Placido Disk

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

Corneal Topography principles
• Videokeratography (Corneal Topography)
A type of computerized imaging technology
• Purpose very detailed description of the
shape and power of the cornea
• Surface 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

Methods
Small Cone
Color Coded Map
ANGLE
RING
DISTANCE
RoC
0
1
311
10320
Large Cone
Computer
Unit
0
…
0
1
2
…
30
1
381
…
4182
312
10320
…
8880
10320
1
2
384
10330
…
…
…
…
1
30
4184
8890
…
…
…
359
1
311
10320
…
…
…
…
359
30
4185
8880
Topographic results as exported in
ASCII raw data file.
Categories of Videokeratographers
• 2 General Categories
◦Curvature 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 placido
cone disk that fit very close to the
eye when the imaging is performed.

Categories of Videokeratographers –
Curvature based systems
• Using 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 cornea
◦We 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 systems
◦Orbscan, manufactured by Orbtek,
acquires data from the diffuse reflection of
a slit beam of light scanned across the
cornea.

Categories of Videokeratographers
• Elevation based systems
◦Optical Interferometry
based Videokeratography
•Devices with a sinusoidal
grading is reflected from
the surface of the cornea.
From this data, the
principle of interferometry
is used to localize the
outer surface of the eye.
Corneal Topography Principles
• Keratometric diopter from radii of curvature as :
K = 1 - 1.3375 / RoC(m).
• The simplification ignoring
◦The refracting surface is air-tear interface
◦Not 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.

Corneal Topography Principles
Inaccuracies generated by conversion of curvature to
power (SKI=Standard Keratometric Index)
Assumptions made in converting
ROC to power
Conversion formula Spherical optics
Effects
Inaccurate outside the central cornea
SKI Normal posterior curvature
SKI Normal thickness cornea
SKI Uniform cornea refractive index &
Not recognize of different refractive
properties of epithelium and stroma
Inaccurate for very steep or very flat
corneas(High myopia or high
Hyperopia)
Inaccurate following excimer laser
photorefractive keratectomy
Inaccurate in situation as following
refractive surgery
Comparison of three cornea topographic devices
Instruments
Keratometer
Photokeratoscope
Computer
videokeratoscope
Examples
Von Helmoltz,
Javal-Schiotz
Corneascope
TMS, Eye-Sys
Number of points
4
Many
6000-11000
Area
Annulus of 3 mm radius
70% surface
95% of surface 9-11mm
diameter
Dioptric Range
30-60 D
infinite
8-110 D
Focusing
Superimposition/alignment
of two mires(easy)
Subjective focusing of single
image (difficult)
Overlap laser or
croshairs(easy)
Mires
Four objects
12 rings
15-38 rings
Record
Two numbers
Still photography
Stills from video
Method
Measurement
Observation
Quantitative
Topographic information
None
Quantitative
Quantitative
Accuracy
Excellent(for spheres)
Poor
Good
Sensitivity
Moderate
Low(3DC)
0.25 D or better
Reproducibility
Excellent
Moderate
Good(0.5D)

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

Color encoding
Scales II
• 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 keratometric
values from their different
color scales.
Based upon the distribution
of corneal curvatures within
the population. Steepest
areas are depicted in
warmer colours and flattest
areas in cooler colours.
Population
+3SD
+1SD
Mean
-1SD
-3SD
Slope
Steep
Average
Flat
Curvature(
mm)
7.0
7.5
7.8
8.0
8.7
Power
(D)
48.0
45.0
43.5
42.0
39.0
Color
Red
Orange/y
ellow
Yellow/G
reen
Green/
Light
Blue
Blue
Various types of topographic representations
Standard/Normalized scale/Diopter Map

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

Various types of topographic representations Standard/Normalized scale vs
Absolute Scale/Diopter Map
Mild
Astigmatism
Various types of topographic representations Standard/Normalized
scale/Diopter Map – 3-D representation

Various types of topographic representations Standard/Normalized
scale/Diopter Map – Enchanced Height Map
Ablation profiles - Algorithm Simulation - Spherical
Myopia
With a 6mm OZ No Blend Zone
6.0
DENSITY MAP
SHOT PATTERN
Ablation pattern for low myopia:
-1.00D sphere
6.0mm zone with no blend

Ablation Profiles - Algorithm Simulation - Myopic
Astigmatism
5.5mm circular O.Z. with 1mm blend of flat axis
ablation zone
1.0
BZ
7.5
5.5
oz
SHOT PATTERN
1.0
BZ
DENSITY MAP
Ablation pattern for myopic
astigmatism:
-1.00 -2.00 x030
5.5mm zone with 1.0mm blend
The Normal Cornea

Apex: Point with the
smallest radius of
curvature
Mean Diff between keratometric
axis and corneal sighting
center:0.38+/-0.10mm
Mean Diff between keratometric
axis and apex 0.62+/-0.23mm