corneal topography

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  • 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

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