Transcript ORB-SCAN-1390

Corneal topography
orbscan
S.A.A MORTAZAVI MD
Associate professor of ophthalmology
Feiz hospital 1390
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ORBSCAN SYSTEM
 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
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Orbscan imaging

Forty slit images are acquired in two 0.7
second periods

Each of the 40 slit images triangulates one
slice of ocular surface

Distance between data slices average 250
microns
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ORBSCAN
 Orbscan
I only slit scan topography
 Orbscan
II the placidodisc added in
orbscan I
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ORBSCAN

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
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Elevation

Orbscan measure elevation

Elevation is important the only complete
scaler measure of surface shape

Both slope & curvature can be
mathematically derived from a single
elevation map
BEST FIT SPHERE (BFS)

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
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Topography quad map
 The
upper left : anterior float
 The
upper right : posterior float
 The
lower left : keratometric pattern
 The
lower right : pachymetry map
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NORMAL BAND SCALE

Highlights the abnormal areas in the
cornea in orange to red colors

The normal areas are all shown in green

Helpful in generalized screening in
preoperative examination
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AXIAL MAP

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
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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

K readings of more than 48 D are an
indication of potential keratoconus
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Middle box

Keratometric readings

White to white distance in mm

Angle kappa readings

The thinnest point of cornea

irregularity within the central 3 mm & 5
mm
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PACHYMETRY MAP
The orbscan measures thickness from the
tear film layer to descemet’s membrane and
is thicker than that obtained with
ultrasound
 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

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PACHYMETRY MAP
 Thinnest
 In
point <470 micron
pathological corneas, thinnest point is
often displaced inferotemporal
 Difference
of >100 microns from the
thinnest point to the values at 7mm
optical zone
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ELEVATION MAP
 The
green colour is referred as
refrence sphere (at sea level )
 The
warmer colours are above this
level and the cooler colours are below
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ANTERIOR ELEVATION MAP

Looking at a proper scale in the cornea ,
can see height differences

Compare the height of the actual cornea
to a best fit sphere
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
 55
D elevation as an absolute cut off
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ELEVATION DATA
 The
difference between the highest
and lowest points is a potential
keratoconus indicator if over 100
microns (Rousch criteria)
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DIAGNOSTIC CRITERIA

Power map changes

Posterior elevation maps

Pachymetry

Composite/integrated topography
information
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POWER MAP
 Mean
corneal power >45D
 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
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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
POSTERIOR ELEVATION MAP
 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
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POSTERIOR ELEVATION MAP
 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
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Posterior float difference

Greater than 50 micron generally
accepted as abnormal

In corneas thinner than normal over 40
as abnormal
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

Posterior float difference;40 to 50 microns
seems to be the maximum difference
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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
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 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
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Risks of ectasia indices
Number of abnormal maps
 Posterior float difference >0.050
 3mm & 5mm irregularity
 Peripheral thickness changes
 Astigmatism variance between eyes
 Steep k’s –mean power map

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Three step rule

One abnormal map ; perform with
caution

Two abnormal map ; with concern
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Three abnormal map ;contraindicated
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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

Inferotemporal displacement of the highest
point
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Abnormal tear film

Can significantly distort the readings
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The significant change in surface quality
and validity of the dry eye
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