Corneal Topography

This comes from a lecture I did in 2013. You can download the original slides here.

Topography Basics

Using a Placido ring based topographer (eg. Medmont E300), what is measured?

• The spacing of the reflected rings
• Corneal slope (i.e. first derivative) is derived easily

What can be calculated from the raw data?

• Tangential/axial Radius of Curvature (mm)
• Tangential/axial power (D)
• Corneal height (um)

Reference points

Three main reference points:

1. Geometric centre of cornea
2. Patient’s line of sight
3. Instrument axis

Note the line of sight and geometric centre may not coincide. You should ask the patient to shift their line of sight away from the instrument axis, so that geometric centre of the cornea corresponds with the instrument axis.

Tangential Radius of Curvature

• Radius of the circular arc that best approximates the curve at that point

Axial RoC

• Also known as axial sagittal height
• Geometrically, just the distance from a point to the instrument axis, along the surface normal

Comparison

Small scale variation

• Localized corneal variations are more visible in tangential than axial maps
• We can see why (mathematically) from a simple example

Tangential vs axial

• Tangential:

• Axial: Simple geometric formula involving y’