Consultant's Corner
Setting the Scale – Corneal Topography
by Nicole Edwards, NCLE-AC
Consultation Supervisor
When sending topography maps to Art Optical, please make sure the scale is set to the normalized setting. A normalized scale will include only the range of curvature from the flattest to steepest values.
A standard or absolute scale setting uses a set scale and will skew the color range of the corneal curvature. Some topographers use a standard scale setting that is similar to a keratometer (39.00 to 52.00 D). If the corneal curvature is greater than 52.00 Diopters as it is in many cases of keratoconus, you will not be able to determine the apex position or how large of an area the actual apex steepness covers. The opposite would be true for post refractive surgical corneas. You will not be able to clearly determine how flat the central cornea really is, or the size of the area it covers. For topographers with absolute scales, the scale may be set as wide as 30.00 to 100.00 D, providing a skewed color range and making lens design more difficult.
It is also important to use an axial map and not a tangential map. An axial map with a normalized scale is best for transplants, keratoconic patients and traumatized corneas. The normalized scale sets the dioptric values and colors around the average corneal curvature for that individual patient.
A tangential map captures extreme corneal curvature values and offers a more detailed view of the cornea in both flat and steep areas. Tangential maps are more commonly used on post refractive patients.
Also, be sure to take the topography of each eye individually. If the scale is not reset for the left eye, the topographer will use the same scale setting as the right eye. If the OD is a cone and the OS a transplant, the OS information will be skewed and will make it very difficult to design a good fitting, functional GP lens.
Using topography for irregular corneas and post surgical patients is ideal. Just make sure that the scale is set properly and that the correct map is selected for use in contact lens design.
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