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Corneal tomography

Corneal tomography: definition

Corneal tomography is the measurement of corneal thickness in all the analyzed surface. It is accomplished by scans (slots, Scheimpflug camera) or partial coherence tomography systems.

Before the spread of elevation and tomography of corneal topographers, thethickness the cornea could be appreciated by an ultrasound to ultrasound, using a small probe that was affixed to the cornea after local anesthesia of it. This measure is called Ultrasonic product. This method is old and still considered by some to be a reference ("gold standard") method. It can be used to corroborate topographic data, but is not as interesting as the modern tomographic techniques because it only provides "local" data, and is relatively dependent on ' operator'. Indeed, the conventional pachymetres are taking measures in a given point, where the probe touches the cornea.

Interest of corneal tomography

The topographers Orbscan and Pentacam perform one complete measurement of the corneal surfaceoptical - without touching the cornea, which allows the continuous reading of the corneal thickness and to variations, and not simply of one or several isolated points. Determine the thickness of all the points of the cornea can make sure that the area where the 'thinner' point is not missed. It also lets you appreciate the way the cornea thickness varies between the Center and the edges of it (thickness gradients). The precise and extensive measurement of corneal thickness and its variations is fundamental for the detection of Keratoconus frustes forms; She plays an essential role in preventing the post ectasia LASIK. The presence and/or the conjunction of the tomographic following must do evoke the presence of a fragile cornea (Keratoconus very beginner subclinical risk, still called Keratoconus frustrates): point the thinner ('windows point') shifted into lower temporal, rapid variation of the thickness of the cornea between the Center and edges (faster central thinning), tomographic difference between one eye and the other (important central product average difference between the right and left of the same patient eye (: 530 microns vs 500 microns for example).

Topography tomography keratoconus fruste

Interest of the tomographic map (Orbscan topography in Quad Map view). The patient's right eye is reached (top left) advanced Keratoconus. The left eye (map right) presents a topography specular Placido previous 'normal' (indices of Klyce Maeda, topographer OPD scan, Nidek). So you would think that this eye is 'normal '. The context (other affected eye of Keratoconus) we enseinge that this eye has a very beginner form of Keratoconus (subclinical form). This form is so little pronounced screening based on the anterior indices are negative. However, inspection of the tomographic map reveals the presence of a marked lower cancer thinning (red arrow). In this patient, he is the first step towards a possible evolution towards a more proven form of Keratoconus subclinical, with damage detectable from earlier topography. If a ultrasonic ultrasound measurement of the thickness is made on that eye, it may overestimate the corneal thickness (the thickness at the geometric center is higher than that of the end which is shifted).

It appears so much better to perform a (tomographic) optical measurement of corneal thickness as part of a review of refractive surgery, not because it provides data such as the point of minimum thickness (point windows) and its location. Automated testing software SCORE Analyzer includes two maps to assess the profile of the average thickness of the cornea, and thinning speed between the periphery and the point the thinner.

The measurement of corneal thickness can also be accomplished by thereview OCT (optical coherence tomography). This review is especially interesting to estimate the distance in depth of some elements such as the interface of a LASIK flap (where is deducted as the thickness of the flap, and the residual wall posterior). Its accuracy is still a little less than that of optical tomography.


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