Corneal Tomography

Topography versus tomography

Topography reflects rings off the tear film and maps the front surface curvature. Tomography images the cornea in cross section and reconstructs both surfaces and the thickness between them. Most early keratoconus shows on the back surface first, and a thinning cornea shows on the thickness map, neither of which a front-surface map can see. For screening before laser surgery, tomography is therefore widely recommended, and topography a useful complement.

What the maps show

• Curvature maps of the front and back surfaces, showing astigmatism and localized steepening
• Elevation maps, which compare each surface with a best-fit sphere and reveal a cone as an island of elevation
• A thickness map with the thinnest point and the rate at which thickness changes toward it, one of the most sensitive keratoconus indicators
• Combined indices that score the likelihood of keratoconus and, for refractive candidates, the risk of ectasia after surgery
• Anterior chamber depth and angle for ICL planning and glaucoma assessment

Uses

Screening every laser vision correction candidate for keratoconus and ectasia risk; diagnosing, staging and monitoring keratoconus, where change between scans defines progression and the need for cross-linking; confirming stability after cross-linking; planning toric lenses and detecting irregularity before cataract surgery; assessing scars, dystrophies and grafts; and designing specialty contact lenses.

Comparing scans over time

Each scan is stored and compared with all previous ones, and the software reports change in the key indices beyond the instrument's known variability. This is how progression is judged in keratoconus and how a cornea is confirmed stable before and after surgery. Consistency matters: contact lenses out beforehand, the same instrument where possible, and a good tear film.