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TOPOGRAPHY ORBSCAN. S.A.A. Mortazavi MD. Associate Professor of Ophthalmology Isfahan University of Medical Sciences. ORBSCAN SYSTEM. Use the principle of projection

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Topography orbscan

TOPOGRAPHYORBSCAN

S.A.A. Mortazavi MD.

Associate Professor of Ophthalmology

Isfahan University of Medical Sciences


Orbscan system
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


Orbscan imaging
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


Orbscan
ORBSCAN

  • Orbscan I only slit scan topography

  • Orbscan II the placidodisc added in orbscan I


Orbscan1
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


Best fit sphere bfs
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


Normal band scale
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


Axial map
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


Pachymetry map
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


Pachymetry map1
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


Elevation map
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


Elevation data
ELEVATION DATA

  • The difference between the highest and lowest points is a potential keratoconus indicator if over 100 microns (Rousch criteria)


Posterior elevation map
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


Posterior elevation map1
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

  • In corneas thinner than normal over 40 as abnormal


Orbscan diagnostic parameters for k c
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


Risks of ectasia indices
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


Three step rule
Three step rule

  • One abnormal map ; perform with caution

  • Two abnormal map ; with concern

  • Three abnormal map ;contraindicated


ضمن عرض پوزش

بدلیل حجم بالای LECTUER ادامه اسلایدها امکان پذیر نمیباشد در صورت نیاز به ادامه لطفا به واحد سمعی و بصری مرکز آموزشی درمانی فیض مراجعه و یا با شماره تلفن 03114476010 داخلی 392تماس حاصل نمائید

با تشکر


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