Keratometry - An Overview

Introduction

Keratometry is a type of electromagnetic test. It allows knowing the exact curvature and radius of the cornea. It helps to obtain the value of corneal astigmatism (fault in the shape of a person's eye). Keratometry is an objective test because it records exact values that the patient can not manipulate. Keratometry is a diagnostic test for the diagnosis of corneal astigmatism. A person has astigmatism when the results differ significantly from the standard value. Keratometry helps adapt contact lenses and calculate the exact power of intraocular lenses. Astigmatism of the eye is of different types, such as the east type of astigmatism, anterior astigmatism, and posterior astigmatism. The cornea's refractive power is measured by keratometry, expressed as a radius of curvature (millimeters) or optical energy. The first keratometer was invented by Hermann von Helmholtz. There are different types of keratometers, such as surgical or operating keratometers, as well as automated keratometers.

How to Perform Keratometry?

Keratometry is done with the help of a keratometer. It is also known as a keratoscope or an ophthalmometer. A keratometer is a barrel-shaped machine that projects light on the anterior chamber of the cornea. The cornea is evaluated by how images are projected onto it, which helps determine its curvature. The keratoscope examines the curvature of the cornea.

The patient sits at the time of the test facing the keratometer, and the chin rests on the machine's chinrest. The patient must look into the equipment when a beam of light travels through the ophthalmometer. A convex mirror reflects light. The way light reflects on the mirror aids in determining the cornea's curvature. Keratometry results are required for the treatment of corneal astigmatism. The ophthalmologist can plan a refractive surgery according to the result obtained by keratometry. Patients have to sit motionless for a few minutes during the test. It is a painless and non-invasive test.

What Are the Different Types of Keratometers?

The different types of keratometers are as follows:

1.Javal-Schiotz Keratometer:

It is a two-position instrument. The Javal-Schiotz keratometer is based on the Javal-Schiotz principle. A fixed image, a doubling size, and an adjustable object size are needed to obtain corneal curvature. It uses two self-illuminated objects. The instrument needs to stay focused to get accurate results.

2. Bausch and Lomb Keratometer:

This is a one-position keratometer. It is based on Bausch and Lomb. It works mainly with a fixed object instead of a fixed image.

3. Manual Keratometer versus Automated Keratometer:

Manual keratometers give:

• A clear reading of the tear film present on the cornea.

• A dynamic image (3-dimensional) of the corneal surface.

• Clear visualization of the corneal surface.

• Manual fine adjusting produces accurate measurements.

Automated keratometer provides:

• A produces a static image of the corneal surface.

• Adjustments of readings can be made by computer.

As eye operations are very critical, keratometry requires exact measurement. Errors in measurements are responsible for causing huge issues. Automated keratometers are comparatively easy for beginners, but manual keratometers give a more accurate reading. A manual instrument is used for fine adjustments. Both the patient and the doctor should be satisfied with the reading values.

What Is the Procedure of Keratometry?

It involves two-steps:

1. Instrument adjustment.

2. Patient adjustment.

Instrument Adjustment:

• The instrument calibration should be done before use.

• A white paper is kept in front of an objective lens and a black line is drawn sharply on it.

• The keratometer is then calibrated with steel balls of the known radius of curvature.

• The steel ball is placed before the keratometer.

• Mirrors are focused by clockwise and anticlockwise movement of the eyepiece.

Patient Adjustment:

• Set the instrument and the patient's chair at a reasonable height for the patient and the examiner.

• Instruct the patient to place his forehead against the forehead rest and his chin on the chin rest and adjust for the patient.

• The eye is not going to examine; it should be covered with an occluder.

• Chin is raised or lowered till the patient's pupil and projective knob are at the same level.

What Are the Uses of Keratometry?

There are several uses of keratometry. These are the following:

• It helps in the measurement of corneal astigmatism.

• It helps estimate the radius of curvature of the cornea, which helps improve contact lens fitting.

• It helps to assess the anterior corneal surface.

• The integrity of the cornea can be assessed with the help of keratometry.

• Estimation of the tear film.

• Assists in the detection of keratoconus (thinning of the cornea, which leads to corneal blindness).

• It is also used to assess pre-surgical and post-surgical astigmatism.

• Assists in estimating corneal scarring.

• Assess the refractive index of the eye.

• Monitoring of progressive myopia.

What Are the Limitations of Keratometry?

Apart from wide use, it has some limitations also. These are the following:

• Measurement of keratometry is based on the idea that the cornea is symmetrical or spherical, with the two principal meridians separated by ninety degrees.

• Loss of accuracy when the cornea is flat or very steep.

• The presence of irregularity in the cornea hinders accurate measurement.

• A faulty lid position can cause errors in keratometry.

What Is the Maintenance of the Keratometer?

• When not in use, keep the instrument covered.

• Cleaning the body and the optics (removal of dust).

• Regular lubrication of all running parts of the machine is a must.

• Replacement of bulb is necessary when required.

• Regularly check the power switch for uninterrupted on-off and the power cable, replacing them when necessary.

• Check the calibration every day before use.

Conclusion

Keratometry is a type of electromagnetic test. It is a routine investigation done in day-to-day procedures. It allows knowing the exact curvature and radius of the cornea. It helps to obtain the value of corneal astigmatism. It is a painless and non-invasive test. It helps in the measurement of the parameters of the cornea. As eye operations are very critical, keratometry requires exact measurement. Errors in measurements are responsible for causing huge issues. It is beneficial for ophthalmologists to monitor progressive disease.