TOPOGRAPHY-ORBSCAN
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Transcript TOPOGRAPHY-ORBSCAN
TOPOGRAPHY
ORBSCAN
S.A.A. Mortazavi MD.
Associate Professor of Ophthalmology
Isfahan University of Medical Sciences
2/22/2013
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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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ضمن عرض پوزش
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