Keratometer: Principles, Procedure & Clinical Applications in Optometry

This presentation provides a comprehensive overview of the Keratometer, an essential clinical instrument in optometry and ophthalmology. It covers the principle of operation, types of keratometers, procedure for measurement, clinical uses, advantages, limitations, and interpretation of results. The PPT is designed for optometry students, educators, and eye care professionals to understand the importance of keratometry in diagnosing corneal conditions and fitting contact lenses.

1. KERATOMETER Ms. Reshu Yadav Assistant Professor SHS CSJM University

2. Keratometry  Keratometry is a measurement of curvature of the anterior surface of cornea across a fixed chord length, usually 2- 3 mm, which lies within the optical spherical zone of cornea. Expressed in Dioptric power. Keratometer also called as Ophthalmometer.

3. PRINCIPLE Keratometry is based on the fact that the anterior surface of the cornea acts as a convex mirror & the size of the image formed varies with its curvature. Greater the curvature of cornea, lesser is the image size

4. KERATOMETER It is a instrument used to perform a keratometry by measuring the central corneal curvature. Principle- Constant object size with variable image size. Types of keratometer- Manual keratometer Automated keratometer

5. BAUSCH & LOMB KERATOMETER Principle - It is based on the principle of constant object size & variable image size.

6. OPTICAL SYSTEM & OTHER PARTS The Object : Circular mire with two plus(+) & two minus signs(-). Lamp illuminates the mire by means of a diagonally placed mirror. Light from the mire strikes the patient’s cornea & produces a diminished image behind it. This image becomes the object for the remainder of optical system.

7. 2. Objective lens- Focuses light from the image of the mire (new object) along the central axis. 3. Diaphragm and doubling prisms : 4 aperture diaphragm is situated near objective lens. Beyond the diaphragm are two doubling prisms, one with its base up & other with its base out. Prisms can be moved independently, parallel to the central axis of instrument

8. Light passing through left aperture of diaphragm is made to deviate above the central optical axis by a base-up prism. Light passing through right aperture is deviated by base-out prism, placing the second image to the right of the central axis. Light passing through upper & lower apertures does not pass through either prism & an image is produced on the axis.

9. The image-doubling mechanism is unique in Bausch and Lomb keratometer, in that double images are produced side by side as well as at 90 degree from each other. This allows  the measurement of the power of cornea in two meridian, without rotating the instrument. Therefore, it is also known as ‘one-position keratometer. 4. Eyepiece lens : Enables examiner to observe magnified view of the doubled image.

10. PROCEDURE OF KERATOMETRY   Instrument adjustment: The Instrument is calibrated before use. Mires are focused by clockwise & anticlockwise movement of eyepiece through trial & error When mires are in focus, the calibration is complete.

11. 2. Patient adjustment : Patient is seated in front of the instrument. Chin on chin rest & head against head rest. Eye not being examined is covered with occluder. Chin is raised or lowered till patient’s pupil & projective knob are at the same level.

12. 3. Focusing of mire: After adjusting the instrument & patient, the mire is focused in the center of cornea. Note that the central image is doubled, indicating that instrument is not correctly focused on the corneal image of the mire.

13. MEASUREMENT OF CORNEAL CURVATURE Instrument is correctly focused on corneal image so that central image is no longer doubled. To measure curvature in horizontal meridian, plus(+) signs of central & left images are superimposed using horizontal measuring control & reading is noted. To measure curvature in vertical meridian, minus(-) signs of central & upper images are coincided with the help of vertical measuring control & reading is noted. In presence of oblique astigmatism, two plus signs will not be aligned. The entire instrument rotated till they are aligned.

14. INTERPRETATION OF FINDINGS 1. Spherical cornea – No difference in power b/w 2 principal meridian & Mires seen as perfect sphere. 2. Astigmatism - Difference in power b/w 2 principal meridian. Horizontally oval mires in WTR astigmatism. Vertically oval mires in ATR astigmatism.

15. 3. Oblique astigmatism – The principal meridian b/w 30 degree-60 degree & 120 degree -150 degree. Irregular anterior corneal surface –Irregular mires & Doubling of mires. Keratoconus - Jumping of mires on attempt to adjust the mires. Minification of mires in advanced cases (K >52 D) due to increased amount of myopia. Oval mires due to large astigmatism. Irregular, wavy & distorted mires in advanced keratoconus.

16. Range of Keratometer- Range 36.00 to 52.00 D. Normal values - 44.00D to 45.00 D 

17. CLINICAL USE OF KERATOMETER It helps in measurement of corneal astigmatism error. It helps to estimate radius of curvature of the anterior surface of cornea Use in contact lens fitting. Monitors shape of the cornea in Keratoconus Keratoglobus. Assess refractive error in cases of hazy media. IOL power calculation. To monitor pre- & post-surgical astigmatism.

18. LIMITATION OF KERATOMETRY It measures the refractive status of a very small central area of cornea (3-4mm), ignoring the peripheral corneal zones.  It loses accuracy when measuring very flat or very steep cornea.

19. SOURCE OF ERROR IN KERATOMETRY Improper calibration Positioning of the patient. Excessive tearing Improper focussing of the corneal image.