Accreditation Slides Topography

8/15/2019 Accreditation Slides Topography

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Corneal TopographyCorneal Topography

DefinitionDefinition

•• Corneal topography can be defined as theCorneal topography can be defined as themeasurement of corneal shape.measurement of corneal shape.

•• This has been done in some form forThis has been done in some form forcenturies.centuries.

•• Modern systems allow an enormousModern systems allow an enormousamount of information to be collectedamount of information to be collectedabout the cornea.about the cornea.

Normal corneal shapeNormal corneal shape

•• Aspheric surface which Aspheric surface whichflattens towards theflattens towards the

periphery.periphery.

•• i.e.i.e. ProlateProlate ellipse.ellipse.•• Rate of flatteningRate of flattening

varies for individuals.varies for individuals.

•• Usually possible to findUsually possible to find2 axes of symmetry.2 axes of symmetry.

Why measure cornealWhy measure corneal

shape?shape?

•• Establishing models of normal corneal shape.Establishing models of normal corneal shape.•• Screening for abnormalities.Screening for abnormalities.•• Measuring the shape for different applications:Measuring the shape for different applications:

– – Standard contact lens fitting.Standard contact lens fitting.

– – Ortho K Ortho K

– – Refractive surgery.Refractive surgery.

•• Assessing change over time. Assessing change over time.


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Traditional methodsTraditional methods

•• TraditionallyTraditionallymeasured with ameasured with a

keratometerkeratometer..

•• Measures the centralMeasures the central3mm.3mm.

•• Known as theKnown as the “ “corneal capcorneal cap” ” ..

•• Corneal cap assumedCorneal cap assumedto have ato have a spherosphero--

cylindricalcylindrical shape.shape.

DoesDoes keratometrykeratometry meetmeet

our aims?our aims?

•• Establish model of Establish model of normal corneal shape?normal corneal shape? – – No, only measuresNo, only measures

centre and assumes itcentre and assumes itis spherical.is spherical.

•• Screening forScreening forabnormalities?abnormalities? – – Not particularly well,Not particularly well,

e.g. earlye.g. early keratoconuskeratoconusetc.etc.

DoesDoes keratometrykeratometry meetmeet

our aims?our aims?

•• Measuring the shape for different applications?Measuring the shape for different applications? – – Standard contact lens fitting?Standard contact lens fitting?

•• Not bad but problems if patient isNot bad but problems if patient is “ “abnormalabnormal” ”

– – Ortho K?Ortho K?•• No, need peripheral measurements for lens design.No, need peripheral measurements for lens design.

– – Refractive surgery?Refractive surgery?•• No, no information about what will be needed in theNo, no information about what will be needed in the

periphery.periphery.

•• Assessing shape over time? Assessing shape over time?•• Not possible for peripheral shape changes which are oftenNot possible for peripheral shape changes which are often

just as important as central changes. just as important as central changes.

Modern MethodsModern Methods

•• Projection techniqueProjection technique – – Used mainly by ophthalmologistsUsed mainly by ophthalmologists

particularly in refractive surgery.particularly in refractive surgery.

•• Reflection techniqueReflection technique – – Most common type in use in optometricMost common type in use in optometric

practice.practice.


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Projection systemsProjection systems

•• ExpensiveExpensive•• Not widely used inNot widely used in

Ortho K Ortho K

•• Do allow anterior andDo allow anterior andposterior curvatureposterior curvaturemaps.maps.

•• Also possible to Also possible tomeasure cornealmeasure cornealthickness.thickness.

ReflectionReflection

Videokeratoscopy Videokeratoscopy

•• PlacidoPlacido mires aremires arereflected onto thereflected onto thecornea.cornea.

•• Video image of the Video image of thereflection isreflection isanalysed.analysed.



•• Mires close together = steep corneaMires close together = steep cornea



•• Mires far apart = flatter corneaMires far apart = flatter cornea


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Coloured mapsColoured maps

•• Coloured map isColoured map isgenerated.generated.

•• Warmer colours =Warmer colours =steeper corneasteeper cornea

•• Cooler colours =Cooler colours =flatter corneaflatter cornea

•• Normal shape isNormal shape iswarmer colours in thewarmer colours in thecentre and coolercentre and coolercolours in thecolours in theperiphery.periphery.

Limitations of usingLimitations of using

reflectionreflection

•• Important to remember that you are notImportant to remember that you are notdirectly measuring corneal shapedirectly measuring corneal shape

•• Actually measuring tear film shape over Actually measuring tear film shape overthe cornea.the cornea.

•• Therefore affected by dry eye, staining, airTherefore affected by dry eye, staining, aircon, fluorescein etc.con, fluorescein etc.



Large cone:Large cone:

•• Comfortable workingComfortable workingdistance for practitioner & distance for practitioner & patient.patient.

•• Loss of information asLoss of information asnose & brow in the way.nose & brow in the way.

•• Larger instrument.Larger instrument.

Different design principles:Different design principles:

Large cone v small cone.Large cone v small cone.



Small cone:Small cone:

•• Smaller instrument.Smaller instrument.•• Minimal loss of Minimal loss of

information due toinformation due to

nose & brow.nose & brow.

•• More invasive.More invasive.

Different design principles:Different design principles:

Large cone v small cone.Large cone v small cone.


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Axial, tangential Axial, tangential

or height?or height?

•• There are 3 basic methods of calculatingThere are 3 basic methods of calculatingthe corneal shape.the corneal shape.

– – Axial Axial

– – TangentialTangential

– – HeightHeight

•• Each has different advantages andEach has different advantages anddisadvantages.disadvantages.

Axial Maps Axial Maps

•• Axial maps are based on Axial maps are based ona spherically biaseda spherically biasedalgorithm.algorithm.

•• Are good at simulating Are good at simulatingthe refractive power of the refractive power of thethe “ “corneal capcorneal cap” ” ..

•• However as the corneaHowever as the corneabecomes increasinglybecomes increasinglyaspheric towards theaspheric towards theperiphery, axial maps doperiphery, axial maps donot represent thenot represent thecurvature effectively.curvature effectively.

Tangential mapsTangential maps

•• Tangential mapsTangential mapscalculate the curvature atcalculate the curvature ateach point with respect toeach point with respect to

its neighbouring points.its neighbouring points.

•• As it is not biased to an As it is not biased to anartificial spherical axis it isartificial spherical axis it ismore accurate in themore accurate in theperiphery and can showperiphery and can showlocalised irregularities.localised irregularities.

Height mapsHeight maps

•• TopographicalTopographicalmaps of the earthmaps of the earth

use contours of use contours of height above seaheight above sea

level.level.

•• Cannot use anCannot use anabsolute for theabsolute for the

cornea.cornea.


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Height mapsHeight maps

•• For the cornea theFor the cornea theheight is calculatedheight is calculated

from a referencefrom a reference

sphere.sphere.

Axial, tangential Axial, tangential

or height?or height?

•• Axial used to determine whether eye is Axial used to determine whether eye is “ “normalnormal” ” and potential VA.and potential VA.

•• Tangential more useful in looking for peripheralTangential more useful in looking for peripheralirregularities or determining position of OK irregularities or determining position of OK lenses.lenses.

•• Height data is often used to transmit data viaHeight data is often used to transmit data viamanufacturer software.manufacturer software.

•• So we often use all of them for the sameSo we often use all of them for the samepatient.patient.

Difference mapsDifference maps

•• Difference maps are used to show the differenceDifference maps are used to show the differencein topography from one measurement toin topography from one measurement toanother.another.

•• Used to assess the progression in variousUsed to assess the progression in variouscorneal conditions such ascorneal conditions such as keratoconuskeratoconus..

•• Also used to determine the change in the cornea Also used to determine the change in the corneafollowing refractive surgery or orthokeratology.following refractive surgery or orthokeratology.

•• Warmer colours now = steeper, cooler coloursWarmer colours now = steeper, cooler colours= flatter.= flatter.

Difference mapsDifference maps

Original map 1 day follow up Difference between

the two


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Different scalesDifferent scales

•• Absolute scale Absolute scale – – Same colour is always equal to the same curvature.Same colour is always equal to the same curvature.

– – Very wide range of curvatures covered. Very wide range of curvatures covered.

– – Will usually represent normal cornea as yellow/greenWill usually represent normal cornea as yellow/greenin centre changing towards blue in the periphery.in centre changing towards blue in the periphery.

– – Allows comparisons of eyes. Allows comparisons of eyes.


•• Relative or normalised scaleRelative or normalised scale – – Set number of colours are represented on theSet number of colours are represented on the

map.map.

– – Allows detailed analysis of specific areas. Allows detailed analysis of specific areas.

– – Cannot compare to other eyes if differentCannot compare to other eyes if different

relative scales have been chosen.relative scales have been chosen.


•• Adjusted scale: Adjusted scale: – – Operator can define a scale that they wish toOperator can define a scale that they wish to

use.use.

– – Should be used with care.Should be used with care.


Patient 1: sim K’s 7.98@101, 8.19@11 Patient 2: sim K’s 6.86@93, 7.02@3

Absolute scale


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Patient 1: sim K’s 7.98@101, 8.19@11 Patient 2: sim K’s 6.86@93, 7.02@3

Normalised scale


Absolute Normalised

Beware of using normalised maps too soon.

Always assess whether normal on absolute before usingnormalised to look for detail.

What to look forWhat to look for

•• Spherical shape.Spherical shape. – – Cornea flattens equallyCornea flattens equally

to the periphery in allto the periphery in allmeridians.meridians.


•• With the ruleWith the ruleastigmatismastigmatism

– – “ “bowtiebowtie” ” shapeshape

– – Bowtie verticalBowtie vertical

– – Spectacle cyl isSpectacle cyl is

horizontal minus.horizontal minus.

– – Some slightSome slightasymmetry is normal.asymmetry is normal.


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•• Against the rule Against the ruleastigmatismastigmatism

– – “ “bowtiebowtie” ” shape.shape.

– – Horizontal bowtieHorizontal bowtie

– – Spectacle cyl is verticalSpectacle cyl is vertical

minus.minus.


•• KeratoconusKeratoconus – – Usually inferior nasalUsually inferior nasal

steepening but notsteepening but notalways.always.

– – Always has a flattening Always has a flatteningin the oppositein the oppositemeridian.meridian.

– – Quality of vision willQuality of vision willdepend on thedepend on theregularity within theregularity within thepupil.pupil.


•• Pellucid marginalPellucid marginaldegeneration.degeneration.

– – Often confused withOften confused withkeratoconuskeratoconus as it has aas it has asimilar effect on vision.similar effect on vision.

– – UnlikeUnlike keratoconuskeratoconus thethe

steepest parts of thesteepest parts of thecornea are usuallycornea are usuallyalong the inferioralong the inferior

oblique meridians.oblique meridians.

Corneal topography andCorneal topography and

contact lensescontact lenses

•• Screening and pre assessment.Screening and pre assessment.

•• Choosing a design.Choosing a design.

•• Calculating lens parameters.Calculating lens parameters.

•• Fitting unusual designs and unusualFitting unusual designs and unusualcorneas.corneas.


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Screening & Screening &

Pre assessmentPre assessment

•• Pre screen patients for:Pre screen patients for: – – Keratoconus.Keratoconus.

– – WarpageWarpage..

– – Assess shape after refractive surgery. Assess shape after refractive surgery.

– – Other corneal irregularities.Other corneal irregularities.

Choosing a designChoosing a design

•• Patient 1Patient 1 – – Corneal cylCorneal cyl

of 2.00Dof 2.00D

– – Do we needDo we needa toric RGP?a toric RGP?

– – Can assessCan assessby eye orby eye orcould usecould usefluoresceinfluoresceinsimulation.simulation.


•• SphericalSphericalRGP designRGP design


•• Toric RGPToric RGP


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Calculating parametersCalculating parameters

•• Traditionally done by taking readings atTraditionally done by taking readings atset positions and calculating design.set positions and calculating design.

•• Time consuming.Time consuming.

•• Now software programs can do thisNow software programs can do thisautomatically for you and even createautomatically for you and even create

custom designs.custom designs.



Topography is

transferred to

a software

program andthe patients

prescription is

entered.


Software

program

calculates the

parameters

and generatesthe simulated

fluorescein

pattern.


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Fitting unusual designsFitting unusual designs

•• E.g. Orthokeratology:E.g. Orthokeratology: – – Topography used to calculate peripheralTopography used to calculate peripheral

curves.curves.

– – Topography is used to assess the changes inTopography is used to assess the changes in

the cornea post wear.the cornea post wear.


Ortho K Ortho K

Import topographical data


Ortho K Ortho K

Fill in Patients Rx + HVIDFill in Patients Rx + HVID


Ortho K Ortho K

Send data to manufacturerSend data to manufacturer


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Ortho K Ortho K

Lab calculates lenses and orderLab calculates lenses and order


Ortho K Ortho K

•• Assessing the corneal changes post wear Assessing the corneal changes post wear

Bulls eye Smiley face

Lateral decentration Central I slands

Frowny Face

Fit of lenses is amended

based on the post wear topography to improve

the result.

SummarySummary

•• Topography expands our knowledge of Topography expands our knowledge of corneal shape.corneal shape.

•• We can increase our first time successWe can increase our first time successwith contact lenses.with contact lenses.

•• We can fit unusual corneas and unusualWe can fit unusual corneas and unusuallenses.lenses.

•• We can save time and provide a betterWe can save time and provide a betterservice for our patients.service for our patients.

Thank you for listeningThank you for listening

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Accreditation Slides Topography

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