Corneal topography

corneal topography for under graduates and postgraduates

1. Corneal Topography Sana Azam M Phil. (VS) 3 rd semester PICO HMC Peshawar

2. Contents Introduction D evelopment of different systems for corneal measurements Different systems/techniques used for corneal topography Normal topographic map Indications for corneal topography Interpretations of corneal topography maps Corneal aberrometry Corneal topography in different clinical conditions

3. Introduction Corneal topography is a non-invasive imaging technique used to map the surface curvature of cornea

4. Overview of corneal dimensions Cornea comprises 2/3 rd of optical power of eye Refractive index 1.376 Vertical diameter is 10.6mm Horizontal diameter is 11.7mm

5. Overview of corneal dimensions (cont.) Radius of curvature Anterior surface :7.7mm Posterior surface :6.9mm

6. Overview of corneal dimensions (cont.) Zones of cornea 3-4 mm………. Apical zone 4 -8 mm………. Para-central 8-11 mm……….. peripheral 11-12 mm……….. limbal

7. Overview of corneal dimensions (cont.) Corneal thickness C entral 0.52 - 0.57 mm Peripheral 0.66 - 0.76mm Limbus 1.2mm

8. Principle of Corneal topography Cornea acts as a convex mirror Image formed is inversely related to curvature of mirror

9. Development of different systems for corneal measurements 1. Scheiner’s method 2. Keratometer 3 . Keratoscopy 4 . Rasterstereography 5. Interferometry

10. Development of different systems for corneal measurements 1. Scheiner in 1619 analyzed corneal curvature by matching image of window frame reflected onto subject’s cornea with the image produced by one of his calibrated spheres

11. Development of different systems for corneal measurements (cont.) 2. Keratometer described by Helmholtz in 1854 measures the radius of curvature of anterior corneal surface from 4 reflected points approximately 3mm apart

12. Development of different systems for corneal measurements (cont.) 3 . Keratoscopy involves evaluation of topographic abnormalities of corneal surface by direct observation of images of mires Advances in Keratoscopy are: Placido disc Keratoscopy Photokeratoscopy Videokeratoscopy

13. Development of different systems for corneal measurements (cont.) 3. Keratoscopy Placido disc Keratoscopy was introduced by Antonio Placido in 1880 Disc consists of black and white mires with a central hole Distortion in corneal shape will be observed as deviation from evenly spaced concentric mires

14. Development of different systems for corneal measurements (cont.) 3. Keratoscopy b. Photokeratoscopy The technique was given by Gullstrand in 1896 The system consists of a photographic film camera attached to Keratoscope There are 9-15 rings covering 55-75% of corneal area Closer the rings, steeper will be the cornea

15. Development of different systems for corneal measurements (cont.) 3. Keratoscopy c. Videokeratoscopy consists of a television camera attached to it Its mires cover about 95% of corneal area

16. Development of different systems for corneal measurements (cont.) 4 . Rasterstereography uses calibrated grid which is projected on fluorescein stained tear-film

17. Development of different systems for corneal measurements (cont.) 5 . Interferometry comprises a reference sphere that is compared to tested surface Interference fringes are produced as a result of difference in 2 shapes This difference is then interpreted as contour map of surface elevation

18. Techniques used for corneal measurements Specular reflection technique Placido disc system Interferometry based systems Diffuse reflection technique Rasterstereography Scattered light slit based system ORBSCAN II

19. Topography of normal cornea

20. Characteristics of corneal shape Corneal surface is not a perfect sphere and can be studied in terms of; Eccentricity e Shape factor P Asphericity parameter Q

21. Characteristics of corneal shape Eccentricity is the degree of peripheral flattening of cornea (e =0.41-0.58) Shape factor/ p-value mathematically defines corneal asphericity p = 1 – e2 e = √ 1-p e2 =1-p e2 is sometimes taken as asphericity parameter Q

22. Eccentricity

23. Characteristics of corneal shape C ornea progressively flattens out towards periphery by 2-4 D Nasal area flattens more than temporal

24. Topographic patterns in normal cornea Regular patterns Round 23% Oval 21% Steepening Superior Inferior T hese patterns are considered normal w.r.t to keratoconic cornea however for their presence astigmatism is necessary to present

25. Topographic patterns in normal cornea (cont.) Astigmatic patterns Symmetric bow-tie 18% Symmetric bow-tie with skewed axis Symmetric bow-tie without skewed axis Asymmetric bow-tie 32% Asymmetric bow-tie with superior steepening Asymmetric bow-tie with inferior steepening Asymmetric bow-tie with skewed radial axis Irregular astigmatism 7%

26. Indications for corneal topography To diagnose corneal diseases i.e keratoconus To guide CL fitting To evaluate the effect of keratorefractive procedures To guide the removal of tight sutures after corneal surgery To determine keratometry values in order to calculate required IOL power

27. Corneal topography examination & interpretations

28. Pre-requisites for good topographic examination Patient should be seated facing the bowl and adjustments should be done for proper alignment Exactly center and focus the mires on cornea Observe if there are tear film irregularities Observe if there are any artifacts induced by nose or eyelids

29. Interpretations of corneal topography maps Interpretations of maps are based on analysis of: Raw Photokeratoscope image Color coded scales Topographic displays Basic topographic index

30. Interpretations of corneal topography maps 1. Raw photokeratoscope image This image is based on unprocessed data and verifies the reliability of resulting topographic map

31. Interpretations of corneal topography maps 2. Color-coded Scales Hot colors = steepness Cool colors = flatness

32. 2. Color-coded Scales (cont.) Absolute Scale Normalized Scale Each color represents 1.5D interval between 35-50D and above this range colors represent 5D intervals In this scale cornea is divided into 11 colors spanning the eye's total dioptric power Doesn't show subtle changes of curvature Identifies minute topographic changes Easier to read Can magnify subtle changes

33. Interpretations of corneal topography maps 3. Topographic displays Axial maps Instantaneous map Refractive map Elevation map Difference map Relative map Irregularity map

34. 3.Topographic displays 3.1 A xial map Axial or corneal power map is a 24 color representation of dioptric power of cornea at various points on cornea Radius of curvature is measured 360 times for each P lacido ring image from center to vertex S agittal algorithm averages the data points from first to next ring and so on

35. 3. Topographic displays 3.2 Tangential Map In this scale ring curvature is measured along the tangents which are projected from center vertex 360 degrees This map is best indicator of corneal shape but poor indicator of corneal power

36. 3. Topographic displays 3.3 Elevation Map Elevation of a point on corneal surface displays the height of point on corneal surface relative to spherical reference surface Distinguishes localized elevation from otherwise steep cornea Helpful in determining ablation depth after refractive surgeries

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38. 3. Topographic displays 3.4 Refractive Map / Asphericity M ap Displays refractive power of cornea Relates corneal shape to vision by considering the effects of spherical aberrations Spherical cornea has cooler colors in center with increasing hotter colors towards periphery

39. 3. Topographic displays- Refractive Map Useful in understanding the effects of refractive surgery and determining the optical zone for RGP lenses

40. 3. Topographic displays 3.5 Irregularity Maps Same as elevation map but reference surface is a toric surface Difference between corneal surface and reference toric surface represents the part of cornea that cannot be optically corrected

41. Topographic displays – Irregularity Maps Hotter colors represent higher value of distortion in units of wave-front error which can be translated into diopters of distorted power

42. 3. Topographic displays 3.6 Difference Map Displays the changes in certain values between two maps Use to monitor any type of change i.e. recovery from contact lens-induced warpage

43. 3. Topographic displays 3.7 Relative Map Displays some values by comparing them to an arbitrary standard i.e. sphere, asphere, normal cornea 3.8 OD/OS comparison map It allows simultaneous comparison between both eyes

44. 4- Basic topographic indices Basic topographic indexes include: Sim -K reading Min-K reading Corneal eccentricity index Average corneal Power Surface regularity index Surface asymmetry index

45. 4. Basic topographic indices 4.1 Sim -K reading It provides the power and axis of steepest and flattest corneal curvatures Common uses are contact-lens fitting, refractive surgery, assessing irregular corneal shape

46. 4.Basic topographic indices 4.2 Min-K reading Minimum Meridional power from rings 7, 8, and 9 The average power and axis are displayed

47. 4.Basic topographic indices 4.3 C orneal eccentricity index Estimates eccentricity of central cornea and is calculated by fitting an ellipse to corneal elevation data Positive value is for prolate surface Negative value is for oblate surface Zero value is for perfect sphere T his value is used for contact lens fitting

48. 4. Basic topographic indices 4.4 Average corneal power This is the area-corrected average corneal power in front of pupil Helpful in determining central corneal curvature when calculating appropriate intraocular lens power

49. 4. Basic topographic indices 4.5 Surface regularity index (SRI) It measures the regularity of corneal surface that correlates with best spectacle corrected visual acuity assuming the cornea to be only limiting factor SRI index increases with increase in irregularity in corneal surface It measures optical quality

50. 4. Basic topographic indices 4.6 Surface asymmetry index The index of asphericity indicates how much the curvature changes upon movement from center to periphery of cornea When cornea becomes less symmetric, the index differs more from zero

51. 5. Corneal aberrometry & wave-front maps Corneal aberrometry is measure of aberrations that occur during refraction through cornea Wavefront aberration is deviation of resulting wavefront from ideal wavefront Greater the difference between resulting and ideal wavefront, more worse will be the image quality

52. 5. Corneal aberrometry & wave-front maps Zernike polynomials are used to define and quantify monochromatic aberrations Zernike terms are defined using double index notation Radial order (n) Angular frequency (m )

53. Corneal aberrometry terminologies (cont.) Low order aberrations 1 st order is not related to wavefront & is corrected by prisms 2 nd order = spherical / astigmatism defocus = corrected by spectacles , CL High order aberrations ( not corrected by spectacles/CL) 3 rd order = coma/ trefoil defects 4 th order = spherical aberrations

54. 5. Corneal aberrometry & wave-front maps (cont.) Wavefront maps measure the pathway difference between measured wavefront and reference wavefront in microns at pupil entrance Each color represents a specific degree of wavefront error in microns

55. 5. Wavefront Maps (cont.) In order to evaluate the impact of aberrations on vision quality following quantitative parameters are defined; Peal to valley error (PV error) Root mean square error (RMS) Strehl ratio Point spread function (PSF) Modulation transfer, phase transfer, optical transfer function

56. 5. Wavefront Maps (cont.) 5.1 PV error is measure of distance from lowest to highest point in wavefront It doesn't represent the extent of defect so is not a best measure of optical quality

57. PV error

58. 5. Wavefront Maps (cont.) 5.2 RMS Error is statistical measure of deviation of corneal wavefront from the ideal wavefront I t describes the overall aberration and indicates how bad aberrations are

59. 5. Wavefront Maps (cont.) 5.3 PSF measures how well a point object is imaged on output plane (retina) through optical system In small pupils (1mm) PSF is affected by diffraction I n large pupil aberrations are source of image degradation

60. 5. Wavefront Maps (cont.) 5.4 S trehl ratio represents the ratio of maximum intensity of actual image to maximum intensity of fully diffracted image both being normalized to same integrated flux

61. 5. Wavefront Maps (cont.) 5.5 Modulation transfer, phase transfer, optical transfer function defines the ability of optical system to affect the image of grating by reducing the contrast and is known as modulation transfer function (MTF) or By shifting the image sideways called phase transfer function (PTF) Optical transfer function is made up of MTF and PTF

62. 6 . Corneal topography in different clinical conditions 6 .01 Keratoconus Keratoconus appears as an area of increased corneal power surrounded by concentric area of decreasing power Inferior-superior power asymmetry Skewed radial axis

63. 6 . Corneal topography in different clinical conditions 6 .02 Pellucid Marginal Degeneration (PMD) Inferior corneal thinning between 4 & 8 o’ clock position Flattening of vertical meridian Against the-rule astigmatism

64. 6 . Corneal topography in different clinical conditions 6 .03 Keratoglobus is a bilateral disorder in which entire cornea is thinned out most markedly near limbus Corneal topography shows; Peripheral steepening Asymmetrical bow-tie configuration

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66. 6 . Corneal topography in different clinical conditions 6 .04 Terrien’s Marginal Degeneration Flattening over the area of peripheral thinning Relative steepening of corneal surface 90 degrees away from midpoint of thinned area Against the-rule or oblique astigmatism

67. 6 . Corneal topography in different clinical conditions 6 .05 Pterygium Flattening of cornea at axis of lesion Marked WTR astigmatism even more than 4D

68. 6 . Corneal topography in different clinical conditions 6 .06 Post-radial keratotomy (post-RK) RK corrects myopia by placing series of radial incisions leaving a clear optical zone Typical topographic pattern is polygonal shape Incisions cause flattening of central cornea surrounded by bulging ring of steepening

69. 6 . Corneal topography in different clinical conditions 6 .07 Post-astigmatic Keratotomy Incisions are placed on steepest meridian to cause flattening of that particular meridian 90 degree apart meridian becomes steeper

70. 6 . Corneal topography in different clinical conditions 6 .08(a) Post-photorefractive keratotomy in Myopia Laser beam flattens central cornea to correct myopia thus giving the cornea an oblate shape

71. 6 . Corneal topography in different clinical conditions 6 .08(b) Post –PRK in Hypermetropia Laser beam flattens mid-peripheral cornea in hyperopia giving it a prolate profile 6 .08(c) Post-PRK in Astigmatism The zone on which laser beam is directed will appear oval

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73. 6 . Corneal topography in different clinical conditions 6 .09 Post LASIK In LASIK laser beam ablates the tissue under superficial corneal flap Topographic pattern for myopia is oblate Topographic pattern for hyperopia is prolate

74. 6 . Corneal topography in different clinical conditions 6 .10 Post-laser thermal keratoplasty Holmium laser heats the corneal stromal collagen in a ring around the outside of pupil causing localized flattening The surrounding zone will become steeper Typical topography pattern is central corneal steepening and ring of flattening around it

75. 6 . Corneal topography in different clinical conditions 6 .11 Post INTACS INTACS are inserted into periphery of cornea to correct small degrees of myopia or hyperopia by changing orientation of collagen lamellae

76. 6 . Corneal topography in different clinical conditions 6 .12 Post-keratoplasty Sutures usually induce a central bulge in corneal graft Prolate configuration is common

77. 6 . Corneal topography in different clinical conditions 6 .13 CL induced corneal warpage Corneal warpage is topographic change in cornea caused by mechanical pressure exerted by CL use Common topographic patterns are: Peripheral steepening Central flattening Furrow depression Central molding/irregularity Pseudokeratoconus