CORNEAL TOPOGRAPHY Wide acceptance as a clinical examination procedure with the advent of modern...
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CORNEAL TOPOGRAPHY Wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery Measure a greater area of the cornea with a much higher number of points and produce permanent records
CORNEAL TOPOGRAPHY Wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery Measure a greater area of the
CORNEAL TOPOGRAPHY Wide acceptance as a clinical examination
procedure with the advent of modern laser refractive surgery
Measure a greater area of the cornea with a much higher number of
points and produce permanent records
Slide 2
Topographic techniques Reflection perform indirect measurement
of the corneal surface Reflection perform indirect measurement of
the corneal surface Projection visualize the corneal surface
directly Projection visualize the corneal surface directly
Slide 3
PLACIDODISC Illuminates the cornea by sending a mire of
concentric rings Illuminates the cornea by sending a mire of
concentric rings A videocamera captures the corneal reflex from the
tear layer A videocamera captures the corneal reflex from the tear
layer A computer & software perform the analysis of the data
through different algorithms A computer & software perform the
analysis of the data through different algorithms
Slide 4
Keratometry and corneal topography with placidodisc systems
were originally invented to measure anterior corneal curvature
Keratometry and corneal topography with placidodisc systems were
originally invented to measure anterior corneal curvature The
problem in the placidodisc system is that cannot perform a slit
scan topography The problem in the placidodisc system is that
cannot perform a slit scan topography
Slide 5
TOPOGRAPHY corneal topography plays a critical role in
refractive surgery decisions corneal topography plays a critical
role in refractive surgery decisions conventional axial &
tangential topography are not enough to demonstrate a healthy
cornea conventional axial & tangential topography are not
enough to demonstrate a healthy cornea can not decide any type of
laser refractive surgery based only on surface topographic
evaluation can not decide any type of laser refractive surgery
based only on surface topographic evaluation 5
Slide 6
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Slide 7
Artifacts of topography Artifacts of topography Shadows on the
cornea from large eyelashes or trichiasis Shadows on the cornea
from large eyelashes or trichiasis Ptosis or non-sufficient eye
opening Ptosis or non-sufficient eye opening Irregularities of the
tear film layer (dry eye) Irregularities of the tear film layer
(dry eye) Too short working distance of the small placidodisc cone
Too short working distance of the small placidodisc cone Incomplete
or distorted image (pathology) Incomplete or distorted image
(pathology)
Slide 8
ORBSCAN SYSTEM Use the principle of projection Use the
principle of projection Forty scanning slit beams (20 from the left
and 20 from the right with up to 240 data points per slit ) to scan
the cornea and measure independently the X,Y & Z locations
Forty scanning slit beams (20 from the left and 20 from the right
with up to 240 data points per slit ) to scan the cornea and
measure independently the X,Y & Z locations 8
Slide 9
Orbscan imaging Forty slit images are acquired in two 0.7
second periods Forty slit images are acquired in two 0.7 second
periods Each of the 40 slit images triangulates one slice of ocular
surface Each of the 40 slit images triangulates one slice of ocular
surface Distance between data slices average 250 microns Distance
between data slices average 250 microns
Slide 10
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Slide 11
ORBSCAN Orbscan I only slit scan topography Orbscan I only slit
scan topography Orbscan II the placidodisc added in orbscan I
Orbscan II the placidodisc added in orbscan I
Slide 12
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Slide 13
ORBSCAN The images used to construct the anterior corneal
surface,posterior corneal surface,anterior iris and anterior lens
surfaces The images used to construct the anterior corneal
surface,posterior corneal surface,anterior iris and anterior lens
surfaces Data regarding the corneal pachymetry and anterior chamber
depth Data regarding the corneal pachymetry and anterior chamber
depth 13
Slide 14
Elevation Orbscan measure elevation Orbscan measure elevation
Elevation is important the only complete scaler measure of surface
shape Elevation is important the only complete scaler measure of
surface shape Both slope & curvature can be mathematically
derived from a single elevation map Both slope & curvature can
be mathematically derived from a single elevation map
Slide 15
BEST FIT SPHERE (BFS) The computer calculates a hypothetical
sphere that matches as close as possible to the actual corneal
shape being measured The computer calculates a hypothetical sphere
that matches as close as possible to the actual corneal shape being
measured Compares the real surface to the hypothetical sphere
showing areas above the surface of the sphere in warm colours and
areas below the surface in cool colours Compares the real surface
to the hypothetical sphere showing areas above the surface of the
sphere in warm colours and areas below the surface in cool colours
15
Slide 16
Topography quad map The upper left : anterior float The upper
left : anterior float The upper right : posterior float The upper
right : posterior float The lower left : keratometric pattern The
lower left : keratometric pattern The lower right : pachymetry map
The lower right : pachymetry map
Slide 17
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Slide 18
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Slide 19
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Slide 20
NORMAL BAND SCALE Highlights the abnormal areas in the cornea
in orange to red colors Highlights the abnormal areas in the cornea
in orange to red colors The normal areas are all shown in green The
normal areas are all shown in green Helpful in generalized
screening in preoperative examination Helpful in generalized
screening in preoperative examination
Slide 21
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Slide 22
AXIAL MAP Provides detailed keratometric information across the
diameter of the cornea Provides detailed keratometric information
across the diameter of the cornea K readings are between certain
values the cornea must be neither too steep nor too flat K readings
are between certain values the cornea must be neither too steep nor
too flat 22
Slide 23
AXIAL MAP To create a good quality corneal flap in LASIK if
either extremes (too steep or too flat) is the case, this can lead
to surgical flap complications To create a good quality corneal
flap in LASIK if either extremes (too steep or too flat) is the
case, this can lead to surgical flap complications K readings of
more than 48 D are an indication of potential keratoconus K
readings of more than 48 D are an indication of potential
keratoconus 23
Slide 24
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Slide 25
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Slide 26
Middle box Keratometric readings Keratometric readings White to
white distance in mm White to white distance in mm Angle kappa
readings Angle kappa readings The thinnest point of cornea The
thinnest point of cornea irregularity within the central 3 mm &
5 mm irregularity within the central 3 mm & 5 mm
Slide 27
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Slide 28
PACHYMETRY MAP The orbscan measures thickness from the tear
film layer to descemets membrane and is thicker than that obtained
with ultrasound The orbscan measures thickness from the tear film
layer to descemets membrane and is thicker than that obtained with
ultrasound Adjustment factor (acoustic factor),the default setting
is 92% Adjustment factor (acoustic factor),the default setting is
92% Provides a reading showing the thinnest point of the cornea
that may not necessarily be the central reading Provides a reading
showing the thinnest point of the cornea that may not necessarily
be the central reading
Slide 29
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Slide 30
PACHYMETRY MAP Thinnest point 100 microns from the thinnest
point to the values at 7mm optical zone 30
Slide 31
ELEVATION MAP The green colour is referred as refrence sphere
(at sea level ) The green colour is referred as refrence sphere (at
sea level ) The warmer colours are above this level and the cooler
colours are below The warmer colours are above this level and the
cooler colours are below 31
Slide 32
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Slide 33
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Slide 34
ANTERIOR ELEVATION MAP Looking at a proper scale in the cornea,
can see height differences Looking at a proper scale in the cornea,
can see height differences Compare the height of the actual cornea
to a best fit sphere Compare the height of the actual cornea to a
best fit sphere
Slide 35
Posterior map The highest elevation value as a keratoconus
indicator or at least as a screen for patients may be at risk of
developing keratectasia The highest elevation value as a
keratoconus indicator or at least as a screen for patients may be
at risk of developing keratectasia 55 D elevation as an absolute
cut off 55 D elevation as an absolute cut off 35
Slide 36
ELEVATION DATA The difference between the highest and lowest
points is a potential keratoconus indicator if over 100 microns
(Rousch criteria) The difference between the highest and lowest
points is a potential keratoconus indicator if over 100 microns
(Rousch criteria) 36
Slide 37
DIAGNOSTIC CRITERIA Power map changes Power map changes
Posterior elevation maps Posterior elevation maps Pachymetry
Pachymetry Composite/integrated topography information
Composite/integrated topography information 37
Slide 38
POWER MAP Mean corneal power >45D Mean corneal power >45D
In addition to steep corneal curvatures-the bowtie or broken bowtie
appearance indicative of early keratoconus In addition to steep
corneal curvatures-the bowtie or broken bowtie appearance
indicative of early keratoconus Central corneal asymetry a change
within central 3mm optical zone of the cornea of more than 3D
Central corneal asymetry a change within central 3mm optical zone
of the cornea of more than 3D 38
Slide 39
Irregularity in central cornea Greater than 1.5 D in 3 mm zone
and Greater than 1.5 D in 3 mm zone and greater than 2.0 D in 5 mm
zone is considered abnormal and cause for concern greater than 2.0
D in 5 mm zone is considered abnormal and cause for concern
Slide 40
POSTERIOR ELEVATION MAP Many surgeons think the first sign of
keratoconus appears on the posterior surface of the cornea Many
surgeons think the first sign of keratoconus appears on the
posterior surface of the cornea 3.13% of population screened for
laser surgery had posterior ectasia criteria by orbscan, despite
having axial topography classified as normal 3.13% of population
screened for laser surgery had posterior ectasia criteria by
orbscan, despite having axial topography classified as normal
40
Slide 41
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Slide 42
POSTERIOR ELEVATION MAP The most common reference surface for
viewing elevation maps is the best fit sphere The most common
reference surface for viewing elevation maps is the best fit sphere
A best fit sphere (BFS) >55D on the posterior profile,
indicative of posterior ectasia A best fit sphere (BFS) >55D on
the posterior profile, indicative of posterior ectasia 42
Slide 43
Posterior float difference Greater than 50 micron generally
accepted as abnormal Greater than 50 micron generally accepted as
abnormal In corneas thinner than normal over 40 as abnormal In
corneas thinner than normal over 40 as abnormal
Slide 44
Posterior elevation map The location of the steepest part of
the posterior float should be relatively central, but is a more
concern it be located away from the center and in an area of
corneal thinning The location of the steepest part of the posterior
float should be relatively central, but is a more concern it be
located away from the center and in an area of corneal thinning
Posterior float difference;40 to 50 microns seems to be the maximum
difference Posterior float difference;40 to 50 microns seems to be
the maximum difference
Slide 45
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Slide 46
Correlation of signs of the highest point Highest point on the
posterior elevation coincides with the highest point of anterior
elevation, the thinnest on pachymetry and the point of steepest
curvature on the power map Highest point on the posterior elevation
coincides with the highest point of anterior elevation, the
thinnest on pachymetry and the point of steepest curvature on the
power map 46
Slide 47
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Slide 48
Although high posterior elevation and ratio between two
elevation maps rarely used as exclusion criteria alone, but by
considering these together, more conclusive information can be
obtained Although high posterior elevation and ratio between two
elevation maps rarely used as exclusion criteria alone, but by
considering these together, more conclusive information can be
obtained 48
Slide 49
Risks of ectasia indices Number of abnormal maps Posterior
float difference >0.050 3mm & 5mm irregularity Peripheral
thickness changes Astigmatism variance between eyes Steep ks mean
power map
Slide 50
50
Slide 51
Three step rule One abnormal map ; perform with caution One
abnormal map ; perform with caution Two abnormal map ; with concern
Two abnormal map ; with concern Three abnormal map ;contraindicated
Three abnormal map ;contraindicated
Slide 52
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Slide 54
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Slide 55
Composite/integrated information Similarly between anterior
& posterior profiles a forward bending of areas shown above the
BFS and association with the thinnest point on the cornea Similarly
between anterior & posterior profiles a forward bending of
areas shown above the BFS and association with the thinnest point
on the cornea Inferotemporal displacement of the highest point
Inferotemporal displacement of the highest point 55
Slide 56
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Slide 57
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Slide 58
Abnormal tear film Can significantly distort the readings Can
significantly distort the readings The significant change in
surface quality and validity of the dry eye The significant change
in surface quality and validity of the dry eye
