Extended keratometer and keratometry procedure

This presentation includes the function of the keratometer and keratometry procedure.

1. keratometry Presented by – Rahul Deb Bera ( M.Optm 2 nd yr ) VIDYASAGAR COLLEGE OF OPTOMETRY & VISION SCIENCE RD 1

2. contents Definition Principle Parts of Keratometer Procedure of keratometry Interpretation of findings Clinical uses Limitations Sources of error RD 2

3. Keratometry Kerato = Cornea Metry = Measurement RD 3

4. definition Keratometry is 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. Keratometer also called as Ophthalmometer . AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;160. RD 4

5. principle Keratometry is based on the fact that the anterior surface of the cornea acts as a convex mirror and the size of the image formed varies with its curvature. Greater the curvature of cornea, Lesser is the image size . From image size formed by anterior surface of cornea( 1 st Purkinje image) – radius of curvature of cornea can be calculated. AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;160 RD 5

6. Therefore, B’P/A’B’ = BP/AB B’P = v F/ i = u/o A’B’ = i r/2i = u/o ( since F = r/2) BP = u r = 2ui/o AB = o U (BP) is constant for any instrument AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;160. RD 6

7. Relationship between radius of curvature and dioptric power of cornea - D = n-1/r D = 0.3375/r D – Dioptric power of cornea n – Refractive index of cornea r – Radius of curvature of cornea (m) AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;160. RD 7

8. Doubling Principle : Because of involuntary eye movement image formed on cornea would be constantly moving. To overcome this Doubling technique is used. Two prism ( Base Up prism in left diaphragmatic aperture & Base Out prism in right diaphragmatic aperture) is introduced into the optical system so that 2 images are formed. The prism is moved until the images touch each other. RD 8

9. Keratometer is based on 2 concepts : Variable Doubling Fixed Doubling Eg . Bausch and Lomb keratometer Eg . Javal-Schiotz Ha RD 9

10. Parts of Bausch and lomb keratometer RD 10

11. Optical system of keratometer AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;162. RD 11

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21. Total area of upper & lower apertures = Area of each the other two apertures Upper and lower apertures also act as Scheiner’s disc doubling the central image, whenever the instrument is not focused precisely on central mire image. Thus image – doubling mechanism is unique in Bausch and lomb keratometer , in that double images are produced side by side as well as 90 degree from each other. This allows the instrument of the power of cornea in two meridian , without rotating the instrument. “it is called one position keratometer ” AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;163. RD 21

22. Procedure Instrument calibration and eye piece focusing 2. Patient Adjustment : Seated in front of the instrument. Chin on chin rest and head against head rest. Eye not being examined is covered with occlude. Chin raised or lowered till patient’s pupil and projective knob are at same level . AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;163-164. RD 22

23. Instruct the patient to : Keep eyes open wide and blink normally. Try not to move the head nor speak. Look at the reflection of own eye in the keratometer barrel. AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;163-164. RD 23

24. 3. Focusing of mires : The central image is doubled, indicating that Instrument is not correctly focused on the Corneal image of the mire. AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;163-164 . RD 24

25. To measure curvature in horizontal meridian, plus sign of central and left images are superimposed using horizontal measuring control. RD 25

26. To measure curvature in vertical meridian, minus signs of central and upper imaged are coincided with the help of vertical measuring control. RD 26

27. In presence of oblique astigmatism, two plus signs will not be aligned entire instrument rotated till they are aligned. RD 27

28. Interpretation of findings SPHERICAL CORNEA No difference in power between 2 principle meridian. Mires seen as perfect sphere. ASTIGMATISM Difference in power between 2 principle meridian. Horizontally oval mires in with the rule astigmatism. Eg . 45.00D @ 90/ 43.25D @180 Vertically oval mires in against the rule astigmatism. Eg . 42.00D @ 90/44.50D @180 Oblique astigmatism -> principal meridian b/w 30 – 60 & 120 – 150 . AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;164. RD 28

29. IRREGULAR ANTERIOR CORNEAL SURFACE Irregular mires Doubling of mires KERATOCONUS Pulsating mires ( inclination & jumping of mires on attempt to adjust the mires) Minification of mires in advanced cases ( k > 52D ) due to increased amount of myopia. Oval mires due to large astigmatism. Irregular, wavy & distorted mires in advanced keratoconus. AK Khurana,Theory and practice of optics and refraction 2 nd edition.2008;164. RD 29

30. BAUSCH & LOMB KERATOMETER Range - 36.00 to 52.00 D Normal Values – 44.00 to 45.00 D Extended Keratometer - Place +1.25 D lens in front of aperture to extend range to 61 D ADD 9 D Place -1.00 D lens in front of aperture to extend range to 30 D SUBSTRACT 6 D RD 30

31. Clinical uses of keratometers It helps to measure corneal astigmatic error. Helps to estimate radius of curvature of the anterior surface of cornea. IOL power calculation. To monitor pre & post surgical astigmatism. It is used to monitor the shape of cornea in keratoconus and keratoglobus . RD 31

32. Limitations of keratometry Measurements of keratometer based on false assumption that the cornea is a act like a convex mirror, whereas the cornea in reality is aspheric. Measure refractive status of small central cornea ( 3-4 mm) Loses accuracy when measuring very flat or very steep cornea. One position instruments assume regular astigmatism. RD 32

33. Sources of errors in keratometry Improper calibration Faulty positioning of the patient. Excessive tearing. Abnormal lid position. Improper focusing of central image. RD 33