Corneal Topo Atlas Imp

Topography / Tomography 2010 update

www.athensvision.eu Anast Charonis

“Anterior Segment Topology-2010 ”

1.Placido based technology with arc step reconstruction τοπογραφία ανάκλασης

2. Scanning Slit / transverse and rotationalτοπογραφία προβολής

3. Wavefront analyzers/ Corneal Wavefrontαµπερροµετρική τοπογραφία

• Dynamic Corneal Imaging (Grabner et al) Indentation Topography- Elasticity

• Ocular Response Analyzer (Reichert)Ocular hysteresis and IOP- Viscoelasticity

• Electronic Speckle Pattern Interferometry (ESPI) –Measures out of Plane corneal displacement

• Manual Keratometry

• Classical Corneal Topography– Placido based principles

• Keratometry, Axial, Tangential, Power Maps• Keratometric / physiological corneal indices• Corneal asphericity/ related clinical relevance

• Corneal Tomography (Elevation Topography)– Transverse Slit Scanning/ Rotational Scheimpflung

• Wavefront basics• Corneal Wavefront

GARBAGE IN

GARBAGE OUT

ALWAYS CHECK THE RAW DATA FIRST!!

Quiz #1How do you diagnose irregular astigmatism without a topographer?

• Direct ophthalmoscope

• Retinoscope

• Lohne keratoscope

• Keratometer

The Scheiner Principle -1619

keratometry

• Measures the curvature of the cornea along the 2 principal meridiens at two points 3-4 mm apart on the paracentral cornea

• Assumes the rest of the cornea is perfectly spherocyndrical

Scheiner > Helmholtz > Javal-Schotz

J Schwiegerling

Corneal topography

• Placido disc technology – Large target Eyesys, Keratograph, Dicon CT

200, Eyemap-Alcon, atlas topographer- (H-Z)– Medium target Eyesys Vista, Shin-Nippon-

Reichert CTS– Small Target Tomey TMS-3/4, Optikon Scout

Portable,Magellan-Nidek

Curvature measures bending.

A small radius circle has a large curvature.

• Small radius• Large

curvature• Fast bend

• Large radius• Small

curvature• Slow bend

R

1=κCurvature Radius of curvature

Turner 1999

Keratometry K-quiz#2 true or false

1. K Determines curvature by measuring the size of a reflected ‘mire’

2. Doubling of image avoids problems with eye movements

3. In the B&L keratometer the mire has constant size, the image doubling is variable

4. In the Javal-Schiotz ophthalmometer the image doubling is constant and the mire separation is variable.

5. The radius scale used in keratometers is not exact

yes

yes

yes

yes

NO

Principle of keratometryQuiz #3

• If you examine a “55d” cornea the specular reflected image of the cornea is going to be different from that of a ‘40d’ cornea.– Is it going to be larger or smaller ?

• The mire image of the B&L keratometer is always constant and measures the curvature at specifically 3mm around the cornea apex. Yes or No

Keratoscope: Placido's disc

• Disc with white reflecting rings(also

called mires) and convex lens in the center

• Held in front of the eye and illuminated

• Examine the shape of the reflected rings off the pre-corneal tear film handle

Reflecting

rings

Viewing

Aperture

with lens

Measure Surface Slope directly using Specular Reflection

“Normal” eye

equally spaced

symmetrical rings

Corneal astigmatism

oval-shaped rings

Aspherical cornea

non-equidistant rings

Corneal topography

• With a keratometer one typically measures up to 4 points of the central corneal surface

• Computer-analysis of a picture from Placido's disc can measure the curvature at thousands of points on the whole cornea

Although there is no universally accepted color scale in corneal topography, spectral directions are standardized.

Color Scale: Surface Curvature

• sharp• fast bend• short radius

• flat• slow bend• long radius

(+ +)

(+)

Red

BlueMin

Max

In all curvature maps, red is sharp (large curvature, small radius), and blue is flat (small curvature, large radius).

Turner 1999

Astigmatism: Axial Curvature Map

The bow-tie is actually an axial artifact

Axial CurvatureTurner 1999

K-AXIS

Astigmatism: Meridional Curvature Map

You can get axial curvature from a meridional map. The axial value at any point is the average meridional curvature along a radial, from the map center to the point of interest.

Meridional CurvatureTurner 1999

Keratoconus: Axis-Based MapsAxial Curvature Meridional Curvature

Turner 1999

Quiz #4Axial vs Tangential Curvature

K axis

A

B

C

Point A: tangential power>axial: increased steepness

PointB: tangential=axial evtl spheric cornea

Point C= tangential <axial evidence of increased prolateness

PLACIDO >>> Perspective ReflectionMeridional/ Axial > Principal- Mean

Turner 1999

Keratoconus: Anterior Quad Map

Elevation (sphere) Mean Curvature

Axial Curvature Meridional Curvature

Turner 1999

pre-cornealwavefront

post-lenticular

wavefront

pre-retinal

wavefront

Curvature Vs Optics

Surface Geometry Curvature or Slope

Material Properties Refractive index

Optical Incidence Light ray incoming angle

Curvature, slope, elevation, thickness, and depth are all geometric properties. They quantify the

size, shape, and location of the refracting surfaces. No optics is involved.

Optical properties quantify net changes in the optical wavefronts traversing one or more refracting

surfaces.

curvature

depth

thickness

Turner 1999

Raytrace Power MapAxial Curvature [K] Refractive(optical) Power

This eye has with-the-rule astigmatism. Axial curvature, although expressed in keratometric

diopters, is NOT an optical property.

Raytrace power maps areoptical. Notice that on every meridian, raytrace power increases peripherally. This is a

manifestation of the spherical aberration caused by insufficient corneal prolateness.

Turner 1999

•oblate shape factor S, prolate shape factor E, and asphericity Q. The relations between these shape factors

are

•S = 1 - E = 1 + Q

•Asphericity is the negative of the prolate shape factor.

Keratometry K-quiz#5 true or false

• The axial map of a perfect sphere of 7,5mm ROC in 0,5step scale is monochromatic

• The curvature tangential map of a perfect sphere of 7.5mm ROC in 0,5step is trichromatic

• The axial optical power map of a perfect sphere of 7,5mm ROC in 0,5step is trichromatic

• The axial optical power map of a perfect sphere of 7,5mm ROC in 0,5step scale is hexachromatic

no

no

yes

yes

quiz#6true or false

• The virgin normal cornea shape can be described as a prolate ellipsoid

• The median value of the Q factor of the virgin cornea is ~-0,2.

• The spherical aberration of the normal young eye is very close to zero.

• The corneal spherical aberration of the normal young eye is not zero, it would theoretically be zero if the q factor was ~-0,52

yes

yes

yes

yes

Quiz 7

Keratoconic patients benefit the most from aspheric lenses when they have cataract surgery True or False ??

Aligned Curvatures: Axial Curvature

Axial Curvature [D]Anterior Cornea

Axial Curvature [D]Posterior Cornea

PosteriorHorizontal

VerticalF = 49.7S = 52.1

AnteriorF = -5.8

S = - 6.8

CornealF+F = 43.9 S+S = 45.3

∆ = 2.4 ∆ = 1.0 ∆ = 1.4

Turner 1999

True corneal power, Quiz # 8• The Physiological index (PI)of the cornea is

1,337 and is used in modern topography systems

FALSE: The Keratometric Index is used because the corneal power Is estimated only by measurements of the front corneal surface

The KI however is 1,337

KI is not the refractive index of anything N -= 1.3375 is an approximation

Ant power (1,376-1.00) 1000/r =376/rPosterior power (1,336-1,376) 1000/r = -40/r

True corneal power, Quiz # 9

• The ratio of ROC between anterior and posterior corneal surface is stable and is ~0,882 (that’s why KI is valid in IOL calculations)

• The ratio of ROC changes after RxSurgery

because the ROC of the posterior cornea remains unchanged and the “compensating” Keratometric index is not valid.

• Nevertheless all Scheimpflung posterior power images are (in 2010) still invalid!

Placido based assumptions• The precorneal tear is perfect • The corneal apex, line of sight and the

VK normal are all the same

• There are no curvature discontinuities

• The central cornea curvature is actually not measured

Tear Stability Analysis System-TSAS

WOC 2006

Placido curvature may be ambiguous.

CF

C

F

dist

ant p

atte

rndi

stan

t pat

tern

convex surface

concave surface

R > 0

image is small

and erect

image is small

and inverted

R < 0

Placidoimages

Turner 1999

Quiz #10

Which patient may have

A/ warpageB/ keratoconus

C/ astigmatism

Curvature topography is

Reference Axis dependant

This increases the false

Positives for KC

M Belin

Curvature limitations

• “pseudokeratoconus” curvature abnormalities occur when the line of sight, apex and VK normal do not line up

• ‘keratoconus”maps have been created in normal aspherical surfaces with angular decentrations as small as 5 degrees

Quiz#11 Centering topography maps

• Proper centration is not an issue anymore with advanced reconstruction algorithms

• Advanced Topography systems can be utilized to center the ablation zone during Excimer laser surgery

• Wavefront hyperopic corrections are very sensitive in proper centration on the correct axis so ‘angle kappa” considerations are extremely important.

wrong

correct

wrong

Placido technology: discussion

� 2 >>3 dimensions� Curvature >>> Height

“Realize that Placido disk topography represents twenty-first-century technology applied to eighteenth-

century science, in that it digitizes a Placido disk measurement, similar to the idea of installing GPS

positioning on a horse and buggy’

WCC 2005 Michael W Belin

Triangulation maps complex surfaces.

Topographic map of a Lincoln head

penny.

Elevation scale is 5 microns

per step.

Spatial resolution is about 0.25 mm.

Turner 1999

The Orbscan Unit Paradigm

• 1993 exciting idea

• 1996 curiosity in university practices

• 1997-00 toy for graduate students• 2001 B&L acquisition, Orbscan II,

a very successful story

• 2004 Do not perform Refractive Surgery without an Orbscan

X Eykarpides

Scanning slits measure several surfaces.

Turner 1999

Triangulation locates points in space

triangulatedspace pointcomplex object

Video

Camera

calibrated slit-beamsurface diffusely

reflectedcamera ray

Curvature, Height and Power, Pachymetry

• ORBSHOT placido reflective technology

• ORBSCAN slit scan technology measures multiple surfaces

• ORBSCAN II Integrates both technologiesslit scan + reflective

Use of “Corneal Topography”in Refractive Surgery

• Determine Fruste Keratoconus

• Determine OZ centration• Measurement of the functional OZ

• Estimation of the pupillary size

• WW size – AC depth

• Pachymetrical maps

March 1997 Orbscan I Concept 55

Fundamentals of Measurement

• Beam and Camera Calibration• Specular vs Diffuse Reflection• Edge Detection • Raytrace Triangulation• Eye Tracking• Surface Fitting• Backscatter Measurement

Turner 1999

0,7 sec/ pass240 data points per slit

Overlapping central 5 mmIncreases accuracy &

ResolutionEye Tracker

March 1997 Orbscan I Concept 58

Eye Tracking• 40 slit images are acquired in two 0.7 second

periods.• During acquisition, involuntary saccades

typically move the eye about 50 microns.• Eye movement is measured from anterior

reflections of the stationary slit-beam and other light sources.

• Eye tracking data permits saccadic movements to be subtracted from the final topographic surface.

Resolution

4µ centrally

8µ periphery

8

Maps, 3-D Views, and Section Profiles

Surface topography can be viewed in several different ways: as conventional color contour maps, as 3-D renderings, and a section profiles.

Reading Corneal Elevation Maps

John A. Vukich, MD

Global Perspective

Relevant features at the local level can be lost due to scale

.John A. Vukich, MD

Global Perspective

12,000 mi 12,000 microns

The elevation topography of both globes is small in comparison to the entire surface

12,000 Kilometers

The average lasik ablation is less than 100 microns. On the scale of the entire cornea this represents only .01%.

John A. Vukich, MD

Close-Fitting Reference Surfaces

Corneal topography differs from terrestrial topography in that the reference surface is not some fixed “mean sea-

level”, but is movable.For the cornea, a reference surface (typically, a sphere) is constructed by fitting the reference

surface as close as possible to the data surface.

A best-fit minimizes the square difference (always a positive number) between the two surfaces, but only within

a specified region known as the fit-zone.

Fit-zoneReference surface (sphere)

Data surface(cornea)

Topographic maps of terrestrial landscapes are displayed in the form of constant-elevation contours, measured from the

“mean sea-level” of the earth.

John A. Vukich, MD

Elevation Topology: Central Hill

Sharp center

Flat periphery

John A. Vukich, MD

Elevation Topology: Central SaddleJohn A. Vukich, MD

Relative elevation measures height difference from a best-fitting reference sphere.

Color Scale: Relative Elevation

In all elevation maps, green is the reference surface or zero level. Red is high and positive. Blue is low and negative.

• high• anterior to the

reference surface

• low• posterior to the

reference surface

reference

(+)

(-)

anterior

posterior

Red

BlueMin

Max

Turner 1999

Elevation (sphere) Elevation (sphere)Turner 1999

Toric Patterns

Central Saddle

When toricity dominates, a central saddle will form.

John A. Vukich, MD

Elevation (sphere)10 mm fit-zone

Elevation (sphere)5 mm fit-zone

Both maps contain exactly the same information. They look different, because altering the fit-zone changes the size and alignment of the

reference sphere.

7.16 mm radius 6.88 mm radiusJohn A. Vukich, MD

Astigmatism: Elevation vs. Axial Map

Axial CurvatureElevation (sphere)

Elevation and axial curvature maps look different, almost reversed.

On the axial map, the steep axis follows the sharpest curvature (red line segments) and aligns

to the red-shifted bow-tie.On the relative elevation map, the steep axis dips

below the reference surface and runs from “sea” to “sea”.

Turner 1999

Που γίνεται η φωτοκερατεκτοµή στον αστιγµατισµό? (-/+ κύλινδρος) Quiz # 13

Όχι!

Crossed Curvatures: ElevationElevation (sphere)Anterior Cornea

Elevation (sphere)Posterior Cornea

In this case posterior astigmatism augments anterior astigmatism. This case is atypical and

occurrences are relatively rare.

Turner 1999

Orbscan technology quiz 14

• Orbscan II directly measures elevation data utilizing specular reflection technology

• Orbscan II measurements are accurate because a passive eye tracker is utilized

• The actual data-acquisition time per scan is 0,7 secs

wrong

wrong

wrong

• Anterior Symmetric, Posterior Symmetric• Anterior Symmetric, Posterior Asymmetric• Anterior Asymmetric, Posterior Symmetric• Anterior Asymmetric, Posterior Asymmetric• Decentered Apex Symmetric• Decentered Apex Asymmetric

Topographic Types of Astigmatism - New Concept

Daniel S. Durrie, MD

Example - ASPS


Example - ASPA


Example - DAS


Example - DAA


Posterior Cone: Recommended Quad

Elevation, Anterior Elevation, Posterior

Mean Curvature, Anterior Thickness, Cornea

Quiz #15

Do you think that this elevation topography set may hide a cone???

Posterior Cone: Recommended Quad



Toric Keratoconus: Recommended Quad



Quiz 16A/ This is not keratoconus

B/ Elevation topography can’t show

the cone because of extreme toricity

C/ once the astigmatism is filtered out

The cone presents itself (mean c)

Θα κάνατε laser σε αυτόν τον κερατοειδή? quiz #17

244µ

413µ

Study case

• 46 y.o presents 33 months postop lasik with «sudden» loss of vision in her left eye. – 24 months ago she was sc 10/10(BSCVA +0,25

sph)– 17 months ago she was sc 9/10 (BSCVA -0,50-

050x110)– Today she was 2/10- (BSCVA 9/10 with –3,25-

0,75 x115)

Quiz22 # 66 y.o with cat, wants a multifocal lens

Is she eligible for an Alcon- Restor??

Old Rx: -3.00-2.25x180

New RX: -5,25-3.00x 005 7/10-

Elevation topography Quiz #23

• Posterior cornea astigmatism may counter balance anterior astigmatism so the overall refractive astigmatism is less

• What was previously known as “lenticularastigmatism” usually with the orbscan is proven to be posterior corneal

• Misaligned elevation topographies are generally considered safer in respect to postoperative ectasia

yes

yes

No

Scheimpflug Tomography Quiz 24

• A substantial advantage of the rotating slit (compared to the Scanning slit) is the acquisition of a common central reference point in each scan

• Curvature measuring sensitivity in placidosystem is 20x compared to Scheimplug

• Motion artifact always exist with Scheimpflug(less in a dual system) but much less in placido

• Scheimpfug increased classification agreement between normal and abnormal among “experts”

yes

yes

yes

NO

Retinal image quality Quiz 25

• Pupil size is a major player and is defractionlimited

• WFE is also strongly pupil diameter dependant

• Scatter is the main reason why cataracts degrade visual performance

• The optical quality of the cornea can also be expressed in Zernicke terms (Corneal wavefront)

The wavefront error (WFE) is the error between the actual wavefront (red) and the ideal

wavefront (yellow) as a function of location within the pupil.

RAA

The shape of the aberrated WF is a fundamental description of the optical

quality of the eye.

Quiz 26

• What is the shape of the wavefront with a Z1

1, Z 2

0

Zernicke error ?

Corneal Topo Atlas Imp

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