Corneal TopographyWavefront Analysis and

Aberrometry

Dr.C.Suriyakumar

• Corneal topography refers to study of the shape of the corneal surface

• It is useful now in patients with corneal surface abnormalities undergoing contact lens fitting and keratorefractive procedures

Optical principle and nomenclature of techniques

• Most of the techniques employed to study the shape of corneal surface are based on the fact that the anterior surface of cornea acts as a convex mirror and that size of the image formed varies with its curvature.

• Various techniques 1.keratometry 2.keratoscopy 3.rasterstereography 4.interferometry

Keratometry

• Helps to measure the radius of curvature of the anterior corneal surface from four reflected points approximately 3mm apart.

• It provides no useful information regarding corneal surface central or peripheral to these points

Keratoscopy

• Refers to the evaluation of topographic abnormalities of the corneal surface by direct observation of images of mires reflected from the surface of cornea.

1. Placido-disc keratoscope 2. Photokeratoscopy 3. Videokeratoscopy

Placido-disc keratoscope

• In 1880 Antonio Placido developed first keratoscope – Placido disc

• Gross method of qualitative assesment of corneal surface

• Disadvantages # small degrees of abnormalities are not easily

identifiable # cannot be used in epithelial defects and

stromal ulcers

Photokeratoscopy

• Photographic film camera is attached to a keratoscope

• Nidek PKS 1000 is the current model has 9-15 rings which cover 55-75% of the corneal surface

Videokeratoscopy

• Television camera is attached to a keratoscope.

• Computer assisted videokeratoscopy has become synonymous with the corneal topography.

• It covers approximately 95% of the corneal surface.

Rasterstereography

• Uses a direct image on the corneal surface.• It projects a calibrated grid onto the

fluorescein stained tear film.• The advantage is that it includes information

across the whole cornea.• The projected nature of the test does not

allow interference due to corneal surface or stromal defects.

Interferometry

• Uses the technique of light wave interference.• This includes both holography and moire

fringe techniques.• This method is not in widespread clinical use.

Basic units of corneal topography system (CTS)

• Projection device• Video camera• computer

Different types of CTS

• Placido-disc topography systems• Slit-imaging topography systems• Digital rasterstereography based topography

systems• Laser halographic interferometry based

topography systems

Placido-disc topography systems

• Historically first.• 8-32 concentric rings projected on cornea• Numbered from inside out, diameter should be

mentioned• Limitations - data at the central zone have to be interpolated - quality of tear film is critical - data are less accurate

Commercially available Placido-disc CTS

1. LSUCTS2. CMS3. Computerized corneal topographer EH 2704. EyeSys 2000 corneal analysis system5. TMS-1 Topographic Modelling system

LSUCTS

• Projects 11 rings 1980 mire coordinates for imaging of cornea.

• Displays four basic types of diagrams 1.raw keratoscopic data 2.three dimensional wire model of corneal surface

power distribution 3.dioptric point surface power 4.colour coded contour map of corneal surface

power

CMS

• Videokeratography has 32 rings covering from apex to limbus.

• CMS with attachments gives topography as well as pachymetry.

• Simulated K value, surface asymmetry index(SAI) and surface regularity index(SRI) have been developed for determining axis and power of the corneal cylinders.

Computerised corneal topographer EH270

• Has different video displays depending on the needs and desires of the operating person.

• Zoom magnification upto 8 times, corneal contour map showing whole of the cornea, cross sectional view of the cornea, three dimensional display, etc.

EyeSys 2000 corneal analysis system

• 16 ring videokeratoscopic device with fast image processing time of 3 secs.

• Analyses 6500 data points and provides colour coded contour map plots.

TMS-1 Topographic modelling system

• Utilises 31 projected rings providing 7000 data points.

• Corneal coverage is 0.02 – 11.00 mm with an accuracy of 0.10 D.

• It is an exclusive refractive surgery planning programme.

Slit-imaging topography systems

• Use scanning slits similar to slit lamp in principle.

• Advantage-measure all surface of anterior segment

• Limitations – relatively long scanning time, clinical accuracy has not been fully established for posterior corneal segments.

• Types – ORBSCAN, PENTACAM.

ORBSCAN

• Most recent of commercially available CTS.• In addition to anterior surface measures

posterior surface also.• Corneal thickness can be measured at any

point on cornea.• ORBSCAN – II . Considered the best available

CTS. Combines slit scanning with placido-disc system.

PENTACAM

• Comprehensive anterior aegment analyser.• Perform following 5 functions1.Scheimpflug image of anterior segment2.Three dimensional anterior chamber analyser3.Pachymetry4.Corneal topography5.Cataract analyser

Digital rasterstereography based topography systems – PAR CTS

• Calibrated grid pattern is used.• Sodium fluorescein is added to the tear film

and is excited by the blue light.• Whole cornea and part of sclera are analysed.• The projected nature of the test does not

allow interference due to corneal surface or stromal defects.

Laser halographic Interferometry based topography system

• Corneal Lens Analysis System(CLAS) unit.• Applies three dimensional imaging to the

analysis of corneal surface changes.• Analyses optical aberrations from reflecting

surface.

Display of topographic data

• Numerical power plots • Keratometry view• Photokeratoscopic view• Profile view• Colour coded topographic maps

Numerical power plots

• The corneal curvature of specific areas is shown in dioptre values

• 10 concentric circular zones with 1mm interval in colour as colour maps

Keratometric view

• It depicts keratometric reading in two principle meridia( K 1 & K2) in three different zones.

• Central 3mm, intermediate 3-5mm, peripheral 5-7mm.

Photokeratoscopic view

• It depicts the actual black & white photograph of the placido rings.

• It helps in confirming the proper patient fixation and in identifying the eye captured.

PAR CTS profile view• It shows the graphical

plotting along the XY axis of the steepest and the flattest meridia of the cornea and the difference between the two in dioptres.

• Grey zone is pupillary area, grey band is straight in symmetrical eye and slag in astigmatism.

Colour coded topographic map

• Most useful and most commonly used display formation.

• For interpretation following parameters should be considered

1. colour codes 2.scale used 3.quantitative indices

Colour codes

• Hot colour – red, indicates steep portions of cornea.

• Cool colours – blue, indicates flat portions.• Red- orange- yellow- green- purple- blue

denotes progressively lessening refractive power.

Scale used

• Absolute scale – each colour represents 1.5 D interval between 35 & 50D, where as above and below this range colours represent 5D interval.

• Disadvantage – it does not show subtle changes of curvature.

• Normalised scale – in it the cornea is divided into 11 equal colours.

• Advantage – it shows more detailed description of the surface

• Disadvantage – colours of two different maps cannot be compared.

Quantitative indices

• These include the following 1.predicted visual acuity based on corneal

shape 2.simulated K reading 3.minimum K reading 4.surface regularity index 5.surface asymmetry index 6.point spread function

Corneal topographic patterns in normal corneas

• 10 different topographic patterns seen in normal eyes.

• Regular pattern – round, oval, superior & inferior steepening

• Astigmatic pattern – symmetrical and orthogonal, asymmetrical and orthogonal and irregular

Formats for display of datas on colour maps

1. Corneal power map( saggital&axial)2. Tangential map3. Elevation map4. Refractive power map5. Irregularity map6. Trend and time display7. Difference display map8. Rt /Lt eye compare map

Corneal power map24 colour representation of diopteric power at various points on the cornea

Tangential map

• Better geographical representation of cornea

• Best indicator of corneal shape than power

Elevation map

• Displays height of the point on the corneal surface relative to a spherical reference surface.

• Helps in distinguishing localised elevation from steep corneal area.

• Elevation data appear more relevant for calculation of ablation depth & optical zones in laser refractive surgery.

Elevation map

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.

Refractive power map

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.

Irregularity map

Trend and time display

• In this changes ocurring in topography with time can be displayed in chronological order.

Trend and time display

Difference display mapIt exibits comparative difference in two given topographic maps

Rt/Lt eye comparision mapAllows comparision of both eyes simultaneously

Clinical applications

• Early diagnosis of corneal diseases such as keratoconus.

• Topography and contact lenses.• Topography in RK.• Post keratoplasty astigmatism.• Photorefractive keratectomy and LASIK.• IOL power calculation.

Wave front analysis and Aberrometry

• Aberration – deviation from ideal optical system.• Lower prder aberrations – myopia,

hypermetropia, regular astigmatism.• Higher order aberrations – spherical aberration,

chromatic aberration, coma, decentring, oblique aberration and centring.

• Two ways to analyse aberrations – ray aberrations, wave aberrations

Ray aberrations

• Ray aberrometry is based on snell’s law.• Ideally all rays from a single object point

converge to a single image point.• An imaginary plane is located near the desired

image point and the intersection of each ray with the plane is plotted as a spot diagram.

Wavefront analysis

• Wavefront refers to any isochronic surface associated with a specific object point.

• Thus, the amount of time required for light to travel from a specified object point to the wavefront is equal for all rays.

• The rays are perpendicular to the wavefront.• Thus, given the shape of the wavefront, the direction

of any light can be calculated.• Conversely from the direction of light rays the shape of

wavefront can be calculated

Wave aberrations

• It refers to the optical path difference(OPD) between the actual image wavefront and the ideal spherical wavefront.

• Since the wavefront aberration is the difference between two surfaces, therefore, its surface is usually shaped somewhat like a potato chip.

Zernick’s polynomials and Zernick’s terms

• The monochromatic aberrations are defined and quantified in terms of zernick’s polynomials(ZPs), consisting of zernick’s terms (ZT) and associated spot diagrams.

• Using the wavefront sensor method, the aberrations upto 10th order of ZP have been determined.

Wavefront aberrometry

• It refers to measurement of aberrations in the optical system of the eye by wavefront analysis.

• Three main modifications of aberrometer1.Hartmann-shack aberrometry2.Tscherning aberroscope3.Ray-tracing aberrometry

Hartmann-shack aberrometry

• Currently most commonly used• A single laser beam is projected as a spot on the

retina and the reflected bundle of rays passes through the optical system of the eye.

• It is picked up by an array of small lenslets, which focus these rays into spots on an array of a CCD camera.

• It is analysed and the wavefront and its deformation generated.

Tscherring aberroscope

• Basically an ingoing retinal imaging aberrometer.

• A bundle of equidistant light rays is shone on cornea,which are imaged on the retina.

• Like indirect ophthalmoscopy, a CCD camera linked to a computer is used analyse the pattern of spots on retina.

Ray-tracing aberrometry

• In this system, pattern of one ray, rather than all rays at the same time, is analysed.

• A point incident on the retina and the location of its conjugate focus is analysed with referrence to the ideal conjugate focus point.

• The wavefront map is calculated from the many such points measured separately.

Thank U