2. • 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
3. ZONES OF CORNEA
3-4 mm
7-8 mm
11 mm
12 mm
Central
Optical
Limbal
Peripheral
Transitional
Paracentral
Mid-peripheral
Central Optical
Paracentral
Peripheral
Limbal
5. The range of powers found in the normal
cornea range from 39 D found at peripheral cornea,
close to the limbus, to 48 D found at corneal apex.
6. Cornea - Power calculation
P = N2 – N1
R
P : Power of corneal surface
N1 : Refractive Index of 1st
medium
N2 : Refractive Index of 2nd
medium
R : Radius of Curvature in metres
12. 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
13. 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
14.
15.
16. 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
17. 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
25. 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
39. KERATOMETRY
KEY 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.
40.
41. Calculation of Radius of Curvature
R = 2x h’/h
R : radius of curvature
x : distance from object to focal point
h’ : image height
h : object height
42. 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)
43. 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
44. • 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 −−=
45. 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
46. Keratometry
Limitations & 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
47. • 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
48. Need of Keratoscopy
?
?
?
?
?
?
?
?
? •
Most corneas are aspheric,
flattening peripherally.
Keratoscopy samples a large
area of the corneal surface
→ can assess asphericity and
other surface variations
49. 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
50. 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”
51. 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.
62. 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
63. • 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
68. 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
69. 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 done
after topography
70. Computerized Corneal Topography
Indications & 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)
71. 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
74. 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
75. 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 assessing
the skewing of semi-meridia
76. PHOTOKERATOSCOPIC VIEW
• Depicts actual black & white photograph of Placido
rings captured by video camera.
• Helps in confirming proper patient fixation
77. 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
78. COLOUR-CODED
TOPOGRAPHIC MAPS
• Most widely used
• Most useful
• Quick interpretation possible
• User-friendly
Louisiana State University Color-Coded Map
1987 by Stephen Klyce
79. Interpretation of a colour map:
1. Colour Codes:
– Hot colours: red-orange
– steep portions
– Cool colours: blue-purple
– flat portions
1. 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
84. 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
85. 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
86. 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
1. 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
87.
88.
89. Ring Verification Map Axial Dioptre Map
3D Reconstruction Map Tangential Dioptre Map
90. Formats for display of data on color maps:
3. Elevation Map
• Distinguishing localized elevations from otherwise steep corneal
area
• They are difference measurements
• “Red is Raised”, “Blue is Below”
3. 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
92. Formats for display of data on color maps:
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
5. Trend & Time Display
• Chronological display of changes
5. Difference Display Map
6. OD/OS Compare Map
7. Fourier Analysis :
• extract spherical, cylindrical, prismatic and irregular aberrations
93.
94. Axial Dioptre Map in 2002 Axial Dioptre Map in 2003
Difference Dioptre Map
95.
96. Other Overlays that can be added
• Pupil Margin
• Grids
• Optical Zone
• Eye Image
• Keratometric Mires
99. 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
100.
101.
102.
103. 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
104. 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
105.
106.
107. • 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
108. Several Classifications
CLINICO-TOPOGRAPHIC :
1. Keratoconus:
• One or more of clinical signs
• Asymmetrical bow-tie with skewed radial axis pattern (AB/SRAX)
1. Early Keratoconus:
• No Slit-lamp findings, but scissoring reflex on retinoscopy
• AB/SRAX pattern
1. Keratoconus Suspect:
• Only an AB/SRAX pattern
Aym.Bow-Tie with skew
109. 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
111. MORPHOLOGY OF ECTASIA
Nipple - Shaped
Small, central ectasia
Less than 5.0mm
High WTR astigmatism
360O
normal peripheral cornea
Oval- Shaped
Varying degree of Inferior mid-periphery steepening.
Island of normal/flatter than normal cornea exactly
located 180O
away .
Globus- Shaped
Affects largest area.
All mires within the ectatic cornea
No island of normal mid-peripheral cornea.
112. 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
113.
114.
115. 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 keratoconus
KISA% > 60 to 100% is Suspect
116. 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
117. A case of Unilateral Keratoconus (Right Eye)
accurately diagnosed by Humphrey Pathfinder Analysis
CIM, SF, TKM : if values in green color code range : normal
119. 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 Map
Posterior
Float
3D-reconstruction
120. 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.
124. 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
125. • Topographical abnormalities classified as:
• Central irregular astigmatism
• Loss of radial symmetry
• Reversal of normal topographic pattern
• Keratoconus like images
127. 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.
132. 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
133. 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
134. 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
135. 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
136. 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
137. 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
138.
139. 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
141. Central island post LASIK
Degraded laser optics
External hydration
Beam blockage by
photodisrupted tissue
Tends to resolve by
18months after surgery
144. REGULAR ASTIGMATISM
Bow-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
145. 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
146. IRREGULAR ASTIGMATISM
Classification:
• 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
149. IRREGULAR ASTIGMATISM
Classification:
• With Defined Pattern
– Central Island: increase in central power of ablation zone at least 3D and
1.5mm surrounded by areas of lesser curvature
159. • 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)
160. • Technique of lightwave interference
• Interference fringes cover entire ocular surface
• Includes : holography and moire’s fringe tachnique
• Applies 3-dimensional imaging
165. 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
166.
167.
168. • 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 source
in all directions as a sphere,
and the line that connects
the points upon the surface
of this propagating wave is
called a wavefront
169. • A lens can be used to change the shape of wavefronts.
Here, plane wavefronts become spherical after going
through the lens.
170. Wavefront Aberration
The deviation of a wavefront in an optical system
from a desired perfect planar wavefront
Ab-erratio : going off track or deviation
171. 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 Free
vs.
Aberration Affected Optical Systems
172. 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
180. • 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)
181. 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