1. NORMAL CORNEA
The highest diopter of human eye,about 43D at corneal apex
Average radius curvature of 7.8 mm
Is not absolutely transparent,scatters about 10%of the incident light

2. CORNEAL GEOGRAPHY
The central zone (4mm diameter)almost spherical and called apex, responsible for the high definition vision
the paracentral zone where the cornea begins to flatten
The peripheral zone
the limbal zone

3. Normal cornea
A normal prolate cornea is steep in center and flat in periphery
the center of the normal cornea is steeper than the best fit sphere & midperiphery is flatter than reference sphere
Central cornea is red on a normal elevation map and midperiphery appears blue

4. SUPER VISION
Developing new tools and extremely promising laser surgical techniques that have proven to increase the human being’s VA by reducing corneal aberrations
Topographic & aberrometer linked LASIK are on the way to achieve this goal of better than normal vision
Regularizing the corneal shape by means of reduction of halos,glare & other optical aberrations
The influence of other dioptric surfaces (vitreous,lens,..)and interfaces still has to be ascertained

5. Posterior corneal power
Is negative and much smaller than anterior corneal power.
The posterior surface reduces corneal power

7. Instruments to measure the corneal surface
Keratometry
Keratoscopy
Computerized videokeratoscopy (topography)

8. CORNEAL TOPOGRAPHY
Wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery
Measure a greater area of the cornea with a much higher number of points and produce permanent records

9. Topographic techniques
Reflection perform indirect measurement of the corneal surface
Projection visualize the corneal surface directly

10. PLACIDODISC
Illuminates the cornea by sending a mire of concentric rings
A videocamera captures the corneal reflex from the tear layer
A computer & software perform the analysis of the data through different algorithms

11. Keratometry and corneal topography with placidodisc systems were originally invented to measure anterior corneal curvature
The problem in the placidodisc system is that cannot perform a slit scan topography

12. TOPOGRAPHY
corneal topography plays a critical role in refractive surgery decisions
conventional axial & tangential topography are not enough to demonstrate a healthy cornea
can not decide any type of laser refractive surgery based only on surface topographic evaluation

13. ORBSCAN
Combines both slit scan and placido images to give a very good composite picture from topographic analysis

16. Artifacts of topography
Shadows on the cornea from large eyelashes or trichiasis
Ptosis or non-sufficient eye opening
Irregularities of the tear film layer (dry eye)
Too short working distance of the small placidodisc cone
Incomplete or distorted image (pathology)

17. ORBSCAN II Is a fully integrated multidimensional diagnostic system
Acquires over 9000 data points in 1.5 seconds to meticulously map the entire corneal surface
Analyze elevation & curvature measurement on both the anterior & posterior surfaces of the cornea

18. 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

19. 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

20. ORBSCAN A three dimensional slitscan topographic
Orbscan detect the abnormalities on the posterior surface of the cornea

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

24. ORBSCAN II
Is based on slit scanning technology in addition to traditional placido-based technique
Placidodisc improve the accuracy of the curvature measurements and give information on axial keratometric readings

25. 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

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

27. ORBSCAN
Slit beam scanners and triangulation are used to derive the actual spatial location of thousands of points on the surface
Each beam sweep across the cornea gives information on corneal elevation or height from the anterior corneal surface , posterior surface & iris

28. 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

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

34. 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

36. POWER MAP = AXIAL MAP A familiar sagital map from placido system
The mean power map determines the location of a surface abnormality
Normal astigmatism appears a classic bowtie

37. 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

38. 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

42. 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

44. Pachymetry
Ultrasound provides a reading of corneal thickness from bowman’s membrane to descemet’s membrane
Orbscan provides a reading from the precorneal tearfilm to the endothelium,slightly thicker readings can be expected

45. 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

47. PACHYMETRY
Provides thickness information the cornea from limbus to limbus
The relationship between pachymetry readings can be looked,100 micron should be a cut-off criteria

48. pachymetry
Central pachymetry compare to peripheral readings considered abnormal if : -the peripheral readings are not at least 20 micron thicker The thinnest reading less than 30 micron thinner

49. 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

50. ELEVATION MAPS The anterior elevation map ;the top left hand map
The posterior elevation map ;the top right hand map
Slit scanning provides elevation data , and also can create a 3D interpretation of the cornea

51. ELEVATION MAP
The green colour is referred as reference sphere (at sea level )
The warmer colours are above this level and the cooler colours are below

54. 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

55. 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

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

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

58. 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

59. 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

60. ELEVATION MAPS
A ratio can be calculated between the posterior and anterior surfaces ,which gives an indication of the relative difference in curvature between the two maps

61. 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

63. 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

64. POSTERIOR ELEVATION MAP
Increased forward shift of the posterior corneal surface is common after myopic LASIK and correlates with the residual corneal thickness

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

66. 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

68. 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

70. 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

71. 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

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

77. 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

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