Keratometer

Keratometer is an ophthalmic instruments and has a very important role in optometry field specially for IOL power calculation, Contact lens fitting, to rule out corneal pathology and its progression ie Keratoconus, PMCD.

1. KERATOMETRY Shishir Shukla Optometrist, Ocularist MGM Eye Institute

2. Keratometer It is an optical instrument which is use to measure the central corneal curvature. Kerato = cornea Metry = measurement

3. History Christopher Scheiner in 1619. Ramsden in 1796. Developed in 1854 by Hermann Von Helmholtz. Later improved by Javal Schiotz & others. The term keratometer is a registered trademark of Bausch & Lomb now.

4. Helmholtz Assumption Helmholtz proposed that the cornea was spherical in shape and this hypothesis was the basis of the design of the keratometer The central cornea is spherical and becomes flatter towards the periphery

5. Principle of Keratometer The anterior surface of cornea acts as a convex mirror and the size of the image formed varies with its curvature .

6. Principle of Keratometer The cornea is comparable to a convex mirror in that it reflects the illuminated object of known size, i.e. the mire (the target), forming an image which is virtual, small and erect. r= radius of curvature d= distance between object and cornea O= object size I = image size r = 2d x I/O

7. Target Size 64 mm(6.4cm) used in keratometer. Object distance from patient eye each 75 mm(7.5cm).

8. Object and image size Fixed object size : Image size varies h (object size)constant Variable doubling B & L keratometer Variable object size : Image size constant h’(image size)constant Fixed doubling Javal Schiotz ophthalmometer

9. Configuration of mire(object ) used in B&L keratometer

10. Doubling principle Difficult to estimate the image height because of continuous movement of the eye Prisms used to split the central image into two images The distance between these images are independent of the eye movement

11. The Doubling Principle + + + + + + + + + + + A B Adjustment of prism is made to align the doubled images

12. Optical design Illumination System An observation System

13. Optical system of the B +L Keratometer Sinners disk

14. Skeleton of keratometer

15. The plus and minus signs are the extremities of the image and acts as reference points Plus sign – Horizontal meridian Minus sign – Vertical meridian

16. Eye piece Vertical adjustment control Focus control Chinrest height control Body lock Horizontal power drum Vertical power drum Axis rotation grip PARTS OF KERATOMETR

17. Eye piece Measurement control Focus control Vertical adjustment control Chinrest height control Head rest Chin rest Occluder Axis drum Power switch Axis rotation grip

18. Types of keratometer One position keratometer. Fixed object size variable image size Exam. B & L keratometer Two position keratometer. Variable object size Fixed Image size Exam. Javal schiotz ophthalmometer .

19. Clinical procedure of the keratometry Focus the eyepiece. Instruct the patient. Position the patient. Adjust the instrument. Locate the principal meridian. Coincide the plus sign by horizontal drum. Coincide the minus sign by vertical drum Record the measurements .

20. Preliminary adjustments Position a white background in front of occluder Turn the eyepiece cap anticlockwise Switch on the instrument lamp & view through eyepiece :a blurred cross will be seen Slowly turn the eye cap in clockwise direction till the cross is well focused

21. Target

22. Configuration of mire used in B&L keratometer

23. Examiner’s view before focus

24. Examiner’s view before alignment

25. Keratometry reading K1 46D@90 K2 45.50@180 K1 46.00@120 K2 46.50@30

26. Examiner’s view after alignment

27. Range of Keratometer Bausch and Lomb 36 D(9.38mm) - 52 D(6.49) Extended Keratometry. If > 52.0 D add +1.25Dsp lens add +9.00 D to K reading If < 36D add – 1.00Dsp add – 6.00 D to K reading

28. Uses of keratometer • Measurement of corneal astigmatism. • Estimate radius of curvature of cornea which helps in contact lens fitting. • Assess integrity of cornea and/or tear film. • Detection of irregular astigmatism keratoconus/ pterygium/corneal scarring. • Assess refractive error in cases with hazy media (Rough estimate, comparison of two eyes). • IOL power calculation.

29. Sources of error in keratometry Improper calibration. Faulty positioning of patient. Lack of proper fixation by patient. Reduced visual acuity of examiner. Accommodative fluctuation by examiner Localized corneal distortion Abnormal lid position Improper focusing of the corneal image

30. Disadvantage Central 3.0 mm of cornea can be measured. Irregular corneal surface can not be measured. Cannot measure more than 52D & less than 36D. Difficult to perform for nystagmus pt.