corneal topography

CORNEAL TOPOGRAPHYCORNEA

L

DR. MRINMAYEE GHATAKDO, FMRF (Sankara Nethralaya)

Consultant OphthalmologistKota Eye Hospital & Research Foundation

Kota – INDIA

[email protected]

• Anterior 1/6th of eyeball

• Measures 10.6mm Vertically and 11.7mm horizontally

• NOT SPHERICAL, typically described as prolate ellipsoid

• Central 4mm (optical zone or apical zone or corneal cap)

supposed to be spherical

ZONES OF CORNEA

3-4 mm

7-8 mm

11 mm

12 mm

Central Optical

Limbal

PeripheralTransitional

ParacentralMid-peripheral

Central OpticalParacentralPeripheralLimbal

CORNEA - Curvature• Central 4mm :

– 7.8mm anterior surface– 6.5mm posterior surface

The range of powers found in the normalcornea range from 39 D found at peripheral cornea,close to the limbus, to 48 D found at corneal apex.

Cornea - Power calculation

P = N2 – N1 R

P : Power of corneal surfaceN1 : Refractive Index of 1st mediumN2 : Refractive Index of 2nd mediumR : Radius of Curvature in metres

Power : N2-N1 / R

6.5

Sphericalvs

AsphericalSurface

Christopher Scheiner (1619)

HISTORICAL EVOLUTION• 1619 : 1st reported description of corneal curvature by Christopher

Scheiner

• 1796 : Jesse Ramsden built the 1st device exclusively for keratometry

• 1854 : Herman von helmholtz modified Ramsden’s instrument : termed it Ophthalmometer

• 1881 : 1st practical keratometer for clinical use by Javal & Schiotz

• 1932 : modified and improved version by Bausch & Lomb

KERATOMETRY

PRINCIPLE• Observation of 1st Purkinge’s Image• Based on geometry of aspherical convex reflecting surface (cornea)• Object of known size and distance is reflected off the corneal

surface to determine the size of the reflected image with a measuring telescope

• Calculates the refracting power on the basis of an assumed index of refraction

Advantages of Keratometry

• Accuracy and reproducibility for measuring regular corneas within normal range of curvatures (40 -46 D)

• Good for fitting CL and IOL power calculation• Ease of use• Low cost• Minimal maintenance requirements

TYPES OF KERATOMETER

• B & L– Object size constant– Amount of doubling varied to produce the image

of fixed size

• Javal Schiotz– Amount of image doubling is constant– Measures the object size needed to produce an

image of fixed size

BAUSCH & LOMB (Reichert) KERATOMETER

Eye piece

Vertical Knob

Horizontal Knob

Focussing Knob

Chin Rest

Head Rest

Chin Height Knob

Keratometer height Knob

Lock

AP rotation axis scale

Patient’s view of B&L keratometer mire

Examiner’s view

• Most keratometers have two prismatic doubling systems• (one horizontal and one vertical)

Schiener’s Discs

PROCEDURE

• Focusing the eyepiece• Aligning the instrument• Positioning of the patient• Explaining the patient• Aligning and focusing the mires on cornea• Measurement of axis• Measurement of both curvatures

Oblique astigmatism

JAVAL-SCHIOTZ KERATOMETER

OPTICAL SYSTEM

Patient’s view of mires

Examiner’s view of the doubled mire image

Unapproximated mires Approximated mires

Horizontal meridian

Vertical meridian

Unapproximated mires

Approximated mires

Oblique Astigmatism

Unaligned mires Aligned but unapproximated mires

Approximated mires

KERATOMETRYKEY POINTS:

• Focus the eyepiece before beginning the measurement.

• Let the patient blink normally to keep the cornea smooth.

• Make sure the patient is comfortable while positioned at the instrument.

• Loosely lock the instrument to avoid accidentally misaligning it during the measurement.

• Keep the mires centered and focused at all times.

Calculation of Radius of Curvature

R = 2x h’/hR : radius of curvaturex : distance from object to focal pointh’ : image heighth : object height

Power Calculation

P = N2 – N1 R

In keratometers, N2 = 1.3375 (assumed R.I. of cornea) N1 = 1.000 (air)

P = 1.3375 – 1.000 = 0.3375 = 337.5 __ _ R (in mtrs) R (in mtrs) R (in millimetres)

RANGE OF KERATOMETRIC READING

• Dioptric Power : 36D to 52D• Radius of Curvature : 6.5mm to 9.38mm

• Can be extended upto :• Lower Limit : 30D (5.6mm) with -1.0D lens• Upper Limit : 61D (10.9mm) with +1.25D lens

• Objective method for determining curvature of the cornea.

• To estimate the amount and direction of corneal astigmatism

• The ocular biometery for the IOL power calculation

• To monitor pre and post surgical astigmatism.

• Differential diagnosis of axial versus refractive anisometropia.

• To diagnose and monitor keratoconus and other corneal diseases.

• For contact lens fitting by base curve selection

KLAP 9.05.2

Problems in Measurement• Measurement Problems:– Measures only central 3 mm of cornea– Corneal epithelial irregularity render defocussing– Very steep cornea: reading exceed range– Post-refractive surgery readings are inaccurate

KeratometryLimitations & Assumptions

o Calculations are based on the geometry of a spherical reflecting surface: the cornea is described as a prolate (flattening) ellipsoid (true apical radius steeper)

o Quantitative data are based on only four points within the central 3 millimeters of the cornea (gross qualitative indication of corneal regularity between them)

o The formula approximates the distance of image as the distance of focal point from the object

o Power in diopters depends on an assumed index of refraction

• Keratoscope: instrument that projects multiple concentric rings (mires) on the cornea

• Keratoscopy: direct visualization of the rings• Photokeratoscope: when a still camera is

added to photograph the mires• Videokeratoscope: when a video camera is

added

Need of Keratoscopy

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? •Most corneas are aspheric, flattening peripherally.

Keratoscopy samples a large area of the corneal surface

can assess asphericity and other surface variations

KERATOSCOPY - HISTORICAL

• 1870 : 1st clinical use : Placido : studied the corneal surface by

observing the shape of the concentric rings reflected off the

cornea

• 1880 : Javal : recognised importance the recording the image

photographically

• 1896 : Gullstrand : developed 1st Photokeratoscope

Placido Disc: the Original Corneal Topographer

Placido Disc: observer views the pattern of concentric white rings (mires) reflected from the patient’s cornea through a central +2 D lens.

Very “qualitative”

Images formed by Placido Disc

• Based on the overlay of concentric mires on the cornea.– The closer the mires, the steeper the axis.– The wider the rings, the flatter the axis.

Overlay of Mires

Safety PinFlieringa Ring

Barret Plastic LollypopCylindrical Keratoscope

Qualitative Methods of Keratoscopy

Maloney Conical Keratoscope Klein Hand-held

internally illuminated Keratoscope

Astigmatism control enforcer with

Applanation tonometer

Nidek Sun Photokeratoscope PKS-1000

Limitations of Placido Disc System

•It misses data on the central cornea

• It is only able to acquire limited data points

• It measures slope not height

• It is difficult to focus and align

• In most topographers, the patient is exposed to high light

• Computerized VideoKeratoscopy

• Capturing the keratoscopic details onto a video and displaying

data analysed with mutiple algorithms

• Measures larger area with more points

• Produce permanent reproducible records

• One of the most important developments in diagnostic

instrumentation

Present DayPDB Video-Keratoscopes

The Real Need – Analysing each & every point over

cornea

Types of Computerized Topographers

Basics of Computerized Corneal Topography

Sequence of events• Projecting Placido Disc Ring Pattern onto

patient’s cornea• Achieving centration of mires• Instantaneous capturing of reflected mires by

high resolution digital video-camera• 256 circumferential points on each ring are

identified• Analysis of each point is done and processed data

is displayed onto computer screen in various formats e.g. color maps

Key Points

• Avoid all eye drops, particularly local anaesthetics as they decrease TBUT

• Explain the patient & make comfortable• Ask patient to blink normally• Other contact procedures on cornea

(tonometry, A-scan) should be doneafter topography

Computerized Corneal TopographyIndications & Uses

• Preoperative and postoperative assessment of the refractive patient• Preoperative and postoperative assessment of penetrating keratoplasty• Irregular astigmatism• Corneal distrophies, bullous keratopathy• Keratoconus (diagnostic and follow-up)• Follow-up of corneal ulceration or abscess • Post-traumatic corneal scarring• Contact lens fitting• Evaluation of tear film quality• Reference instrument for IOL-implants to see the corneal difference

before and after surgery• To study unexplained low visual acuity after any surgical procedure

(trabeculectomy, extracapsular lens extraction, …).• Preoperative and postoperative assessment of Intacs™ corneal rings

(intrastromal corneal rings)

READING OF TOPOGRAPHICAL DATA• Check the name of the patient, date of exam and examined

eye.• type of measurement (height in microns, curvature in mm,

power in D)• Check the scale & step interval• study the map (type of map, form of abnormalities)• Evaluate statistical information• Compare with topography of the other eye• Compare with the previous maps

• Numeric power plots• Keratometry view• Photokeratoscopic view• Profile view• Colour-coded topographic maps– Most useful

NUMERIC POWER PLOTS• Corneal curvature showed in dioptre values• 10 concentric circular zones with 1mm interval• Also shows Value radius of curvature of each of the 10

concentric zones• Average overall corneal curvature also displayed

KERATOMETRIC VIEW• Depicts K-readings in 2 principle meridia in 3

different zones simultaneously.– Central : 3mm– Intermediate : 3-5 mm– Peripheral : 5-7 mm

• Important for assessingthe skewing of semi-meridia

PHOTOKERATOSCOPIC VIEW• Depicts actual black & white photograph of Placido

rings captured by video camera.• Helps in confirming proper patient fixation

PROFILE VIEW• Graphical plotting along the X-Y axis of the

steepest and flattest meridia and difference between the two.

• Grey zone denotes the pupillary area.– Symmetrical eye : straight line tracing– Asymmetrical eye : apparent slag seen

COLOUR-CODED TOPOGRAPHIC MAPS

• Most widely used• Most useful• Quick interpretation possible• User-friendly

Louisiana State University Color-Coded Map

1987 by Stephen Klyce

Interpretation of a colour map:

1. Colour Codes:– Hot colours: red-orange

– steep portions– Cool colours: blue-purple

– flat portions

2. The Scale used:– Absolute Scale: routine practice / screening

• 35-50D : each color = 1.5D interval• <35D or >50D : each color > 5D interval

– Normalized Scale: more minute details• 11 equal colours spanning ‘that’ eyes’ dioptric power

ABSOLUTE SCALE

RELATIVE SCALE

Corneal Topographic Patterns:• Depending on corneal curvature• Rabinowitz et al in 1996 described 10 different patterns:

• REGULAR PATTERNS :– Round– Oval– Steepening : Superior or Inferior

• ASTIGMATIC PATTERNS:– Symmetrical & Orthogonal : (Bow-Tie Effect)

• With or without skewed axis– Asymmetrical & Orthogonal:

• With superior steepening• With inferior steepening• Bow-tie with skewed radial axis

– Irregular : no pattern and non-orthogonal

Aym.Bow-Tie with skew

Round Oval Sup.Steep Inf.Steep

Sym.Bow-Tie Sym.Bow-Tie with Skew Asym.Bow-Tie with Inf.Steep

Asym.Bow-Tie with Sup.Steep

Formats for display of data on color maps:

1. Ring Verification: raw data

2. Corneal power map: (Axial)• Original & most stable and most commonly used map• 24-colour representation of dioptric power• Curvature measured 360 times for each placido ring image• Sagittal algorithm averages data from between rings• Evaluate overall characteristics and helps in classification

3. Tangential map: (Instantaneous Curvature Map)• Better geographical representation than axial map• Tangents are projected outwards from centre vertex 360 degree• Ring curvature measured along tangent• Best indicator of corneal shape >> ectatic conditions• Poor indicator of corneal power >> never calculate K values

Ring Verification Map Axial Dioptre Map

3D Reconstruction Map Tangential Dioptre Map


3. Elevation Map• Distinguishing localized elevations from otherwise steep corneal

area• They are difference measurements• “Red is Raised”, “Blue is Below”

4. Refractive Power Map• Takes into account spherical aberrations• Illustrates refraction of light in true dioptres• Useful in determining optical zone for RGP lenses and refractive

corneal surgery

5. 3D Reconstruction Map

Elliptical Elevation Map

Best Fit Sphere


5. Irregularity Map• Displays distortion of cornea using elevation map with toric

reference• Hotter colours represent higher value of distortion• Helps to quickly assess if cornea is causing poor VA

6. Trend & Time Display• Chronological display of changes

7. Difference Display Map8. OD/OS Compare Map9. Fourier Analysis :

• extract spherical, cylindrical, prismatic and irregular aberrations

Axial Dioptre Map in 2002 Axial Dioptre Map in 2003

Difference Dioptre Map

Other Overlays that can be added

• Pupil Margin

• Grids

• Optical Zone

• Eye Image

• Keratometric Mires

Other Software Application & Displays

• Multiple Display Option

• Keratoconus Pathfinder Application

• Contact Lens Fitting Application

Artefacts of Topography Map

• shadows on the cornea from large eyelashes or trichiasis

• ptosis or non-sufficient eye opening• irregularities of the tear film layer (dry eye, mucinous

film, greasy film)• too short working distance of the small Placido disk

cone

Normal Cornea

• wide spectrum of normality

• nasal cornea is flatter than temporal.

• physiological astigmatism of around 0.75 diopter.

• can take on many topographic patterns commonly:

– With the rule astigmatism : vertical bow-tie

– Against the rule astigmatism : horizontal bow-tie

• Enantiomorphism : mirror image

Normal Cornea

• Small changes in corneal shape do occur throughout life:

– In infancy the cornea is fairly spherical

– In childhood and adolescence, probably due to eyelid

pressure on a young tissue, cornea becomes slightly

astigmatic with-the-rule

– In the middle age, cornea tends to recover its sphericity

– Late in life, against-the-rule astigmatism tends to develop

• Provides evidence even before SLE can diagnose• Most sensitive method to distinguish:– True Early keratoconus– Asym Bow-tie or Inf. Steepening due to contact lens

warpage

• “Keratoconus Suspect” Patients:– Specially to diagnose & follow progression

Several Classifications

CLINICO-TOPOGRAPHIC :

1. Keratoconus:• One or more of clinical signs• Asymmetrical bow-tie with skewed radial axis pattern (AB/SRAX)

2. Early Keratoconus:• No Slit-lamp findings, but scissoring reflex on retinoscopy• AB/SRAX pattern

3. Keratoconus Suspect:• Only an AB/SRAX pattern

Aym.Bow-Tie with skew

Keratoconus Fruste• Called “form fruste” • 1st described by Amsler in 1937.• Extremely mild form of keratoconus• Central or para-central zone of irregular astigmatism of

unknown etiology.• The most striking hallmark - lack of progression

Several Classifications

CENTRAL CORNEAL POWER– Mild keratoconus : < 48D– Moderate keratoconus : 48 - 54D– Advanced keratoconus : >54D

• PACHYMETRY– Normal cornea : > 543 Microns– Early keratoconus : ~ 506 Microns– Moderate keratoconus : ~ 473 Microns– Advanced keratoconus : ~ 446 Microns

MORPHOLOGY OF ECTASIA Nipple - Shaped

Small, central ectasiaLess than 5.0mm

High WTR astigmatism360O normal peripheral cornea

Oval- ShapedVarying degree of Inferior mid-periphery steepening.Island of normal/flatter than normal cornea exactly

located 180O away .

Globus- ShapedAffects largest area.

All mires within the ectatic corneaNo island of normal mid-peripheral cornea.

Typical Topographic pattern of Keratoconus

• High central corneal power• Steeper inferior cornea (AB/SRAX – diagnostic value)• Large difference between the power of corneal apex and

corneal periphery• Often a disparity of the central powers between the two

corneas of a given patient• Typical pattern of progression of steepening - rotational

KISA% index for Keratoconus• Central K : descriptive of central steepening• I-S values: inferior-superior dioptric asymmetry• AST index : degree of regular corneal astigmatism (SimK1 – SimK2)• SRAX index : expression of irregular astigmatism• KISA% is product of all of the above:KISA% = (K) x (I-S) x (AST) x (SRAX) x 100

300

KISA% > 100% is keratoconusKISA% > 60 to 100% is Suspect

Humphrey Atlas Pathfinder Corneal Analysis System

• Corneal irregularity measurement (CIM):– Represent the irregularity of corneal surface

• Normal CIM: 0.3 to 0.60 microns• Borderline CIM: 0.61 to 1.0 microns• Abnormal CIM: 1.1 to 5.0 microns

• Shape factor (SF):– Represents the degree of corneal asphericity or eccentricity

• Normal Shape Factor: 0.13 to 0.35• Borderline Shape Factor: 0.02 to 0.12 and 0.36 to 0.46 • Abnormal Shape Factor: 0.47 to 1.0

• Mean toric corneal measurement (TKM):– Two values are calculated at the apex of the flattest meridian and

their mean determined. The mean value of apical curvature.• Normal TKM: 43.12 to 45.87D• Borderline TKM: 41.12 to 43.00 D. and 46.00 to 47.25 D.• Abnormal TKM: 36.00 to 41.75 D. and 47.37 to 60.00

A case of Unilateral Keratoconus (Right Eye)accurately diagnosed by Humphrey Pathfinder Analysis

CIM, SF, TKM : if values in green color code range : normal

Videokeratoscopic Pseudokeratoconus

• Contact Lens Wear• Technical errors• Dry spot formation• Early PMD• Previous ocular surgery

PRIMARY POSTERIAL CORNEAL ELEVATION

• Early presenting sign in keratoconus• Preoperative analysis of PPCE to detect a posterior corneal

bulge is important to avoid post LASIK keratectasia

Elevation MapPosteriorFloat

3D-reconstruction

PELLUCID MARGINAL DEGENERATION• Very steep contour in the peripheral peri-limbal cornea• High power radiating in towards the center from the inferior meridians• “Butterfly” or a "lazy C" or a “kissing pigeon” configuration• Area of flattening down the center of the cornea• High against-the-rule astigmatism.

Butterfly appearence

PELLUCID MARGINAL DEGENERATION

TERRIEN’S MARGINAL DEGENERATION

• prominent flattening of the corneal contour• High against-the-rule astigmatism

KERATOGLOBUS

CONTACT LENS WARPAGE• Harstein : 1st to note CL induced corneal changes

• WARPAGE: All CL induced changes in corneal topography, reversible or irreversible, that are not associated with corneal edema

• Signs & Symptoms:– Mostly asymptomatic– Changes in refraction and K readings over a period of time– Changes in curvature and distortion of mires– Central irregular astigmatism– Loss of normal progressive flattening from the center to the periphery

• Very commonly confused with keratoconus

• Topographical abnormalities classified as:• Central irregular astigmatism• Loss of radial symmetry• Reversal of normal topographic pattern• Keratoconus like images

• Inaccurate topography causes hazards in patients posted for LASIK– Soft CL causes:

• Topographic steepening (with keratoconus-like image)• Increased myopia• Central corneal thinning

– RGP CL causes: • Topographic flattening• Decreased myopia• Central corneal thinning.

CL Warpage – Special parameters

– Simulated Keratoscopic Readings;• Average powers of the steepest (SimK1) and the

flattest meridia (SimK2)

– Surface Asymmetry Index;• Centrally weighted sum of the differences in corneal

power between corresponding points on mires located 180º apart

– Surface Regularity index:• Calculated on the basis of the local regularity of the

surface over the corneal area within pupillary area.

CONTACT LENS WARPAGE

Contact Lens Fitting

Contact Lens Fitting in Keratoconus

• Superior Alignment Fitting Technique for Early Keratoconus• The Intra-Palpebral Three Point Touch Fitting Technique for

Early Keratoconus• Aspheric Lens Designs for Early Keratoconus

PENETRATING KERATOPLASTY

• Making decisions about trephination and graft size

• Identifying thin areas to be avoided in the graft-host junction

• Choosing a suturing technique

• Managing selective suture removal or adjustment

• Deciding on the need for a relaxing incision in astigmatism

• Correcting refractive errors by a excimer laser procedure

• Guide the post PKP fitting of a contact lens

Cataract Surgery• Preoperative Use:

– Most useful for IOL calculation in eyes with irregular surfaces– Evaluation of astigmatism, previous refractive surgery– Decision taking on type of surgery– Planning for site & type of incision– Has shown that smaller, temporal & scleral incision for phaco cause

less induced astigmatism• Intra-operative Use:

– to reduce surgically induced astigmatism– Wound closure– Application of sutures and adjustment

• Postoperative Use:– To identify tight sutures and adjust accordingly– Evaluating and managing Post-op refractive suprises– Determine causes of poor post-op vision

REFRACTIVE SURGERY

• Should be performed in every case Pre-op:– To develop a surgical / ablation profile– To detect pre-existing corneal abnormalities

• Post-op uses to evaluate:– Decentration– Multifocality– Regression– Induced astigmatism– Central islands

RADIAL KERATOTOMY (RK)

• Most useful in evaluating Post-RK problems:– Irregular astigmatism– Glare, halos (induced spherical aberrations)– Diurnal changes in refraction & vision (dumble-

shaped or split optical zones)– Multifocality due to regional change in curvature

with time

ASTIGMATIC KERATOTOMY (AK)

• Pre-op Evaluation of:– Astigmatism (specially asymmetric)– Calculating best position & configuration of relaxing

incision

• Post-op evaluation reveals:– Longer incision : more steepening of un-incised meridian– Incision closure to limbus: less flattening– Deeper incision : more effect

PHOTOREFRACTIVE KERATECTOMY

• Laser ablation of cornea to flatten/steepen cornea

• VKS used for evaluation of:– Ablation profile– Decentration– Regression and stabilization– Multifocality and induced aberration– Central islands diagnosis and follow-up

Myopic & hyperopic LASIK

Central island post LASIKDegraded laser optics

External hydration

Beam blockage by

photodisrupted tissue

Tends to resolve by

18months after surgery

PTERYGIUM

with-the-ruleastigmatism caused by localized flattening of the cornea central to the leading apex of the pterygium

CORNEAL ULCER

REGULAR ASTIGMATISMBow-tie pattern : most common pattern

(even 50 % of normal corneas exhibit it)Simulated K readings have good correlation with K readings

Bow Tie•Vertical•Horizontal

IRREGULAR ASTIGMATISM

• Rarely occurs naturally• Common causes:

• Dry eye• Corneal scars• Ectatic corneal degenerations• Pterygium• Trauma• Surgery (cataract surgery, PKP, and refractive surgery)

• It represents the remainder after subtracting sphere & cylinder from corneal power map

IRREGULAR ASTIGMATISMClassification:• With Defined Pattern

– Decentered Ablation: decentered myopic ablation in more than 1.5mm in central cornea

– Decentered Steep: decentered hyperopic ablation in more than1.5mm in central cornea

– Central Island: increase in central power of ablation zone at least 3D and 1.5mm surrounded by areas of lesser curvature

– Central Irregularity: more than one area of <1.0mm and <1D in central myopic ablation zone

– Peripheral Irregularity: similar to central island extending to periphery of ablation zone in one meridian

• With Undefined Pattern– More than one areas of >3.0mm in central 6mm cornea


– Decentered Ablation: decentered myopic ablation in more than 1.5mm in central cornea


– Decentered Steep: decentered hyperopic ablation in more than 1.5mm in central cornea


– Central Island: increase in central power of ablation zone at least 3D and 1.5mm surrounded by areas of lesser curvature


– Central Irregularity: more than one area of <1.0mm and <1D in central myopic ablation zone


– Peripheral Irregularity: similar to central island extending to periphery of ablation zone in one meridian

IRREGULAR ASTIGMATISMClassification:• With Undefined Pattern

– More than one areas of irregularity >3.0mm in central 6mm cornea

Scanning Slit Technology

• ORBSCAN

ORBSCAN

40 slit scanning (20 from each side)

Measurable parameters in ORBSCAN

• Projection based corneal topography• A grid of horizontal and vertical bars of light (0.2mm apart) is projected

onto the flourescein stained tear film• Pattern is directly observed and measured• Entire corneal, limbal and interpalpebral conjunctival surfaces• Can even measure epithelial defects• Defines elevation points (not curvature)• Produces a true topographic map (elevation map)

• Technique of lightwave interference• Interference fringes cover entire ocular surface• Includes : holography and moire’s fringe tachnique• Applies 3-dimensional imaging

Various Topographers available

Haag-Streit ®Keratograph CTK 922

EysSys

ASTRAMAX™ 3-D Stereo Topographer (Lasersight®)

Zeiss Humphrey Systems® ATLAS™

DICON® CT200

KERATRON™ Corneal Topographer

The Scheimpflug principle:It is a geometric rule that describes the orientation of the plane of focus of an

optical system (such as a camera) when the lens plane is not parallel to the image plane

• A ‘WAVEFRONT’ is a locus, or a line or a wave of point having the same phase

• Relates to light’s property of moving in a uni-directional manner through space

• Light waves emanate from a single point sourcein all directions as a sphere,and the line that connectsthe points upon the surfaceof this propagating wave iscalled a wavefront

• A lens can be used to change the shape of wavefronts. Here, plane wavefronts become spherical after going

through the lens.

Wavefront AberrationThe deviation of a wavefront in an optical system

from a desired perfect planar wavefront

Ab-erratio : going off track or deviation

Perfect Optical System

For any point P the output wavefront is a convergent

spherical wavefront

Real Optical System

For a object point corresponds several image points that form

together a blurred image

Aberration Freevs.

Aberration Affected Optical Systems

ABERROPIA

• a refractive error that results in a decrease in the visual quality that can be attributable to high order aberration

• Not caused by:– Lower order aberrations : myopia/hyperopia/astigmatism– Eye diseases : cornea, lens, retina

• Measured by Zernike Polynomials:– Complex methametical calculation

Need of AberrometryWavefront Technology

VARIOUS TYPES OF ABERROMETERS

Type 1 AberrometryHartmann Shack Sensor

Principle of the Hartmann-Shack aberrometer

Type 2 AberrometryTscherning aberrometer

Type 3 AberrometryIngoing Adjustable

Aberrometer

Type 4 AberrometrySlit Skiascopy

• Point Spread Function (PSF):– Gives an indication of what happens to a spot of light when it reaches

the retina– Expresses the effect of the aberration on the retinal image and

consequently on the quality of the image

• Root Mean Square (RMS):– Sq. Root of total aberration relative to the reference sphere– High value >0.3microns indicates Higher Order Aberrations (HOA)

If you can imagine light as a solid plane when it enters the eye, the Zernike polynomials illustrate how that flat plane is distorted by a specific aberration.

Lower Order Aberrations

Higher Order Aberrations

Bausch & LombZYWAVE Aberrometer

EYE TRACKING in LASIK

Thanks… (References as below)

corneal topography

Healthcare

peripheral cornea

power of corneal surface

cornea curvature central

corneal topography cornea

image doubling

zones of cornea

normal cornea range

cornea power calculation