Topography Topics

Topography Difference Maps

topography topics

Topography Difference Maps

November 2000

A simple method of assessing surgical and healing changes of the cornea after any corneal surgery is to engage a topography difference map. The usefulness of this monitoring tool is not limited to corneal surgery ­ you can also use it to monitor changes induced by contact lens wear over time. Topography difference maps are generated by mathematical subtractions between two selected displays to document how a cornea heals, changes or responds. Refractive surgeons may find refractive power difference maps useful in documenting the actual power changes attributed to the cornea resulting from surgery. I find curvature difference maps useful in detecting how a cornea responds to contact lens wear.

Healing Trends

Topography difference maps are largely accepted for following corneal surgical healing trends. Typically a difference display shows two serial topography maps for a selected eye, and the dioptric power difference between the two (Figure 1). Some manufacturers have a "healing trend" software module which may be optionally purchased that includes a five-map display of three serial topography maps and the respective two difference maps calculated from them.

Lens Wear

I find curvature difference maps useful for monitoring the corneal shape of post refractive surgery patients fit with RGPs After a myopic kerato-refractive procedure, RGPs may induce spectacle blur after lens removal due to the uneven tear layer and pressure distributions on the cornea. I find this occurs most frequently after radial keratotomy (RK), which can weaken and destabilize the cornea. Figure 2 shows the topography of a post-RK patient prior to and after fitting with a reverse geometry RGP lens. The lens provided good vision,but it decentered superior-temporally and resulted in over five diopters of focal steepening within the pupillary zone, documented on the difference map.

Maintaining a well centered lens with even tear layer distribution is essential to preventing spectacle blur after lens removal. Figure 3 shows the topography of a post ALK (automated lamellar keratoplasty) patient prior to and after fitting with a reverse geometry lens. The corneal shape remained stable after lens fitting, documented on the difference map by no appreciable central or overall power changes.

Figure 1. High astigmatism after penetrating keratoplasty significantly minimized after astigmatic keratotomy (AK). The difference map shows that the greatest shape change occurs as flattening in the vertical meridian at the location of the AK incisions.

Figure 2. Difference display of an RK patient prior to and after fitting with an RGP lens.

Figure 3. Difference display of an ALK patient prior to and after fitting with an RGP lens.

Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.