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November 03, 2018
The most critical factor in achieving a successful result in topography based contact lens fitting and orthokeratology is the reliability and repeatability of the pre-fit topography. In this series of blogs, we will look at the essential steps you need to follow to capture good topography maps.
Placido-disc corneal topographers reflect a series of concentric rings of the corneal surface back to the camera at the centre of the disk or cone. The topographer treats the cornea as a convex mirror and in reality measures the surface tear film rather than the epithelium itself.
If the tear film is inconsistent, the topographer won't accurately measure and calculate the true shape of the cornea. A poor tear film due to dry eyes, disrupted epithelium or corneal scarring can cause a topographical error called 'ring jam' which occurs when the mirror surface (tear film or cornea) is inconsistent, causing the Placido rings to break or intersect. Ring jam can result in analysis miscalculation or extrapolation errors. This type of assumption or guess results in completely invalid corneal topography interpretation.
The example below shows the effect of ring jam. Notice both the surface asymmetry index (SAI) showing asymmetry in four perpendicular meridians and the surface regularity index (SRI) which measures central corneal regularity show abnormal values.
The example below shows the same cornea recaptured with no ring jam. Notice how both the SAI and SRI index values are now within normal limits.
Ring Jam due to dry eyes can be resolved by instilling artificial tears prior to topography capture to help 'even' or 'smooth' the tear film layer. Use artificial tears with low viscosity and allow the eyes to blink for a minute or two to establish a stable tear layer.
Disrupted epithelium limits the accuracy of the instrument in reading the shape of the cornea. First, treat the cause of the disrupted epithelium before attempting corneal topography capture. Treatment can entail discontinuing current contact lens wear, managing the inflammation with corticosteroid - and antibiotic - drops and copious lubrication using both artificial tears and ointments.
Tip: If disrupted epithelium is unavoidable, or scarring is significant, for example keratoconus, it is helpful to place a soft contact lens on the eye and then attempt corneal topography. The soft lens help to eliminate most of the 'ring jam' otherwise observed. Note the soft lens will smooth out curvature readings and finite shape changes, but the contour of the cornea will be cleaner(2).