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Placido Based Corneal Topography. Mohammad Ghoreishi , MD Isfahan University of Medical Sciences Isfahan, Iran. Topography. Topography is the science of describing or representing the features of a particular surface. Corneal topography. Computer- assisted videokeratoscopy

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slide1

Placido Based Corneal Topography

Mohammad Ghoreishi, MD

Isfahan University of Medical Sciences

Isfahan, Iran

topography
Topography

Topography is the science of describing or representing the features of a particular surface

corneal topography
Corneal topography
  • Computer- assisted videokeratoscopy
  • The most accurate and powerful tool for measurement of corneal surface
the importance of anterior corneal surface
The importance of anterior corneal surface
  • The anterior cornea is the major refractive surface of the eye
  • Average refractive power is 43 D

(+49 D for anterior surface and –6 D for posterior surface)

parameters for description of corneal shape
Parameters for description of corneal shape
  • Surface slope
  • Radius of curvature
  • Power
  • Elevation
curvature and power
Curvature and power
  • Curvature
    • A measurement of shape of the cornea
    • Geometric property
  • Power
    • A measurement of refractive effect of the cornea
    • Functional property
  • Radius of curvature is more accurate than power
slide7

Conversion of curvature to power

  • Snell’s law
  • p = (n2-n1)/r
  • Where n1 is the refractive index of the first medium (air = 1) and n2 is the refractive index of the second medium (cornea = 1.376)
slide8

ANSI: Geometry and Optics

  • Curvatureis always geometric (a property of surface shape) and can only be applied to surfaces individually.
  • Poweris always optical (a property of the surface shape, refractive index, and illumination incidence) and can be applied to any sequence of surfaces.
  • Although the conventional color maps (axial and tangential) are expressed in diopters (the unit of optical power), they actually display surface curvature. Henceforth these “power” maps will be called curvature maps.
standard keratometric index
Standard keratometric Index
  • Refractive index of corneal stroma is 1.376
  • However, the curvature of posterior corneal surface is not easy to measure
  • Therefore the Standard kexratometric Index (SKI=1.3375 ) is an approximation to take account both corneal surfaces
  • p = (1.3375-1)/r
  • P = 0·3375/r
inaccuracy of power
Inaccuracy of power
  • Conversion formula assumes:
    • Spherical optics
    • The curvature of posterior cornea to be normal
    • Cornea to be normal thickness
    • Cornea to have uniform RI
radius of curvature
Radius of curvature
  • Global (axial or sagital)
  • Local (tangential or instantaneous)
global radius of curvature
Global Radius of curvature
  • Calculates the curvature of the cornea radially at points along each of the meridians
  • It measures the perpendicular distance from the tangent at a point to the optical axis
  • The algorithms have a spherical bias
slide13

Radius of Curvature

Global / Axial / Sagital

Local / Instantaneous / Tangential

X

X

Y

Y

r

r

X

X

r

r

Y

Y

methods of measurement of corneal shape
Methods of measurement of corneal shape
  • Keratometry
  • Keraoscopy
  • Photokeratoscopy
  • Videokeratoscopy
photokeratoscopes
Photokeratoscopes

No elliptic distortion of the mires due to irregular astigmatism

slide20

Reflection-based Topography

Optics of convex mirrors

measuring radius of curvature
Measuring radius of curvature

Magnification=M = I/O = v/u

v = u X I/O

v= r/2

r/2 = u X I/O

r = radius of curvature = 2u X I/O

slide22

Measurement of Topography

Radius of curvature / Power

evaluation of mires and color map
Evaluation of mires and color map

Pear-shape distortion of mires representing irregular astigmatism due to pterygium

Color map shows irregular pattern

slide25

Color coding

  • The most useful form of data presentation is a color-coded corneal contour map
  • Steep areas are depicted as "hot colors," such as reds and browns, and flat areas as "cool colors," such as blues and greens
color scales
Color scales
  • Absolute scale
  • Normalized scale
  • Adjustable
slide28
The normalized scale should NOT be used for Keratoconus Suspects It emphasizes features that are not clinically significant
axial map
Axial Map
  • Axial curvature (formerly termed sagital curvature) measures the curvature at a certain point on the corneal surface in axial direction relative to the center
  • The axial map shows the corneal curvature calculated with respect to the optical axis of the cornea.
  • Axial topography is similar to manual keratometry in that it is most accurate within the central portion of the cornea.
slide31

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