1. Placido Based Corneal Topography
Mohammad Ghoreishi, MD
Isfahan University of Medical Sciences
Isfahan, Iran

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

3. Corneal topography Computer- assisted videokeratoscopy
The most accurate and powerful tool for measurement of corneal surface

4. 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)

5. Parameters for description of corneal shape
Surface slope
Radius of curvature
Power
Elevation

6. Curvature and power Curvature Power
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

7. 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)

8. ANSI: Geometry and Optics
Curvature is always geometric (a property of surface shape) and can only be applied to surfaces individually.
Power is 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.

9. 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= ) is an approximation to take account both corneal surfaces
p = ( )/r
P = 0·3375/r

10. 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

11. Radius of curvature Global (axial or sagital)
Local (tangential or instantaneous)

12. 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

13. Radius of Curvature X X Y Y r r r r Global / Axial / Sagital
Local / Instantaneous / Tangential X X Y Y r r X X r r Y Y

14. Methods of measurement of corneal shape
Keratometry
Keraoscopy
Photokeratoscopy
Videokeratoscopy

15. Keratometer

16. Photokeratoscopes
No elliptic distortion of the mires due to irregular astigmatism

17. Placido-type light cone of videokerastoscope

18. Mires images

19. Components of videokerastoscopes

20. Reflection-based Topography
Optics of convex mirrors

21. 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

22. Measurement of Topography
Radius of curvature / Power

23. Mires and color map

24. Evaluation of mires and color map
Pear-shape distortion of mires representing irregular astigmatism due to pterygium
Color map shows irregular pattern

25. 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

26. Color map

27. Color scales
Absolute scale
Normalized scale
Adjustable

28. The normalized scale should NOT be used for Keratoconus Suspects It emphasizes features that are not clinically significant

29. A case of subclinical keratoconus plotted using scales with different step intervals

30. 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.

31. ضمن عرض پوزش
بدلیل حجم بالای LECTUER ادامه اسلایدها امکان پذیر نمیباشد در صورت نیاز به ادامه لطفا به واحد سمعی و بصری مرکز آموزشی درمانی فیض مراجعه و یا با شماره تلفن داخلی 392 تماس حاصل نمائید
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