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Keratometry

Keratometry. Walter Huang, OD Yuanpei University Department of Optometry. Keratometry. Definition “Kerato” = cornea “metry” = measurement of. Keratometer. Anatomy. Schematic. Keratometer.

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Keratometry

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  1. Keratometry Walter Huang, OD Yuanpei University Department of Optometry

  2. Keratometry • Definition • “Kerato” = cornea • “metry” = measurement of

  3. Keratometer

  4. Anatomy

  5. Schematic

  6. Keratometer • It is used to determine the curvature (i.e., steepness or flatness), refracting power, and toricity (i.e., astigmatism) of the anterior central cornea in each of the two principal meridians • The keratometer is used to measure the front surface corneal radii • It is used to assess the integrity of the cornea and/or tear film

  7. “K” Reading Uses • Fitting and evaluating contact lenses • Evaluating the corneal topography for any distortions or irregularities • Patients with irregular corneas are often not correctable to 20/20 vision • Determining whether the cornea or axial length is the cause of refractive error • Evaluating the corneal healing process after surgery or injury affecting the cornea

  8. How Does It Work? • The cornea is both a convex refracting surface and a convex mirror • An object of known size is reflected from a known distance to the corneal surface • The size of the reflected image is measured with a telescope • The refracting power of the cornea is calculated based on an assumed index of refraction (1.3375)

  9. Calibration Should be done regularly to ensure the accuracy of “K” readings Mount a 5/8 inch steel ball bearing at the position close to that normally of the patient’s eye The steel ball has a known radius of curvature, which upon proper calibration of the keratometer , can be correctly read

  10. Preparation • Focus the eyepiece of the keratometer for the examiner’s eye • Turn on the power • Set the adjustable eyepiece as far counter-clockwise as possible • Place a white sheet of paper in front of the instrument’s objective lens to retroilluminate the reticle (i.e., cross hairs) • Turn the eyepiece clockwise until the reticle is first seen in sharp focus

  11. Preparation • Adjust the instrument for the patient • Adjust the height of the patient’s chair and the instrument to a comfortable position for both the patient and the examiner • Unlock the instrument controls • Instruct the patient to place his chin in the chin rest and his forehead against the forehead rest and adjust for the patient

  12. Preparation • Align the instrument for the patient • Raise or lower the chin rest until the patient’s outer canthus is aligned with the hash mark on the upright support of the instrument • From outside the instrument, roughly align the barrel with the patient’s right eye by raising or lowering the instrument and by moving it to the left or right until a reflection of the mire is seen on the patient’s cornea

  13. Procedure • Instruct the patient • Keep eyes open wide and blink normally • Try not to move the head or speak • Look at the reflection of own eye in the keratometer barrel

  14. Procedure Look into the keratometer and refine the alignment of the image of the mires (three circles) on the patient’s cornea Focus the mires and adjust the instrument so that the reticle is centered in the lower right hand circle Lock the instrument in place

  15. Procedure • Focus the keratometer • Out of focus

  16. Procedure • Focus the keratometer • In focus with the reticle centered

  17. Procedure Adjust the horizontal and the vertical power wheels until the mires are in close apposition To locate the two principal meridians of the patient’s cornea, rotate the telescope until the two horizontal plus signs of the mires are perfectly continuous with one another

  18. Procedure • Locate the principal meridians • Off the principal meridian in one direction

  19. Procedure • Locate the principal meridians • On the principal meridian

  20. Procedure • Adjust the horizontal power wheel until the plus signs of the mires overlap into one image • The primary meridian is closest to 180 degrees

  21. Procedure • Adjust the vertical power wheel until the minus signs of the mires overlap into one image • The secondary meridian is 90 degrees from the primary meridian

  22. Procedure • Determine the refractive power • Neither horizontal meridian nor vertical meridian is measured

  23. Procedure • Determine the refractive power • Horizontal meridian is measured (+ sign)

  24. Procedure • Determine the refractive power • Vertical meridian (- sign) is measured • Endpoint is reached

  25. Procedure Throughout the procedure, adjust the focus and recenter the reticle as needed

  26. Recording Record for each eye separately Record to two significant digits to the nearest eighth diopter using one of three methods Remember that it is the power meridian, NOT the axis, being recorded in keratometry

  27. Recording Method 1 Horizontal Power/Horizontal Meridian Vertical Power/Vertical Meridian Example 1 OD 41.25/020 42.87/110 OS 44.12/010 43.50/100

  28. Recording Method 2 Horizontal Power at Horizontal Meridian / Vertical Power at Vertical Meridian Example 2 OD 41.25 at 020 / 42.87 at 110 OS 44.12 at 010 / 43.50 at 100

  29. Recording Method 3 Horizontal Power/Vertical Power at Vertical Meridian Example 3 OD 41.25/42.87 at 110 OS 44.12/43.50 at 100

  30. Recording • Record the amount of corneal astigmatism in diopters (i.e., the difference in power between the two principal meridians) • Record the type of astigmatism • WTR – more power in the vertical meridian • ATR – more power in the horizontal meridian • OBL – major meridians within 15。of 045 and 135 • Irregular – two principal meridians are not 90。apart

  31. Recording • Record the conditions of the mires • Mires clear and regular • Mires irregular and distorted

  32. Interpretation The difference in power between the two principal meridians is the amount of corneal astigmatism In Optometry, the astigmatism is corrected by the minus cylinder lens From the K readings, the meridian of least refracting power indicates the position of the minus axis of the correcting cylinder

  33. Interpretation • Example 4 OD 42.50D at 180 / 44.50D at 090 • Corneal astigmatism = 2.00D • Correcting cylinder = -2.00DC x 180 • WTR astigmatism!!

  34. Interpretation • Example 5 OS 42.75D at 180 / 42.00D at 090 • Corneal astigmatism = 0.75D • Correcting cylinder = -0.75DC x 090 • ATR astigmatism!!

  35. Expected Findings Average K readings are 43.00D to 44.00D The two principal meridians are expected to be 90。apart

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