Measurement of corneal curvature


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Measurement of corneal curvature

  1. 1. Measurement of Corneal curvature Principle of Keratometry Gauri S. Shrestha M.Optom, FIACLE
  2. 2. Keratometers <ul><li>Measure the front surface corneal radii </li></ul><ul><ul><li>Apparent front corneal surface curvature </li></ul></ul><ul><li>Give the total corneal power </li></ul><ul><ul><li>on the assumption that the back surface of the cornea has 10% of the power of the front surface, using a calibration/refractive index of 1.3375 </li></ul></ul><ul><li>Main function – </li></ul><ul><ul><li>measurement of the radius of curvature of the central portion ( 3 mm- optic cap) </li></ul></ul>
  3. 3. Keratometers: uses <ul><li>CL fitting and verification </li></ul><ul><li>Monitoring corneal shape </li></ul><ul><li>Calculate the power of intraocular Lens </li></ul><ul><li>Helpful to determine ocular refraction </li></ul>
  4. 4. History <ul><li>In 1619 , Scheiner (glass sphere of known radii - corneal curvature) –( compared size of the bars in a window- lens & cornea). </li></ul><ul><li>In 1796 , Ramsden – Invetor of keratometer with three essential elements </li></ul><ul><li>In 1854 , Helmholtz improved Ramsden’s design for laboratory use </li></ul><ul><li>In 1881 , Javal & Schiotz modified Helmholtz’s instrument for clinical use </li></ul><ul><li>In 1980 , the development of autokeratometer </li></ul>
  5. 5. Principle of Keratometry: Optical <ul><li>Measures angular size of reflected image (1st Purkinje image) formed by anterior corneal surface </li></ul><ul><li>Image is formed at a specific distance from the object plane </li></ul><ul><li>Virtual image formation behind the cornea </li></ul>
  7. 7. PRINCIPLES OF KERATOMETRY OPTICAL <ul><li>Referring to triangles: </li></ul><ul><li>h/h' = - f/x = - r/2x </li></ul><ul><li>For a mirror, f=r/2 </li></ul><ul><li>But... x = d (Most instrument working distances are large relative to the corneal radii of curvature) </li></ul><ul><li>r =-2dh’/h where: r = corneal radius of curvature </li></ul><ul><li> d = object to image distance </li></ul><ul><li> h' = image height </li></ul><ul><li> h = mire separation </li></ul><ul><li> f = corneal focal length </li></ul>
  8. 8. Optical Principle of Keratometry <ul><li>As, d & h are constant or measurable </li></ul><ul><li>? h’ measurement = challenge </li></ul><ul><li>Due to presence of involuntary miniature eye movement during fixation of a eye </li></ul><ul><ul><li>The image formed by anterior surface of the cornea also moves : Not possible to measure </li></ul></ul><ul><ul><li>To over come obstacle – Doubling principle </li></ul></ul>
  9. 9. DOUBLING PRINCIPLE <ul><li>Measurement of image height </li></ul><ul><li>Doubling device: Plano prism </li></ul><ul><li>Displacement = image height </li></ul><ul><li>Axes determination by scissors distortion </li></ul><ul><li>Prism is moved along the optical axis until two images are just touching. </li></ul><ul><li>At this point, the prismatic displacement is exactly equal to the size of the image. </li></ul><ul><li>The larger the image size is ,the greater the amount of doubling must be to achieve contact of two image </li></ul>
  10. 10. DOUBLING SYSTEMS <ul><li>TYPES </li></ul><ul><ul><li>Fixed doubling </li></ul></ul><ul><ul><li>Variable doubling </li></ul></ul><ul><ul><li>Divided doubling </li></ul></ul><ul><ul><li>Full doubling </li></ul></ul>
  11. 11. DOUBLING SYSTEMS <ul><li>FIXED DOUBLING – B & L , Topcon & Magnon </li></ul><ul><ul><li>VARIABLE </li></ul></ul><ul><ul><ul><li>Image size and mire separation </li></ul></ul></ul><ul><ul><li>FIXED </li></ul></ul><ul><ul><ul><li>Object height and doubling device distance </li></ul></ul></ul><ul><li>VARIABLE DOUBLING - Haag streit & Javal Schiotz </li></ul><ul><ul><li>Fixed </li></ul></ul><ul><ul><ul><li>image size & mire separation </li></ul></ul></ul><ul><ul><li>Variable </li></ul></ul><ul><ul><ul><li>object size & doubling device distance </li></ul></ul></ul>
  12. 12. DOUBLING SYSTEMS <ul><li>DIVIDED DOUBLING </li></ul><ul><ul><li>Telescope objective- two separate fields: </li></ul></ul><ul><ul><ul><li>separate transmission of reflected beam </li></ul></ul></ul><ul><li>FULL DOUBLING </li></ul><ul><ul><li>Full aperture </li></ul></ul><ul><ul><li>Beam splitting device </li></ul></ul>
  13. 13. Objective Fixed graticule
  14. 14. Instrumentation <ul><li>The position of image U’V’ within the cornea prevents its direct measurement by superimposing a scale </li></ul><ul><ul><li>Keratometer incorporates a long focus microscope (or short focus telescope) whose objective O forms a second image in a plane accessible for measurement </li></ul></ul><ul><li>q’/q remains constant </li></ul><ul><li>A fixed graticule G is placed in the predetermined image plane at a distance q’ from p’. So conjugate object distance q is also fixed </li></ul>
  15. 15. <ul><li>Move keratometer until U’’V’’ is seen sharp focus on graticule </li></ul><ul><li>Since the test object UV is fixed in the instrument and moves with it, distance d attains its predetermined value simultaneously </li></ul>
  16. 16. Optical System of Keratometer
  17. 17. Types of Keratometers <ul><li>Two-position keratometers: </li></ul><ul><ul><li>require rotation about the axis to measure each of the principal meridians. </li></ul></ul><ul><ul><li>Ex: Javal-Schiotz Keratometer manufactured by Haag Streit & Sbisa </li></ul></ul><ul><ul><li>Fixed amount of image doubling & object height adjusted. </li></ul></ul>
  18. 18. Types of Keratometers <ul><li>One-position keratometers: </li></ul><ul><ul><li>simultaneous doubling of perpendicular pairs of mires is produced by doubling devices in each of the corresponding meridians </li></ul></ul><ul><ul><li>the principal meridians are assumed to be at right angles to each other. </li></ul></ul><ul><ul><li>Ex: Sutcliffe type keratometer manufactured by B & L or Magnon </li></ul></ul><ul><ul><li>Doubling device variable & object height constant. </li></ul></ul>
  19. 19. Keratometers <ul><li>Haag-Streit/Gambs. </li></ul><ul><li>Rodenstock keratometer C-BES & CES. </li></ul><ul><li>Topcon OM-4 Ophthalmometer </li></ul><ul><li>Zeiss Oberkochen keratometer. </li></ul><ul><li>B&L keratometer </li></ul><ul><li>Humphrey keratometer </li></ul>
  20. 20. Keratometers Humphrey Keratometer B & L Keratometer
  21. 21. Topcon OM-4 Ophthalmometer
  22. 22. Javel’s keratometers : Varieties of Mires
  23. 23. <ul><li>Humphrey Auto Keratometer </li></ul><ul><li>Canon Auto Keratometer </li></ul>Automated Keratometry
  24. 24. <ul><li>Focuses the reflected corneal image on to an electronic photosensitive device, which instantly records the size and computes the radius of curvature. </li></ul><ul><li>No Doubling device is needed. </li></ul><ul><li>Measures the angle size in many meridians so it computes angle as well as power in many meridians. </li></ul><ul><li>Absence of annoying glare of the brightly illuminated mires. </li></ul>Automated keratometry
  25. 25. To expand the range of measurement <ul><li>Range from 36 Ds to 52 Ds </li></ul><ul><li>If K reading is very high </li></ul><ul><li>For very high </li></ul><ul><ul><li>Place +1.25 Ds trial lens over eye piece – increase range by 9 D or </li></ul></ul><ul><ul><li>Multiply k reading by 1.185 </li></ul></ul><ul><ul><li>Ex: if with +1.25 D, dial reading = 49 D, Actual K = +58 D </li></ul></ul><ul><ul><li>Precise 49 D x 1.185 = + 58.07 Ds </li></ul></ul>
  26. 26. To expand the range of measurement <ul><li>For very low </li></ul><ul><ul><li>Place – 1.00 Ds trail lens over the eye piece – shift 6 D </li></ul></ul><ul><ul><li>Multiply k reading by 0.840 </li></ul></ul><ul><ul><li>Ex: With –1.0Ds, dial reading = +38 Ds , Actual reading = +32 Ds </li></ul></ul><ul><ul><li>Accurate, +38 x 0.840 = + 31.92 Ds </li></ul></ul>
  27. 27. Expanded ranges of Keratometer
  28. 28. Keratometry Calibration <ul><li>Calibration: </li></ul><ul><ul><li>Image-forming rays are reflected from the cornea at a height of at least 1 mm from the vertex. </li></ul></ul><ul><li>Calibration is done with precision spherical steel/glass balls of known radii. </li></ul>
  29. 29. Keratometry <ul><li>Calibration Index: </li></ul><ul><ul><li>The keratometer uses a specific refractive index to account for both the front and back surface corneal curvatures. </li></ul></ul><ul><li>The calibration index adopted is normally 1.332 or 1.3375. </li></ul>
  30. 30. CALIBRATION FORMULA <ul><li>Calibration </li></ul><ul><li>Formula </li></ul><ul><li>r = 2dh‘/h where: </li></ul><ul><li>r = radius </li></ul><ul><li>d = image distance </li></ul><ul><li>h' = image height </li></ul><ul><li>h = mire separation </li></ul>
  31. 31. Measurement <ul><li>Calculate the approximate radius of curvature: </li></ul><ul><ul><li>r = (d 2 +4S 2 ) / 8S where; </li></ul></ul><ul><ul><li>d = corneal diameter </li></ul></ul><ul><ul><li>S = limbal plane to apex distance </li></ul></ul><ul><li>Calculate corneal power from measured radius </li></ul><ul><ul><li>K = (n 1 -n 2 ) / r </li></ul></ul>
  32. 32. Measurement <ul><li>Javal’s rule: </li></ul><ul><li>A t = p (A c ) + K where; </li></ul><ul><li> A t = Total refractive astigmatism </li></ul><ul><li> A c = Corneal astigmatism </li></ul><ul><li> p = Approx 1.25 </li></ul><ul><li> K = 0.50 D against the rule astig </li></ul><ul><li>Substituting the values: </li></ul><ul><li>A t = 1.25 (A c ) – 0.50 x 90 0 </li></ul><ul><li>Grosvenor modification of Javal’s rule: </li></ul><ul><ul><ul><li>A t = 1.00 (A c ) – 0.50 x 90 0 </li></ul></ul></ul>
  33. 33. Measurement <ul><li>Measuring Rigid CL Radii </li></ul><ul><ul><li>Utilizes a prism or mirror attachment on the headrest. </li></ul></ul><ul><ul><li>45 diopter prism is used and positioned at the headrest so that light can be reflected back from the contact lens surface to the instrument. </li></ul></ul>
  34. 34. Procedure <ul><li>Focus the eye piece </li></ul><ul><li>Position the patient & Instruct </li></ul><ul><li>Adjust the instrument </li></ul><ul><li>Locate the principle meridian </li></ul><ul><li>Make the instrument </li></ul><ul><li>Record the measurement </li></ul>
  35. 35. Mires in keratometer Focusing keratometric target Locating the principle meridians Determining the corneal refractive power
  36. 36. PROBLEMS OF MEASUREMENT <ul><li>Fixation </li></ul><ul><li>Accuracy required </li></ul><ul><li>Instrument errors </li></ul><ul><li>Operator variability </li></ul>
  37. 37. SOURCES OF ERROR <ul><li>Instrumentation </li></ul><ul><ul><li>Assumptions </li></ul></ul><ul><ul><li>Uses paraxial theory </li></ul></ul><ul><ul><li>Assumes focal length = image distance </li></ul></ul><ul><ul><li>Object source comes from a flat plane </li></ul></ul><ul><ul><li>Reproducibility </li></ul></ul><ul><ul><li>Assumes the cornea is sphero-cylindrical </li></ul></ul>
  38. 38. SOURCES OF ERROR <ul><li>Operator </li></ul><ul><ul><li>Focusing </li></ul></ul><ul><ul><li>Proximal accommodation </li></ul></ul><ul><ul><li>Alignment errors </li></ul></ul><ul><ul><li>Orientation of instrument </li></ul></ul><ul><li>Patient </li></ul><ul><ul><li>Corneal distortions cause mire distortions </li></ul></ul><ul><ul><li>Fixation steadiness </li></ul></ul>
  39. 39. PHOTOKERATOSCOPE/ PLACIDO DISK <ul><li>Photokeratoscope/Placido Disc </li></ul><ul><ul><li>Used to study the corneal contour over a large </li></ul></ul><ul><ul><li>area, thus providing more information. This is </li></ul></ul><ul><ul><li>its advantage over the keratometer. </li></ul></ul><ul><ul><li>The Placido disc has more complete mire locus. </li></ul></ul><ul><ul><li>Can also provide qualitative information. </li></ul></ul>
  40. 40. Placido Disc <ul><li>Qualitative assessment. </li></ul><ul><li>Gross changes only. </li></ul>
  41. 41. Sun Nidek Photokeratoscope <ul><li>A polaroid camera or video is attached to record images of the mires on the patient’s cornea. </li></ul>
  42. 42. PHOTOKERATOSCOPE/ PLACIDO DISK <ul><li>Principles of the Placido Disc </li></ul><ul><li>Utilizes corneal reflections (Purkinje image #I)of bright rings. </li></ul><ul><li>Viewing is done through the central magnifying lens. </li></ul><ul><li>Outer rings subtend a wider angle, creating a defocus of peripheral mire images. </li></ul>
  43. 43. Images <ul><li>Seen Through the Photokeratoscope / Placido Disc </li></ul><ul><ul><li>Elliptical - astigmatism. </li></ul></ul><ul><ul><li>Distorted - scars, irregularity. </li></ul></ul><ul><ul><li>Asymmetric - keratoconus. </li></ul></ul>
  44. 44. Images
  45. 45. Disadvantages of the Placido Disc <ul><li>Clinically significant amounts of corneal cylinder (up to 3.00 D) can be present in a cornea and may not be diagnosed. </li></ul><ul><li>Subtle irregularities cannot be detected. </li></ul><ul><li>The centrally positioned lens obstructs the central mire reflections. </li></ul><ul><li>There is limited depth of focus during photography because of the curved image surface. </li></ul><ul><li>Anatomy of the nose or orbit may limit field size and restrict the corneal area that can be examined. </li></ul>