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Keratometry (ophthalmometry, corneal topography) is a non-invasive, painless diagnostic test that is performed when it is necessary to measure the curvature and breaking power of the cornea. Among the indications for the study are:

  • Diagnosis of vision defects and the need to select eyeglasses/contact lenses
  • diagnosis of astigmatism
  • Preparation for cataract surgery (need to calculate the power of the artificial implantable lens)
  • Before planned laser vision correction procedures and after such procedures to evaluate the achieved changes in corneal curvature

In the diagnosis of certain corneal diseases (e.g. corneal cone[keratoconus], giant cornea [megalocornea], globular cornea[keratoglobus], small cornea [microcornea], flat cornea [corneaplana] or scleral cornea).

Manual keratometry is the gold standard for assessing the power and axis of astigmatism in toric lens calculations. The keratoscope (Placido disc), which is a disc with black and white circles, allows you to observe the shape of the circles’ reflections on the cornea. Specialists also still use Javal’s ophthalmometer (keratometer), a handheld keratometer that is used to measure corneal curvature in two main sections. With these measurements, the magnitude and axes of astigmatism can be accurately determined.

Ophthalmologists also have a computerized (digital) videokeratograph at their disposal – an image of Placido’s disc projected onto the surface of the cornea is used for the test, which is then recorded by a camera, transmitted to a computer and analyzed. The keratometer projects images onto the cornea and analyzes the position of their reflections relative to each other. The central part of the cornea acts as a spherical convex mirror.

If an illuminated object of known size is positioned at a known distance from the cornea and the size of the reflected image can be measured, its radius of curvature can be determined. Since the eye is constantly moving, measuring the size of an image using a linear scale is difficult. The use of two prisms placed with their bases facing each other will produce two images separated by a fixed distance, which is not affected by small movements of the eye (the principle of doubling).

Corneal topography makes it possible to accurately assess the surface of the eye and detect many anterior segment pathologies, and thus properly prepare the patient for surgery. There is a perception that it is the test that determines whether a patient can be admitted to surgery. A cornea that is too thin may or may not be a contraindication to the procedure. Corneal thickness is measured during an examination called pachymetry (optical measured by non-contact or touch using an ultrasound probe). If the cornea is too thin, but the other (very many!) eye parameters checked by the specialist, which are planned to be operated on with the laser, are correct, the laser can be programmed in such a way that such an operation can be carried out successfully.

Corneal tomography also allows us to determine the depth of the anterior chamber, the corneal-ocular angle, the size of any shape changes, scars, inhomogeneities. During computerized keratometry, the doctor can see:

  • The surface of the eye: thus gaining additional information about each corneal curvature and its features. On this basis, he assesses whether the planned correction is possible and will bring the results expected by the patient.
  • The posterior surface of the cornea: it should be healthy for a decision to operate.
  • Corneal thickness: determines whether all methods are possible – that is, superficial methods, LASIK and microlens laser vision correction.
  • The depth of the anterior chamber and the diameters of the cornea: this is important when implanting phakic lenses.

The results of the study are a corneal map, a cross-section of the corneal surface, and a map of digital corneal curvature values.

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