What is Keratometry ?
Keratometry is a procedure commonly used to measure the curvature of the cornea. It is performed by an ophthalmologist, using an instrument called a keratometer or ophthalmometer. Keratometry sessions do not require any special preparation on the part of the patient, and the procedure is usually painless, unless the patient experiences difficulty sitting still. The original keratometer was developed in 1880, although the design has been greatly refined since then.
In the keratometr procedure, an instrument is used to measure the degree of curvature in the cornea, looking for the shallowest and steepest curve. Most modern keratometers use computers and optical sensors, rather than forcing the doctor to perform calculations, and the computer can compare the patient’s cornea with a database of known measurements and specific criteria. Ultimately, numbers will be created so that keratometr data can be entered into a patient’s chart or given to other physicians.
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A keratometr provides the following information:
- The radius of curvature of the corneal surface.
- The direction of the main meridian of the eye. (astigmatism with or against the rule)
- The axis and degree of corneal astigmatism.
- The occurrence of any form of corneal deformity.
How to work keratometer ?
Keratometr is a technique used to measure the anterior curvature of the cornea, which when damaged or misaligned causes a vision problem called astigmatism. A keratometr works by reflecting an image from two points in the cornea, and uses the relationship between the size of the object, the size of the reflected image, The distance between the object and the keratometr. The result is the minimum and maximum curvature values for the cornea.
There are two types of keratometers: single position Helmholtz keratometrs, which are more common, and can adjust image size; and Javal-Schiotz keratometrs, two-position machines that adjust the size of the object. Some all-in-one automated optical test systems include keratometry capabilities, while stand-alone keratometrs are usually manual. The latter is often preferred by more experienced ophthalmologists and technicians who can reduce error when reviewing the surface of the cornea.
What is a K reading in keratometry ?
Keratometr is called the measurement of corneal curvature. In this, corneal curvature determines or reflects the strength of the cornea, and differences in power in the cornea also result in astigmatism, which is why keratometr measures astigmatism. It is obtained by means of various types of instruments or manually or through automatic methods to measure it. Its measurements can be refined. Such as topographies, which measure a cornea at a wider number of points, or it is measured over a more limited area of the cornea, for example a manual keratometr or IOL master also measures axial length and other ocular parameters.
Uses a keratometer
- Evaluation of the degree of astigmatism.
- Fitting of glasses and contact lenses.
- Analysis of patients with keratoconus.
- Determination of intraocular lens power for patients with glaucoma.
Working Principle of Keratometry?
The main purpose of a keratometr is to detect the optical refractive power of the cornea (which is a convex refracting surface). When an object of known size is placed at a known distance from the surface of the cornea in such a way that the size of the reflected image the object is measured by a telescope.
Here the radius of curvature of the cornea is given
R = 2D (I/O)
R: The radius of curvature of the cornea in metres.
d: distance of the objects from the cornea.
I: Image size.
O: The size of the object.
Now, the refractive power of cornea can be given by the formula
d = (n-1) / r
D: Dioptric refractive power of the corneal surface.
n: refractive index of the instrument (n=1.3374 generally)
A keratometr displays the curvature of the cornea in terms of diopters or millimeters and diopters of power. If the readings are shown in milliamps then the dioptric power can be obtained by the equation shown above.
Different types of keratometers
- Javal and Schiotz Keratometr
- bosch and lomb keratometr
- Helmholtz Doubling Principle Keratometr
- Native American Optical Keratometr
The Javal–Schiotz keratometer is based on the Javal–Schiotz principle that works with a fixed image, a doubling size, and an adjustable object size to obtain the curvature of the corneal surface. It is a two-position device and uses two self-illuminated objects. One of the objects is a red square and the other is a green staircase design.
These objects are designed to be placed on the circumferential path so that a certain distance is maintained from the eyes. It is important for the instrument to focus on getting accurate results. This device, like many autofocus devices, works on the Skinner principle. The reflected light rays coming into it (converging in nature) are observed through two or more discrete symmetrical holes.
Bausch and Lomb Keratometer
The Bausch and Lomb keratometer is based on the principle of Bausch and Lomb working with a fixed object rather than a fixed image while working. This is a position keratometr and the size of the image can be varied. The reflected light rays coming inside it (converging in nature) pass through a Skinner disc with four discrete holes. And in this two prisms are aligned vertically with each other. In this, the powers of the major and minor axes can be independent without adjusting the orientation of the instrument.
Helmholtz Doubling Principle Keratometer
Helmholtz Doubling Principle Keratometr:- This principle consists of two rotating prisms that make a particular angle with each other. Each prism have parallel surface. This doubling is dependent on the angle of the two prisms.
Native American Optical Keratometer
Original American Optical Keratometer:- Here, bi prisms are used
What is a normal keratometry reading?
Result: The average K was 43.56, with a range of 38.24 to 50. The average axial length was 24.03, with a range of 18.3 to 31.90 and more than 90% of the axial lengths were between 22.5 and 26.5 mm.
What is the purpose of keratometry?
The principle of keratometry is that the anterior corneal surface behaves as a convex mirror and the shape of the image changes with curvature. Keratometers have various clinical uses, such as refractive error estimation, IOL power calculation, corneal astigmatism measurement, contact lens fitting, etc.
What is a normal K reading?
Shows the range of K-readings in the general population, averaging about 7.70 to 7.90 mm, or 43 to 44 D. Falling outside these parameters indicates a sharper or flatter than average central cornea.
How is keratometry done?
The patient will sit in front of a measuring instrument called an ophthalmometer or keratometer. This is a barrel-shaped machine that shines light onto the anterior corneal chamber to check how images are magnified on the cornea and thereby measure its curvature.
What is KH and KV in keratometry?
Axial length and keratometry measurements were obtained for each prism. The raw values K1, K2, and meridian provided by the IOL master were used to calculate Kh (horizontal component) and Kv (vertical component) for each measurement obtained using equations for calculating points on the ellipsoid.
What is the formula for keratometer?
The keratometer index used in the “Holladay formula” is 4/3. The Haggis formula considers AL, ACD and Rmean together with the formula constant triplet a0/a1/a2. In the context of multiple linear regression 3 formula constants are used to calculate ELP from ‘ELP = a0+a1 ACD+a2 AL’.