+ +

Corneal topography: curvature maps

Specular topography of the cornea

The first cards obtained in topography were maps of curvature; the curve was calculated from the computerized study of the reflection of a disc or a dome of Placido. This pattern consists of alternating bright and dark rings. The cornea is comparable to a convex mirror, which it is possible to calculate the curvature by studying the digital image obtained by the reflection of the light emitted by the Placido disc. The patient sets the center of the Dome (the center of the rings). Clear disc rings are illuminated (they are white rings observed on the recorded image). The review focuses on the central corneal reflection) image of Purkinje ). A mathematical algorithm to calculate the local curvature (several thousand points) of the cornea from the relative displacement on the image of the concentric rings.

map of corneal topography of curvature Placido

Topographic maps of a same corneal anterior surface established axial from the reflection of the mires of Placido (top), image one diopter, the other in mm mode. The curvature is expressed in mm (left) with a no ladder of 0.10 mm and diopters of curvature (right), converted using a value of the refractive index (k and non-physical) of 1.337 for the cornea (no scale: 0.50 D). Topographer OPD SCAN (Nidek)

Topography of curvature

Whenever interpreting a corneal topographical map of curvature (this curvature be axial, tangential or average), always remember that you actually look at the "radii of curvature.. Millimetre scale maps in general attribute a more warm color (orange, red) that the radii of curvature measured are small (and therefore the cornea is more curved locally). Conversely, the cool colors (blue) correspond to less curved areas. The use of cards in radius of curvature is naturally preferred in contactology, when it comes to guide the choice of the radius of a lens to test. At the top of the cornea, the radius of curvature is usually between 7.2 mm (arching corneas) and 8.4 mm (flat corneas), with an average close to 7.8 mm in a healthy population. The study of the spatial form (tri-dimensional) of the cornea statement of the topography of elevation.

In refractive surgery, maps of curves are often captioned diopters... which is rather a unit dedicated to the measurement of the optical power (vergence). The top of the cornea corneal curvature is usually between 40 D (flat corneas) and 47 D (curved corneas).

Increased localized anterior corneal curvature, especially when it is located in the half lower part of the corneal surface, can lead to the existence of a beginner Keratoconus. The topography of specular curvature is a review very sensitive to measure the variations of the anterior corneal curvature.

The diopters in topographies of curvature to match it to a map of the vergence of the cornea? No! In all case not on the entire corneal surface.

 Diopters of curl vs diopters of vergence

When the scale of a map of curvature is in diopters, must keep in mind that it's 'diopters of curls', and not of diopters of optical power. These diopters are certainly calculated as the inverse of the radius of curvature (expressed in meter) multiplied by the difference between the k (equal to 1.33) index and the index of the air (equal to 1).  In the vicinity of the top of the cornea (and only at this place), the diopters of curvature are equal to the strengths of optical power, if you measure a 'standard' cornea, which we presume as the cornea after a proportional to the anterior cornea curvature.

On average, the posterior reduces the optical power of the anterior cornea by 10% (this is related to the gradient index of negative refraction between aqueous and corneal stroma). THEindex k is a minus index (the actual physical refractive index of the corneal stroma is close to 1,376). This choice is intended to compensate for the lack of measurement of the posterior face of the cornea: indeed, when using the value n = 1.33 instead of n = 1,376, we get a reduced optical power by 10%.

Why this choice of a index reduced, intended to make it more realistic to a calculation of optical power, for the calculation of the rendering of maps of curvature? This goes back to the days where there was no simple clinical method to measure the curvature of the rear face of the cornea (and so on in meurer optical power). Must thereby pay attention to this: axial or instant maps labelled diopters are maps of curvature, and no optical power.  To get a map of corneal optical power, choose a representation in 'refractive power '.

Thus, for a map of curvature, an area represented by 'hot' colors is simply an area or the curvature is more important (the radii of curvature are smaller). Conversely, a cooler color area is an area where the curvature is smaller (the radii of curvature are larger).

A common error is to extrapolate the tri dimensional representation of the cornea from curvature information; view the red zones such as the summits or bumps, and blue areas like hollows or valleys leads to an erroneous interpretation. It is natural to apprehend a topographic map as well: indeed, the characteristic of topography is to represent a 'form', and not data of curvatures. But the history of corneal exploration wants topography 'of curvature"has preceded the topography of elevation, and introduced a color representation code borrowed from the land topography, or warm colours correspond to"higher"areas, and not"more curved. Thus, curvature maps show that... the curvature.

The representation of the optical power of the anterior surface of the cornea is provided by the card of refractive power of the cornea. They are obtained through a "ray tracing" virtual on the corneal surface.

Learn more about:

the corneathe geometry of the cornea
the optical power of the cornea

screening of Keratoconus indices

infra clinical Keratoconus

 

Leave a comment

You can ask questions or comment on this content: for this, use the "comments" form located below. The questions and comments of a general interest will be processed and published, and the information provided on the relevant pages should be clarified or supplemented.

Your e-mail address will not be published. Required fields are indicated with *