What a Difference a Scale Makes!
In the consultation department at Art Optical we continually see an increase in the number of topographies we receive every year – and we’re glad of it! Topographies help us to more accurately design and/or troubleshoot lenses for your patients, no matter the situation or the diagnosis.
The type of map and scale settings are crucial to how we design lenses and each topographer utilizes different terminology. The type of map that we use in most scenarios are typically called Axial or Sagittal. The preferred scale setting is typically referred to as Normalized, Auto Scale or Relative. Most topographers have a default scale setting that will cause the colors on the topography to be concentrated either too blue/green or too orange/red, depending on the corneal diagnosis. If the correct scale is selected, the scale will adjust accordingly to show only as flat and as steep as each particular cornea gets and will show the full rainbow of colors.
This article aims to introduce the reader to a variety of topographers and their terminology and show examples of exactly why the correct scale is so important. This article does not contain examples of every topographer available so do check the manual of your topographer for the relevant types of maps and scale settings if it is not listed here.
Topographer: Medmont
Map: Axial
Scale setting: Normalized
Example: (NOTE: All our consultants have the Medmont software available so full Medmont files can be exported to us if you prefer.)
Medmont
Topographer: Nidek
Map: Axial
Scale setting: Normalized
Example:
NIDEK
Topographer: Oculus-Pentacam
Map: Axial/Sagittal Curvature [Front]
Scale setting: Relative
Example:
Oculus
Topographer: Tomey
Map: Standard
Scale setting: Normalized
Example(NOTE: From what we have observed, the eyes have to be selected separately in order for each scale to be normalized individually. If kept together, the scale will normalize to only one of the eyes, leaving the other eye with an incorrect scale.):
Tomey
With the Absolute scale, both eyes look distinctively like low cones with the OD cone slightly more compact than the OS cone.
This is the OD map with a Normalized scale. The look of this eye didn’t change much, but there is more definition in the size of the cone now that we have the full spectrum of colors present.
This is the OS map with a Normalized scale. The look of the cone is significantly more compact than the Absolute scale indicated – more compact even than the OD.
Topographer: Visionix
Map: Axial
Scale setting: Normalized
Example:
Visionix
This maps Absolute scale is set with such a wide range – 28.75 Diopters to 61.25 Diopters – that the entirety of the cornea falls into the green color range. Looking at the keratometric values within the map, we can see that this is a normal cornea. A scale change was still requested in order to see a better representation of the corneal toricity.
This is the same map, simply adjusted to a Normalized scale. This confirms that this is a normal cornea, but this scale setting allows us to see that the cylinder is limbus to limbus, something that would have been hard to decipher from the Absolute scale.
Topographer: Zeiss/Humprey/Atlas
Map: Axial
Scale setting: Standard Palette/Auto Scale
Example:
Z/H/A
With the standard default scale setting, the cornea looks almost like a nipple cone due to the extremely small apex; however, nipple cones are usually more centered and the Ks are typically much steeper.
Topographer: Oculus-Keratograph
Map: Axial/Sagittal Curvature
Scale setting:Relative
Example:(Note: This topographer has a larger color-range than other topographers and even when the correct scale is selected the corneal curvature will rarely span the full range of colors. However, the correct scale will provide a better idea of how flat and how steep the cornea actually extends.):
Oculus
The Absolute scale on this topography is ranging from 34.5mm flat to 3.8mm steep – that’s 9.75 Diopters flat to 88.75 Diopters steep. Most corneas will never reach a 9.75 D value!
The Relative scale adjusts to a much more reasonable flat-to-steep range – 36.5 Diopters to 54.0 Diopters. The extreme ends of the scale, the pale blue and the dark grey/black, aren’t on this map but we still have a better idea about the true range of highs and lows for this cornea with the adjusted scale.
