The curvature maps are the most basic format of how most people interpret corneal topographic maps. The Axial and Tangential curvature maps represent the most common formats. The Mean Curvature map is even more sensative that tangential map.
Axial Curvature Maps
Axial is a term used to describe the sagittal reading used in corneal topography. The axial map has also been referred to as a "power map." This description is not quite correct as the map shows the curvature of the cornea measured in diopters, as opposed to the "true" refractive power of the cornea. This is the map that is most common to users of corneal topography systems. The axial map is a simple way of describing the overall shape of the cornea, and even though the values are given in diopters, the axial map is actually a calculation of curvature instead of power. Although these two values are intimately related, there is not a one to one correlation.
Axial maps naturally incur more smoothing when measuring the corneal surface, and may not be able to calculate more subtle changes that occur in the periphery of the cornea, as can another type of curvature map, the tangential map. Axial and tangential curvature maps which incorporate an advanced arc step algorithm, as the Humphrey system does, are able to minimize this error significantly.
Tangential Curvature Maps
The tangential map is another type of curvature map. Tangential maps have also been called "local curvature" or "instantaneous rate of curvature" maps due to the fact that they are able to calculate corneal curvature based on a tangent to the normal. Tangential maps are much more sensitive to local or immediate changes on the corneal surface, and are able to show transitions which may be occurring on the cornea with much greater sensitivity. This is accomplished through less smoothing, however the sensitivity also yields more noise which can affect the dioptric power calculations to some degree.
Tangential maps are best used in identifying or locating a corneal pathology. Locating the exact position is important when determining quality of vision complaints, as pupil involvement in the irregular or pathological area will directly affect the patient's visual acuity and quality of vision.
Mean Curvature Maps
The mean curvature maps is similar to tangential map with the big main difference that tangential map is calculated in only one meridan. The Mean curvature map combines the information from horizontal and vertical meridian as a "Mean" curvature map. The Mean Curvature maps are more practical format of tagnetial maps.
Page last edited 07/04/11