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TOPOGRAPHYORBSCAN

S.A.A. Mortazavi MD.Associate Professor of OphthalmologyIsfahan University of Medical Sciences

2/22/2013

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 locations2

ORBSCAN SYSTEM

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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

Orbscan imaging

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Orbscan I only slit scan topography

Orbscan II the placidodisc added in orbscan I

ORBSCAN

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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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ORBSCAN

2/22/2013

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 colours8

BEST FIT SPHERE (BFS)

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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

NORMAL BAND SCALE

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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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AXIAL MAP

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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

PACHYMETRY MAP

2/22/2013

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 zone17

PACHYMETRY MAP

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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 MAP

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The difference between the highest and

lowest points is a potential keratoconus

indicator if over 100 microns (Rousch

criteria)

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ELEVATION DATA

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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

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• A best fit sphere (BFS) >55D on the posterior

profile , indicative of posterior ectasia

• Greater than 50 micron generally accepted as

abnormal

• In corneas thinner than normal over 40 as abnormal

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POSTERIOR ELEVATION MAP

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• 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

Orbscan diagnostic parameters for K.C

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• 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

Risks of ectasia indices

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One abnormal map ; perform with caution

Two abnormal map ; with concern

Three abnormal map ;contraindicated

Three step rule

2/22/2013

پو�زش عرض ضمن

باالی حجم در LECTUERبدلیل نمیباشد پذیر امکان اسالیدها ادامهآموزشی مرکز بصری و سمعی واحد به لطفا ادامه به نیاز صورت

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