Corneal topography is a micro-precise examination method of the cornea. Corneal topography creates a kind of map of the surface of the cornea. The representation is possible in color and image.
Below you will find further information and selected corneal topography specialists.
The cornea is very important for the eye. Firstly, it protects the inside of the eyeball from mechanical stimuli. It must always have a high degree of clarity in order to allow light to pass through unhindered. Furthermore, the curvature of the cornea ensures that light is refracted before the rays fall on the lens of the eye.
The cornea protects the iris and the sensitive inside of the eye @ Firn /AdobeStock
Depending on the system, the procedure records 5,000 to 10,000 points on the corneal surface. The ophthalmologist uses the results to determine the condition of the cornea. This enables him to identify possible pathological changes.
Precise measurement by corneal topography is also of great importance for the correction of refractive errors. The examination provides the basis for comparing other findings and enables precise evaluation of the results.
Diagnosis using corneal topography is used to detect and monitor pathological changes in the cornea.
Doctors should definitely carry it out before operations that change the overall refractive power of the eyes(refractive surgery).
After refractive surgery, corneal topography can be used for further monitoring. It provides precise information about the anatomical condition of the cornea, which is necessary for the fitting of contact lenses.
Corneal topography of the eye should be used in the following cases:
Corneal topography is suitable for determining the degree of astigmatism. Astigmatism leads to a refractive error in those affected, which in turn impairs vision.
Depending on the type of curvature, the defect can have different consequences:
It is particularly important to differentiate between severe corneal curvature and the more pronounced and progressive type(keratoconus).
Diseases of the eyelids, such as eyelid malpositions or eyelid tumors, can also lead to changes in corneal topography. For this reason, doctors should rely on corneal topography in the case of eyelid diseases and before operations on the eyelid.
A corneal topography examination is also recommended for corneal injuries or infections. These impairments can result in scarring, which leads to blurred vision and increased sensitivity to glare. Corneal topography can be used to measure deviations in the cornea with micrometer precision.
Cataracts lead to a cloudy lens and can severely impair vision, even leading to blindness. An effective treatment for the disease is the implantation of an artificial lens.
Corneal topography is a necessary examination before the artificial lens is implanted. It can be used to precisely determine the strength of the lens. Corneal topography provides clarity as to whether the patient is suitable for artificial lens implantation.
Keratoconus is an excessive curvature of the cornea, which is also characterized by an acute conical deformation.
The congenital disease often occurs in young adulthood. The visual acuity of those affected decreases. There are always noticeable changes in the strength values for glasses and contact lenses.
The corneal topography of the eye makes the exact gradation of the corneal curvature visible. Keratoconus can only be diagnosed in the early stages using this procedure.
Corneal transplantation on the eye enables early diagnosis and treatment. Initially, conservative visual aids are available for treatment; if the cornea is severely thinned, a corneal transplant may be considered.
If the cause of visual deterioration is unclear, corneal topography can help with the diagnosis. It can rule out or confirm whether the condition of the cornea is causing the visual impairment.
Contact lenses can be fitted precisely on the basis of the corneal topography. The individual characteristics of the cornea must also be taken into account when selecting suitable lenses. This also applies to complicated corneal shapes and challenging corneal surfaces.
Corneal topography is performed without touching the cornea and does not cause any further discomfort for the patient. It only takes a few minutes.
The patient places their head on a chin-forehead rest so that the ophthalmologist can take the measurement on the eye using a keratograph. During the corneal topography, the patient fixates a luminous dot on the device.
The keratograph is an instrument for recording and evaluating corneal topography @ coldwaterman /AdobeStock
Placido-based and Scheimpflug-based techniques are available for corneal topography on the eye.
The examination involves computer-based scanning of the cornea through a Placido disk onto the patient's anterior corneal surface. This projection consists of black and light rings that alternate regularly. The video camera of the keratograph records an image of the reflected cornea.
The images are then analyzed on the computer. The doctor analyzes the distances between the rings on the corneal surface after projection. Asymmetrical shapes indicate pathological changes to the cornea.
Placido disk @ Anna /AdobeStock
In this corneal topography procedure, the ophthalmologist uses a camera to take images from different viewing directions. He determines the shape of the front and back surface of the cornea. This produces a map of the corneal thickness. The map shows the thinnest part of the cornea and the curvature in the steepest axis.
The computer-controlled system evaluates the data using various methods. The system measures various parameters and values in order to precisely record and map the corneal surface :
In a healthy eye, an even curvature of the cornea ensures sharp vision. The corneal topography measures this curvature as a radius of curvature in mm or as a curvature equivalent in diopters.
The height data is a three-dimensional model of the cornea. Here too, computer-controlled systems are used for the mathematical calculation.
The height data are recorded as coordinates z (x,y). These values allow precise statements to be made about the condition of the cornea.
Refractive imaging enables an assessment of the corneal surface. This is based on the focal lengths. These can be calculated using the law of refraction (light beam tracking). The unit for focal lengths is the diopter.
The original size of the cornea can be calculated on the basis of the refractive power data determined by Fourier analysis. The method was developed by the French physicist Jean-Baptiste Fourier (1768-1830).
The computer-controlled procedure calculates individual components from the image.
The following information enables a precise analysis of the cornea:
These components form the basis for determining the Fourier indices. These values allow the ophthalmologist to precisely characterize the corneal surface.
Zernike analysis is used to measure the wavefront error.
The wavefront aberration is a fine irregularity in the contour of the eye. The scientific model represents the light optically in wave form.
These wavefronts can usually be described using Zernike polynomials. The Dutch physicist Frits Zernike (1888-1966) developed a mathematical model that determines the deviation between the real and ideal wavefront.
Each individual polynomial has the name of the disturbance it relates to.
The most important Zernike polynomials include
The computerized corneal topography system on the eye performs the Zernike analysis based on the elevation data.
Corneal indices summarize the data from the corneal topography in a meaningful way. These parameters provide information about the surface shape of the cornea.
The data also allows classification.
The corneal indices are based on the following values:
These indices enable the ophthalmologist to recognize even minor deviations in the eye.
The computer-controlled corneal topography systems combine the measurement of the cornea and the display of the results. As a rule, ophthalmologists can select different views:
The computerized system processes the measured data and displays it in numerical form. The doctor can then compare the values with older findings and compare them with standard values.
Computerized corneal topography systems provide a clear representation of the cornea in three dimensions. This form of representation allows the curvature of the corneal surface to be visualized.
The ophthalmologist can use graphics to show his patient the nature of the corneal surface. Various buttons make the radii of curvature recognizable.
In this overview, the radii of curvature are given different colors.
The results of the corneal topography are available to the ophthalmologist in the following form:
The possibilities depend on the computerized system used. In most cases, the doctor can quickly get an overview of the individual values. The program often helps to classify the measured values.
For the following conditions, the doctor must observe and individually assess the patient's changes:
In the case of astigmatism, two planes of incidence are usually measured:
The difference between these two main planes is the degree of astigmatism. Experts state this either as the difference in corneal radii or as the difference in refractive power.
The axis is the angle of the plane of incidence to the horizontal plane. For a visual aid, doctors specify the refractive power of astigmatism in diopters and the type of correction in cylinders. The position of the astigmatism is represented by an axis.
If keratoconus is present, the ophthalmologist usually determines the distribution of the central radii of curvature. The patient's cornea is characterized by a tilt. If the keratoconus has progressed, the astigmatism usually increases.
Based on the results of the corneal topography, the ophthalmologist determines whether the patient meets the requirements for artificial lens implantation. Excessive corneal curvature or corneal dystrophy can be exclusion criteria for such an operation.
Artificial eye lenses @ Mohammed /AdobeStock
The results of the corneal topography provide information on whether there is a deviation and consequently a deterioration in vision due to the cornea. Injuries to the surface of the cornea can also be detected.
The results of the corneal topography provide valuable information for the fitting of all types of contact lenses.
The patient's individual values facilitate the selection of suitable lenses. The use of astigmatic or dimensionally stable contact lenses is possible thanks to the precise measurement results. A corneal topography can also be used to monitor the compatibility of the contact lenses.
