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Corneal topography

Corneal topography: definition

Corneal topography to collect information related to the curvature or terrain (elevation) of the cornea, with the projection and analysis of the reflection of a light pattern illuminating or sweeping the cornea. The images collected are analyzed way automated by software, and color maps are provided to the practitioner for interpretation.

A section is devoted to the principles of corneal topography

Interest of the corneal topography

The topography and the tomography of the cornea are the essential and mandatory reviews before any refractive surgery. The cornea is the part of the eye which will receive the beam of the laser in the surgical treatment of the optical defect (LASIK or PKR). It is therefore essential to evaluate certain characteristics of the cornea as his curvature, its regularity and sound thickness during the initial assessment.

 

Corneal topography

Review by topography and tomography (measurement of corneal thickness - product) with the Orsbcan II topographer

 

This review is essential during the pre-surgery, because it allows to check theabsence of contraindication to corneal origin)subclinical keratoconus ) and choose the technical details of the surgery (LASIK or PKR, etc.)

The technical platform of the Rothschild Foundation has several corneal topographers (Orbscan, Pentacam, OPD SCAN, iTrace, Topolyzer Vario), whose additional data to establish a precise mapping of each eye's cornea.

Corneal topography: principles

The functioning of the so-called corneal topographers specular is based on certain principles that have common la projection of an image of reference on the cornea and the analysis of its reflection. The analysis of the reflection of a sight made of concentric circles on the cornea is certainly the oldest and documented method. This focus, made up of concentric rings, black and white, was invented by the Portuguese ophthalmologist Antonio Placido in 1880. It is today located on a hemispherical dome at the front of the instrument. When measuring, this fire is lit and his image is projected onto the cornea while the patient fixes a point located in the Center. For a decade, corneal topography combined with specular analysis an analysis in elevation, with the collection of images in the corneal wall section (scan by) light slots, or rotating camera Scheimpflug).

Corneal topography by specular reflection

A digital photograph of the image of the corneal reflection of the mires is then performed and the appliance computing curvature at each point of the device according to the deformation of the mires (this distortion is induced by the) geometry of the cornea explored). The use of the topographer «» OPD scan III"(Nidek) for example allows a detailed analysis of the variations of the anterior face of the cornea curvature (this topographer also allows the collection of data aberrometriques). It is equipped with a software built around a neural network capable of automated diagnosis (software corneal navigator, indices of Klyce & Maeda). The screening of Keratoconus, pellucid marginal degeneration are possible with this technology.

Placido OPD SCAN III

Dome of the informed OPD SCAN III a retro Placido disc equipped topographer

The calculation of curvature maps is accomplished from the scanned image of the corneal reflection of Placido disc.

maps of corneal topography Placido curls

Calculation of cards of corneal topography of curvature from the capture of reflecting a Placido disc.

Topography by scanning by light slots

The topographer Orbscan (Bausch and Lomb / Technolas perfect vision) also allows to study changes in the curvature of the cornea. He realizes a scanner with light slits which 'scan' the cornea, allowing to rebuild tridimensionnellement the volume corneal, and study changes in the corneal relief not only past but also posterior (the appliance allows maps of elevation of the front anterior and posterior of the cornea).

horny light slot elevation orbscan topography

Example of a capture of the cup of the corneal dome by a slot balayante (topographer Orbscan)

Thickness "point by point" of the cornea (optical product) can be calculated. This thickness is simply the 'distance' between the anterior and posterior of the cornea faces. It is also provided with a Placido disc which allows a direct collection of data from the anterior curvature of the cornea.

Topography by camera rotating Scheimpflug

The topographers Pentacam (Oculus), TMS 5 (Tomey) and Galilei (Ziemer) have a system of camera Scheimpflug for the study of the thickness and elevation of the cornea. They are also equipped with a disc of Placido (except the Pentacam).

Cup topographer pentacam scheimpflug

Map obtained by the topographer Pentacam (Oculus): several cuts of the cornea and anterior segment are obtained by Scheimpflug imaging and allow to reconstruct the volume of the anterior segment, the cornea, and calculate its thickness point by point and

As with the Orbscan (Rotary) scanning by a line of light allows to establish records of corneal tomography, interested in maps of thickness corneal at each point, and the study of its variations. Early detection of infra-clinical forms of Keratoconus is partly based on the collection of these data.

Rendering of corneal topography maps

Regardless of the information gathered in topography and corneal tomographythey are made in the form of multiple colorful cards. In general, for maps of curvature, the warm colors (red, orange, ect.) reflect a bend high, and vice versa for cool colors. Digital signs are also available: they vary according to the instruments (index of regularity, the Asphericity value, etc.). Some clues are specially designed to detect early forms of Keratoconus, representing a formal contraindication for LASIK.

Corneal topography and refractive surgery

In refractive surgery, corneal topography maps review, faced with the degree of lens to bring correction and the clinical context, allows to validate the operative indication of LASIK or instead of track a possible corneal anomalies that may be a contraindication to the operation, such as the presence of a thin cornea and/or a beginner Keratoconus suspicion (beginner form of Keratoconus infra-clinic, also called "fruste Keratoconus": all these entities could be due to a minor corneal deformation) related to repeated and vigorous eye rubbing).

The topography and the tomography of the cornea are particularly critical investigations in the choice of technique (operation with a stream cutting = LASIK, or surface laser = PKR photoablation technique). In fact, the screening of 'at risk' corneas for LASIK is essential; It is compatible with the presence of an unknown beginner Keratoconus signs in patients candidates for refractive surgery. A LASIK patients may get complicated of ectasia (induced Keratoconus), even if surgery is perfectly done (end flap, depth of ablation laser moderate, etc...).

Learn more about: the corneas at risk of ectasia screening

Some examples of corneal topography maps :

Topography maps corneal enantiomorphisme

Topographic maps or maps of the right eye and left eye are juxtaposed, and which have a high degree of enantiomorphisme (mirrored symmetry). Upstairs: cards Orbscan mode Quad Map. Bottom: axial topography maps (OPD scan). These cards are from normal eyes. Note the symmetry between right eye and left eye mirrored (this property is called "enantiomorphisme")

One of the elective indications of corneal topography is the screening of Keratoconus infra clinical. Keratoconus is a form of degeneration of the cornea, and is a formal contraindication for LASIK surgery. We meet all stages in practice, and the early stages can be diagnosed only through the realization of a topography of the cornea.  Here is a typical example of infra clinical Keratoconus  (card Orbscan)

introduced keratoconus subclinical topography

The preoperative Orbscan card inspection, we noted several arguments for a form infra-clinic of Keratoconus. * 1: vertical asymmetry of Camber: the lower hemi-cornea is more arched, but the visualization of this hyper-cambrure is partly hidden by the use of a too large scale. * 2: lower the horizontal "promontory" of positive elevation offset, reflecting early "Ectatic" deformation of the front, whose map is dominated by a mixture of toricite and asymmetry. * 3 accentuation of asphericity prolate to the posterior side of the cornea, and offset of the central island of positive elevation in temporal turned * 4: pronounced lower offset from the end point, at the centre of a concentric thinning area.

Some clues or automated screening software to increase the sensitivity and specificity of screening of Keratoconus subclinical (ex:) SCORE Analyzer).

 

The study of theastigmatism corneal for surgery of cataract with implant-rings is another common application of the corneal topgographie. The morphology of corneal diopter governs its optical properties. As soon as we look at the optical quality of the cornea, it is appropriate to perform a corneal topography. This review is repeated after refractive surgeryto check good centering and the regularity of the treatment laser issued.

 

12 responses to "corneal topography"

  1. […] k is a term used in corneal topography. It is defined as the axis that connects the center of curvature of the cornea to the point of [...]

  2. ZAARATE says:

    I thank very much Dr DAMIEN GATINEL of the effort he made to document us on the use of recent equipment that provides .thank you DR ZAARATE Algerian OPHTHALMOLOGIST ophthalmology

  3. Macdonald mohamed says:

    Hello hope not to bother the master but would like to know how is defined the best fit sphere for each eye thanks

  4. Dr. Damien Gatinel says:

    The calculation of the reference sphere (best fit sphere) is done for each surface studied of the cornea (anterior and posterior). Although the algorithms are 'owners', it comes to find the theoretical sphere whose radius and position to minimize the gap with the corneal surface of interest (minimization of the residual variance). The sphere is then used 'zero level' to represent the elevation, in microns.

  5. Sammy said says:

    Thank you to Dr. Ganeshan for all the information he puts at our disposal.
    I am a technician specializing in the maintenance of ophthalmic and optical equipment.
    Thank you very much for orienting us more;

  6. De Falco Dominique says:

    Hello, could you please tell me what this topographic data means:
    Ks 7.4 @ 20. Kf 7,49 @ 110. MinK 7,53 @ 86 Avek 7.45. Cyl 0, 58D. SRI 0.42
    Es: 0.58/Em: 0.61. Pva. 20/20-20/25. SAI: 0.85
    Is this a keratoconus that requires quick surgery?
    With all my thanks
    Well cordially
    Dominique

  7. Dr. Damien Gatinel says:

    These numbers are relative to the central curvature of your cornea (K – > Kerratometrie, F for flat, s for steep, Min for minimum, Ave for average, Cyl for cylinder – astigmatism, SRI: Surface regularity index. E for Eccentricity (asphéricité), PVA for potential visual acuity, and SAI: Surface asymmetry index. It is not possible to tell you whether you have a kéatocone or not on the basis of these figures alone.

  8. MAHY Isabelle says:

    Doctor.

    I allow myself to leave a comment here because I do not know where or to whom to address.

    Since 2014, I see bad.... great characters!!

    For example: I do not see the number of a license plate of a car if I am next to the vehicle... while I read very well this number of the second floor!

    The ophthalmologists consulted do not find any explanation for this.... they tell me they don't know any case Similar...

    I made a brutal detachment of the glazed in both eyes (one in 2017, the other in 2018; The floating bodies are really annoying but are unrelated to this problem dating back to 2014.

    I underwent several eye backgrounds (with and without three-mirror glass) and a O.C.T.

    I have no retina problems, not glaucoma, not macular degeneration.
    I have "crystalline corresponding to my age" (57 years) but not affected by cataract.

    Vision from afar: Right Eye: – 4.25 Left Eye: – 2.25
    Additional: + 2.50 for both eyes.
    Astigmatism Right eye

    I am convinced that, to my problem, there is an undetected physical cause.
    I do not have medical training, I just intuition that "something is no longer bent normally in my eyes".... maybe the cornea.......

    Am I entitled to demand a topography of the cornea? Another additional exam?

    I beg you, give me your opinion, please because my problem makes my life difficult...

    In advance thank you; Happy year 2019 to you,

    Isabelle

  9. Dr. Damien Gatinel says:

    A useful test would be to make a small character reading at a low distance (corresponding to the apparent size of the plates when you look at them from the second floor for example). At the same distance, do you see better the small or the large characters? In principle, there is actually not too much explanation except a decompensated hyperopia, which can reduce the quality of close-range vision.

  10. MAHY Isabelle says:

    Doctor.

    I thank you very much for taking the time to answer me.
    (I see your answer only today due to a computer concern.)

    At A low distance, I see the small print better than the big ones.

    (when the characters are large, there is "too much light in and between the letters"; When they are small, they are much more "compact", I see them more sharply.

    Thanks again.
    Sincerely,

    Isabelle

  11. A D says:

    Hello

    A surgeon proposes to correct a problem of decentrage of the laser of my previous operation against myopia using the topography of the eye to guide the laser and make the cornea less asymetric.
    What to think of this kind of operation, is this rare? dangerous?

    Cdlt

  12. Dr. Damien Gatinel says:

    In this context, the use of the corneal topography to guide the retouching is logical and quite indicated. Since the shape of the cornea has become peculiar due to the decentrement, it is necessary to establish a "tailor-made" pattern of shooting, and its characteristics will be established from the corneal topography (the so-called "guided Topo" treatment).

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