvature-based Devices
• The normal corneal outer surface is smooth;
corneal irregularities being neutralized to some
extent by the tear film layer. The anterior surface
acts as an almost transparent convex mirror; it
reflects part of the incident light. Many devices
have been developed to measure the curvature of
the anterior corneal surface using the first Purkinje
image. These noncontact devices use light target
(in different shapes) and a microscope or other
optical systems. These curvature-based devices
are either quantitative or qualitative, and are either
reflection-based or projection-based.

15. Keratometry
• A keratometer is a quantitative reflection-based instrument.
It measures the corneal radius in the central 3 mm by
measuring the size of the reflected image, and converting
the image size into corneal radius using the following
mathematical relationship:
• r = 2 x a x (i/o) where
• r: anterior corneal radius
• a: distance from mire to cornea (75 mm in keratometer)
• i: image size
• o: mire size (64 mm in keratometer)
• The keratometer can convert from corneal radius r
(measured in meters) into refracting power RP (in diopters)
using the followingrelationship: RP = 337.5/ r .

16. Keratoscopy or
Photokeratoscopy
• Because of the small area over which the
keratometer can obtain measurements, additional
imaging modalities which provided qualitative
information about the shape of the entire cornea
were needed. While the keratometer only analyses
approximately 6% of the anterior corneal surface,
the keratoscope measures 70% of the total anterior
corneal area (limited by the optical system of the
machine itself). A photokeratoscope is a qualitative
reflection-based instrument.

17. Keratoscopy or
Photokeratoscopy
• The projected light may be a simple flash light or a
Placido disk target. The latter is a series of
concentric rings (10 or 12 rings) or a cone with
illuminated rings lining the internal surface of the
instrument. The rings are concentric, regular and
have uniform interspaces a normal cornea.
According to the changes in the shape of the
reflected rings and the spaces in between, one can
take an idea about the shape of the cornea; for
instance, small, narrow and closely spaced rings
indicate steep regions with small radius of
curvature .

20. Keratoscopy or
Photokeratoscopy
• The use of the photokeratoscope is being abandoned; several
computerized topographers—allowing both qualitative and
quantitative measurements—are being used. Some of the known
disadvantages of the photokeratoscope are:
• It requires assumptions about the corneal shape
• It misses data from the central cornea (not all devices)
• Limited data points
• It measures the corneal curvature and not the height
• It measures only the anterior surface of the cornea
• Defocusing and misalignment
• The patient is exposed to high light
• It is severely affected by tear film disturbances.

21. Computerized Videokeratoscopy
• Modern topographers are based on projected rather
than reflected images. Basically, a projection-based
topographer consists of a Placido disk or cone (small
or large) which projects a mire of concentric light
rings, a video camera that captures the reflected rings
from the tear film layer and a software to analyze the
data. The computer evaluates the distance between
the concentric rings (dark and light areas) in a variable
number of points. The shorter the distance, the higher
is the corneal power, and vice versa. After analyzing
the data, they are plotted by the computer as a color
map.

23. Computerized Videokeratoscopy
• The Placido cone may be large or small according
to the manufacturer. The larger the cone the more
are the rings and the wider is the estimated area.
The mires of most systems exclude the very
central cornea and paralimbal area. The
reproducibility and validity of videokeratography
measurements are mainly dependent on the
accuracy of manual adjustment in the focal plane.

24. ELEVATION BASED
TOPOGRAPHY
• Placido-disk topography has been the standard
methodology used for describing the curvature and
power of the corneal surface. Placido-disk
topography is based on a two dimensional image,
however, and is not capable of accurately
describing corneal shape. In order to accurately
determine corneal shape, a measurement of the Z
coordinate, or elevation, is required.

25. ELEVATION BASED
TOPOGRAPHY
• Several systems have been developed over the
years to measure the X, Y, and Z coordinates of the
cornea in an attempt to determine corneal shape.
Of the systems currently available for use, the
Pentcam (OCULUS GmbH) is the most commonly
encountered. Due to advantages that elevation-
based topography and tomography hold over
placido-disk topography, the Pentacam is rapidly
becoming the standard for corneal imaging,
particularly when screening candidates for
refractive surgery.

26. Scheimpflug-based Devices
• There are four devices adopting the Scheimpflug
principle and using the Scheimpflug camera. These
devices are
• TMS-5 (Tomey, Nagoya, Japan),
• Pentacam® HR (OCULUS, Wetzlar, Germany),
• Sirius® (CSO Florence, Italy),
• Galilei® (Ziemer, Port, Switzerland).

27. PENTACAM
TOMOGRAPHY
THANKS TO MAZEN M SINZAB

28. PENTACAM TOMOGRAPHY
• Placido-based or curvature-based systems rely on
the data collected from anterior surface of cornea,
such systems can be reflection-based or projection-
based. Without the information about the posterior
surface, complete pachymetric evaluation of the
cornea is not possible. Ultrasonic pachymetry can
give us central and few paracentral measurements,
but full pachymetric map is mandatory in modern
refractive surgeries .

29. PENTACAM TOMOGRAPHY
• Moreover, the posterior surface of cornea is being
more seen as a sensitive indicator of corneal
ectasia and can often be abnormal in spite of a
normal anterior corneal surface. It is now
recognized that while refractive power of cornea is
mostly determined by the anterior surface, the
biomechanical behavior of the cornea is at least
equally determined by both surfaces.
• In the curvature-based systems the elevation map
of the anterior surface is derived from the
curvature map, while it is directly calculated in the
elevation-based systems.

30. Principle of a traditional camera.
All planes are parallel

31. Principle of a Scheimpflug-based
camera. The three planes intersect
at one point

32. SCHEIMPFLUG CAMERA

34. The Pentacam system.

36. Description of the Unit
• The OCULUS Pentacam is a rotating Scheimpflug
camera . The rotational measuring procedure
generates Scheimpflug images in three dimensions,
with the dot matrix fine-meshed in the center due
to the rotation. It takes a maximum of 2 seconds to
generate a complete image of the anterior eye
segment. Any eye movement is detected by a
second camera and corrected for in the process to
some extent. The Pentacam calculates a 3-
dimensional model of the anterior eye segment
from as many as 25000 (HR: 138000) true elevation
points .

37. Description of the Unit
• Topography and pachymetry of entire anterior and
posterior surfaces of cornea from limbus to limbus
are calculated and depicted. The analysis of the
anterior eye segment includes a calculation of the
chamber angle, chamber volume and chamber
depth . In a moveable virtual eye, images of the
anterior and posterior surface of the cornea, the
iris and the anterior and posterior surfaces of the
lens are generated. The densitometry of the lens is
automatically quantified.

38. Description of the Unit
• The Scheimpflug images taken during the examination
are digitalized in the main unit and all image data are
transferred to PC. When examination is finished, the
PC calculates a 3D virtual model of the anterior eye
segment, from which all additional information is
derived. The Scheimpflug law states: To get a higher
depth of focus, move the three planes, provided that
the picture plane, the objective plane and the film
plane have to cut each others in one line or one point
of intersection. Advantages of the Scheimpflug camera
include higher depth of focus and sharp picture .

39. Pentacam vs Galilei vs Sirius
• The Pentacam combines a rotating Scheimpflug
camera with a static camera to acquire multiple
photographs of the anterior eye segment.
• The Galilei Dual Scheimpflug Analyzer integrates a
Placido disc and a dual rotating Scheimpflug
system for corneal topography and three-
dimensional analysis of the anterior eye segment.

41. Pentacam vs Galilei vs Sirius
• The dual camera configuration captures two
Scheimpflug slit images from opposite sides of the
slit beam and simultaneously tracks decentration
due to eye movements . The height data obtained
from two corresponding slit images are averaged to
improve the measurements of corneal elevation
and thickness. The Sirius Scheimpflug Analyzer
integrates a Placido disc and a mono rotating
Scheimpflug system for corneal topography and
three-dimensional analysis of the anterior eye
segment.

42. Pentacam vs Galilei vs Sirius
• The aim of integration of Placido disc into Galilei
and Sirius is to enhance the analysis of the anterior
corneal surface. Three devices have high
repeatability. The Pentacam and Galilei could be
considered interchangeable contrary to Sirius. The
Pentacam has more repeatability and
reproducibility than Galilei in measuring the
curvature, astigmatism and corneal wavefront, and
vice versa in measuring thickness, although both
have good correlation and agreement with each
other and with ultrasound pachymetry.

43. Pentacam vs Galilei vs Sirius
• On the other hand, the Pentacam is twice more
precise than Galilei but as equally precise as Sirius
in measuring Sim-K (anterior curvature), while the
three devices have same precision in measuring
the curvature of the posterior surface. All maps in
the Pentacam and Galilei are nearly comparable,
while the elevation maps are displayed in Sirius in
a special manner that makes them incomparable
with those in the Pentacam and Galilei.

44. PACHYMETRIC MAPS
• The remaining map on Pentacam four-map view is
called topometric map, or pachymetric map. This map
not only determines central or paracentral corneal
thickness, as has been traditionally determined by
ultrasound, but it also 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. It also provides
invaluable data when ruling out subclinical
keratoconus (FFKC), as it distinguishes whether
thinnest point corresponds with corneal apex.

48. corneal tomography
• corneal tomography
consisting of two parts:
• corneal parameters on
the left side,
• 4-view refractive
composite map on the
right side.

52. Corneal Parameters

53. Corneal Parameters Qs
• 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.

54. Corneal Parameters Q-val
• Q-val: Value of Q which 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) is found in oblate corneas (e.g.
after > –3 D myopic photoablation and after radial
keratotomy (RK). Over minus Q (<–1) is found in
hyperprolate corneas (e.g. after > +3 D hyperopic
photoablation and in keratoconus (KC). Both oblate
and hyperprolate corneas produce spherical
aberrations.

56. Corneal Parameters
• 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 is > 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.

57. Corneal Parameters
• 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. When Km is <
40 D, free-flap complication may occur; while Km > 46
D may result in a button-hole complication.
• K-max: Maximum curvature power of the whole
anterior surface of the cornea expressed in diopters
(D). Normal K-max is < 49 D, the normal difference in K-
max between the two eyes is < 2 D, and the 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.

58. Corneal Parameters
• 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.

59. Corneal Parameters
• 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.

60. Corneal Parameters
• 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.

61. Corneal Parameters
• Anterior Chamber Volume (ACV), Angle (ACA) and
Depth (ACD):
• Anterior Chambers with ACV < 100 mm3, ACA < 24°
or 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° and ACV ≥ 100 mm3.

62. Corneal Maps

63. Corneal Maps
• 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.

65. 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) .

66. The ANTERIOR SAGITTAL map
• The two segments (a) and (b) are equal in size, and
their axes are aligned. Note 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; in this case, the S-I difference should be < 2.5
D. 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.

68. 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 .

72. Abnormal patterns
• They include the following:
• 1. Round (R) .
• 2. Oval (O) .
• 3. Superior Steep (SS) .
• 4. Inferior Steep (IS) .
• 5. Irregular (Irr) .

73. Abnormal patterns
• 6. Abnormal Symmetric Bowtie (SB) . K READING IS
HIGH .
• 7. Symmetric Bowtie with Skewed Radial Axis
(SB/SRAX). The angle between the axes of the two
lobes is >22° .
• 8. Asymmetric Bowtie/Inferior Steep (AB/IS); the I-S
difference is >1.5 D .
• 9. Asymmetric Bowtie/Superior Steep (AB/SS); the S-I
difference is >2.5 D .
• 10. Asymmetric Bowtie with Skewed Radial Axis
(AB/SRAX). The angle between the axes of the two
lobes is >22° .

107. The 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.

108. The Elevation Maps

109. 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 surface
are displayed as zeros, i.e. exactly like the sea
level .

110. 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 .

113. 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

114. RELATIONSHIP BETWEEN
REFERENCE BODY AND CORNEA

115. 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. Its advantage consists
in the very good approach to the real course of, e.g.
astigmatic corneal surface.
• Spherical Body
• It is better than the previous bodies in highlighting corneal
irregularities 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.

117. 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” . On the other hand, 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.
• Unfortunately, very early stages of keratoconus (KC) are
difficult to recognize on the float shape due to distance
optimized adjustment.

118. NO FLOAT AND FLOAT LOCATION

119. Which Reference Body Should we
Use?
• 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 .
• On the other hand, 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 .

120. The Enhanced Spherical
Reference Body

121. WHICH REFERENCE BODY & MODE
• The most important are best fit sphere (BFS)
which describes (qualifies) the shape of the
measured surface, and best fit toric ellipsoid
(BFTE) which estimates (quantifies) the parameters
of that surface. For routine use in refractive
surgery screening, the ideal diameter of the RS is 8
mm and the ideal mode is the float mode.

122. Shape (BFS float mode):
• The normal
shape of a
cornea with
regular
astigmatism is
the symmetric
hour glass .
Vertical steeper
meridian is
shown as
depressions –ve
value .

124. Abnormal shapes include
• Abnormal shapes include
• a. Skewed hourglass . Normally, it can be seen with
large angle Kappa and misalignment during taking
the capture, otherwise it indicates an abnormal
distorted cornea.
• b. Tongue-like extension and irregular hourglass .
They are seen in abnormal distorted corneas.
• c. Isolated island . It is encountered in abnormal
distorted corneas with central or paracentral
protrusion.

125. Abnormal shapes include

126. Abnormal shapes include

129. Parameters (BFTE float mode):
• Look at the highest plus values within the central
5-mm zone . Abnormal values are > 12 μm and > 15
μm on the anterior and posterior elevation maps
respectively.

132. Parameters (BFS float mode):
• look at values corresponding to the TL using the
BFS float mode. This can be done by pointing with
the cursor at the TL symbol on the elevation maps
and left click on the mouse to display the
corresponding values.

134. Cone location (BFS mode):
• In ectatic corneal disorders, the cone can be
localized by the BFS float mode , and can be
quantified by the BFTE float mode . On the BFS, the
cone can be central, paracentral or peripheral
when its apex is located within the central 3-mm
zone, between 3-mm and 5-mm, or outside the
central 5-mm zone, respectively .

137. Cone location (BFS mode):
• Whenthe cone is peripheral, the elevation map
takes “kissing birds” sign . Classifying cone
location is important for the treatment of KC.

164. Corneal Thickness Map

165. Main Elements
• The computer displays the thickness map in two patterns:
• 1. Five values: A central value representing the central thickness,
and four values around at the 5 mm central circle .
• 2. Distributed values all over the cornea : The distributed pattern
is more important and valuable.
• However, those are not the main elements of the corneal
thickness map. The main elements are three locations that
appear on the main page : the thinnest location, the corneal apex
and the pupil center. These three locations are displayed with
their coordinates, where the corneal apex is the origin point (zero
point). 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 “+0.3, -0.5” position, i.e. this point is
located 0.3 mm temporal to and 0.5 mm inferior to corneal apex.

170. The Pachymetry Map
• The pachymetry map has three main landmarks :
cornea apex (orange arrow), Thinnest location TL
(red arrow), and the 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 .

175. Abnormal shapes include
• a. Horizontal displacement of the TL .
• b. Dome shape. The TL 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 .

180. Thickness Profiles
• These profiles are only displayed in the Pentacam.
There are two pachymetry profiles:
• Corneal Thickness Spatial Profile (CTSP) and
• Percentage Thickness Increase (PTI).
• The former describes the average progression of
thickness starting from the TL to corneal periphery
in relation to zones concentric with the TL. The
latter describes the percentage of progression of
the same.

181. Thickness Profiles
• The normal profile is a 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 TL 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.

183. 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.

198. Corneal Topometry
• Corneal topometry measures the slope of the
cornea. Corneal surface may take one of the four
main shapes: spheric, aspheric oblate, aspheric
prolate or aspheric hyperprolate . Q-value is
positive (> 0) when the cornea is oblate, negative
(< 0) when the cornea is prolate or hyperprolate,
and = 0 when the cornea is spheric. The normal
value is [–1 , 0]. In KC, Q-value is highly negative;
and after high myopic photoablation, Q-value is
positive. Abnormal Q-value causes spherical
aberrations. The least spherical aberrations are
found when Q-value = –0.27.

199. Steps of Reading
Corneal Tomography

204. Study the
parameters
carefully

205. Study the corneal parameters and focus on the
parameters of anterior corneal surface, corneal
thickness and anterior chamber
“Quality of the image (QS) is OK for both surfaces.
K-readings are within the normal range; both K2 and
K-max are < 49 D and (K-max—K2) is < 1 D. The
amount and axis of TA should be compared with MA.
Q-value of both surfaces is within the normal range
[–1 , 0]. TL thickness is > 500 μm. Difference in
thickness between the TL and pachy apex is < 10
μm. There is no vertical displacement of the TL .
Angle kappa is not significant ; x-coordinate is < 200
μm in absolute value. ACV is > 100 mm3, ACA is
normal and > 30°, ACD is normal (> 2.1 mm) but < 3.0
mm.”

207. • Quality of the image (QS) is OK for both surfaces.
K-max is > 49 D and (K-max—K2) is > 1 D. The
amount and axis of TA should be compared with
MA. Q-value of both surfaces is within the normal
range [–1 , 0], but on the upper limit indicating a
mild hyperprolate cornea. TL is < 500 μm.
Difference in thickness between the TL and pachy
apex is < 10 μm. The TL is < –500 μm in absolute
value. Angle kappa is not significant; x-coordinate
is < 200 μm in absolute value. ACV is 240 mm3,
which is very high; ACA is wide 39.2°; and ACD is
3.73 mm, which is quite deep.”

208. “The anterior sagittal map: SB, no
SRAX, I-S = 1 D, which is normal.”

210. • The anterior sagittal map: although S-I is normal (1
D), it is an abnormal pattern, which can be
considered either superior steep or junctional with
significant SRAX.”

212. • “The anterior elevation map shows normal shape in
BFS mode and normal values within the central 5-
mm zone in BFTE mode.”

214. • “The posterior elevation map shows tongue-like
extension in BFS mode and abnormal values (> 15
μm) within the central 5-mm zone in BFTE mode.”

216. • “The pachymetry map: concentric pattern, I-S is
normal (< 30 μm), and there is no horizontal or
vertical displacement in the TL.”

218. • “The pachymetry map: thin cornea, abnormal I-S (>
30 μm), and there is an abnormal inferior
displacement of the TL (690 μm).”

221. Ectatic Corneal
Disorders

222. Topographic & Tomographic Features of
Ectatic Corneal Disorders
• Apart from keratoglobus, ectatic corneal disorders
consist of four major entities:
• KC, PMD, FFKC and Pellucid-like Keratoconus
• (PLK). These disorders can be classified
morphologically and tomographically.

223. Morphologic Classification
• There are three morphologic patterns of a cone .
• a. Nipple cone .
• b. Oval cone .
• c. Globus cone .
• These patterns are better described by the
tangential map. In mild cases, cone morphology
may be indeterminate .

228. Tomographic Classifications
• Tomographically, an abnormal cornea can be
classified according to the elevation maps,
pachymetry map or curvature map. Table
summarizes tomographic classification. When more
than one of the following criteria is found, any of
the above-mentioned patterns is considered as
frank KC, FFKC, early stage KC, or at least a case
of suspicion according to the severity and amount
of the following criteria.

229. Tomographic Classifications
• On the sagittal map:
• a. Steep K ≥ 49 D.
• b. K-max ≥ 49 D.
• c. Difference in K-max between the two eyes ≥ 2 D.
• d. K-max—Steep K ≥ 1 D.
• e. SRAX > 22°.
• f. S-I ≥ 2.5 D.
• g. I-S ≥ 1.5 D.

230. Tomographic Classifications
• On the elevation maps:
• a. Skewed or irregular hourglass, isolated island or
tongue-like extension.
• b. Plus values > 12 μm on the anterior elevation
map (within central 5-mm zone).
• c. Plus values > 15 μm on the posterior elevation
map (within central 5-mm zone).

231. Tomographic Classifications
• On the pachymetry map:
• a. Dome-shaped, Bell-shaped or Globus.
• b. S-I > 30 μm.
• c. TL < 470 μm, or < 500 μm in case of
abnormalities.
• d. Thickness @ pachy apex—thickness @ TL > 10
μm.
• e. Y coordinate value of the TL > –500 μm.
• f. Difference in thickness between the two eyes @
the TL > 30 μm.

232. Tomographic Classifications
• On thickness profiles:
• a. Average > 1.1.
• b. Quick slope.
• c. S-shape.
• d. Flat shape.
• e. Inverted.

233. Forme Fruste Keratoconus
• Forme Fruste Keratoconus (FFKC) is a subclinical
disease and is not a variant of KC. Although
clinicians use many other terms such as mild KC,
early KC and subclinical KC, their exact meanings
and applications are less certain. These terms are
not universally accepted. The diagnosis of KC is
clinically aided by tomography, while the diagnosis
of FFKC is only by tomography or even beyond
tomography by measuring corneal biomechanics or
other considerations.

234. Forme Fruste Keratoconus
• Recently, there are two definitions of this disease:
• 1. In a patient with a normal cornea in one eye and
KC in the other, the normal cornea can be
considered FFKC. If both eyes are normal but the
patient has a first- or second-degree relative with
KC, that patient may be considered to have FFKC.
In other words, it is the potential to develop ectasia
after keratorefractive surgery.
• 2. Another definition of FFKC is a cornea with
abnormal tomography that is not distinct enough to
be classified clearly as one of the ectatic disorders

235. Pellucid Marginal Degeneration and
Pellucid-like Keratoconus
• Pellucid marginal degeneration (PMD) is an ectatic
corneal disorder characterized by peripheral
inferior corneal thinning observed with slitlamp
biomicroscopy and Scheimpflug image. Pellucid-
like keratoconus (PLK) is a different entity; it is KC
that has some features of PMD as will be discussed
bellow.
• Differentiation between these two entities is
important for the right plan of treatment. PMD and
PLK can be differentiated by their tomographic
features on the curvature, elevation and
pachymetry maps.

238. Pellucid Marginal Degeneration and
Pellucid-like Keratoconus
• Curvature map: The anterior sagittal curvature
map shows a claw pattern . This is seen in both
PMD and PLK.
• Elevation maps: There are two important things
related to each other that can be identified on the
elevation maps, peripheral location of the cone and
the "kissing birds" sign. Neither of these two signs
is a hallmark of PMD or PLK.

239. Pellucid Marginal Degeneration and
Pellucid-like Keratoconus
• Pachymetry map: In PMD, the pachymetry map
shows a characteristic pattern known as "bell-
shape" pattern . This pattern is a hallmark of PMD
and absent in PLK .
• • Pachymetry profiles: In KC, PLK and in PMD, the
curve deviates from the normal range rapidly and
usually before the 6-mm zone . The S-shape is an
indicator of ectatic corneal disorders . In advanced
cases of PMD, the curve usually follows an inverted
course .

243. QUICK 12 POINT READING ON
PENTACAM

248. pentacam

273. BFS>8 ANT ; >18 POST

274. DECISION
PRK
NO RISK DO PRK OR LASIK

275. K READINGS

276. K READINGS

277. K READINGS

278. K READINGS

279. K READINGS

280. K READINGS

281. K READINGS

282. K READINGS effect on flap properties

283. Some typical
presentations

284. Keratoconus
• Keratoconus is a bilateral, noninflammatory
degeneration of the eye characterized by
paracentral corneal ectasia and steepening, high
keratometry values, and often against the- rule
astigmatism. The thinning is reported to originate
in the stroma of the cornea.

285. Keratoconus
• Early keratoconus presents on topography as
inferior or central steepening, with or without a
difference compared with the fellow eye. Mild
astigmatism may be present. Several indices have
been developed to differentiate these eyes from
normal eyes, such as the inferior-superior (I-S)
difference, but a high false-positive rate exists.
Although these indices may be helpful, the
diagnosis remains largely dependent on pattern
recognition on the part of the clinician.

289. Keratoconus
• keratoconus is a disease that progresses anteriorly
through the cornea, starting at the posterior
surface and progressing toward the anterior
surface. Therefore, the anterior elevation map
appearance is similar to that of the posterior
surface, but this is seen later in the disease
process. WE consider anterior elevation to be
abnormal if it is > +4 μm at the thinnest point or
• > +6 μm at the anterior apex.

292. Keratoconus
• Keratoconus is a disease process that is first
evident in the posterior corneal surface; it then
moves anteriorly through the corneal surface until
it is evident anteriorly. Therefore, posterior
elevation maps are useful in detecting the earliest
cases of subclinical keratoconus. As with anterior
elevation maps, we have chosen to use a best fit
sphere as the reference surface . WE consider
posterior elevation of > +19 μm at the thinnest
point or > +6 μm at the posterior apex to be
abnormal .

295. Keratoconus
• Eyes with keratoconus, or subclinical keratoconus,
typically have thinner corneas than normal eyes.
Keratoconic eyes also have a more progressive
increase in corneal thickness from the center to the
periphery. In other words, there is a more rapid
increase in thickness when moving from the center to
the periphery in eyes with keratoconus than there is in
normal eyes. Furthermore, the thinnest point of a
keratoconic eye typically is inferior to the center of the
cornea, which is known as inferior displacement. The
pachymetric map on the Pentacam is useful to detect
these differences in eyes with possible keratoconus .

298. Keratoconus
• The Pentacam offers the ability to view measured
tomographic data in many different formats, depending
on the needs of the clinician. The four-map view is the
“standard” view of the Pentacam. The four-map
refractive view presents 4 maps that are most useful to
clinicians screening patients for refractive surgery.
This view is useful because it shows the traditional
axial power map, anterior elevation map, posterior
elevation map, and pachymetric map. Each map
provides valuable data regarding the health and
structure of the cornea. When viewed together as a
group, a tremendous amount of data are available to
the clinician on one page.

305. Pellucid Marginal Degeneration
• Similar in findings to keratoconus, pellucid
marginal degeneration is characterized by inferior
peripheral thinning of cornea due to an idiopathic,
noninflammatory condition. Generally thought to be
a distinct condition from keratoconus, many
experts now consider PMD to be the same process
as keratoconus but occurring in a different area of
the cornea, thus generating a unique topographic
appearance .

306. Pellucid Marginal Degeneration
• Although high amounts of against-the-rule
astigmatism are usually found, typically there is
normal central corneal thickness and an intact
central epithelium and, thus, lack of corneal
scarring. Corneal topography remains the gold
standard for diagnosis. Although the crab-claw
appearance may be present in patients’ topography
maps for both keratoconus & PMD additional
elevation maps and locations of corneal thinning
can differentiate the 2 degenerations.

307. Pellucid Marginal Degeneration
• Pellucid marginal degeneration typically has a 1-
to 2-mm wide band, or strip, of thinning that is
more peripherally located, whereas keratoconic
patients usually have an inferior temporal spot or
area of thinning in a cone shape.

316. PRK or LASIK
• Eyes with a history of prior excimer laser surgery,
either PRK or LASIK, have similar appearances. In
eyes with prior myopic treatment, there is central
flattening relative to the periphery. This results in
flatter simulated K readings on the axial power
maps. The opposite is true of eyes with prior
hyperopic treatment because tissue is removed
from the periphery to induce a relative steepening
centrally. Anterior elevation maps are particularly
important in eyes with prior excimer laser surgery,
as excimer laser procedures remove tissue from
the anterior corneal surface .

333. Post Refractive Surgery Ectasia
• Ectasia is described as progressive thinning and
steepening of the cornea, resulting in irregular
astigmatism and loss of best-corrected visual
acuity Topographically, post refractive surgery
ectasia resembles keratoconus. The hallmarks are
inferior steepening on the axial power map, as well
as elevation abnormalities noted first posteriorly
and later anteriorly. However, the presence of
central flattening from myopic excimer laser
surgery complicates topographic interpretation.

347. THANK
YOU
DR DINESH
DR SONALEE