2. Corneal topography orbscan
S.A.A MORTAZAVI MD Associate professor of ophthalmology Feiz hospital 1390

3. 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

4. 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

6. ORBSCAN Orbscan I only slit scan topography
Orbscan II the placidodisc added in orbscan I

8. 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

9. Elevation Orbscan measure elevation
Elevation is important the only complete scaler measure of surface shape
Both slope & curvature can be mathematically derived from a single elevation map

10. 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

11. Topography quad map The upper left : anterior float
The upper right : posterior float
The lower left : keratometric pattern
The lower right : pachymetry map

15. 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

17. AXIAL MAP
Provides detailed keratometric information across the diameter of the cornea
K readings are between certain values the cornea must be neither too steep nor too flat

18. 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

21. Middle box Keratometric readings White to white distance in mm
Angle kappa readings
The thinnest point of cornea
irregularity within the central 3 mm & 5 mm

23. 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

25. 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

26. 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

29. ANTERIOR ELEVATION MAP
Looking at a proper scale in the cornea , can see height differences
Compare the height of the actual cornea to a best fit sphere

30. Posterior map
The highest elevation value as a keratoconus indicator or at least as a screen for patients may be at risk of developing keratectasia
55 D elevation as an absolute cut off

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

32. DIAGNOSTIC CRITERIA Power map changes Posterior elevation maps
Pachymetry
Composite/integrated topography information

33. POWER MAP Mean corneal power >45D
In addition to steep corneal curvatures-the bowtie or broken bowtie appearance indicative of early keratoconus
Central corneal asymetry a change within central 3mm optical zone of the cornea of more than 3D

34. Irregularity in central cornea
Greater than 1.5 D in 3 mm zone and
greater than 2.0 D in 5 mm zone is considered abnormal and cause for concern

35. 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

37. POSTERIOR ELEVATION MAP
The most common reference surface for viewing elevation maps is the best fit sphere
A best fit sphere (BFS) >55D on the posterior profile , indicative of posterior ectasia

38. Posterior float difference
Greater than 50 micron generally accepted as abnormal
In corneas thinner than normal over 40 as abnormal

39. Posterior elevation map
The location of the steepest part of the posterior float should be relatively central , but is a more concern it be located away from the center and in an area of corneal thinning
Posterior float difference;40 to 50 microns seems to be the maximum difference

41. Correlation of signs of the highest point
Highest point on the posterior elevation coincides with the highest point of anterior elevation , the thinnest on pachymetry and the point of steepest curvature on the power map

43. Although high posterior elevation and ratio between two elevation maps rarely used as exclusion criteria alone , but by considering these together , more conclusive information can be obtained

44. 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

46. Three step rule One abnormal map ; perform with caution
Two abnormal map ; with concern
Three abnormal map ;contraindicated

50. Composite/integrated information
Similarly between anterior & posterior profiles a forward bending of areas shown above the BFS and association with the thinnest point on the cornea
Inferotemporal displacement of the highest point

53. Abnormal tear film Can significantly distort the readings
The significant change in surface quality and validity of the dry eye