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.
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.
The calculation of curvature maps is accomplished from the scanned image of the corneal reflection of 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).
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).
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 :
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)
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.