TOPOGRAPHY

1. DR.SALEH ALGHAMDI

2.  Topos: Place
 Graphien: To write
 Description or presentation of the features of a
place (the cornea) in detail
 Diagnostic tool adjunctive to clinical diagnosis

3.  Keratometry (1728) von Helmholtz / Javal –
Schiotz
 Placido disc (1880)
 Keratoscope (1911) – Gullstrand
 Computerized video keratography (1988) Klyce
 Slit topographer (orbscan – pentacam)
 Corneal aberrometry

4.  The topographer uses
the 1st Purkinje image of
the anterior corneal
surface as a convex
mirror
 Mires are the image of
the rings
 The topographer
computes and analyzes
the shapes and
relationships of the
mires

6.  The topographer uses placido disk technology, to plot
approximately 7000 data points over the entire corneal
surface
 Combining video imaging techniques and computer
processing algorithms the topographer captures the image
of reflected rings of light from the cornea and analyzes
thousands of data points to plot the corneal contour, shape
and refractive power

7.  Calculations are done in terms of mm radius of
curvature for each point
 Mm is converted into Diopter
 Shape of the cornea is transformed into color
coded maps representing the Axial power of
the cornea (the warmer the color, the higher the
power and vice versa)

8.  Absolute: 9-101 D with
relatively large
increments/steps; gives the
overall quality of the
cornea; used for screening
(gross picture)
 Normalized /adjustable:
lower range, smaller scale
(0.25-0.75 D
increments/steps),
sensitive to subtle changes.
Good for details and
detection of keratoconus
suspect cases; much
affected by noise such as
nebulae and dryness.

9.  Assigns a specific color to each diopteric value
 Allows direct comparison of images from
different eyes, or from SIGNIFICANT
curvature changes in one eye (e.g. pre- vs. post-
refractive surgery status)
 Downside: the diopteric range is greatly
expanded; hence, clinically significant
irregularities may become somewhat obscured

10.  Subdivides the cornea into diopteric intervals
based on its actual curvature range
 Actual colors are not specific to a certain
diopteric value, but rather are relative to that
particular patient’s eye
 So, two significantly diopterically different
eyes might have maps that look similar, if the
curvature is more or less similar

11. Clinically, it is probably best to use normalized
maps when evaluating one particular eye, and use
absolute maps when comparing two different eyes
or comparing the same eye over time
So, Absolute Color Scale is used for serial comparison
purposes. Otherwise, the “normalized” or
“Individual” scale setting will not be useful since
the colors will not associate with the same
diopteric power from exam to exam. Therefore,
comparisons are misleading

13. 1. Axial Curvature Map
2. Instantaneous Curvature Map
3. Refractive Power Map
4. Elevation Map
5. Eye Image
6. OPD Map
7. Wavefront Total Map
8. Wavefront HO Map

14.  The traditional curvature map
 Shows the general surface shape of the cornea
 It is derived from curvature (millimeters)
measurements converted into Diopters of
Power
 The misuse of the term Power Map over the
years has led to confusion when comparing it
to the Refractive Power Map

15.  The AXIAL map generates corneal curvature at all points and expresses it
in Dioptric Power
 Calculated by forcing the center of each curve fit at each measurement
point on the cornea through the camera’s optical axis and then converts
the curvature to diopteric powe using the keratometer formula to give a
K reading for measured points
 Corneal index of refractions (n’) = 1.3375
Given radius @ a specific point = 7.67 mm
Corneal Power = n’ - n = = 1.3375 -1 = + 44 D
r 0.00767m
 It tends to underestimate changes in the mid and peripheral zones
 AXIAL traditionally has been used to assess the central 4mm zone

16.  Warm colors such as red and orange show
steeper areas; cool colors such as blue and
green denote the flatter areas.
 The axial map gives a global view of the
corneal curvature as a whole. Its downside is
its tendency to ignore minor variations in
curvature
 Because of its limitations and central bias,
clinicians have decided to look at additional
topography maps to assess corneal power

19.  Displays the cornea as a topographical
illustration, using colors to represent changes
in diopteric values
 Uses different calculation method (based on
angle theta calculation)
 More acurately determines the peripheral
corneal configuration
 More closely represents corneal curvature over
axial map
 Recognizes sharp power transitions easily

20.  It is the better map to use for defining
transition zones from the 4 to 12 mm zone
 Effectively defines points of curvature change,
resulting in clearly defined, small or
"instantaneous" curvature changes
 All pre- and post- corneal refractive surgery
cases are best viewed with this map (clear
transition zones)
 However, like the axial map, it underestimates
the refractive power because data is collected
more at the periphery

22.  Both Axial and Iinstantaneous Maps calculate
Dioptric Power from corneal curvature (thru
different calculations):
 Radius of curvature of 7.5mm = 45 Diopters
and 8.0mm = 42 Diopters
 The shorter or smaller (mm) curve = Steeper
cornea = Greater Diopters = Warmer Colors
 The longer or larger (mm) curve = Flatter
cornea = Lower Diopters = Cooler Colors
 The AXIAL map does not reveal the transition
zones as well as the Instantaneous Map

23.  Uses Snell’s Law to quantify the true Refractive
Power of the cornea at each point using a ray
tracing calculation
 Like curvature maps, the Refractive Power
map displays power and allows the clinician to
see changes in corneal power over the surface
in units that directly correlate with the patient’s
refraction

24.  The central portion of the refractive map is
most important. This area overlies the pupil, so
aberrations here almost invariably impact
visual performance
 This view identifies central islands in patients
who have undergone PRK or LASIK

25.  Normal Corneas = 44 Diopters = Set as Green
color
 Steeper Corneas = Greater than 46 Diopters
(Suspect Keratoconus if greater than 47-48 D)
 Flatter Corneas = Less than 42 Diopters (most
post-op LASIK-Myopia)

27. Axial and instantaneous curvature maps denote
“curvature” in terms of steepness and flatness
without any indication of the “direction”.
Refractive power maps generate dioptric power
values. A cornea that is steep with higher dioptric
values does not indicate whether the steepness is
“upward” (Keratoconus) or “downward” (tissue
removal or astigmatic shape). Curvature maps and
power maps are not shape maps. Elevation maps
“are” shape maps

28.  Shows the measured height from which the
corneal curvature varies (above or below) from a
computer-generated reference surface. Warm
colors depict points that are higher than the
reference surface; cool colors designate lower
points
 This map is most useful in predicting
fluorescein patterns with rigid lenses. Higher
elevations (reds) represent potential areas of lens
bearing, while the lower areas (greens) will likely
show fluorescein pooling
 Spherical elevation map compares the cornea to
a best fit sphere.Elliptical elevation map compares
the cornea to a ellipse(a better method)

29.  So, some points are higher than the best-fit
sphere and some points are lower than the
best-fit sphere. Some clinicians refer to this as
the deviation from the best-fit sphere map. As
such, the higher points are traditionally
displayed in warmer colors and the lower
points are displayed in cooler colors. The green
color is set at “0” elevation. The Elevation Map
uses a scale in microns of height, not diopters!

30. Reference surface (sphere)
Fit-zone

31. Higher elevations are associated with
Keratoconus, LASIK flap edges and hinges,
LASIK and PRK transition zones, Central
islands, Corneal scars, RK & AK corneal
incisions, corneal suture points and other
conditions

32. Lower points are associated with normal
astigmatism, tissue removal, corneal trauma
and irregular surface tissue conditions
associated with corneal transplants.

33.  Clinically significant height changes:
A localized area of 1mm with an associated abrupt
change in elevation of 15 microns or more! A cone can
be suggested with a height increase of 15 microns in a
localized area progressing to 50 microns and beyond
for advanced Keratoconus
Typically, the Axial, IROC or Refractive Power Map
can indicate 47 Diopters and greater for this very same
patient. So, all maps can be used to formulate a clinical
diagnosis = greater “steepening” in the “upward”
direction (cone)

34.  Displays:
1. 2- Dimensional color display
2. 3-Dimensional color display
3. Wire plot display

36.  The shape of the cornea is known using the
Elevation Map. The rate of change in curvature
is computed using the Axial or Instantaneous
radius of curvature maps
 Axial or Instantaneous Map = Steep or Flat
curvature converted into Diopters
 Elevation Map = Higher or lower than the best
fit sphere in Microns of height

37. This is the actual
image of the eye
when the
measurement is
taken. By looking at
the actual eye,
conditions such as
corneal or cataract
opacification can be
identified. Also
displays Photopic
and Mesopic images
in addition to
Placido Ring image

38. The OPD (Optical
Path Difference)
map plots the
refractive error
distribution of
TOTAL eye
aberrations, lower
and higher order, in
Diopters. This map
allows the clinician
to easily determine
the refractive status
and visual quality of
the eye with one
quick look

39. The K values
 Emimeridians: the
power of the
principle meridia
(steepest and flatest
90° apart) @ 3, 5 and 7
mm giving the
quality of sloping and
all in all regularity of
the corneal surface

40.  Simulated (Sim) K:
Analogue to
keratometer
readings
Displays the
spherocylinder
power of the whole
cornea giving the
steepest and flattest
meridia irrespective
to the angle in
between

41.  Many many many indices for prediction of ectasia
that vary between different machines !!!!!!!
 Surface Regularity index (SRI)
 Surface Asymmetry index (SAI)
 Skewing of Steepest Radial Axis (SRAX)
 KC (%)
 KCS (%)
 Prolate Shape Factor (PSF)
 Oblate Shape Factor (OSF)

42.  Contact lens fitting (warpage!).
 Postoperative (keratoplasty, caratct extraction,
etc…) astigmatism.
 Perioperative refractive evaluation.
 Keratoconus screening, diagnosis, evaluation
and postoperative follow up (ICRS and
collagen cross-linking)

44.  Experience !
 History
 Corneal topography can help you differentiate
corneal warpage from true keratoconus. The
key differentiating indicator is shape factor or
eccentricity
 Keratoconic eyes generally have high shape
factors (more than 0.6), while eyes with contact
lens-induced distortion typically show low
prolate (less than 0.1) to oblate (0 to -0.1) shape
factors

46.  Normal cornea:
Spherical/regularly astigmatic with symmetrical
bow tie configuration
 Suspicious cornea:
-Inferior/superior steepening
-Asymmetrical bowtie
-Skewed radial axis
-Combination.
-Steep >48.0D

50.  Myopic ablation:
- Oblate with hot ring

51.  Hyperopic ablation:
-Hyper-prolate, might look keratoconus-like

52.  High K values.
 Localized steepening

53.  Variant of 1ry corneal ectasia.
 Progressive astigmatism against the rule.
 Srax / lazy 8 / butterfly appearace
 Crab claw appearance

56. • Give elevation and curvature information.
• Anterior and posterior cornea surface’s.
• Full cornea thickness
• Scans the eye using light slits that are projected
at a 45-degree angle.
• 40 slits in total.
• Processing and construction of elevation maps
of the anterior & posterior cornea.
• Pachymetry: Diferences in elevation between
the anterior and posterior surface.

58. Reference surface (sphere)
Fit-zone

61.  Prediction of keratectasia utilizing posterior
surface information is debatable:
1. Anterior/posterior radii of curvature>1.2.
2. Posterior best fit sphere>52D.
3. Pachymetry difference @ 7mm zone>100µm.
4. Thinnest point is highest posterior point.
5. Highest posterior point>45µm above post BFS.

62.  80% of aberration on anterior corneal surface
(RMS in µm)

63.  Dryness
 Small surface irregularities
 Periphery
 Post-refractive surgery change of index of
refraction

64.  Using corneal topography for diagnosis:
 Sim K
 Shape of the topography
 Clinical data (including refraction)

65. Thank You !!