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 â 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 Ramsdenâs 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
Purkingeâs 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
â Object size constant
â Amount of doubling varied to produce the image
of fixed size
⢠Javal Schiotz
â Amount of image doubling is constant
â 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 hâ/h
R : radius of curvature
x : distance from object to focal point
hâ : image height
h : object height
42. Power Calculation
P = N2 â N1
R
In keratometers, N2 = 1.3375 (assumed R.I. of cornea)
N1 = 1.000 (air)
P = 1.3375 â 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 ââ=
45. Problems in Measurement
⢠Measurement Problems:
â Measures only central 3 mm of cornea
â Corneal epithelial irregularity render defocussing
â Very steep cornea: reading exceed range
â 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
?
?
?
?
?
?
?
?
? â˘
Most corneas are aspheric,
flattening peripherally.
Keratoscopy samples a large
area of the corneal surface
â 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 patientâs cornea
through a central +2 D lens.
Very âqualitativeâ
51. Images formed by Placido Disc
⢠Based on the overlay of concentric mires on
the cornea.
â The closer the mires, the steeper the axis.
â The wider the rings, the flatter the axis.
62. Limitations of Placido Disc System
â˘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
patientâs 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, âŚ).
⢠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.
â Central : 3mm
â Intermediate : 3-5 mm
â 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.
â Symmetrical eye : straight line tracing
â 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:
â Hot colours: red-orange
â steep portions
â Cool colours: blue-purple
â flat portions
1. The Scale used:
â Absolute Scale: routine practice / screening
⢠35-50D : each color = 1.5D interval
⢠<35D or >50D : each color > 5D interval
â Normalized Scale: more minute details
⢠11 equal colours spanning âthatâ eyesâ dioptric power
84. Corneal Topographic Patterns:
⢠Depending on corneal curvature
⢠Rabinowitz et al in 1996 described 10 different patterns:
⢠REGULAR PATTERNS :
â Round
â Oval
â Steepening : Superior or Inferior
⢠ASTIGMATIC PATTERNS:
â Symmetrical & Orthogonal : (Bow-Tie Effect)
⢠With or without skewed axis
â Asymmetrical & Orthogonal:
⢠With superior steepening
⢠With inferior steepening
⢠Bow-tie with skewed radial axis
â 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
⢠âRed is Raisedâ, âBlue is Belowâ
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:
â With the rule astigmatism : vertical bow-tie
â Against the rule astigmatism : horizontal bow-tie
⢠Enantiomorphism : mirror image
104. Normal Cornea
⢠Small changes in corneal shape do occur throughout life:
â In infancy the cornea is fairly spherical
â In childhood and adolescence, probably due to eyelid
pressure on a young tissue, cornea becomes slightly
astigmatic with-the-rule
â In the middle age, cornea tends to recover its sphericity
â 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:
â True Early keratoconus
â Asym Bow-tie or Inf. Steepening due to contact lens
warpage
⢠âKeratoconus Suspectâ Patients:
â 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 âform frusteâ
⢠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 â 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 â 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):
â 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):
â 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):
â 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
⢠âButterflyâ or a "lazy C" or a âkissing pigeonâ 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:
â Mostly asymptomatic
â Changes in refraction and K readings over a period of time
â Changes in curvature and distortion of mires
â Central irregular astigmatism
â 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 â Special parameters
â Simulated Keratoscopic Readings;
⢠Average powers of the steepest (SimK1) and the
flattest meridia (SimK2)
â Surface Asymmetry Index;
⢠Centrally weighted sum of the differences in corneal
power between corresponding points on mires located
180Âş apart
â 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:
â Most useful for IOL calculation in eyes with irregular surfaces
â Evaluation of astigmatism, previous refractive surgery
â Decision taking on type of surgery
â Planning for site & type of incision
â Has shown that smaller, temporal & scleral incision for phaco cause
less induced astigmatism
⢠Intra-operative Use:
â to reduce surgically induced astigmatism
â Wound closure
â Application of sutures and adjustment
⢠Postoperative Use:
â To identify tight sutures and adjust accordingly
â Evaluating and managing Post-op refractive suprises
â Determine causes of poor post-op vision
135. REFRACTIVE SURGERY
⢠Should be performed in every case Pre-op:
â To develop a surgical / ablation profile
â To detect pre-existing corneal abnormalities
⢠Post-op uses to evaluate:
â Decentration
â Multifocality
â Regression
â Induced astigmatism
â Central islands
136. RADIAL KERATOTOMY (RK)
⢠Most useful in evaluating Post-RK problems:
â Irregular astigmatism
â Glare, halos (induced spherical aberrations)
â Diurnal changes in refraction & vision (dumble-
shaped or split optical zones)
â Multifocality due to regional change in curvature
with time
137. ASTIGMATIC KERATOTOMY (AK)
⢠Pre-op Evaluation of:
â Astigmatism (specially asymmetric)
â Calculating best position & configuration of relaxing
incision
⢠Post-op evaluation reveals:
â Longer incision : more steepening of un-incised meridian
â Incision closure to limbus: less flattening
â Deeper incision : more effect
138.
139. PHOTOREFRACTIVE KERATECTOMY
⢠Laser ablation of cornea to flatten/steepen
cornea
⢠VKS used for evaluation of:
â Ablation profile
â Decentration
â Regression and stabilization
â Multifocality and induced aberration
â Central islands diagnosis and follow-up
141. Central island post LASIK
ď§Degraded laser optics
ď§External hydration
ď§Beam blockage by
photodisrupted tissue
ď§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
â˘Vertical
â˘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
â Decentered Ablation: decentered myopic ablation in more than 1.5mm in
central cornea
â Decentered Steep: decentered hyperopic ablation in more than1.5mm in
central cornea
â Central Island: increase in central power of ablation zone at least 3D and
1.5mm surrounded by areas of lesser curvature
â Central Irregularity: more than one area of <1.0mm and <1D in central
myopic ablation zone
â Peripheral Irregularity: similar to central island extending to periphery of
ablation zone in one meridian
⢠With Undefined Pattern
â More than one areas of >3.0mm in central 6mm cornea
149. IRREGULAR ASTIGMATISM
Classification:
⢠With Defined Pattern
â 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 moireâs 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 âWAVEFRONTâ is a locus, or a line or a wave of point
having the same phase
⢠Relates to lightâs 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:
â Lower order aberrations : myopia/hyperopia/astigmatism
â Eye diseases : cornea, lens, retina
⢠Measured by Zernike Polynomials:
â Complex methametical calculation
180. ⢠Point Spread Function (PSF):
â Gives an indication of what happens to a spot of light when it reaches
the retina
â Expresses the effect of the aberration on the retinal image and
consequently on the quality of the image
⢠Root Mean Square (RMS):
â Sq. Root of total aberration relative to the reference sphere
â 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