Keratometry and Topography

• What is the major refractive element of the eye?

  The anterior ocular surface with tear film
• Why is measuring corneal shape important?

  • Screening tool
  • Contact lens fitting and evaluation
  • Stability of cornea and tear film
  • Aids in diagnosis/monitoring of corneal pathology
  • Gold standard for pre- and post-refractive surgery evaluation
  • Estimating astigmatism when refractive results are poor
  • IOL determination for cataract surgery

  
• What is the most common instrument used to quantify corneal shape?

  Keratometer

  
• What does a keratometer measure?

  Anterior corneal radius of curvature

  
• How is corneal power calculated?

  By change in refractive index divided by radius of curvature

  
• What is the formula for corneal power in diopters (D)?

  K (diopters) = (1.3375 - 1) / r
• What does a K value less than 41.0 D indicate?

  A flat cornea
• What is indicated by a K value greater than 46.0 D?

  A steep cornea
• What are the limitations of modern keratometry?

  • Assumes refractive index = 1.3375
  • Assumes cornea is spherical or toric
  • Assumes cornea is symmetrical
  • Estimates average curvature of central 3 mm
  • Does not measure peripheral cornea
  • Limited in measuring irregular corneal surfaces

  
• What was the first known keratoscope target described?

  In 1619 by Scheiner
• What did Cuignet develop in 1820?

  The keratoscope
• What was the purpose of the ophthalmalometer developed by Helmholtz in 1854?

  To measure the local slope of the cornea
• What concept did Placido develop in the 1800s?

  Using a disc with concentric rings
• What are the two systems of videokeratoscopy?

  • Placido disc system
  • Elevation based system

  
• How does the Placido disc system work?

  It projects illuminated annular rings onto the cornea

  
• What does the elevation based system measure?

  Anterior and posterior corneal shape and corneal thickness

  
• What technology does the Pentacam use?

  Rotating Scheimpflug camera

  
• What does the Orbscan combine?

  Slit scanning and Placido system

  
• What does the Visante system combine?

  OCT and Placido system
• What are the differences between axial and tangential maps in keratometry?

  • Axial: Measures curvature at a point relative to the center, simpler, less accurate
  • Tangential: Measures curvature relative to other points, more accurate, better for defects

  
• What do the terms absolute and normalized refer to in keratometry maps?

  • Absolute: Same scale for every map, easy comparison
  • Normalized: Different scale for each map, adapts to range of corneal power
• What do shape factor and eccentricity measure in corneal analysis?

  • Shape Factor: Corneal asphericity; negative SF indicates flatter center
  • Eccentricity: 0.0 = circle, 1.0 = maximum flattening; average = 0.55
• What does the Corneal Irregularity Measurement (CIM) indicate?

  • Represents corneal surface irregularity
  • Predicts irregular astigmatism or visual distortions
  • Higher CIM values indicate higher probability of ocular pathology

  
• What does Toric Keratometry Mean measure?

  • Uses elevation data to compare toric reference to actual cornea
  • Provides accurate toric representation of a patient's cornea

  
• What is the effect of Excimer PRK on corneal power?

  • Initial reduction in corneal power (~4D)
  • Regression over 6 months to within 1D of original power
• What are the ranges for normal, borderline, and abnormal values in Toric Keratometry Mean?

  • Normal: 43.10D to 45.90D
  • Borderline: 41.80D - 43.00D and 46.00D - 47.20D
  • Abnormal: 36.00D - 41.70D and 47.3D - 60.0D
• What types of astigmatism are listed in the study material?

  1. Asymmetrical astigmatism
  2. Against the rule astigmatism
  3. Irregular astigmatism
  4. Oblique astigmatism
  5. With the rule astigmatism
• What are the effects of scarring on corneal power after Excimer PRK?

  • Initial reduction in corneal power (~4D)
  • Regression over 6 months to within 1D of original power