TOPOGRAPHY
ORBSCAN
S.A.A. Mortazavi MD.
Associate Professor of Ophthalmology
Isfahan University of Medical Sciences
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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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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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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
 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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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 point <470 micron
 In 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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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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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
•
A best fit sphere (BFS) >55D on the posterior profile ,
indicative of posterior ectasia
•
Greater than 50 micron generally accepted as
abnormal
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In corneas thinner than normal over 40 as abnormal
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Orbscan diagnostic parameters for K.C
• Post BFS >55D
• Difference pachymetry between thinest point & thickest
point in 7 mm zone >100 µ
• Diff >45µ
• Mean k >47 D
• I-S index >1.5D
• Distance from corneal apex to thinest point >0.9 mm
• Thinest point <470 µ
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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
 Three abnormal map ;contraindicated
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