CORNEAL TOPOGRAPHY

PRESENTED BY

BRAHAMDEV MANDALAND RIDDHI MISTRY

HARIJYOT COLLEGE OF OPTOMETRY

Anterior surface of cornea:

horizontal diameter: 11.7 mm vertical diameter :11 mm Posterior surface of cornea:11.5mm

Thickness of cornea:

In centre :0.5 to 0.6 mm In periphery :1 to 1.2mm

Radius of curvature: center : 5 mm anterior : 7.8 mm

Dimensions of cornea

posterior :6.5 mm

Refractive index : 1.376

Refractive power :

anterior surface : +48.33 D

posterior surface : - 5.88 D

Keratometer :

measurement of anterior surface of cornea.

Topography :

The study of the shape of the cornea.

Methods of measurement

Introduction: Refers to the study of shape of corneal

surface.

It helps in determining the refractive status of the eye, since the anterior surface of cornea along with the tear film layer forms the major refractive element of the eye

Corneal topography:

3D view

Aids in diagnosis various cornea condition and disease.

Used in laser refractive surgeries.

• Keratoconus diagnosis and monitoring

• Refractive surgery

–Pre-surgical keratoconus screening –Troubleshooting unsatisfactory outcomes • Contact lens fitting –Lens selection

–Orthokeratology –Monitoring corneal warpage

Why corneal topography?

Fringe projection

Principle

Have the patient sit at a comfortable level.

Inform the patient on what test you are doing.

Try to have a good tear film.

Let the patient know they cannot move doing this test.

How to take Topographies

Placido disc corneal topography Digital rasterstereography based

topography systems Laser halographic interferometry based

topography system Slit scanning corneal topography system Scheimpflug imaging(pentacam) High speed anterior segment optical

coherence tomography Very high frequency ultrasound

Corneal topography system

o It is originally limited to evaluation of the anterior corneal surface, now include the imaging of back surface of cornea and direct evaluation of elevational changes of both anterior and posterior corneal surface.

1. Placido disc corneal topography

LSUCTS CMS Computerized corneal topographer EH 270 Eyesys 2000 corneal analysis system TMS – 1 topographic modelling system Astramax

Commercially available placido disc topography system

Display in following formats

A. Numerical power plots

B. Keratometry view

C. Photokeratoscopic views

D. profile views

E. colour –code topographic maps

Display of Placido-Disc corneal Topography system Data

Cornea curvature of specific areas is shown in dioptre values .

The data are displayed in 10 concentric circular zones with 1 mm interval between each.

The numerical value are displayed in colour scale being used .

The display also shows the average dioptric value of each of the 10 concentric circular zones individually along with the average overall corneal curvature.

A Numerical power plots

It depicts the keratometric reading in two principal meridian(k1 and k2) in three different Zones simultaneously.

The three zones measured are central 3mm zones (as in a conventional keratometry ), intermediate

3-5 mm zones and peripheral 5-7 mm zone. It is important map for assessing the skewing of

semi –meridia.

B .keratometric view

It depicts the actual black and white photograph of the placido rings captured by the video camera.

This view helps in confirming the proper patient fixation and in identifying the eye captured .

The reflected rings on the cornea are situated more towards the limbus on one side than the other ,and on the nasal side the distance between the rings is comparatively narrower.

C. photokratoscopic view

It shows the graphical plotting along the x y axis of the steepest and the flattest meridia of the cornea and the difference between the two in dioptres .

The display button shows the astigmatic difference (difference plot, Delta map) between the flat and steep meridia.

A grey zone in this difference plot denotes the pupillary area.

D profile view

Colour –coded contour maps of the cornea are the most useful and most commonly used display formation.

E colour –coded topography map

While interpreting colour –coded contour maps of the cornea ,following parameters should be consider :

(1)colour: These are used as follows: Hot colour , i.e red and its various hues represent the steep

portions of cornea.

Cool colour, i.e blue and its various hues represent the flat portion of cornea.

So the colour red –orange –yellow-green –purple-blue denote progressively lessening refractive power.

The colour intensity is relative, meaning that an area of 45D is less red as compared to an area power.

Interpretataion of a colour map

2.The scale used:

It is very important to know the scale used before interpreting a colour map.

The two apparently similar maps may in fact show markedly different cornea depending upon the scale used .

The commonly used scales are absolute and normalized scales:

Absolute scale:

In it ,each colour represents a 1.5 D interval between 35 and 50D ,whereas above and below this range ,colour represent 5D intervals .

This scale is useful in routine practice(e.g Preoperative screening).

Disadvantage of absolute scale is that it does not show subtle changes of curvature and thus can miss subtle local changes (e.g. early keratoconous)

Normalised scale:

In it cornea is divided in to 11 equal colour spanning that eye’s total dioptric power.

In this scale ,more minute topographic details within an individual cornea are appreciated.

Advantage :

It shows more detailed description of the surface than the absolute scale.

Disadvantage:

The colour of two different maps cannot be compared directly and have to be interpreted based on the keratometric values from their different colour scales

:

Quantitative indices : These include : Predicted visual acuity based on corneal

shape Simulated keratometric reading (simxk) Minimum Keratometry reading Surface regularity index Surface asymmetry index Point spread function (psf)

Refractive power map

Regular Pattern Round (23%) Oval(21%) Steepening superior steepening Inferior steepening Astigmatism patterns Symmetrical and orthogonal ,i.e. bow –tie effect (18%) Symmetrical bow –tie with non –skewed axis Symmetrical bow –tie with skewed axis

Cornea topographic patterns in normal cornea

Asymmetrical and orthogonal (31%)

Asymmetrical bow –tie superior steepening

Asymmetrical bow –tie inferior steepening

Asymmetrical bow –tie skewed radial axis

Irregular ,i.e. no pattern and non -orthogonal

1. Corneal power map• 24 colour representation of dioptric

power.• The radius of curvature is measured

360 time for each placido ring image from center to vertex.

Formats for display of data on color maps

• 2.Tangential map:• Tangents are projects outwards from the

centre vertex 360.• This is best indicator of corneal shape,but is

a poor indicator of corneal power.

• 3.Elevation map:• It helps in distinguishing localized elevation

(steep because of projection) from otherwise steep corneal area.

• For remembering • - red is increase steep cornea.• - blue is increase flat cornea.

Tangenital map

4.Refractive power map: Taking in to account of shperical

aberrations. It illustrates how the corneal curvature

refracts light in true diopters of power not curvature.

Refractive power map

5. irregularity map: shows areas on cornea that are hot in

color . The hotter color represents the higher value

of distortion. This map allows the surgeon to quickly

assert ,if the cornea is causing poor visual acuity.

Irregularity map

In this ,changes occurring in topography with time can be displayed in chronological order.

Trend & time display

It exhibits the comparative difference in two given topographic maps.

Difference display map

Allows comparison of both eyes simultaneously.

Right / left eye ( OD/OS)compare map

Role of early diagnosis of corneal disease.

Topography and contact lens.

Topography in keratoconus.

Topography in RK.

Topography IN PRK AND LASIK.

Topography in post keretoplasty astigmatism Iol power calculaion

Clinical application of corneal topography

To quantify irregular astigmatism and corneal warping associated with contact lens wear.

To diagnosis early keratoconus.

To evaluate change in corneal shape after refractive surgery. corneal grafting or cataract examination

INDI CATION

Algorithms for power calculation . The correlation between corneal curvature and power.

Data is averaged across meridia.

The formulae employed for power calculation are centered on the corneal apex and not on the more revant line corneas.

Placido- disc –based computerized videokeratographic instruments have problems of critical focus and inability to measure highly irregular corneas

Limitations

THANK YOU