NORMAL CORNEA The highest diopter of human eye,about 43D at corneal apex The highest diopter of...
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NORMAL CORNEA The highest diopter of human eye,about 43D at corneal apex The highest diopter of human eye,about 43D at corneal apex Average radius curvature
NORMAL CORNEA The highest diopter of human eye,about 43D at
corneal apex The highest diopter of human eye,about 43D at corneal
apex Average radius curvature of 7.8 mm Average radius curvature of
7.8 mm Is not absolutely transparent,scatters about 10%of the
incident light Is not absolutely transparent,scatters about 10%of
the incident light
Slide 2
CORNEAL GEOGRAPHY The central zone (4mm diameter)almost
spherical and called apex, responsible for the high definition
vision the paracentral zone where the cornea begins to flatten The
peripheral zone the limbal zone
Slide 3
Normal cornea A normal prolate cornea is steep in center and
flat in periphery A normal prolate cornea is steep in center and
flat in periphery the center of the normal cornea is steeper than
the best fit sphere & midperiphery is flatter than reference
sphere the center of the normal cornea is steeper than the best fit
sphere & midperiphery is flatter than reference sphere Central
cornea is red on a normal elevation map and midperiphery appears
blue Central cornea is red on a normal elevation map and
midperiphery appears blue
Slide 4
SUPER VISION Developing new tools and extremely promising laser
surgical techniques that have proven to increase the human beings
VA by reducing corneal aberrations Topographic & aberrometer
linked LASIK are on the way to achieve this goal of better than
normal vision Regularizing the corneal shape by means of reduction
of halos,glare & other optical aberrations The influence of
other dioptric surfaces (vitreous,lens,..)and interfaces still has
to be ascertained
Slide 5
Posterior corneal power Is negative and much smaller than
anterior corneal power. Is negative and much smaller than anterior
corneal power. The posterior surface reduces corneal power The
posterior surface reduces corneal power
Slide 6
6
Slide 7
Instruments to measure the corneal surface Keratometry
Keratometry Keratoscopy Keratoscopy Computerized videokeratoscopy
(topography) Computerized videokeratoscopy (topography)
Slide 8
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 9
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 10
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 11
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 12
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 12
Slide 13
ORBSCAN Combines both slit scan and placido images to give a
very good composite picture from topographic analysis
Slide 14
14
Slide 15
15
Slide 16
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 17
ORBSCAN II Is a fully integrated multidimensional diagnostic
system Is a fully integrated multidimensional diagnostic system
Acquires over 9000 data points in 1.5 seconds to meticulously map
the entire corneal surface Acquires over 9000 data points in 1.5
seconds to meticulously map the entire corneal surface Analyze
elevation & curvature measurement on both the anterior &
posterior surfaces of the cornea Analyze elevation & curvature
measurement on both the anterior & posterior surfaces of the
cornea 17
Slide 18
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 18
Slide 19
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 20
ORBSCAN A three dimensional slitscan topographic A three
dimensional slitscan topographic Orbscan detect the abnormalities
on the posterior surface of the cornea Orbscan detect the
abnormalities on the posterior surface of the cornea
Slide 21
21
Slide 22
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 23
23
Slide 24
ORBSCAN II Is based on slit scanning technology in addition to
traditional placido-based technique Is based on slit scanning
technology in addition to traditional placido-based technique
Placidodisc improve the accuracy of the curvature measurements and
give information on axial keratometric readings Placidodisc improve
the accuracy of the curvature measurements and give information on
axial keratometric readings 24
Slide 25
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
25
Slide 26
Elevation Orbscan measure elevation Orbscan measure elevation
Elevation is important the only complete scaller measure of surface
shape Elevation is important the only complete scaller 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 27
ORBSCAN Slit beam scanners and triangulation are used to derive
the actual spatial location of thousands of points on the surface
Slit beam scanners and triangulation are used to derive the actual
spatial location of thousands of points on the surface Each beam
sweep across the cornea gives information on corneal elevation or
height from the anterior corneal surface, posterior surface &
iris Each beam sweep across the cornea gives information on corneal
elevation or height from the anterior corneal surface, posterior
surface & iris 27
Slide 28
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
28
Slide 29
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 30
30
Slide 31
31
Slide 32
32
Slide 33
33
Slide 34
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 35
35
Slide 36
POWER MAP = AXIAL MAP A familiar sagital map from placido
system A familiar sagital map from placido system The mean power
map determines the location of a surface abnormality The mean power
map determines the location of a surface abnormality Normal
astigmatism appears a classic bowtie Normal astigmatism appears a
classic bowtie
Slide 37
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 37
Slide 38
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 38
Slide 39
39
Slide 40
40
Slide 41
41
Slide 42
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 43
43
Slide 44
Pachymetry Ultrasound provides a reading of corneal thickness
from bowmans membrane to descemets membrane Ultrasound provides a
reading of corneal thickness from bowmans membrane to descemets
membrane Orbscan provides a reading from the precorneal tearfilm to
the endothelium,slightly thicker readings can be expected Orbscan
provides a reading from the precorneal tearfilm to the
endothelium,slightly thicker readings can be expected 44
Slide 45
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 46
46
Slide 47
PACHYMETRY Provides thickness information the cornea from
limbus to limbus Provides thickness information the cornea from
limbus to limbus The relationship between pachymetry readings can
be looked,100 micron should be a cut-off criteria The relationship
between pachymetry readings can be looked,100 micron should be a
cut-off criteria 47
Slide 48
pachymetry Central pachymetry compare to peripheral readings
considered abnormal if : -the peripheral readings are not at least
20 micron thicker The thinnest reading less than 30 micron
thinner
Slide 49
PACHYMETRY MAP Thinnest point 100 microns from the thinnest
point to the values at 7mm optical zone 49
Slide 50
ELEVATION MAPS The anterior elevation map ;the top left hand
map The anterior elevation map ;the top left hand map The posterior
elevation map ;the top right hand map The posterior elevation map
;the top right hand map Slit scanning provides elevation data, and
also can create a 3D interpretation of the cornea Slit scanning
provides elevation data, and also can create a 3D interpretation of
the cornea 50
Slide 51
ELEVATION MAP The green colour is referred as reference sphere
(at sea level ) The green colour is referred as reference 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 51
Slide 52
52
Slide 53
53
Slide 54
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 55
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 55
Slide 56
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) 56
Slide 57
DIAGNOSTIC CRITERIA Power map changes Power map changes
Posterior elevation maps Posterior elevation maps Pachymetry
Pachymetry Composite/integrated topography information
Composite/integrated topography information 57
Slide 58
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 58
Slide 59
Irregularity in central cornea 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 1.5 D in 3 mm zone and greater than
2.0 D in 5 mm zone is considered abnormal and cause for
concern
Slide 60
ELEVATION MAPS A ratio can be calculated between the posterior
and anterior surfaces,which gives an indication of the relative
difference in curvature between the two maps A ratio can be
calculated between the posterior and anterior surfaces,which gives
an indication of the relative difference in curvature between the
two maps 60
Slide 61
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
61
Slide 62
62
Slide 63
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 63
Slide 64
POSTERIOR ELEVATION MAP Increased forward shift of the
posterior corneal surface is common after myopic LASIK and
correlates with the residual corneal thickness Increased forward
shift of the posterior corneal surface is common after myopic LASIK
and correlates with the residual corneal thickness 64
Slide 65
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 66
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 67
67
Slide 68
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 68
Slide 69
69
Slide 70
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 70
Slide 71
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 72
72
Slide 73
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 74
74
Slide 75
75
Slide 76
76
Slide 77
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 77
Slide 78
78
Slide 79
79
Slide 80
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