Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "Topography is the science of describing or representing the features of a particular surface Topography."— Presentation transcript:

1

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

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 – 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 Snell’s law p = (n 2 -n 1 )/r Where n 1 is the refractive index of the first medium (air = 1) and n 2 is the refractive index of the second medium (cornea = 1.376) Conversion of curvature to power

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.  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 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 Refractive index of corneal stroma is 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 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) 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 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 r X r Y X Y Radius of Curvature Global / Axial / SagitalLocal / Instantaneous / Tangential r X r Y X Y

14 Methods of measurement of corneal shape Keratometry Keraoscopy Photokeratoscopy Videokeratoscopy

15 Keratometer

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

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

25 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 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 – 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. 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 با تشکر


Download ppt "Topography is the science of describing or representing the features of a particular surface Topography."

Similar presentations


Ads by Google