WEEK 5: Accommodative Adds and Keratometry

Question 1

What is the equation used to convert radius of curvature to diopters in keratometry?

Answer 1

• The Keratometer will measure radii of curvature and then convert to dioptres based on the following equation;
  ○ D = (n-1)/R
  ○ Where;
  ○ D = dioptric power of the cornea
  ○ n = refractive index of keratometer (1.3375)
  ○ R = radius of curvature in m
  ○ K < 41.0D considered flat
  K > 46.0D considered steep

Question 2

What is prism doubling?

Answer 2

• Small nystagmoid eye movements make measurement of a single reflected image impossible.

Doubling the image via use of a prism allows us to align those two images end to end, and obtain an accurate reading despite these movements.

Question 3

READ KERATOMETRY PROCEDURES

Answer 3

READ KERATOMETRY PROCEDURES

Question 4

If the keratometry readings are 44.00D @160/42.75D @70. What cyl is needed to correct this?

Answer 4

-1.25x70

Question 5

What are the pros of corneal topographers?

Answer 5

Corneal Topographers
• Video capture and computer interpretation of data
• Wider coverage and more detail
• Up to 10mm compared to 3mm with keratometer
• More rings to show contour in the placido disc
• Some models can map anterior and posterior surfaces (ophthalmogy)
• Other features of some models
• Corneal thickness (pachymetry)
• NITBUT
• Meibography
- Analysis software produces colour coded
- Radius variation (asphericity) map
- Power variation (estimated total corneal power) map
• Corneal surface topography map (“contour” map)
• Various programmes exist for interpretation, and change assessment, as well as contact lens specific assessments.
• Varies by brand!

Question 6

Which clinical situation would suit a corneal topographer?

Answer 6

• 3D mapping of corneal surface
• Used in;
• Disease diagnosis
• Specialty contact lens fitting (essential!)
• RGPs
• Keratoconics
• Orthokeratology
• Myopia control
• Refractive surgery and IOL calculations

Question 7

What is the purpose of Javal’s rule?

Answer 7

• Javal’s rule allows us to estimate total astigmatism from keratometry readings.
  It tells us there is an internal contribution of generally against the rule astigmastim (ATR) to the total refractive astigmatism (TRA).