An introduction to understanding elevation-based topography: how elevation data are displayed - a review.

The increased frequency of refractive surgery and the shift towards the correction of higher-order aberrations necessitates a more detailed understanding of corneal shape. Early topography systems were based on Placido technology, as this was initially more intuitive for the general refractive surgeon. Newer computerized corneal modelling has increased our knowledge beyond what was previously possible. Elevation-based systems utilize a direct triangulation technique to measure the corneal surface. Elevation-based Scheimpflug imaging has advantages in that it allows for the measurement of both the anterior and posterior corneal surfaces. Posterior measurements are often the first indicators of future ectatic disease, in spite of completely normal anterior curvature. Examination of the posterior corneal surface can often reveal pathology that would otherwise be missed if one was relying on anterior analysis alone. Although there is little disagreement in diagnosing clinically evident keratoconus, agreement on what constitutes 'form fruste' or preclinical keratoconus remains elusive. The ability of elevation-based topography to analyse both anterior and posterior corneal surfaces adds significantly to our ability to identify eyes believed to be 'at risk'. As more knowledge is gained, it is appreciated that a full understanding of the workings of the human eye requires knowledge obtained from more than just one surface.