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  2. 2. • Cornea is the most powerful refractive element of the eye contributing 43D(70%) of refractive power of eye. • Since the shape of corneal surface determines its refractive power , even a minor modification of its surface can lead to significant alteration of image formed on retina.
  3. 3. • Prolate shape • Anterior surface of cornea: Elliptical • Horizontal diameter: 11.5mm • Vertical diameter:10.6mm • Posterior surface of cornea is circular with average diameter of 11.5mm • Thickness of cornea: • 0.52mm at centre • 0.8mm at periphery • 1mm at limbus • Anterior radius of curvature:7.8mm • Posterior radius of curvature:6.5mm
  4. 4. • CENTRAL ZONE: Approximately 4mm diameter. Also called the apical zone • PARACENTRAL ZONE: 4-8mm diameter. Flatter than the central zone. • Central + Paracentral zone= optical zone. • PERIPHERAL ZONE: 8-11 mm. It is the zone where the normal cornea flattens the most and becomes ashperic. • LIMBAL ZONE
  5. 5. CORNEAL TOPOGRAPHY • Study of shape 0f corneal surface. • Configuration or description of surface • Aspheric – elliptical shape.
  6. 6. METHODS OF MEASUREMENT • Keratometer (Ophthalmometer ) 1) HELMHOLTZ KERATOMETER 2) BAUSCH AND LOMB KERATOMETER 3) JAVAL SCHIOTZ KERATOMETER Keratoscope : • Placido disc • Photokeratoscopy - NIDEK PKS 1000 OR KERACORNEASCOPE • Scanning slit topography- ORBSCAN • Interferometric - HOLOGRAPHY AND MOIR’E fringe techniques (CLAS) • Rasterstereography – PAR corneal topography • Schiempflug imaging - pentacam
  7. 7. PENTACAM • The Pentacam(Oculus Inc) obtains images of anterior segment by rotating Schiempflug camera which is a digital charged coupling device.
  8. 8. KERATOGRAPH ALGORITHMS Process of building a topographic map of cornea from keratoscopic data .  Capture video images of the keratoscope rings.  Measure angular size of points on the rings.  Reconstruct the corneal surface point by point.  Assign dioptric or other descriptors for each surface.  Present surface descriptors in a color topographic map
  9. 9. INTERPRETATION • 1 .Color coding • 2. Scale • 3. Quantitative indices
  10. 10. • Cool colors (black, blue) Flatter surfaces • Warm colors (orange, red, white) Steeper surfaces • Normal (green, yellow) Normal surfaces
  11. 11. SCALE USED • Range: 28.o0 D to 65.00 D • Interval : 1.5 D
  12. 12. QUANTITATIVE INDICES Simulated keratometry (Sim K ) Surface Regularity Index (SRI) Surface Asymmetric Index (SAI) Irregular astigmatic index (IAI ) Differential sector index(DSI) Opposite sector index(OSI) Centre/surround index (CSI ) Analyzed area (AA)
  13. 13. SIMULATED KERATOMETRY(Sim K) • Measure differences in corneal power & compares asymmetry. • Provides the power and location of the steepest and flattest meridians • Equivalent to Conventional Keratometry Reading
  14. 14. • Surface Regularity Index (SRI) -Local fluctuation in central corneal power -Local surface irregularities • Surface Asymmetric Index (SAI) -Measures differences in corneal power between corresponding points at each ring & compares symmetry. - Used to monitor changes caused by contact Lens warpage or keratoplasty
  15. 15. Formats for display of datas on colour maps 1. Corneal power map( saggital&axial) 2. Tangential map 3. Elevation map 4. Refractive power map 5. Irregularity map 6. Trend and time display 7. Difference display map 8. Rt /Lt eye compare map
  16. 16. The ANTERIOR SAGITTAL map • Steep areas are displayed in hot colours (red and orange), while flat areas are displayed in cold colours (green and blue). On the other hand, red segments are displayed on steep areas, while blue segments are displayed on flat areas. The cross point of this segmentation represents apex (anatomical center) of the cornea. Beside the shape of the map, parameters should be studied particularly on the steep axis at the 5-mm central circle. The normal pattern is the symmetric bowtie (SB) .
  17. 17. The ANTERIOR SAGITTAL map • a. In WTR astigmatism, the SB is on or within ―15° of the vertical meridian of the cornea . • b. In ATR astigmatism, the SB is on or within ―15° of the Horizontal meridian of the cornea • c. In oblique astigmatism, the SB is neither vertical nor horizontal . • The SB pattern can be encountered in KC when K readings are abnormally high .
  18. 18. • Corneal power map 24 colour representation of diopteric power at various points on the cornea
  19. 19. Tangential map • Better geographical representation of cornea • Best indicator of corneal shape than power
  20. 20. Refractive power map • It illustrates how the corneal curvature refracts light in true dioptres of power. • Useful for determining the optical zone for rigid gas permeable lenses and in performing refractive corneal surgery.
  21. 21. The Elevation Maps • An elevation map describes the height details of the measured corneal surface by matching it with a reference surface (RS). Points above the RS are considered elevations and expressed in plus values, and those below the RS are considered depressions and expressed in minus values In corneal astigmatism, one meridian is steeper than the other and is located under the RS taking minus values, contrary to the flatter meridian which takes plus values .
  22. 22. Irregularity map • It shows areas in cornea that are hot in colour. • It displays the distorsion of cornea using previous elevation map results with toric reference instead of sphere reference.
  23. 23. Trend and time display • In this changes ocurring in topography with time can be displayed in chronological order
  24. 24. Difference display map It exibits comparative difference in two given topographic maps
  25. 25. • Rt/Lt eye comparision map Allows comparision of both eyes simultaneously
  27. 27. KERATOCONNUS • Keratoconus is a noninflammatory pathologic condition characterized by progressive thinning and protrusion of the cornea. • The thinning process occurs in one particular area so that the surrounding area remains disproportionately thicker. • • Physiologically, the normal cornea is thinner in its center and thicker in the periphery. • The gradual increase of the corneal thickness from the center toward the periphery in healthy eyes falls within a normal range and that this characteristic could lead to a criterion for identifying pathology such as ectasia.
  28. 28. 4MAP ELEVATION AND KERATOCONUS • Use best-fit sphere and float and a scale that is ±75μm • we suggest front elevation first and Look at back elevation and pachymetry next and at curvature last • Normal values for front elevation are less than +12μm. • Differences greater than +15μm typically indicate keratoconus • Between +12 and +15μm are suspicious. • Normal values for posterior elevation are approximately 5μm higher than those for front elevation,
  29. 29. Pellucid Marginal Degeneration • Hallmarked by a thinning of the inferior peripheral cornea • The corneal thinning begins approximately 1.0 to 2.0 mm above the inferior limbus.
  30. 30. • High against the rule astigmatism. • Inferior mid-peripheral steepening at 4 & 8 o’clock position. • Kissing pigeon pattern (diagnostic of PMD)
  31. 31. CORNEAL TOPOGRPHY IN PTERIGYUM • Typical with-the-rule astigmatism is induced. • Bow-tie pattern oriented vertically.
  32. 32. TOPOGRAPHY IN TRAUMATIC CASES Depends upon • Location • Severity(extent & depth) • Type of trauma Flattening along the meridian of laceration & steepening along 90* away
  33. 33. CLINICAL USES Photorefractive Surgery • Preoperative screening • Surgical planning • Assessment of surgical outcomes • Detection & management of complications • Refinement & development of surgical outcome Expected shape of cornea after refractive surgery
  34. 34. Evaluation Of intraocular Surgery • Cataract • Penetrating keratoplasty • Radial Keratotomy
  35. 35. • Diagnosis of Corneal degenerations & dystrophies. • Keratoconus • Keratoglobus • Pellucid Marginal Degeneration • Terrien’s Marginal degenerations
  36. 36. Determination Of • Refraction esp. for poor quality retinoscopic reflex. • Curvature of cornea • Amount and direction of corneal astigmatism • Quality of corneal refracting surface • Stability of corneal refracting surface.
  37. 37. • In cases of trauma • Progressive Myopia • Aphakia or high ametropia • To calculate the power of Intraocular Lens
  38. 38. • Contact lens fitting • Choice of trial lens. • Verification of contact lens parameters • Monitoring corneal shape • Orthokeratology
  39. 39. LIMITATIONS 1) Algorithms for power calculations are based on spherical optical system , while the normal cornea is aspheric. 2) The formulae employed for power calculation are centered on the corneal apex and not on the more relevant line of sight. 3) The keratometric index of refraction (1.3375)employed underestimates the changes in corneal power after procedures like PRK as the actual refractive index of cornea is 1.376
  40. 40. • Thank u