L15 chapter 13 keratometry and keratoscopy 2 2007 2008

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L15 chapter 13 keratometry and keratoscopy 2 2007 2008

  1. 2. Keratometry: Main Points so Far <ul><li>Keratometry uses the anterior cornea as a mirror </li></ul><ul><li>Distant object: h   r AC </li></ul><ul><li>Keratometer Equation: </li></ul><ul><li>Virtual corneal image ( h  ) inaccessible, small and unstable (eye movements), so: </li></ul><ul><ul><li>Use objective lens to focus reflected rays as a real image </li></ul></ul><ul><ul><li>magnify the real image with an eyepiece lens (~ 5  mag) </li></ul></ul><ul><ul><li>split the real image inside the keratometer into two using a half-field prism; adjust prism to “double” images </li></ul></ul>
  2. 3. OBJECTIVE h  ½ h C MIRE F CORNEA ½ h PRISM (P  ) IMAGE PLANE Fig 13.17, Page 13.18 Moving prism toward image plane decreases image displacement ( x ) Previously doubled images are no longer doubled (now overlap) What new corneal radius would this prism position “suit”? What happens if we move the prism?  x h  < h  P
  3. 4. Q1: Based on the previous figure, how could the keratometer prism be used to yield a measure of anterior cornea radius? <ul><li>for shorter corneal radii, the prism would be moved LEFT to double the images </li></ul><ul><li>for shorter corneal radii, the prism would be moved RIGHT to double the images </li></ul><ul><li>It could provide a qualitative comparison only between corneas based on separation or overlap of the images </li></ul>
  4. 5. Fig 13.22, Page 13.27 ILLUMINATED MIRE HORIZONTAL & VERTICAL PRISMS EYEPIECE OBSERVER APERTURE PLATE CORNEAL MIRE IMAGE P H P V OBJECTIVE LENS Schematic View of the B & L Optical System OBJ GRATICULE PLANE Two prisms means two deviated images
  5. 6. Topcon Keratometer What the Clinician Sees V 90 / H 180
  6. 7. B & L: Oriented to Measure r 90 and r 18 0 Question: If most corneas are aspheric, what is one drawback with a keratometer? Answer: only measuring radius at one location (annulus) on cornea; and it is NOT central radius OBJ h' 90 Corneal vertex h' 180
  7. 8. B & L: Oriented to Measure r 60 and r 15 0 h' 60 Corneal vertex h' 150
  8. 9. Q2: What does this appearance indicate? <ul><li>Spherical corneal surface </li></ul><ul><li>Irregular Astigmatism </li></ul><ul><li>Oblique Astigmatism </li></ul><ul><li>With-the-Rule Astigmatism </li></ul><ul><li>Against-the-Rule Astigmatism </li></ul>
  9. 10. B & L: Oriented to Measure r 90 and r 18 0 Question: What does the above appearance indicate? Answer: anterior corneal astigmatism. What type? Against-the-rule OBJ h' 90 Corneal vertex h' 180
  10. 11. Estimation of Total Corneal Power <ul><li>Most keratometers read out both anterior radius and total corneal power. How is this possible? </li></ul><ul><li>It is not! </li></ul><ul><li>Keratometer gives only anterior corneal radius - it cannot measure posterior radius  total corneal power reading is an estimate </li></ul><ul><li>Estimate usually reasonable because the anterior cornea carries so much of the total corneal power (big  n) </li></ul>Page 13.23 OBJ
  11. 12. <ul><li>To see how we could estimate total corneal power from Keratometry (anterior radius alone)  modify the Exact Eye to simulate what the keratometer is measuring </li></ul>Basis of Corneal Power Estimate <ul><li>Effectively  creating a new schematic eye with an anterior cornea only that gives the same total corneal power as the Exact Eye </li></ul>
  12. 13. Basis of Corneal Power Estimate - Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D Page 13.23 n cornea 1.376
  13. 14. Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Based on Keratometry  want anterior surface only n cornea 1.376 r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D
  14. 15. Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Based on Keratometry  want anterior surface only r 2 = +6.8 mm F e (cornea) +43.05 D F 1 = +48.83 D F 2 =  5.88 D
  15. 16. Basis of Corneal Power Estimate - Modified Exact Eye r 1 = +7.7 mm n aqueous 1.336 n air 1.000 Keep true anterior corneal radius - this is what keratometry measures Want single surface cornea to give same +43.05 D as the Exact Eye cornea Using n  = 1.3315, the +7.7 mm Exact Eye anterior corneal radius yields correct total corneal power +43.05 D Why is new n  < 1.336? Only option is to change n aqueous
  16. 17. Estimation of Total Corneal Power <ul><li>Calibration Refractive Index = 1.3315 works for real corneas if: </li></ul><ul><ul><li>anterior : posterior corneal radii are in the same proportion as the SEEE cornea (7.7/6.8) </li></ul></ul><ul><ul><li>central thickness of the cornea is 0.5 mm </li></ul></ul><ul><li>Usually a good estimate, but keratometer cannot verify either of these properties </li></ul>Page 13.24
  17. 18. Calibration Refractive Indices - Real Keratometers <ul><ul><li>Zeiss, Rodenstock 1.332 </li></ul></ul><ul><ul><li>B & L, Haag-Streit (Javal-Schiötz) 1.3375 </li></ul></ul><ul><ul><li>American Optical 1.336 </li></ul></ul><ul><li>B&L and AO index based on corneal back vertex power estimate (using posterior cornea as reference plane) </li></ul>
  18. 19. Calibration Refractive Index - B & L Keratometer <ul><li>Different keratometer calibration refractive indices will give different total power estimates </li></ul><ul><li>Contact lens practice  corneal power estimate used to estimate total corneal astigmatism. </li></ul><ul><li>Astigmatism rarely exceeds 10% of total corneal power (~ +43 D)  0.78 D discrepancy in total power estimate translates to  0.078 D discrepancy in corneal astigmatism </li></ul><ul><li>Intraocular implant design: formula uses total corneal power estimate from keratometry directly  with 1.3375, the SEEE cornea’s in situ power is 0.78 D higher </li></ul>OBJ Page 13.24-25
  19. 20. Intraocular Implant Design <ul><li>Relies heavily on axial length and keratometer readings : </li></ul>OBJ: when applying formula, the basis (n cal ) of the K value must be consistent with the ‘A’ value (design constant) IOL power for emmetropia Constant based on IOL type Axial length in mm Average total corneal power based on keratometry
  20. 21. Corneal Power Estimate - Routine Applications <ul><li>Estimating total corneal astigmatism. </li></ul><ul><li>Estimating total ocular astigmatism: intraocular astigmatism averages 0.5 D atr  for most patients with moderate to high astigmatism, corneal astigmatism is a good predictor of total ocular astigmatism </li></ul><ul><li>Problem with estimates of total ocular astigmatism  keratometry will not identify exceptions to the trend </li></ul>Page 13.25

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