Topography Flashcards
Why are keratometers no good?
Only measure central 3-4mm
Assumes cornea is spherical
What is good about keratometers?
They correlate well with RGP fitting
They are accurate +/- 0.02mm
What two types of topography systems are there?
Slope systems (placido disc systes) Elevation systems e.g. scheimflug, scanning slit, rastersterography
What are the two types of placido disc design?
Large disc (large working distance, less focusing errors) Small cone (short working distance but increased corneal coverage)
What steps are there in the interpretation of a placido image?
1) Image acquisition
2) Image digitisation, editing if required
3) Generation of map
What was the main disadvantage of rastereogrammetry?
You need to render the tear film opaque with either talc of NaF
What are the advantages of elevation systems?
Use robust mathematical calculations
Easy to change elevation data to slope data
Which system is better at looking at unusual corneas?
Elevation system
What is the other name for sagittal maps?
Axial maps
Which map is better for looking at abnormal corneas?
Tangential
Which map gives you the best refraction data?
Sagittal
When would you use difference maps?
Post refractive surgery
Ortho-K
What is the problem with radius of curvature?
If there is a chunk missing with the same RoC, it wouldn’t show up!
What does the tangential map show you>
A mix of refractive and elevation data.
What new applications are there of topography?
Tear film stability.
