Placido Disk Γεώργιος Κουνής – Απρ. 2005 History of Corneal Topography Excimer Surgery Graft, incisional and cataract surgery Contact Lenses Curiosity Motivation Projection Techniques Videokeratoscopy Keratometry Spheres Techniques Height/True Shape Power Curvature Diameter Measurements History 1619 Scheiner Convex mirrors 1882 Placido Placido Disc 1854 Von Helmoltzc 1889 Javal Placido Disc & Kertatometer 1896 Gullstrand Photography&Kertatometer
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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
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