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Placido Disk

Γεώργιος Κουνής – Απρ. 2005

History of Corneal Topography

Excimer Surgery

Graft, incisional andcataract surgery

Contact Lenses

Curiosity

Motivation

Projection Techniques

Videokeratoscopy

Keratometry

Spheres

Techniques

Height/True Shape

Power

Curvature

Diameter

Measurements

History

1619 ScheinerConvex mirrors

1882 Placido Placido Disc

1854 Von Helmoltzc

1889 Javal Placido Disc & Kertatometer

1896 GullstrandPhotography&Kertatometer

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Corneal Topography principles

Videokeratography (Corneal Topography)A type of computerized imaging technologyPurpose very detailed description of the shape and power of the corneaSurface description topographical relief maps of earth

Corneal Topography principles

Multiple light concentric rings are projected on the cornea. The reflected image is captured on charge-coupled device (CCD) camera. Computer software analyzes the data and displays the results in a variety of formats

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Methods

Computer Unit

Color Coded Map

8880418530359

…………

103203111359

………

88904184301

…………

1033038421

1032031211

88804182300

…………

1032038120

1032031110

RoCDISTANCERINGANGLE

Topographic results as exported in

ASCII raw data file.

Small Cone

Large Cone

Categories of Videokeratographers

2 General CategoriesCurvature based systems

Systems that employ a large placido disk which is several inches in diameter and is positioned several inches from the patient's eye where the imaging is performed Systems that employ small placidocone disk that fit very close to the eye when the imaging is performed.

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Categories of Videokeratographers –Curvature based systemsUsing basic optical principles, an object of known size reflected off of a mirror of known power will produce an image of known size. In videokeratography, we know

the size of the circular object that is being reflected off of the corneaWe observe the image of this reflected circle by simply taking a picture of it. The next trick is to determine the power (shape) of the reflecting surface (the cornea).

Categories of Videokeratographers

Elevation based systemsOrbscan, manufactured by Orbtek, acquires data from the diffuse reflection of a slit beam of light scanned across the cornea.

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Categories of Videokeratographers

Elevation based systemsOptical Interferometrybased Videokeratography

Devices with a sinusoidal grading is reflected from the surface of the cornea. From this data, the principle of interferometryis used to localize the outer surface of the eye.

Corneal Topography PrinciplesKeratometric diopter from radii of curvature as :

K = 1 - 1.3375 / RoC(m).

The simplification ignoring The refracting surface is air-tear interfaceNot accounting the oblique incidence of incoming light in the corneal periphery

Miscalculation true corneal refractive index of 1.376 to 1.3375 to correct for some of these factors.

These are keratometric diopters to distinguish from diopters expressing more precisely the true refractive power at certain corneal point.

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Corneal Topography PrinciplesInaccuracies generated by conversion of curvature to power (SKI=Standard Keratometric Index)

Inaccurate in situation as following refractive surgery

SKI Uniform cornea refractive index & Not recognize of different refractive properties of epithelium and stroma

Inaccurate following excimer laser photorefractive keratectomy

SKI Normal thickness cornea

Inaccurate for very steep or very flat corneas(High myopia or high Hyperopia)

SKI Normal posterior curvature

Inaccurate outside the central corneaConversion formula Spherical optics

EffectsAssumptions made in converting ROC to power

Comparison of three cornea topographic devices

Good(0.5D)ModerateExcellentReproducibility

0.25 D or betterLow(3DC)ModerateSensitivity

GoodPoorExcellent(for spheres)Accuracy

QuantitativeQuantitativeNone Topographic information

QuantitativeObservationMeasurementMethod

Stills from videoStill photographyTwo numbers Record

15-38 rings12 ringsFour objectsMires

Overlap laser or croshairs(easy)

Subjective focusing of single image (difficult)

Superimposition/alignment of two mires(easy)

Focusing

8-110 Dinfinite30-60 DDioptric Range

95% of surface 9-11mm diameter

70% surfaceAnnulus of 3 mm radiusArea

6000-11000Many4Number of points

TMS, Eye-SysCorneascopeVon Helmoltz, Javal-Schiotz

Examples

Computer videokeratoscope

PhotokeratoscopeKeratometerInstruments

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Various types of topographic representations Standard/Normalized scale/Diopter Map – Rings over the cornea –Lost measurements

Superimposition of of the color map shows how the topography relates to the whole cornea or focal irregularities

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Normalized maps have different color scales assigned to each map based on the instrument software that identifies the actual minimal and maximal keratometric dioptric value of a particular cornea.

The disadvantage is that the colors of 2 different maps cannot be compared directly and have to be interpreted based on the keratometricvalues from their different color scales.

Blue39.08.7Flat-3SD

Green/Light Blue

42.08.0-1SD

Yellow/Green

43.57.8AverageMean

Orange/yellow

45.07.5+1SD

Red48.07.0Steep+3SD

ColorPower(D)

Curvature(mm)

SlopePopulation

Based upon the distribution of corneal curvatures within the population. Steepest areas are depicted in warmer colours and flattest areas in cooler colours.

Color encoding Scales II

Various types of topographic representations Standard/Normalized scale/Diopter Map

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Various types of topographic representations Standard/Absolute scale/Diopter Map

Myopia

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Various types of topographic representations Standard/Normalized scale vsAbsolute Scale/Diopter Map

Mild Astigmatism

Various types of topographic representations Standard/Normalized scale/Diopter Map – 3-D representation

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Various types of topographic representations Standard/Normalized scale/Diopter Map – Enchanced Height Map

Ablation pattern for low myopia:

-1.00D sphere

6.0mm zone with no blend

DENSITY MAP

SHOT PATTERN

6.0

Ablation profiles - Algorithm Simulation - Spherical Myopia

With a 6mm OZ No Blend Zone

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Ablation Profiles - Algorithm Simulation - Myopic Astigmatism

5.5mm circular O.Z. with 1mm blend of flat axis ablation zone

DENSITY MAP

Ablation pattern for myopic astigmatism:

-1.00 -2.00 x030

5.5mm zone with 1.0mm blendSHOT PATTERN

5.5

oz

1.0

BZ

1.0

BZ

7.5

The Normal Cornea

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Apex: Point with the smallest radius of curvature

Mean Diff between keratometricaxis and corneal sighting center:0.38+/-0.10mm

Mean Diff between keratometricaxis and apex 0.62+/-0.23mm