- 2. CoCornrnealeal ToTopopogrgrapaphyhy andand OOrrbbssccaann IIII M.Khanlari,MDM.Khanlari,MD Noor Vision Correction CenterNoor Vision Correction Center
- 3. ►Topography refers to determiningTopography refers to determining and describing the features of aand describing the features of a surfacesurface
- 4. Description of Corneal surfaceDescription of Corneal surface Corneal surface elevationCorneal surface elevation Corneal surface slopeCorneal surface slope Corneal surface radius ofCorneal surface radius of curvaturecurvature Corneal powerCorneal power
- 5. Methods of MeasurementMethods of Measurement ►Reflection-based methodsReflection-based methods measure themeasure the slopeslope and use this to calculateand use this to calculate radius of curvatureradius of curvature andand powerpower.. However, cornealHowever, corneal elevationelevation can’t be calculatedcan’t be calculated from measurements of slope alone.from measurements of slope alone. ►Projection-based methodsProjection-based methods measure themeasure the elevationelevation and use this to calculateand use this to calculate slopeslope,, curvaturecurvature, and, and powerpower
- 6. Methods of MeasurementMethods of Measurement A.A. Reflection-based methodsReflection-based methods A.A. KeratometryKeratometry A.A. ManualManual B.B. AutomatedAutomated B.B. KeratoscopyKeratoscopy A.A. PhotokeratoscopyPhotokeratoscopy B.B. VideokeratoscopyVideokeratoscopy B.B. Projection-based methodsProjection-based methods A.A. Slit PhotographySlit Photography B.B. RasterstereographyRasterstereography C.C. Moire InterferenceMoire Interference D.D. Laser interferometryLaser interferometry
- 7. Curvature Based Placido Disc VideokeratographyCurvature Based Placido Disc Videokeratography Measures Slopes & CurvaturesMeasures Slopes & Curvatures DirectlyDirectly Specular Reflection of an illuminatedSpecular Reflection of an illuminated conical Placido disk with 16 to 32 ringsconical Placido disk with 16 to 32 rings Maximum corneal diameter that isMaximum corneal diameter that is mapped is 8 to 10 mmmapped is 8 to 10 mm One imageOne image One surfaceOne surface
- 8. Elevation Based Slit Scanning VideokeratographyElevation Based Slit Scanning Videokeratography Measures Elevation DirectlyMeasures Elevation Directly A series of slit lamp beams areA series of slit lamp beams are compiled across the cornea to create acompiled across the cornea to create a profile of the corneaprofile of the cornea Multiple ImagesMultiple Images Multiple SurfacesMultiple Surfaces
- 10. Reading the MapsReading the Maps
- 11. System for studying topographic mapSystem for studying topographic map ► CheckCheck ► NameName ► DateDate ► EyeEye ► ScaleScale ► RangeRange ► StepStep interval (normalized, absolute)interval (normalized, absolute) ► TypeType of measuremnt (curvature, power ,of measuremnt (curvature, power ,heightheight)) ► MapMap ► AxialAxial ► TangentialTangential ► RefractiveRefractive ► ElevationElevation ► Statistical informationStatistical information ► cursor boxcursor box ► IndicesIndices ► Compare previous maps of the same eyeCompare previous maps of the same eye ► Compare with topography of the other eyeCompare with topography of the other eye
- 13. Absolute ScaleAbsolute Scale (standardized)(standardized) Same dioptric step power on every mapSame dioptric step power on every map Allow direct comparison of 2 different mapsAllow direct comparison of 2 different maps AdvantagesAdvantages: Rapid pattern recognition of topographic maps: Rapid pattern recognition of topographic maps DisadvantageDisadvantage: Because the large steps do not show subtle: Because the large steps do not show subtle changeschanges Should be used for routine clinical analysisShould be used for routine clinical analysis
- 14. Normalized ScaleNormalized Scale (relative)(relative) Dioptric power steps based on the patients corneaDioptric power steps based on the patients cornea AdvantageAdvantage:Dioptric steps is smaller and show more detailed changes:Dioptric steps is smaller and show more detailed changes DisadvantageDisadvantage: 2 different maps can not be compared directly: 2 different maps can not be compared directly More sensitive and useful for researchMore sensitive and useful for research
- 15. Color Scale for Surface CurvatureColor Scale for Surface Curvature • sharp • fast bend • short radius • Higher power • flat • slow bend • long radius • Lower power (+ +) (+) Red BlueMin Max GreenGreen andand yellowyellow represent powers characrteristic of normal cornearepresent powers characrteristic of normal cornea
- 16. Absolute scaleAbsolute scale withwith 1.5 D step interval1.5 D step interval Normalized scaleNormalized scale withwith 0.5 D step interval0.5 D step interval
- 17. Topographic MapsTopographic Maps Axial MapAxial Map Tangential MapTangential Map Refractive MapRefractive Map Elevation MapElevation Map
- 18. Axial Map (Axial Map (power map, sagital map)power map, sagital map) ► Axial power:Axial power: A fixedA fixed center of curvature is usedcenter of curvature is used for calculating the powerfor calculating the power at all pointsat all points very simple map andvery simple map and values directlyvalues directly comparable tocomparable to keratometrykeratometry Paxial = (n' – 1) / raxial
- 20. Tangential MapTangential Map (( InstInstantaneous ,local, true , meridional)antaneous ,local, true , meridional) ► Tangential power:Tangential power: A floatingA floating center of curvature is used bycenter of curvature is used by means of a standard formula formeans of a standard formula for calculating powercalculating power ► Measures the power at a certainMeasures the power at a certain point in meridional directionpoint in meridional direction relative to the other points on therelative to the other points on the particular ringparticular ring ► More sensitive for local curvatureMore sensitive for local curvature changechange ► Most useful in following trend inMost useful in following trend in the post surgical or pathologicthe post surgical or pathologic eyes.eyes. Pinst = (n' – 1) / rinst
- 22. Refractive Map (paraaxial )Refractive Map (paraaxial ) ► Paraxial power:Paraxial power: used for centralused for central paraxial rays where theparaxial rays where the sagittal depthsagittal depth is approximately proportional to theis approximately proportional to the curvaturecurvature ► Applies snellen’s law to describe theApplies snellen’s law to describe the cornea’s actual refractive powercornea’s actual refractive power ► Evaluate visual performance in post-Evaluate visual performance in post- refractive surgery patientrefractive surgery patient ► IdentifiesIdentifies central islandcentral island who havewho have undergone PRK or LASIKundergone PRK or LASIK Pref = n/f f = z + y/ tan (θi - θr)
- 23. Quantitative IndicesQuantitative Indices Quantitative reference points to guide andQuantitative reference points to guide and asses the effect of therapyasses the effect of therapy Common indices include :Common indices include : Simulated keratometry values (Sim K)Simulated keratometry values (Sim K) Surface Asymmetry index (SAI)Surface Asymmetry index (SAI) Surface Regularity Index (SRI)Surface Regularity Index (SRI)
- 24. Quantitative IndicesQuantitative Indices ► Simulated keratometry valuesSimulated keratometry values (Sim K)(Sim K) Obtained from data points on miresObtained from data points on mires 77,, 88 andand 99 Provide the power and location of the steepest meridianProvide the power and location of the steepest meridian and flattest meridianand flattest meridian Analogous toAnalogous to keratometerkeratometer Good correlationGood correlation withwith refractive cylinderrefractive cylinder
- 25. Quantitative IndicesQuantitative Indices ► Surface Asymmetry indexSurface Asymmetry index (SAI)(SAI) Centrally weighted summation of differences inCentrally weighted summation of differences in corneal power between corresponding pointscorneal power between corresponding points 180180 degrees apart ondegrees apart on 128128 equally spaced meridiansequally spaced meridians crossing all the mirescrossing all the mires Theoretically isTheoretically is zerozero forfor perfect sphereperfect sphere ,,perfectly spherocylindericalperfectly spherocylinderical regularregular astigmatismastigmatism andand a surface with a powera surface with a power that is radiallythat is radially symmetricalsymmetrical Is sensitive to paracentral keratoconusIs sensitive to paracentral keratoconus
- 26. Quantitative IndicesQuantitative Indices ► Surface Regularity IndexSurface Regularity Index (SRI)(SRI) Summation of local fluctuations in power among 256Summation of local fluctuations in power among 256 equally spaced hemimeridians on the 10 central miresequally spaced hemimeridians on the 10 central mires Approach 0 for a normally smooth corneal surfaceApproach 0 for a normally smooth corneal surface It increases with increasing irregular astigmatismIt increases with increasing irregular astigmatism High correlationHigh correlation withwith best BSCVAbest BSCVA Can predict the optical performance of a normal corneaCan predict the optical performance of a normal cornea
- 27. Kcn Map indices in PentacamKcn Map indices in Pentacam ISV = Index of Surface Variance. IVA = Index of Vertical Asymmetry. KI = Keratoconus Index. CKI = Central Keratoconus Index. Rmin= the smallest radius of curvature IHA = Index of Height Asymmetry. IHD = Index of Height Decentration. ABR = Aberration Coefficient. KKS = Keratoconus Stage. AA = Analysed Area.
- 28. Watch for the BumpsWatch for the Bumps
- 29. Watch for the BumpsWatch for the Bumps A bump on the inferior cornea, associated withA bump on the inferior cornea, associated with thinning, is almost always diagnostic ofthinning, is almost always diagnostic of “KCN”“KCN” A “smile”- shaped rather than round bump, exactlyA “smile”- shaped rather than round bump, exactly at 6 o’clock at 3mm below the center, no moreat 6 o’clock at 3mm below the center, no more than 25than 25 μμm thinner than 3 mm above the center,m thinner than 3 mm above the center, suggestive ofsuggestive of “contact lens“contact lens warpagewarpage””
- 31. Normal CorneaNormal Cornea Flattens progressively from the center to the periphery by 2Flattens progressively from the center to the periphery by 2 to 4 D, asto 4 D, as Nasal hemimeridians flattening more than the temporalNasal hemimeridians flattening more than the temporal hemimeridianshemimeridians Two corneas at one individual normally exhibit non superTwo corneas at one individual normally exhibit non super imposable mirror- image symmetry(imposable mirror- image symmetry(EnantiomorphismEnantiomorphism)) The normal cornea exhibits relative smoothness andThe normal cornea exhibits relative smoothness and absence of significant irregular astigmatism.absence of significant irregular astigmatism.
- 33. Normal corneaNormal cornea ►Naturally occurring corneal topographicNaturally occurring corneal topographic patterns are classified by shape aspatterns are classified by shape as ((Bogan´s studyBogan´s study)) Round (22.6%)Round (22.6%) Oval (20.8%)Oval (20.8%) Symmetrical bowtie (17.5%)Symmetrical bowtie (17.5%) Asymmetrical bowtie (32.1%)Asymmetrical bowtie (32.1%) Irregular (7%)Irregular (7%)
- 34. 20.8%17.5%32.1%7.1%22.6% Pattern AnalysisPattern Analysis ►SphericalSpherical ►OvalOval ►BowtieBowtie SymmetricalSymmetrical AsymmetricalAsymmetrical ►IrregularIrregular
- 35. Regular AstigmatismRegular Astigmatism Most common naturally deviation from the normalMost common naturally deviation from the normal corneacornea Usually with the ruleUsually with the rule As a symmetrical bowtie of high powerAs a symmetrical bowtie of high power
- 36. Irregular AstigmatismIrregular Astigmatism Every deviation from a pure ellipsoidal shape canEvery deviation from a pure ellipsoidal shape can be devastating to vision when is centrally withinbe devastating to vision when is centrally within the pupillary areathe pupillary area Functionally, is that component of astigmatism thatFunctionally, is that component of astigmatism that can not be corrected with spectaclecan not be corrected with spectacle Common causes: dry eye, corneal scars , ectaticCommon causes: dry eye, corneal scars , ectatic corneal degeneration, ptrygium, contact lenscorneal degeneration, ptrygium, contact lens warpage or over wear, trauma ,surgery (cataract,warpage or over wear, trauma ,surgery (cataract, PK and refractive surgery)PK and refractive surgery)
- 37. Corneal topography is particularly mandatory forCorneal topography is particularly mandatory for these corneal conditionsthese corneal conditions Advanced keratoconusAdvanced keratoconus Post-penetrating keratoplastyPost-penetrating keratoplasty Post RK or LASIKPost RK or LASIK
- 39. Rabinowitz and McDonnell CriteriaRabinowitz and McDonnell Criteria ►Central corneal powerCentral corneal power >47>47 ►A difference ofA difference of 3 D or more3 D or more in corneal powerin corneal power comparing points 3 mmcomparing points 3 mm IInferior to the centernferior to the center to points 3 mmto points 3 mm ssuperior to the center (uperior to the center (II--SS)) ►Asymmetry between central corneal powerAsymmetry between central corneal power of fellow eyesof fellow eyes in excess of 1Din excess of 1D
- 40. Keratoconus detection overviewKeratoconus detection overview ► Topographic characteristicTopographic characteristic High irregular astigmatismHigh irregular astigmatism (SRI>1) (CU index<80%) (IAI) (PVA) (PCA) (Tot Astig(SRI>1) (CU index<80%) (IAI) (PVA) (PCA) (Tot Astig)) High central corneal powerHigh central corneal power (Sim K>50 D) (CSI) (DSI) (Eff RP)(Sim K>50 D) (CSI) (DSI) (Eff RP) Inferior cornea, steeper than superior corneaInferior cornea, steeper than superior cornea (IS value>1.4) (SAI>0.5) (OSI)(IS value>1.4) (SAI>0.5) (OSI) Large difference between the apex and the peripheryLarge difference between the apex and the periphery (AA) (HRT)(AA) (HRT) Disparity in the central power between the two eyesDisparity in the central power between the two eyes (>1 D)(>1 D)
- 42. Elevation Based Slit Scanning VideokeratographyElevation Based Slit Scanning Videokeratography • Measures Elevation DirectlyMeasures Elevation Directly • A series of slit lamp beams are compiledA series of slit lamp beams are compiled across the cornea to create a profile of theacross the cornea to create a profile of the corneacornea • Calculates the Full Pachymetry & CurvaturesCalculates the Full Pachymetry & Curvatures • Multiple ImagesMultiple Images • Multiple SurfacesMultiple Surfaces • Evaluates the Shape of the CorneaEvaluates the Shape of the Cornea
- 43. Terrain TopographyTerrain Topography Highlands Lowlands Color-coded Map Sea Level
- 44. Close-Fitting Reference Surfaces • Corneal topography differs from terrestrial topography in that the reference surface is not some fixed “mean sea-level”, but is movable. Higher (Red) Lower (Blue) Sea Level (Green) Sea Level (Green)
- 45. 1. 20 Slits from Left to Right 2. 20 Slits from Right to Left 3. Each slit per 0.7 second 4. Elimination of saccadic movements by eyetracking 5. Large face plate Placido Disc ORBSCAN IIORBSCAN II
- 46. Reading the MapReading the Map
- 47. Color-coded mapsColor-coded maps ►ColorsColors ►Warm colorsWarm colors ►Cool colorsCool colors ►ScalesScales ►RangeRange ►StepsSteps ►OverlaysOverlays ►PupilPupil ►CircularCircular ►GridGrid ►RadialRadial ►Thinnest pointThinnest point ►K-labels & linesK-labels & lines ►NumericalNumerical
- 48. Color-coded mapsColor-coded maps ►DisplaysDisplays ►ProfileProfile ►Two-dimensionalTwo-dimensional ►3-dimensional3-dimensional ►DualDual ►Difference mapDifference map ►Quad mapsQuad maps ►MeasurementsMeasurements ►ElevationElevation ►CurvatureCurvature ►PowerPower ►PachymetryPachymetry ►Anterior chamberAnterior chamber depthdepth
- 49. ColorsColors
- 50. Color Scale for Relative ElevationColor Scale for Relative Elevation • high • anterior to the reference surface • low • posterior to the reference surface reference (+) (-) anterior posterior Red BlueMin Max
- 51. Colors: ElevationColors: Elevation • Profile @ 180° • Profile @ 90° • Points above the best fit sphere • Points below the best fit sphere • Warm Colors • Cool Colors
- 52. Color Scale for Surface CurvatureColor Scale for Surface Curvature • sharp • fast bend • short radius • flat • slow bend • long radius (+ +) (+) Red BlueMin Max
- 53. Colors: Axial PowerColors: Axial Power • Warmer Colors • Higher Power • Cooler Colors • Lower Power
- 54. Color Scale for PachymetryColor Scale for Pachymetry • thick • thin Blue RedMin Max
- 55. Colors: PachymetryColors: Pachymetry • Cooler Colors • Thicker Areas • Warmer Colors • Thinner Areas
- 56. Colors: PachymetryColors: Pachymetry • Thicker Areas • Thinner Areas
- 57. OverlaysOverlays
- 66. ScalesScales
- 67. Scale: PowerScale: Power ►By changing the steps, the range of theBy changing the steps, the range of the scale can be adjusted:scale can be adjusted: 25 – 58 D25 – 58 D with 1D stepswith 1D steps 33 – 50 D33 – 50 D with 0.5 D stepswith 0.5 D steps 37 – 46 D37 – 46 D with 0.25 D stepswith 0.25 D steps
- 72. MeasurementsMeasurements ► Anterior corneal elevation and curvatureAnterior corneal elevation and curvature ► Posterior corneal elevation and curvaturePosterior corneal elevation and curvature ► Full corneal pachymetryFull corneal pachymetry ► Simulated keratometrySimulated keratometry ► White-to-white diameterWhite-to-white diameter ► Pupil size (mesopic)Pupil size (mesopic) ► Anterior chamber depthAnterior chamber depth ► Angle kappaAngle kappa ► Irregularity indexIrregularity index
- 78. DisplaysDisplays
- 79. DisplaysDisplays • Profile • Topographic • 3-dimensional
- 80. Displays: DualDisplays: Dual Axial Power Map Anterior Elevation Flatter areas stand above the best fit sphere Steeper areas fall below the best fit sphere
- 82. Display: Quad MapDisplay: Quad Map
- 84. KCN CRITERIA IN POWERKCN CRITERIA IN POWER MAPSMAPS ►““Broken bow-tie” appearanceBroken bow-tie” appearance ►Central powerCentral power > 47 D> 47 D ►> 3 D> 3 D difference in the 3 mm zonedifference in the 3 mm zone ►> 1 D> 1 D greater power than thegreater power than the fellow eyefellow eye
- 86. KCN CRITERIA IN ELEVATIONKCN CRITERIA IN ELEVATION MAPSMAPS 1.1. Maximum elevationMaximum elevation > 50 µm> 50 µm 2.2. Posterior best fit sphere in diopterPosterior best fit sphere in diopter > 55 D> 55 D 3.3. R anterior BFS / R posterior BFS (mm):R anterior BFS / R posterior BFS (mm): ∀ ≤≤1.211.21 okok ∀ ≥≥1.21 and1.21 and ≤≤ 1.271.27 suspectsuspect ∀ ≥≥ 1.271.27 suggestivesuggestive 4.4. Roush criterionRoush criterion: the relative elevation difference: the relative elevation difference between closebetween close maximamaxima andand minimaminima > 100µm> 100µm 5.5. More thanMore than threethree colors oncolors on 1010µm (µm (antant) and) and 2020 µm (µm (postpost) in the) in the central 3 mm zonecentral 3 mm zone
- 87. ANTERIOR ELEVATIONANTERIOR ELEVATION The maximum elevation measured is 65 µm (>55µm)
- 88. POSTERIOR ELEVATIONPOSTERIOR ELEVATION 59.5 D Maximum elevation= 124 µm
- 89. ANTERIOR & POSTERIORANTERIOR & POSTERIOR ELEVATIONELEVATION R anterior BSF /R posterior BSF > 1.21 - 1.27 R anterior BFS /R posterior BFS (mm) = 1.26 7.41 5.86
- 90. ROUSH CRITERIONROUSH CRITERION Roush criterion: the relative elevation difference between close maxima and minima = 130 µm (>100µm) 130 µm
- 91. COLORS IN ANT CENTRAL 3COLORS IN ANT CENTRAL 3 MM ZONEMM ZONE Seven colors can be counted in the central 3 mm zone with a 10µm step scale (>3 colors)
- 92. COLORS IN POST CENTRALCOLORS IN POST CENTRAL 3MM ZONE3MM ZONE Seven colors can be counted in the central 3 mm zone with a 20µm step scale (>3 colors)
- 93. RELATION BETWEEN 4 MAPSRELATION BETWEEN 4 MAPS Three step rule:Three step rule: ► OneOne abnormal mapabnormal map:: CautionCaution ► TwoTwo abnormal mapsabnormal maps:: ConcernConcern ► ThreeThree abnormal mapsabnormal maps:: ContraindicationContraindication
- 94. 11 ABNORMAL MAPABNORMAL MAP: CAUTION: CAUTION
- 95. 22 ABNORMAL MAPSABNORMAL MAPS: CONCERN: CONCERN
- 96. 33 ABNORMAL MAPSABNORMAL MAPS:: CONTRAINDICATIONCONTRAINDICATION
- 97. RELATION BETWEEN 4 MAPSRELATION BETWEEN 4 MAPS ►The posterior “high point” correlatesThe posterior “high point” correlates well withwell with thethe anterioranterior stesteepestepest pointpoint thethe anterioranterior hhigheighestst ppoioinnt,t, aanndd thethe thinnestthinnest spot on the corneaspot on the cornea
- 98. RECOMMENDED QUAD MAPRECOMMENDED QUAD MAP
- 100. Corneal TopographyCorneal Topography Corneal PathologyCorneal Pathology
- 101. Computer-Assisted VideokeratographyComputer-Assisted Videokeratography ►CapturingCapturing of an image of the cornea afterof an image of the cornea after reflection a target off the corneal surfacereflection a target off the corneal surface ►AnalyzingAnalyzing of the image and converting data toof the image and converting data to curvature values using computer software bycurvature values using computer software by complex algorithmscomplex algorithms ►DisplayingDisplaying of color topographical images byof color topographical images by using pre-determined color scalesusing pre-determined color scales ►ManipulationManipulation of data creation various displaysof data creation various displays
- 102. Corneal Surface ElevationCorneal Surface Elevation ►Data measured in terms ofData measured in terms of height or elevation (orheight or elevation (or sometimes depth) from asometimes depth) from a reference plane describe thereference plane describe the true shape of the cornealtrue shape of the corneal surface.surface. ►Once the true shape hasOnce the true shape has been measured, slope,been measured, slope, curvature, and power can becurvature, and power can be calculated from it.calculated from it.
- 103. Corneal Surface SlopeCorneal Surface Slope ► The gradient of the tangent at aThe gradient of the tangent at a particular point on a curvedparticular point on a curved surfacesurface ► Mathematically, the slope is theMathematically, the slope is the first differential of a curve.first differential of a curve. Therefore, it is a more sensitiveTherefore, it is a more sensitive way of demonstrating smallway of demonstrating small changes in height between twochanges in height between two points on a surface.points on a surface.
- 104. Radius of CurvatureRadius of Curvature ►Slope (Slope (αα) can be converted to) can be converted to radius of curvature (radius of curvature (r)r):: r =r = d/cosd/cos αα wherewhere dd is the distance fromis the distance from the corneal centerthe corneal center ►Corneas with a steepCorneas with a steep surface slope have a smallsurface slope have a small radius of curvature whileradius of curvature while those which are flatter havethose which are flatter have a relatively large radius ofa relatively large radius of curvaturecurvature
- 105. Corneal PowerCorneal Power ► A measure of theA measure of the refractive effect of arefractive effect of a lenslens ► P=(nP=(n22 – n– n11) / r) / r
- 106. Normal CorneaNormal Cornea ►AsphericityAsphericity ►Eccentricity (Prolate and oblate)Eccentricity (Prolate and oblate) ►Toricity ( Regular and Irregular)Toricity ( Regular and Irregular) ►Symmetry (vs Asymmetry)Symmetry (vs Asymmetry)
- 107. Corneal GeographyCorneal Geography ►The corneal surface can be divided intoThe corneal surface can be divided into four anatomic zones.four anatomic zones. • The central zoneThe central zone • The paracentral zoneThe paracentral zone • The peripheral zoneThe peripheral zone • The limbal zoneThe limbal zone
- 111. Best Fit SphereBest Fit Sphere • Anterior Corneal Surface • Best Fit Sphere
- 112. Best Fit SphereBest Fit Sphere
- 113. Raw ImageRaw Image
- 114. Raw Placido ImageRaw Placido Image
- 115. Raw Placido ImageRaw Placido Image
- 118. Triangulation locates points in spaceTriangulation locates points in space triangulated space pointcomplex object Scheim pflug slit-beam projector Video Camera calibrated slit-beam surface diffusely reflected camera ray
- 121. Hybrid Technology of ORBSCAN II 3. Unify triangulated and reflective data to obtain accurate surfaces in elevation, slope, and curvature. 1. Measure surface elevation directly by triangulation of backscattered slit-beam. 2. Measure surface slope directly using specular reflection, supplemented with triangulated elevation.
- 125. Colors: ElevationColors: Elevation Topographic or two dimensional view
- 126. Scale: PowerScale: Power 1.0 D 0.5 D .25 D
- 127. Data Available by One Exam • Four ocular surfaces: Anterior Cornea, Posterior Cornea, Anterior Iris, Anterior Lens • Geometry and shape maps: Relative Elevation, Inclination, Surface Curvature • Distance maps between surfaces: Full Corneal Pachymetry, Anterior Chamber Depth • Optical function maps: Optical Power • Clinical tools: Surgical Planning, Contact Lens Fitting

- The shape of the cornea can be measured and represented in different ways. Each has its own advantages and the use of the most appropriate method for a given clinical situation can enhance the presentation and interpretation of results.
- A. The majority of topography systems in clinical use today are based on the principle of reflection. They measure the slope of the corneal surface and use this information to calculate radius of curvature and power. However, corneal elevation can’t be calculated from measurements of slope alone. New topography systems based on the principle of projection directly measure true corneal shape in terms of elevation, from which slope, curvature, and power can be calculated.
- Curvature Based Videokeratography Within the category of placido based systems, there are two general categories. Those systems that employ a large placido disk which is several inches in diameter and is positioned several inches from the patient&apos;s eye where the imaging is performed. (Figure 7) The second category of placido topographers would be small cone devices that fit very close to the eye when the imaging is performed. (Figure 8) Adequate information can be achieved using either type of device. Occasionally, the physical anatomy of a patient (a deep-set eye or a prominent brow) may make one device preferable to another. A second way in which placido topographers differ from one another is the mathematical algorithms used to determine shape and power from the reflected rings. Each device will make some assumption about the nature of the basic shape or surface to be imaged. Deviations from that assumed surface are what provide the details of each specific eye. How accurately these assumptions apply to each individual will determine how accurate the measurement is. Greater or less appropriate assumptions and fewer points measured will provide less detail and, therefore, less accuracy in the measurement. Optikon 2000, the manufacturer of the Keratron Corneal Analyzer uses an interesting approach to minimize the assumptions and, therefore, improve the accuracy of the instrument. In addition to locating many points along each of the reflected targets, they also collect data that references each of the targets to one other. This increases the resolution and is much like draping a net over the surface instead of just placing several floating independent targets in different locations (Figure 9). This method is called arc step technology. Arc step image
- Elevation Based Videokeratography The second general category of videokeratographers is elevation-based videokeratography. There are an interesting variety of technologies in this category. And although the curvature is not being directly determined, these devices still require some image be reflected from the cornea. One such device is the CTS or Corneal Topography System manufactured by PAR Technologies. This device reflects a three dimensional grid of intersecting lines from the cornea. (Figure 10) A second device, the Orbscan, manufactured by Orbtek, acquires data from the diffuse reflection of a slit beam of light scanned across the cornea. (Figure11) Recently, this manufacturer has also combined a placido ring reflective image with their scanning slit. This marriage of technology should produce some of the most detailed and accurate information about corneal shape.
- Although there is no universally accepted color scale in corneal topography, spectral directions are standardized. In all curvature maps, red is sharp (large curvature, small radius), and blue is flat (small curvature, large radius).
- Normalized scale with 0.5 D step interval. the area of inferior steepening becomes obvious
- Axial power: A fixed center of curvature on the videokeratoscopic axis is used for calculating the power at all points along semi-meridians on the corneal surface Paxial = (n&apos; – 1) / raxial where n&apos; = 1.3375 (keratometric index) and raxial is the distance from the videokeratoscopic axis (fig 10-1) The keratometric index, n&apos;, is an approximated index for total corneal power to compensate for the negative power of the posterior corneal surface Since each curvature measured is referred to the optical axis, axial power has a spherical bias that gives an accuracy of 0.25 D in the central cornea, and up to 3 D in the periphery Provides map that are very simple to interpret and values directly comparable to keratometry although it doesn&apos;t encompass the effects of spherical aberration
- Instantaneous power: A floating center of curvature is used by means of a standard mathematical method for determining the local radius of curvature (fig 10-1) Pinst = (n&apos; – 1) / rinst where n&apos; = 1.3375 and rinst is the radius of curvature for any given point on the corneaThis value is a conversion from the axial value based on a second derivative There is less bias in calculations and offers a more realistic representation of the peripheral corneal topography More sensitive to small measurement error than the axial power calculation leading to &quot;noise&quot; artifact in the color-coded map, and also less repeatability Assumptions used in equations 1 & 2 are not necessarily valid outside the central region
- Paraxial power: can be used for central paraxial rays where the sagittal depth is approximately proportional to the curvature Refractive power (secondary focal point power) for the incoming parallel rays is given as Pref = n/f and f = z + y/ tan (θi - θr) Where f is the distance from the vertex normal to the secondary focal point (intersection of the refracted ray and the videokeratoscopic axis) Z is the dimension along the videokeratoscopic axis from the vertex normal to the surface point θi and θr are the angles of incidence and refraction, respectively (fig 10-2) (color plate 5 & 6) Theoretically valid and provides realistic values for corneal power in the periphery Less familiar to clinicians and not easy to interpret
- ISV = Index of Surface Variance. Gives the deviation of individual corneal radii from the mean value (in statistical terms, a measure of variance). Elevated in cases of astigmatism, scarred cornea, keratoconus etc. IVA = Index of Vertical Asymmetry. Gives the degree of symmetry of the corneal radii with respect to the horizontal meridian as axis of reflection. Elevated in cases of oblique axes of astigmatism. KI = Keratoconus Index. Compares the area of the cornea in which the keratoconus apex occurs most frequently with one in which there are, as a rule, no alterations. Thus in virtually all cases an increased KI index is associated with keratoconus. CKI = Central Keratoconus Index. Increases with severity of central keratoconus. Rmin= Gives the smallest radius of curvature in the entire field of measurement. Usually shows considerable variation in parallel measurements; nevertheless useful for making an overall assessment. IHA = Index of Height Asymmetry. Gives the degree of symmetry of height data with respect to the horizontal meridian as axis of reflection. A useful adjunct to IVA, especially in cases of incipient keratoconus. IHD = Index of Height Decentration. Calculated from a Fourier analysis of height values this index gives the degree of decentration in vertical direction. A sensitive parameter in keratoconus. ABR = Aberration Coefficient. Value of the aberration of the cornea calculated with Zernike analysis. KKS = Keratoconus Stage. Based on topometric measurements, this index follows the clinical findings of Amsler’s classical staging schema. AA = Analysed Area. Gives the area of the corneal surface actually measured (as opposed to merely interpolated) as a percentage of the entire measuring field. Useful in determining which of several parallel measurements might be more reliable.
- Elevation Based Videokeratography The second general category of videokeratographers is elevation-based videokeratography. There are an interesting variety of technologies in this category. And although the curvature is not being directly determined, these devices still require some image be reflected from the cornea. One such device is the CTS or Corneal Topography System manufactured by PAR Technologies. This device reflects a three dimensional grid of intersecting lines from the cornea. (Figure 10) A second device, the Orbscan, manufactured by Orbtek, acquires data from the diffuse reflection of a slit beam of light scanned across the cornea. (Figure11) Recently, this manufacturer has also combined a placido ring reflective image with their scanning slit. This marriage of technology should produce some of the most detailed and accurate information about corneal shape.
- Relative elevation measures height difference from a best-fitting reference sphere. In all elevation maps, green is the reference surface or zero level. Red is high and positive. Blue is low and negative
- Although there is no universally accepted color scale in corneal topography, spectral directions are standardized. In all curvature maps, red is sharp (large curvature, small radius), and blue is flat (small curvature, large radius).
- Although there is no universally accepted color scale in corneal topography, spectral directions are standardized. In all curvature maps, red is sharp (large curvature, small radius), and blue is flat (small curvature, large radius).
- ORBSCAN can compare two different maps and show us the difference between them. Here we have the pachymetry map of an eye before and after LASIK. In the difference map we can see how much ablation has been performed on different spots. Green stands for no change, warm colors show a negative difference (in this case lower pachymetry) and cold colors show positive difference between the two maps.
- An asymmetric and bent bow-tie is the classic pattern associated with keratoconus. A central corneal power over 47 D is a sign of KCN. Classic axis-based keratoconus pattern is an asymmetric and bent bow-tie. Also note the central high power.
- Different studies have suggested the following criteria in elevation maps for the diagnosis of keratoconus by Orbscan
- Elevation maps show a symmetric high cone. Elev: &gt;50, Roush &gt;100 micron, similar Ant & Post
- Posterior elevation map: BFS &gt; 51 D (SOME SAY &gt; 55 D) Elev: &gt;50, Roush &gt;100 micron, similar Ant & Post The posterior best fit sphere in diopters is 59.5 D (&gt;51-55)
- .... definition of Dr. E. Charalambos Red flags: R anterior BSF /R posterior BSF (mm): 1,21 ok 1,21 and 1,27 suspect 1,27 no go
- Elevation maps show a symmetric high cone. Elev: &gt;50, Roush &gt;100 micron, similar Ant & Post
- By changing the scale to a 10 micron step scale, we can count the number of different colors seen in the central 3 mm zone. More than three colors is indicative of KCN. More than 3 colors in ant. (10 micron) and post. (20 micron) Ophthalmology 2002 Jul;109(7):1298-302 Standardized color-coded scales for anterior and posterior elevation maps of scanning slit corneal topography. Tanabe T, Oshika T, Tomidokoro A, Amano S, Tanaka S, Kuroda T, Maeda N, Tokunaga T, Miyata K
- By changing the scale to a 20 micron step scale, we can count the number of different colors seen in the central 3 mm zone. More than three colors is indicative of KCN. More than 3 colors in ant. (10 micron) and post. (20 micron) Ophthalmology 2002 Jul;109(7):1298-302 Standardized color-coded scales for anterior and posterior elevation maps of scanning slit corneal topography. Tanabe T, Oshika T, Tomidokoro A, Amano S, Tanaka S, Kuroda T, Maeda N, Tokunaga T, Miyata K
- Shamsi Sadeqi One abnormal map Thinning
- In this case both the posterior elevation and pachymetry maps show signs suggestive of keratoconus. 1. DBFS=56.3 2. Highest posterior elevation&gt;60 3. Thinnest= 409 microns
- All maps have their particular signs indicative of keratoconus except the anterior elevation; which shows no apparent cone. The highest elevation is about 25 microns and the Roush criterion is not positive.
- Quad map of the same eye with the mean power map instead of the axial The cursor is on the area of maximum elevation, steepening and thinning
- Data measured in terms of height or elevation (or sometimes depth) from a reference plane describe the true shape of the corneal surface. Once the true shape has been measured, slope, curvature, and power can be calculated from it.
- The slope of a curved surface is the gradient of the tangent at a particular point. Mathematically, the slope is the first differential of a curve. Therefore, it is a more sensitive way of demonstrating small changes in height between two points on a surface.
- Corneas with a steep surface slope have a small radius of curvature while those which are flatter have a relatively large radius of curvature Each type of measurement of radius of curvature can be converted to the equivalent type of power measurement Fig: Corneal slope and radius of curvature. A point on the corneal surface is located at distance d from the axis. The corneal slope at this point is at angle α, and the line perpendicular to it is the normal. The radius of curvature (r) is the distance along the normal from the corneal surface to its intersection with the axis, which is given by d/cos α. This is the global/sagittal/axial radius of curvature.
- Power is a measure of the refractive effect of a lens. For optical lenses, measurements of radius of curvature (r, in meters) can be converted to power (P, in diopters) fig 1.2F ref3 N1 is the refractive index of the first medium (air=1, in case of the anterior surface of the cornea) and n2 is the refractive index of the second medium (in this case, cornea=1.367) So dioptric power is derived from the local radii of curvature using one or more methods
- 1-Central 4 mm is approximately spherical but the peripheral to this is aspheric and radially asymmetric 2- Eccentricity (e) is the rate of change in curvature from apex to periphery. This rate is variable at different corneal points. The central cornea best approximated by an ellipse (e ~ 0.50) The normal cornea flattens towards the periphery such that the rate of flattening is very slow (e is smaller) near the apex and rapid (e is larger) near the limbus 3- Meridional changes in curvature: the astigmatic cornea has different radii of curvature in different meridians. Regular: With-the-rule: meridian of greatest refractive power is within 30o of the 90o meridian Against-the-rule: meridian of greatest refractive power is within 30o of the 180o meridian Oblique: meridian of greatest refractive power is between the 30o and 60o, or 120o and 150o meridians Irregular: &quot;Normal&quot;: the two principal meridians are not at right angles to each other, or an otherwise &quot;normal&quot; cornea that does not fit into the above categories Pathological: corneal surface irregularity induced by degeneration, disease, or injury that results in abnormal refractive error 4- Orthogonal meridians Principal meridians are 90o apart Superior - inferior In most normal corneas, the superior and inferior halves of the cornea are symmetrical, but inferior or superior shifts can occur. An inferior shift is suggestive of keratoconus. Nasal - temporal In most normal corneas, the nasal cornea flattens at a faster rate than the temporal cornea. Right eye - left eye In most normals, the right and left corneas are symmetrical, but mirror images of each other
- 1-
- 1- Approximately 4 mm in diameter #Overlies the entrance pupil #Determines high-resolution image formation on the fovea #Cornea is relatively spherical in the central zone 2- An annulus 4 to 7-8 mm diameter # Where the cornea begins to flatten#Topography of it gains optical importance under dim illumination when the pupil dilates # Knowledge of the shape of cornea in this zone facilitates contact lens fitting (+central zone=optical zone)#After RK, it is the site of the paracentral knee 3-Where the cornea flattens the most and becomes more aspherical#The peripheral curvature of a contact lens should be fitted to the shape of the cornea in this region 4- Is a rim 0.5-1 mm wide adjacent to the sclera#Flattest area of cornea Dimensional changes in the limbal Zone as a result of surgery can effect the shape of the more central of the cornea There is a focal steepening to form scleral sulcus Topography have difficulty in imaging of this zone but good images can be obtained using projection techniques
- Elevation map of an eye after LASIK Green stands for no change, warm colors show a negative difference (in this case lower pachymetry) and cold colors show positive difference between the two maps.