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keratometry

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keratometry

  1. 1. KERATOMETRY H.Kangari O.D. Keratometry
  2. 2.  The keratometer is an instrument that is used to measure the front surface (approx 2 - 4 mm radius) curvature of the cornea. Keratometry
  3. 3. Information that provides are:  The  radii of curvature of the cornea. The directions of the principal meridians of the eye, determining whether the astigmatism is with-the-rule or against-therule -- a key factor in contact lens fitting.  The degree of corneal astigmatism.  The presence of any corneal distortion. Keratometry
  4. 4. KERATOMETER Keratometry is essential in the fitting of contact lenses, evaluating and following patients with keratoconus, and in the determination of appropriate intraocular lens implant power for patients undergoing cataract surgery. Keratometry
  5. 5. Optical Principles  Cornea is a convex refracting surface  In order to find the refracting power of the cornea, we need to reflect an object of a known size at a known distance off the corneal surface. Then determine the size of the reflecting image with measuring telescope and calculate the refractive power of the cornea based on the refractive index of n= 1.3375 Keratometry
  6. 6.  The keratometer usually shows the corneal curvature in diopters of power or in millimeters and diopters.  If a millimeter scale is not given, it can be found from tables or by using the surface power formula using the refractive index 1.3375. n-1 Surface power formula: D = ......... R D = the dioptric power of the cornea n = the refractive index of the keratometer used (1.3375) R = the radius of curvature of the cornea in meters  Keratometry
  7. 7. Optics Primary Care Optometry-Grosvenor page 224 Keratometry
  8. 8. FIGURE 1 Keratometry
  9. 9. Keratometer Mechanical Adjustments and Parts  A. Focusing Knob B. Rotating Grip for Locating the Axis C. Chin Rest D. Locking Knob E. Elevating Knob EP. Eyepiece G. Head Rest Adjusting Knob H. Occluding Shield HR. Head Rest J. Chin Rest Adjusting Knob K. Leveling Sight M. Horizontal Measuring Drum N. Vertical Measuring Drum S. Axis Scale Keratometry
  10. 10. Bausch & Lomb Keratometer  One position keratometer: it measures two meridians at the same time  It contains two prisms Keratometry
  11. 11. PROCEDURE  1) Eyepiece Adjustment : turn the eyepiece (EP) counterclockwise fully. Place a white sheet of paper in front of the telescope part. Turn the eye piece clockwise until the specified target in the instrument becomes in sharp focus. Keratometry
  12. 12.  2) Adjust the height of the instrument and align the outer canthus of the patient with the leveling sight sign (item K in Figure 2)  3)Occlude the non-tested eye (with H). Keratometry
  13. 13.  4)Ask the patient to look at the center of the instrument. If you are aligned, you can see the circles in the eye. The patient can see the picture of their eye in the instrument. Keratometry
  14. 14.  5)Adjust the focusing knob( item A in Fig 2) such that you get a single image of the central circle.(A,B in Fig 3)  6) Align the side and upper circles with the central circle by rotating the body of the instrument(using item B in Fig 2) .(C in Fig3) Keratometry
  15. 15.  7) Use the horizontal alignment knob(M) to bring the crosses on each other.(E in Fig 3)  8) Use the vertical alignment knob(N) to bring the crosses on each other.  9) Read the values from the scales (M & N) and the angle from (S). Keratometry
  16. 16. FIGURE 3 Keratometry
  17. 17. Javal-Schiotz Keratometer  Two position Keratometer  Only measures one meridians at one time. Keratometry
  18. 18. Procedure  Follow steps 1-4 of the above instrument  5) Adjust the instrument to see a clear image of the rectangle and staircase targets Keratometry
  19. 19.  6) Rotate the instrument to align the line in rectangle with the line in the staircase target.  7) Turn the measuring knob until staircase and rectangle just overlap.  8) Read the values from the scales. Keratometry
  20. 20. RECORDING  K flattest meridian@angle  Steepest meridian@angle Example 42.00@180 45.00@90 -3.00x180 Mires Clear & Regular Also determine if the mires are clear and regular. Keratometry

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