CORNEAL TOPOGRAPHYCORNEALDR. MRINMAYEE GHATAKDO, FMRF (Sankara Nethralaya)

Consultant OphthalmologistKota Eye Hospital & Research FoundationKota INDIA

dr.mrin@gmail.com

Anterior 1/6th of eyeballMeasures 10.6mm Vertically and 11.7mm horizontallyNOT SPHERICAL, typically described as prolate ellipsoidCentral 4mm (optical zone or apical zone or corneal cap) supposed to be spherical

ZONES OF CORNEACentral OpticalParacentralPeripheralLimbal

CORNEA - CurvatureCentral 4mm :7.8mm anterior surface6.5mm posterior surface

The range of powers found in the normalcornea range from 39 D found at peripheral cornea,close to the limbus, to 48 D found at corneal apex.

Cornea - Power calculation P = N2 N1 R

P : Power of corneal surfaceN1 : Refractive Index of 1st mediumN2: Refractive Index of 2nd mediumR: Radius of Curvature in metres

Power : N2-N1 / R6.5

SphericalvsAsphericalSurface

Christopher Scheiner (1619)

HISTORICAL EVOLUTION1619 : 1st reported description of corneal curvature by Christopher Scheiner1796 : Jesse Ramsden built the 1st device exclusively for keratometry1854 : Herman von helmholtz modified Ramsdens instrument : termed it Ophthalmometer1881 : 1st practical keratometer for clinical use by Javal & Schiotz1932 : modified and improved version by Bausch & Lomb

PRINCIPLEObservation of 1st Purkinges ImageBased on geometry of aspherical convex reflecting surface (cornea)Object of known size and distance is reflected off the corneal surface to determine the size of the reflected image with a measuring telescopeCalculates the

refracting power on the basis of an assumed index of refraction

Advantages of KeratometryAccuracy and reproducibility for measuring regular corneas within normal range of curvatures (40 -46 D)Good for fitting CL and IOL power calculationEase of useLow costMinimal maintenance requirements

TYPES OF KERATOMETERB & LObject size constantAmount of doubling varied to produce the image of fixed size

Javal SchiotzAmount of image doubling is constantMeasures the object size needed to produce an image of fixed size

BAUSCH & LOMB (Reichert) KERATOMETEREye pieceVertical KnobHorizontal KnobFocussing KnobChin RestHead RestChin Height KnobKeratometer height KnobLockAP rotation axis scale

Patients view of B&L keratometer mire

Examiners view

Most keratometers have two prismatic doubling systems(one horizontal and one vertical)

Schieners Discs

PROCEDUREFocusing the eyepieceAligning the instrumentPositioning of the patientExplaining the patientAligning and focusing the mires on corneaMeasurement of axisMeasurement of both curvatures

Oblique astigmatism

Oblique astigmatism

JAVAL-SCHIOTZ KERATOMETER

OPTICAL SYSTEM

Patients view of mires

Examiners view of the doubled mire image

Horizontal meridian

Vertical meridian

Oblique AstigmatismAligned but unapproximated miresApproximated mires

KERATOMETRYKEY POINTS:

Focus the eyepiece before beginning the measurement.Let the patient blink normally to keep the cornea smooth.Make sure the patient is comfortable while positioned at the instrument.Loosely lock the instrument to avoid accidentally misaligning it during the measurement.Keep the mires centered and focused at all times.

Calculation of Radius of CurvatureR = 2x h/h

R : radius of curvaturex : distance from object to focal pointh : image heighth : object height

Power CalculationP = N2 N1 R

In keratometers, N2 = 1.3375 (assumed R.I. of cornea) N1 = 1.000 (air)

P = 1.3375 1.000 = 0.3375 = 337.5 __ _ R (in mtrs) R (in mtrs) R (in millimetres)

RANGE OF KERATOMETRIC READINGDioptric Power: 36D to 52DRadius of Curvature: 6.5mm to 9.38mm

Can be extended upto:Lower Limit : 30D (5.6mm) with -1.0D lensUpper Limit : 61D (10.9mm) with +1.25D lens

Objective method for determining curvature of the cornea.To estimate the amount and direction of corneal astigmatismThe ocular biometery for the IOL power calculation To monitor pre and post surgical astigmatism.Differential diagnosis of axial versus refractive anisometropia.To diagnose and monitor keratoconus and other corneal diseases.For contact lens fitting by base curve selection

Problems in MeasurementMeasurement Problems:Measures only central 3 mm of corneaCorneal epithelial irregularity render defocussingVery steep cornea: reading exceed rangePost-refractive surgery readings are inaccurate

KeratometryLimitations & AssumptionsCalculations are based on the geometry of a spherical reflecting surface: the cornea is described as a prolate (flattening) ellipsoid (true apical radius steeper)Quantitative data are based on only four points within the central 3 millimeters of the cornea (gross qualitative indication of corneal regularity between them)The formula approximates the distance of image as the distance of focal point from the objectPower in diopters depends on an assumed index of refraction

Keratoscope: instrument that projects multiple concentric rings (mires) on the corneaKeratoscopy: direct visualization of the ringsPhotokeratoscope: when a still camera is added to photograph the miresVideokeratoscope: when a video camera is added

Need of KeratoscopyMost corneas are aspheric, flattening peripherally. Keratoscopy samples a large area of the corneal surface can assess asphericity and other surface variations

KERATOSCOPY - HISTORICAL1870 : 1st clinical use : Placido : studied the corneal surface by observing the shape of the concentric rings reflected off the cornea1880 : Javal : recognised importance the recording the image photographically1896 : Gullstrand : developed 1st Photokeratoscope

Placido Disc: the Original Corneal TopographerPlacido Disc: observer views the pattern of concentric white rings (mires) reflected from the patients cornea through a central +2 D lens.Very qualitative

Images formed by Placido DiscBased on the overlay of concentric mires on the cornea.The closer the mires, the steeper the axis.The wider the rings, the flatter the axis.

Overlay of Mires

Safety PinFlieringa RingBarret Plastic LollypopCylindrical KeratoscopeQualitative Methods of Keratoscopy

Maloney Conical KeratoscopeKlein Hand-held internally illuminated KeratoscopeAstigmatism control enforcer with Applanation tonometer

Nidek Sun Photokeratoscope PKS-1000

Limitations of Placido Disc SystemIt misses data on the central cornea It is only able to acquire limited data points It measures slope not height It is difficult to focus and align In most topographers, the patient is exposed to high light

Computerized VideoKeratoscopyCapturing the keratoscopic details onto a video and displaying data analysed with mutiple algorithmsMeasures larger area with more pointsProduce permanent reproducible recordsOne of the most important developments in diagnostic instrumentation

Present DayPDB Video-KeratoscopesThe Real Need Analysing each & every point over cornea

Types of Computerized Topographers

Basics of Computerized Corneal Topography

Sequence of eventsProjecting Placido Disc Ring Pattern onto patients corneaAchieving centration of miresInstantaneous capturing of reflected mires by high resolution digital video-camera256 circumferential points on each ring are identifiedAnalysis of each point is done and processed data is displayed onto computer screen in various formats e.g. color maps

Key PointsAvoid all eye drops, particularly local anaesthetics

as they decrease TBUTExplain the patient & make comfortableAsk patient to blink normallyOther contact procedures on cornea

(tonometry, A-scan) should be doneafter topography

Computerized Corneal TopographyIndications & UsesPreoperative and postoperative assessment of the refractive patientPreoperative and postoperative assessment of penetrating keratoplastyIrregular astigmatismCorneal distrophies, bullous keratopathyKeratoconus (diagnostic and follow-up)Follow-up of corneal ulceration or abscess Post-traumatic corneal scarringContact lens fittingEvaluation of tear film qualityReference instrument for IOL-implants to see the corneal difference before and after surgeryTo study unexplained low visual acuity after any surgical procedure (trabeculectomy, extracapsular lens extraction, ).Preoperative and postoperative assessment of Intacs corneal rings (intrastromal corneal rings)

READING OF TOPOGRAPHICAL DATACheck the name of the patient, date of exam and examined eye.type of measurement (height in microns, curvature in mm, power in D)Check the scale & step intervalstudy the map (type of map, form of abnormalities)Evaluate statistical informationCompare with topography of the other eyeCompare with the previous maps

Numeric power plotsKeratometry viewPhotokeratoscopic viewProfile viewColour-coded topographic mapsMost useful

NUMERIC POWER PLOTSCorneal curvature showed in dioptre values10 concentric circular zones with 1mm intervalAlso shows Value radius of curvature of each of the 10 concentric zonesAverage overall corneal curvature also displayed

KERATOMETRIC VIEWDepicts K-readings in 2 principle meridia in 3 different zones simultaneously.Central : 3mmIntermediate : 3-5 mmPeripheral : 5-7 mmImportant for assessing

the skewing of semi-meridia

PHOTOKERATOSCOPIC VIEWDepicts actual black & white photograph of Placido rings captured by video camera.Helps in confirming proper patient fixation

PROFILE VIEWGraphical plotting along the X-Y axis of the steepest and flattest meridia and difference between the two.Grey zone denotes the pupillary area.Symmetrical eye : straight line tracingAsymmetrical eye : apparent slag seen

COLOUR-CODED TOPOGRAPHIC MAPSMost widely usedMost usefulQuick interpretation possibleUser-friendly

Louisiana State University Color-Coded Map1987 by Stephen Klyce

Interpretation of a colour map:

Colour Codes:Hot colours: red-orangesteep portionsCool colours: blue-purpleflat portions

The Scale used:Absolute Scale: routine practice / screening

35-50D : each color = 1.5D interval50D : each color > 5D intervalNormalized Scale: more minute details

11 equal colours spanning that eyes dioptric power

ABSOLUTE SCALERELATIVE SCALE

Corneal Topographic Patterns:Depending on corneal curvatureRabinowitz et al in 1996 described 10 different patterns:

REGULAR PATTERNS :RoundOvalSteepening : Superior or InferiorASTIGMATIC PATTERNS:Symmetrical & Orthogonal : (Bow-Tie Effect)With or without skewed axisAsymmetrical & Orthogonal:With superior steepeningWith inferior steepeningBow-tie with skewed radial axisIrregular : no pattern and non-orthogonal

Aym.Bow-Tie with skewRoundOvalSup.SteepInf.SteepSym.Bow-TieSym.Bow-Tie with Skew Asym.Bow-Tie with Inf.Steep Asym.Bow-Tie with Sup.Steep

Formats for display of data on color maps:

Ring Verification: raw dataCorneal power map: (Axial)Original & most stable and most commonly used map24-colour representation of dioptric powerCurvature measured 360 times for each placido ring imageSagittal algorithm averages data from between ringsEvaluate overall characteristics and helps in classification

Tangential map: (Instantaneous Curvature Map)Better geographical representation than axial mapTangents are projected outwards from centre vertex 360 degreeRing curvature measured along tangentBest indicator of corneal shape >> ectatic conditionsPoor indicator of corneal power >> never calculate K values

Ring Verification Map Axial Dioptre Map 3D Reconstruction Map Tangential Dioptre Map

Formats for display of data on color maps:

Elevation MapDistinguishing localized elevations from otherwise steep corneal areaThey are difference measurementsRed is Raised, Blue is Below

Refractive Power MapTakes into account spherical aberrationsIllustrates refraction of light in true dioptresUseful in determining optical zone for RGP lenses and refractive corneal surgery

3D Reconstruction Map

Elliptical Elevation MapBest Fit Sphere

Formats for display of data on color maps:

Irregularity MapDisplays distortion of cornea using elevation map with toric referenceHotter colours represent higher value of distortionHelps to quickly assess if cornea is causing poor VA

Trend & Time DisplayChronological display of changes

Difference Display MapOD/OS Compare MapFourier Analysis :extract spherical, cylindrical, prismatic and irregular aberrations

Axial Dioptre Map in 2002 Axial Dioptre Map in 2003Difference Dioptre Map

Other Overlays that can be addedPupil MarginGridsOptical ZoneEye ImageKeratometric Mires

Other Software Application & DisplaysMultiple Display OptionKeratoconus Pathfinder ApplicationContact Lens Fitting Application

Artefacts of Topography Mapshadows on the cornea from large eyelashes or trichiasisptosis or non-sufficient eye openingirregularities of the tear film layer (dry eye, mucinous film, greasy film)too short working distance of the small Placido disk cone

Normal Corneawide spectrum of normalitynasal cornea is flatter than temporal.physiological astigmatism of around 0.75 diopter.can take on many topographic patterns commonly:With the rule astigmatism : vertical bow-tieAgainst the rule astigmatism : horizontal bow-tieEnantiomorphism : mirror image

Normal CorneaSmall changes in corneal shape do occur throughout life:In infancy the cornea is fairly sphericalIn childhood and adolescence, probably due to eyelid pressure on a young tissue, cornea becomes slightly astigmatic with-the-ruleIn the middle age, cornea tends to recover its sphericityLate in life, against-the-rule astigmatism tends to develop

Provides evidence even before SLE can diagnoseMost sensitive method to distinguish:True Early keratoconusAsym Bow-tie or Inf. Steepening due to contact lens warpageKeratoconus Suspect Patients:Specially to diagnose & follow progression

Several ClassificationsCLINICO-TOPOGRAPHIC :

Keratoconus:One or more of clinical signsAsymmetrical bow-tie with skewed radial axis pattern (AB/SRAX)

Early Keratoconus:No Slit-lamp findings, but scissoring reflex on retinoscopyAB/SRAX pattern

Keratoconus Suspect:Only an AB/SRAX pattern

Aym.Bow-Tie with skew

Keratoconus FrusteCalled form fruste 1st described by Amsler in 1937.Extremely mild form of keratoconusCentral or para-central zone of irregular astigmatism of unknown etiology.The most striking hallmark - lack of progression

Several ClassificationsCENTRAL CORNEAL POWERMild keratoconus:< 48DModerate keratoconus:48 - 54DAdvanced keratoconus:>54D

PACHYMETRYNormal cornea:> 543 MicronsEarly keratoconus :~ 506 MicronsModerate keratoconus :~ 473 MicronsAdvanced keratoconus :~ 446 Microns

MORPHOLOGY OF ECTASIA Nipple - ShapedSmall, central ectasiaLess than 5.0mmHigh WTR astigmatism360O normal peripheral corneaOval- ShapedVarying degree of Inferior mid-periphery steepening.Island of normal/flatter than normal cornea exactly located 180O away .Globus- ShapedAffects largest area.All mires within the ectatic corneaNo island of normal mid-peripheral cornea.

Typical Topographic pattern of KeratoconusHigh central corneal powerSteeper inferior cornea (AB/SRAX diagnostic value)Large difference between the power of corneal apex and corneal peripheryOften a disparity of the central powers between the two corneas of a given patientTypical pattern of progression of steepening - rotational

KISA% index for KeratoconusCentral K : descriptive of central steepeningI-S values: inferior-superior dioptric asymmetryAST index : degree of regular corneal astigmatism (SimK1 SimK2)SRAX index : expression of irregular astigmatismKISA% is product of all of the above:

KISA% = (K) x (I-S) x (AST) x (SRAX) x 100 300

KISA% > 100% is keratoconusKISA% > 60 to 100% is Suspect

Humphrey Atlas Pathfinder Corneal Analysis SystemCorneal irregularity measurement (CIM):Represent the irregularity of corneal surfaceNormal CIM: 0.3 to 0.60 micronsBorderline CIM: 0.61 to 1.0 micronsAbnormal CIM: 1.1 to 5.0 micronsShape factor (SF):Represents the degree of corneal asphericity or eccentricityNormal Shape Factor: 0.13 to 0.35Borderline Shape Factor: 0.02 to 0.12 and 0.36 to 0.46Abnormal Shape Factor: 0.47 to 1.0Mean toric corneal measurement (TKM):Two values are calculated at the apex of the flattest meridian and their mean determined. The mean value of apical curvature.Normal TKM: 43.12 to 45.87DBorderline TKM: 41.12 to 43.00 D. and 46.00 to 47.25 D.Abnormal TKM: 36.00 to 41.75 D. and 47.37 to 60.00

A case of Unilateral Keratoconus (Right Eye)accurately diagnosed by Humphrey Pathfinder AnalysisCIM, SF, TKM : if values in green color code range : normal

Videokeratoscopic PseudokeratoconusContact Lens WearTechnical errorsDry spot formationEarly PMDPrevious ocular surgery

PRIMARY POSTERIAL CORNEAL ELEVATIONEarly presenting sign in keratoconusPreoperative analysis of PPCE to detect a posterior corneal bulge is important to avoid post LASIK keratectasia

Elevation MapPosteriorFloat3D-reconstruction

PELLUCID MARGINAL DEGENERATIONVery steep contour in the peripheral peri-limbal corneaHigh power radiating in towards the center from the inferior meridiansButterfly or a "lazy C" or a kissing pigeon configurationArea of flattening down the center of the corneaHigh against-the-rule astigmatism.

Butterfly appearencePELLUCID MARGINAL DEGENERATION

TERRIENS MARGINAL DEGENERATIONprominent flattening of the corneal contourHigh against-the-rule astigmatism

KERATOGLOBUS

CONTACT LENS WARPAGEHarstein : 1st to note CL induced corneal changes

WARPAGE: All CL induced changes in corneal topography, reversible or irreversible, that are not associated with corneal edema

Signs & Symptoms:Mostly asymptomaticChanges in refraction and K readings over a period of timeChanges in curvature and distortion of miresCentral irregular astigmatismLoss of normal progressive flattening from the center to the periphery

Very commonly confused with keratoconus

Topographical abnormalities classified as:Central irregular astigmatismLoss of radial symmetryReversal of normal topographic patternKeratoconus like images

Inaccurate topography causes hazards in patients posted for LASIKSoft CL causes:Topographic steepening (with keratoconus-like image)Increased myopiaCentral corneal thinningRGP CL causes: Topographic flatteningDecreased myopiaCentral corneal thinning.

CL Warpage Special parametersSimulated Keratoscopic Readings;Average powers of the steepest (SimK1) and the flattest meridia (SimK2)Surface Asymmetry Index;Centrally weighted sum of the differences in corneal power between corresponding points on mires located 180 apartSurface Regularity index:Calculated on the basis of the local regularity of the surface over the corneal area within pupillary area.

CONTACT LENS WARPAGE

Contact Lens Fitting

Contact Lens Fitting in KeratoconusSuperior Alignment Fitting Technique for Early KeratoconusThe Intra-Palpebral Three Point Touch Fitting Technique for Early KeratoconusAspheric Lens Designs for Early Keratoconus

POST PENETRATING KERATOPLASTYProposed classification of topographic patterns for Post PK:

Non-astigmatic corneasRegular astigmatism(a) oval(b) prolate symmetric bow tie (PSBT)(c) prolate asymmetric bow tie (PABT)(d) oblate symmetric bow tie (OSBT)(e) oblate asymmetric bow tie (OABT)Irregular astigmatism(a) mixedb) prolate irregular (PI)(c) oblate irregular (OI)(d) horseshoe pattern(e) triple pattern(f) steep/flat (SF)(g) localised steep (LS)(h) unclassified

PENETRATING KERATOPLASTYMaking decisions about trephination and graft sizeIdentifying thin areas to be avoided in the graft-host junctionChoosing a suturing techniqueManaging selective suture removal or adjustmentDeciding on the need for a relaxing incision in astigmatismCorrecting refractive errors by a excimer laser procedureGuide the post PKP fitting of a contact lens

Cataract SurgeryPreoperative Use:Most useful for IOL calculation in eyes with irregular surfacesEvaluation of astigmatism, previous refractive surgeryDecision taking on type of surgeryPlanning for site & type of incisionHas shown that smaller, temporal & scleral incision for phaco cause less induced astigmatismIntra-operative Use:to reduce surgically induced astigmatismWound closureApplication of sutures and adjustmentPostoperative Use:To identify tight sutures and adjust accordinglyEvaluating and managing Post-op refractive suprisesDetermine causes of poor post-op vision

REFRACTIVE SURGERYShould be performed in every case Pre-op:To develop a surgical / ablation profileTo detect pre-existing corneal abnormalities

Post-op uses to evaluate:DecentrationMultifocalityRegressionInduced astigmatismCentral islands

RADIAL KERATOTOMY (RK)Most useful in evaluating Post-RK problems:Irregular astigmatismGlare, halos (induced spherical aberrations)Diurnal changes in refraction & vision (dumble-shaped or split optical zones)Multifocality due to regional change in curvature with time

ASTIGMATIC KERATOTOMY (AK)Pre-op Evaluation of:Astigmatism (specially asymmetric)Calculating best position & configuration of relaxing incisionPost-op evaluation reveals:Longer incision : more steepening of un-incised meridianIncision closure to limbus: less flatteningDeeper incision : more effect

EPIKERATOPLASTY (EPIK)Used to study effect of EPIK on:Keratoconus : flatenning of both ant & post surfacesMyopia : flatenning of ant. Surface onlyAphakia : steeper anterior surfaceBy using VKS, decentration 1st recognized as a complication of refractive surgeryIdentification of astigmatism post surgery

PHOTOREFRACTIVE KERATECTOMYLaser ablation of cornea to flatten/steepen corneaVKS used for evaluation of:Ablation profileDecentrationRegression and stabilizationMultifocality and induced aberrationCentral islands diagnosis and follow-up

Myopic & hyperopic LASIK

Central island post LASIK

Degraded laser opticsExternal hydrationBeam blockage by photodisrupted tissue

Tends to resolve by 18months after surgery

PTERYGIUM

with-the-ruleastigmatism caused by localized flattening of the cornea central to the leading apex of the pterygium

CORNEAL ULCER

REGULAR ASTIGMATISMBow-tie pattern : most common pattern(even 50 % of normal corneas exhibit it)Simulated K readings have good correlation with K readingsBow TieVerticalHorizontal

IRREGULAR ASTIGMATISMRarely occurs naturallyCommon causes:Dry eyeCorneal scarsEctatic corneal degenerationsPterygiumTraumaSurgery (cataract surgery, PKP, and refractive surgery)It represents the remainder after subtracting sphere & cylinder from corneal power map

IRREGULAR ASTIGMATISMClassification:With Defined PatternDecentered Ablation: decentered myopic ablation in more than 1.5mm in central corneaDecentered Steep: decentered hyperopic ablation in more than1.5mm in central corneaCentral Island: increase in central power of ablation zone at least 3D and 1.5mm surrounded by areas of lesser curvatureCentral Irregularity: more than one area of

IRREGULAR ASTIGMATISMClassification:With Defined PatternDecentered Ablation: decentered myopic ablation in more than 1.5mm in central cornea

IRREGULAR ASTIGMATISMClassification:With Defined PatternDecentered Steep: decentered hyperopic ablation in more than 1.5mm in central cornea

IRREGULAR ASTIGMATISMClassification:With Defined PatternCentral Island: increase in central power of ablation zone at least 3D and 1.5mm surrounded by areas of lesser curvature

IRREGULAR ASTIGMATISMClassification:With Defined PatternCentral Irregularity: more than one area of

IRREGULAR ASTIGMATISMClassification:With Defined PatternPeripheral Irregularity: similar to central island extending to periphery of ablation zone in one meridian

IRREGULAR ASTIGMATISMClassification:With Undefined PatternMore than one areas of irregularity >3.0mm in central 6mm cornea

Scanning Slit TechnologyORBSCAN

ORBSCAN40 slit scanning (20 from each side)

Measurable parameters in ORBSCAN

Projection based corneal topographyA grid of horizontal and vertical bars of light (0.2mm apart) is projected onto the flourescein stained tear filmPattern is directly observed and measuredEntire corneal, limbal and interpalpebral conjunctival surfacesCan even measure epithelial defectsDefines elevation points (not curvature)Produces a true topographic map (elevation map)

Technique of lightwave interferenceInterference fringes cover entire ocular surfaceIncludes : holography and moires fringe tachniqueApplies 3-dimensional imaging

Various Topographers available

Haag-Streit Keratograph CTK 922EysSys

ASTRAMAX 3-D Stereo Topographer (Lasersight)Zeiss Humphrey Systems ATLAS

DICON CT200KERATRON Corneal Topographer

The Scheimpflug principle:It is a geometric rule that describes the orientation of the plane of focus of an optical system (such as a camera) when the lens plane is not parallel to the image plane

A WAVEFRONT is a locus, or a line or a wave of point having the same phaseRelates to lights property of moving in a uni-directional manner through spaceLight waves emanate from a single point source

in all directions as a sphere,and the line that connectsthe points upon the surfaceof this propagating wave iscalled a wavefront

A lens can be used to change the shape of wavefronts. Here, plane wavefronts become spherical after going through the lens.

Wavefront AberrationThe deviation of a wavefront in an optical systemfrom a desired perfect planar wavefront

Ab-erratio : going off track or deviation

Aberration Freevs.Aberration Affected Optical Systems

ABERROPIAa refractive error that results in a decrease in the visual quality that can be attributable to high order aberrationNot caused by:Lower order aberrations : myopia/hyperopia/astigmatismEye diseases : cornea, lens, retina

Measured by Zernike Polynomials:Complex methametical calculation

Need of AberrometryWavefront Technology

VARIOUS TYPES OF ABERROMETERS

Type 1 AberrometryHartmann Shack Sensor

Principle of the Hartmann-Shack aberrometer

Type 2 AberrometryTscherning aberrometer

Type 3 AberrometryIngoing Adjustable Aberrometer

Type 4 AberrometrySlit Skiascopy

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Point Spread Function (PSF):Gives an indication of what happens to a spot of light when it reaches the retinaExpresses the effect of the aberration on the retinal image and consequently on the quality of the image

Root Mean Square (RMS):Sq. Root of total aberration relative to the reference sphereHigh value >0.3microns indicates Higher Order Aberrations (HOA)

If you can imagine light as a solid plane when it enters the eye, the Zernike polynomials illustrate how that flat plane is distorted by a specific aberration.

Lower Order AberrationsHigher Order Aberrations

Bausch & LombZYWAVE Aberrometer

Bausch & LombZYWAVE Aberrometer

EYE TRACKING in LASIK

Thanks (References as below)

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