Keratometry is a technique used to measure the anterior curvature of the cornea. It allows the eye doctor to determine the corneal refractive power, which can be expressed as an optical power or as a radius of curvature of the cornea.
Keratometry is an objective assessment that provides accurate results which can't be manipulated by the patient. It is an essential exam before eye surgery and also for adapting contact lenses to fit the patient's cornea.
Also Known As
Keratometry test is a part of the objective assessments of optometry and can be conducted at the eye doctor's office or in an eye clinic by an:
Keratometry provides a dynamic view of the corneal surface and allows the doctor to see the reflections generated by the tear film. It gives him or her better understanding of the cohesion of the precorneal tear film and helps to identify areas of corneal surface compromise or irregularity. The doctor can assess whether the tear film is disrupted, oily, or the cornea has subtle degeneration or dystrophy.
One of the primary reasons for conducting a keratometry examination is to assess the corneal refractive power to detect and treat defects of the cornea, and especially corneal astigmatism. It is a refractive error that occurs due to abnormalities in the curvature and shape of the cornea. Also, there is astigmatism of the anterior and posterior corneal surfaces and astigmatism of the dorsal side of the crystalline. The test also helps to determine the degree of astigmatism.
Keratometry is also a critical examination in cataract surgery because mistakes in measurements can lead to unexpected refractive errors after the operation. It helps to calculate the real power of intraocular lens implants (IOLs) to be used following cataract surgery or clear lens exchanges.
Other uses of keratometer test results include:
- Fit contact lenses well
- Search for irregularities in the shape of the cornea in diseases such as keratoconus
- Determine the location and amount of a patient's astigmatism in their eyeglass prescription
- Define the amount of reshaping of the cornea needed in surgical procedures such as Lasek, Lasik, PRK.
Preparation & Expectation
Keratometry test doesn't require special preparation. It is a quick and painless test that takes about five to ten minutes. There is no discomfort during the procedure.
However, if the patient has dry eye disease or uses contact lenses, they may be required to first undergo a dry eye treatment or stop wearing contact lenses for about one to three weeks before the test.
- Manual keratometry
- Automated keratometry
The patient will sit in front of the measuring instrument called an ophthalmometer or keratometer. It is a barrel-shaped machine that thrusts light onto the anterior corneal chamber to examine how the images are extended onto the cornea and thus measure its curvature. The ophthalmometer examines the curve of the vital corneal meridians, i.e., flat keratometry (K1) and steep keratometry (K2) or the one of higher and lesser power.
The patient must sit with their chin on the chinrest of the instrument. The patient will look into it while a ray of light flows through the device and is reflected in a convex mirror. The keratometry control values or degree of curvature of the vital corneal meridians is measured by the way light is indicated on the mirror.
Manual keratometry is the earlier method of measuring the curve of the cornea. Most of the manual devices take two readings at the flat meridian and two readings at the steep meridian. Computerized (automated) keratometry has become popular in the past two decades. It uses optical sensors and specialized software to compare the patient's cornea with a standard value database.
Keratometry test is an indispensable exam in the diagnosis of corneal astigmatism and it is concluded a patient suffers from the abnormality if the results obtained indicate critical differences in the power of the corneal meridians. The doctor will use the Keratometry test results to plan the refractive surgery for the treatment of the disorder.
The keratometry (K) value is considered to be the mean of K1 and K2. The doctor will be interested in outcomes, such as where the K1 and K2 have a power difference of more than 3.00 D, differing average K readings between eyes are over 1.50 D, very flat or steep average K readings and astigmatism for either eye exceeding 4.00 D.
The doctor will discuss the results of the test with the patient and explain corrective surgery or treatment that might be necessary.
Keratometry test results will also enable the doctor to request for the customizing of the patient's contact lenses and to evaluate the ideal power of a lens implant.
It is important to note that the Keratometry test is based on the assumption that the surface of the cornea is spherocylindrical. Therefore, the test will not provide an accurate result where the corneal surface is not spherocylindrical. Keratometry test doesn't quantify irregular astigmatism and does not give information about the peripheral and paracentral surfaces of the cornea. Moreover, the exam doesn't detect early keratoconus because the changes are restricted to the posterior surfaces of the cornea.
Risks & Complications
Keratometry test is a simple and straightforward eye exam without risks.