Topography orbscan
1 / 29

TOPOGRAPHY ORBSCAN - PowerPoint PPT Presentation

  • Uploaded on

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

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' TOPOGRAPHY ORBSCAN' - arnav

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Topography orbscan


S.A.A. Mortazavi MD.

Associate Professor of Ophthalmology

Isfahan University of Medical Sciences

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 I only slit scan topography

  • Orbscan II the placidodisc added in orbscan I


  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

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

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

  • 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