The computerized topographer can generate various graphical representations. When performing corneal mapping for diagnosis of conditions and/or contact lens fitting, the two most common maps that practitioners use are:

Axial map

Also called the "power" or "sagittal" map, this output is the simplest of all the topographical displays. It shows variations in corneal curvature as projections and uses colors to represent dioptric values. Warm colors such as red and orange show steeper areas; cool colors such as blue and green denote the flatter areas.The axial map gives a global view of the corneal curvature as a whole. Its downside is its tendency to ignore minor variations in curvature.

Tangential map

Sometimes referred to as the instantaneous, local, or "true" map, it also displays the cornea as a topographical illustration, using colors to represent changes in dioptric value. However, the tangential strategy bases its calculations on a different mathematical approach that can more accurately determine the peripheral corneal configuration. It does not assume the eye is spherical, and does not have as many presumptions as the axial map regarding corneal shape.

In fact it is the map that more closely represents the actual curvature of the cornea over the axial map. The tangential map recognizes sharp power transitions more easily than the axial map, and eliminates the "smoothing" appearance that appears on the axial map. This is not universally true for all topographers.

Compared with axial maps, tangential maps yield smaller patterns with details that are more centrally located. Tangential maps also offer a better visualization of the precise location of corneal defects. This display is most useful in following trends in the postsurgical or pathologic eye.

Within the past 10 years, corneal topography has grown from an elaborate and costly device used only for clinical research in large institutions to a critical in-office tool that many optometrists now use daily. Along with advances in computerization and software development, topographers have become smaller, more compact, more affordable and more precise. This primer describes the mechanics, methods of interpreting the data, and indications for performing corneal topography.

The Mechanics Behind Topography

Corneal topography--also known as videokeratography or corneal mapping--represents a significant advance in the measurement of corneal curvature over keratometry. Most corneal topographers evaluate 8,000 to 10,000 specific points across the entire corneal surface. By contrast, keratometers measure only four data points within the cornea's central 3-4mm; the small size of this area can lead to errors in determining precise toricity.

Topography provides both a qualitative and quantitative evaluation of corneal curvature. It does so by utilizing concentric rings, which project onto the cornea to create a virtual image. The device compares this image to the target size, and the computer then calculates the corneal curvature. Although many different systems are available, all share some unifying measurement characteristics.

Bij dit onderzoek wordt een gedetailleerde afbeelding gemaakt van de kromming van het hoornvlies (cornea). Met computer software wordt de bolling van het hoornvlies gemeten en in beeld gebracht.

Deze informatie wordt gebruikt bij bepaalde aandoeningen van het hoornvlies, waaronder keratoconus en bij littekens van het hoornvlies na een ongeval.

Verder vindt corneale topografie plaats voorafgaande aan laserbehandeling van het hoornvlies bij de behandeling van bijziendheid en het bij aanmeten van contactlenzen.

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