pentacam and elevation based topography

1. Bipin koirala
Master’s of Optometry
Himalaya Eye Institute

2.  Introduction
 Elevation based topography
 Interpretation
 Summary
 References

3. Anterior Surface
Dh- 11.5mm
Dv- 10.6mm
Posterior surface
Dh=Dv- 11.5mm
Diameter
Center- 0.52mm
Periphery- 0.8mm
Limbus- 1.2mm
Thickness
Human corneal
surface is aspheric
(PROLATE).
Central optic zone
radii of curvature
Anterior- 7.8mm
Posterior- 6.8mm
Radii of curvature

5. Cornea is the most powerful refractive
element of the eye
Contributes about 43D(70%) of refractive
power of eye out of the total 60D
Even a minor modification on its surface can
lead to a significant alteration of the images
formed on the retina

6. Any distortion in the corneal
surface leads to reduced quality
retinal image.

7. Keratometers and Ophthalmometers have
proven to be useful when one limits the
measurement of corneal power in
Spherocylindrical notations
Keratometer
Keratometer
Mires

8. Topos
Graphein
Place
To Draw
Corneal Topography corresponds to the graphic
representation of the geometrical properties of
corneal surfaces.

9. Preoperative and
Post operative
assessment of
refractive
patient
LASIK
Preoperative and
Post operative
assessment of
penetrating
keratoplasty
CORNEAL TRANSPLANT
Irregular
Astigmatism
ECTATIC CONDITIONS

10. Corneal
dystrophies
& Bullous
keratopathy
CORNEAL DYSTROPHIES
Keratoconus
(diagnostic
and follow-
up)
ECTATIC CONDITIONS
Follow-up of
corneal
ulceration
and abscess
ULCERATION

11. Contact lens fitting
ORTHO K
CORNEO SCLERAL
CONTACT LENS FIT
Evaluation of tear
film quality
ACCESSORY USE
To study unexplained
low visual acuity after
any surgical
procedure
( Trabeculectomy,
Extracapsular lens
extraction,….)
POST SURGICAL USES

12. Post Traumatic
Corneal Scarring
ABBERATION ANALYSIS
Reference
instrument for
IOL-implants to
see the corneal
difference before
and after surgery
TORIC IOL FIT

13. Topography
principles
Projection
based
Reflection
based
Elevation
based

14.  The area of corneal coverage is limited to about 60%
of the corneal surface (i.e. PMD, keratoconus)
 There is no information about the posterior corneal
surface.
 Without measurement of the anterior and posterior
surfaces, Pachymetric maps cant be displayed.

18.  The basic determinant of corneal optics is the shape
of the cornea and lens and the index of refractive of
these structures.
 The traditional curvature maps however, do not
measure shape directly. (indirect measurment)
 True “topography” implies shape and requires the
generation of an X, Y and Z coordinate system.

19.  Placido-based systems create such a coordinate
system from curvature data by making geometric
assumptions about the cornea.
 A better way to determine shape would be to
measure the X,Y and Z coordinates directly (done by
elevation based system)
 Currently, a elevation based systems use a form of
optical cross sectioning to triangulate both the
anterior and posterior corneal surfaces

21. Bausch and Lomb ORBSCAN
•1995, the Orbscan Optical cross-sectioning to measure
elevation
•Combines a slit scanning system and Placdo’s disk
Bon Sirius Topographer
•3D Rotating Scheimpflug Camera & placido based
Topography System.
•Allows precise measurement of the anterior chamber
angle and chamber depth, keratometry, pachymetry
OCULUS PENTACAM
•3D Rotating Scheimpflug Camera
•True elevation based device available today

24.  Scheimpflug technique captures images with a
better spatial accuracy than a traditional camera
containing a coaxial lens and film optical system.
 On the Pentacam, the Scheimpflug camera rotates
180 degrees around a single point of fixation as
the patient focuses on a central light source.
 By rotating around the point of fixation, it reduces
the artifact created by small movements during
image acquisition.

26.  In the ordinary camera the main disadvantage is limited
depth of focus because the picture plane, the objective
plane and the film plane are parallel.
 The Scheimpflug camera has higher depth of focus,
sharp image but distorted.
 The picture plane, the objective plane and the film
plane cut each other in one line or one point of
intersection.

28.  In addition, the Pentacam contains a second
camera that detects any residual eye movements,
which are corrected for by the Pentacam’s software.
 The Scheimpflug camera completes several
rotations before coming to a steady speed lacking
any acceleration or deceleration.
 Capturing images at a fixed speed reduces
vibrations and image artifact that might occur
during acceleration or deceleration.

29.  One centrally located camera also detects pupil
size and orientation, and controls fixation.
 The second while rotating180 degrees it captures
25 or 50 images of the anterior segment to the
level of the iris, and through the pupil to evaluate
the lens

30.  500 true elevation data points are generated per
image to yield up to 25,000 points for each
surface.
 Data points are captured for the center of the
cornea, an area that placido disc topographers and
slit scanning devices are unable to evaluate.
(Based on an elevation map)

31.  A reference sphere / computer calculated
hypothetical sphere
 Used to compare the real surface to the
hypothetical sphere showing areas above the
surface of the sphere in warm color and areas
below the surface in cool color
 BFS diameter is based on average corneal radius
value

33.  While interpreting colour-coded contour maps of the
cornea, following parameters should be considered
 Hot colours, i.e., Red and its various hues represent
the steepness of the cornea.

34.  Cool colours, i.e., blue and its various hues represent
the flat portions of the cornea
 So the colours red-orange-yellow-green-purple-blue
denote progressively lessening refractive power.

35. Scales
Absolute
Normalizd
Adjustable
Klyce/
Wilson
Maguire
/Waring

36. PPENTACAM MAP

37.  24-colour representation of dioptric power
at various points on the cornea.

38. The curvature power of the measured surface in point “a”
is calculated using a tangent line in this point, the normal
in this point intersects the reference axis at point b, ab is
the radius (r) of point “a”, finally the equation is applied to
calculate the power (K) at point a.

39.  The computer displays this power as a colored map
with the used color scale beside
 This is applied on either the front or back surfaces
of the cornea.
 Values of the back corneal surface power are
displayed as negative digits

40. Axial curvature closely approximates the power of the
central 1-2 mm of the cornea
Fails to describe the true shape and power of the
peripheral cornea.

41. For almost sphere cornea the axial curvature remains
almost the same as the curvature is almost the same
from the centre to the periphery.
In fact, corneas are not always spherical.
◦This is not a good descriptor in corneal topography.

42.  A better geographic representation of the cornea than
the axial/saggital map
 Principle depends on tangent circles rather than
straight lines.

43.  This method uses circle tangential to the examined
corneal surface at the points to be measured.
 The radius of the tangential circle is considered as the
curvature radius, on which the equation can be applied.
 Radii of circles differ according to surface changes and
geometrically, the specifications of the tangent circles
are more accurate than those of the tangent lines.
 Consequently, this method can highlight any irregularity
in the cornea whatever small it is

44. Elevation Map
 Elevation is not measured by Placido-based
topographers, but certain assumptions allow the
construction of elevation maps
 Elevation of a point on the corneal surface
displays the height of the point on the corneal
surface relative to a spherical surface.
 The reference surface is mostly considered to be
a sphere, or, to be precise, a Best Fit Sphere (
BFS)

46. • As the cornea has no natural reference surface,it is
imperative to create an artificial one depending on the
mean central radii of the examined surface.
• The distance above and below the reference surface
gives anterior and posterior elevation readings
contd..

47. The BFS (green) is the closest sphere to the corneal surface. Once
the sphere radius and location are positioned, elevation is plotted
as the distance from the surface to the sphere.

48. The relationship between reference body and cornea. Corneal surface
(yellow) has steep center and flat periphery in accordance to this
particular reference body.

49.  Optical power maps (or ‘refractive maps’) are
functional maps and should not be confounded with
curvature maps.
 Since the cornea suffers aberrations, the refraction
varies along the meridians even if the curvature does
not change owing to spherical aberration esp positive
spherical aberration

51. The light rays near center of cornea have a small angle
of incidence relative to surface normal, and thus
refracted angle is also quite small.
Contrarily, light rays in paracentral and peripheral
cornea have a larger angle of incidence relative to
surface normal, and thus refracted angle is larger.

54.  This icon specifies the quality
of the topographic capture &
should be displayed“OK”.
 If we have “data” displayed
in yellow then it means that
some information was missed

55. ≤47.20D

58. • Topographic astigmatism is summated vector
mean of the astigmatism values determined
from a large number of adjacent concentric
Placido rings
• Manifest astigmatism is the vectorial sum of
anterior corneal toricity and internal
astigmatism.

61. ASPHERICITY (Q)
T
orquetti, L

64.  Symmetric shapes
 Asymmetric shapes
 Skew shapes
 Special shapes

78. >22º

81.  Step by step reading pentacam
 Dr Agrawal’s book of corneal topography
 Elevation based Topography