1. KERATOCONUS
   1. ABNORMALLY STEEP CORNEA
2. Images
   1. WTRA
      1. POST LASIK
3. corneal information
   1. Dimensions
      1. HVID => 11.7mm
      2. VVID=> 10.6 mm
      3. T(avg)=> 0.52mm ->0.67
      4. R(Avg)=> 43.27D
      5. Refractive index => 1.376
      6. 'r' apical (Avg)=> 7.8 mm (front)& 6,5 mm( back )
   2. Shape
      1. Elliptical اهليجي
      2. Aspheric
         1. Is the measure of the deviation الانحراف of the peripheral surface curvature from the apical radius of curvature
      3. Prolate متطاوله
   3. Regions
      1. .
4. Definition of CT
   1. Clinical procedure to analyze quality of corneal surface and its regularity
5. Clinical Application
   1. Assessment of corneal pathology
   2. Contact lens fitting
   3. Assessing effecting of contact lenses
   4. Prior to refractive surgery
   5. Assessing effects of refractive surgery
6. Istrumentation
   1. Placido
      1. Placido Disc
         1. Designed by A.Placido in 1882
         2. Black disc with concentric white rings
         3. Diameter – 20 cm
         4. Lens at the peephole
         5. Regular concentric image for normal cornea
         6. Mires appear closer on steeper parts
      2. Placido Images
      3. Disadvantage
         1. Corneal Astigmatism is not quantified
         2. Absence of central mire reflection
   2. Keratometer
      1. Measures the ‘r’ of Optical Cap (central 3mm of cornea)
      2. Helps to monitor only central corneal shape
   3. Videokeratography
      1. Computer software analyses the thousands of data points captured and displays the results in the form of topography map.
      2. The reflected image is captured using a camera.
      3. Multiple placido rings are projected on the cornea
      4. Placido Disc based (Reflection based)
         1. Eg :Topography Modeling System
      5. Elevation based (Projection based)
         1. Eg : ORBSCAN
      6. Color Coding
         1. Steep
            1. Hot colors
            2. Yellow
            3. Orange
            4. Red'
         2. Intermediate / Reference
            1. Green
         3. Flat
            1. Cool Colors
            2. Light
            3. Dark blue
         4. Images
   4. Orbscan
      1. Floating Topic
      2. Anterior surface – Placido & Post. surface – slit scan
      3. Displays in the form of elevation maps
      4. Over 9000 data points
      5. Combination of reflective corneal topography & Optical slit scan method
      6. Corneal reference
         1. Hypothetical sphere
            1. Best Fit Spher concept
         2. Computer compares the real surface with BFS
         3. Above Best Fit Spher
            1. Steeper
            2. Hot colors (Y,O,R)
         4. Equal to Best Fit Spher
            1. Green
         5. Below Best Fit Spher
            1. Flatter
            2. Cool colors (B,V)