corneal topography- basics

2.  Cornea is the most powerful refractive
element of the eye contributing 43D(70%) of
refractive power of eye.
 Since the shape of corneal surface
determines its refractive power , even a
minor modification of its surface can lead to
significant alteration of image formed on
retina.

3.  Prolate shape
 Anterior surface of cornea: Elliptical
 Horizontal dia: 11.5mm
 Vertical dia:10.6mm
 Posterior surface of cornea is circular with
average diameter of 11.5mm
 Thickness of cornea:
 0.52mm at centre
 0.8mm at periphery
 1mm at limbus
 Anterior radius of curvature:7.8mm
 Posterior radius of curvature:6.5mm

4.  CENTRAL ZONE: Approximately 4mm
diameter.
 Also called the apical zone
 PARACENTRAL ZONE: 4-8mm diameter.
Flatter than the central zone.
 Central + Paracentral zone= optical zone.
 PERIPHERAL ZONE: 8-11 mm. It is the zone
where the normal cornea flattens the most
and becomes ashperic.
 LIMBAL ZONE

5.  Corneal topography refers to study of the
shape of corneal surface

6.  Placido based
 Elevation based
 Interferometric
 Rasterstereography
 Schiempflug imaging

7.  Devised by Helmholtz,who originally used the
term ophthalmometer
 Calculations are based on the geometry of a
spherical reflecting surface
 Based on fact that anterior surface of cornea
acts as convex mirror and size of image
varies with curvature
 Helps to measure the radius of curvature of
anterior corneal surface from 4 reflected
points within central 3mm of cornea

8.  Helmholtz keratometer
 Bausch and lomb keratometer
 Javal schiotz keratometer

9.  I/O=v/u
 Since image is formed near F so
v=F=r/2
therefore r can be calculated as
2uI/O
Thus for known object size ,
measurement of image size will
help determine radius of
curvature.
In all keratometers u is constant
being focal distance of viewing
telescope

10.  To overcome the natural movements of
patients eye for accurate measurements, the
principle of image doubling is used.
 The readings are obtained by aligning the
images with one another

11.  Fixed object size
 Image size is
adjusted to
measure corneal
curvature
 Uses rotationg
glass plates to
obtain doubling of
images

12.  Also based on
principle of
constant object
size and variable
image size

14.  Based on principle
of variable object
and constant
image size.
 Doubling of image
is achieved by
wollaston prism

15.  Assumes cornea to be spherical
 Details of only central 3mm ignoring
peripheral corneal zones
 Loses accuracy when measuring very steep or
flat corneas
 Small corneal irregularities preclude the use
due to irregular astigmatism.

17.  consists of equally
spaced alternating
black and white
rings with a hole in
the centre to
observe patients
cornea.
 Utilizes corneal
reflections
(purkinje image )
of bright rings

18.  The steeper the
cornea the closer
reflected rings of
placido disc lie to
one another.

19.  DISADVANTAGE:
1. small degree of abnormality of corneal
shape are not easily identifiable.
2. Cannot be used in corneas with epithelial
defects and stromal ulcers etc because of
nonreflection of image by the cornea.
3. Anatomy of nose and orbit may limit field
size and restrict the corneal area that can
be examined

20.  A photographic film camera attached to a
keratoscope.
 Current photokeratoscopes(eg Nidek PKS
1000 or keracorneoscope) have 9-15 rings
which cover 55-75% of corneal surface.
 Closer the line steeper is the corneal surface
and the farther apart the lines flatter is the
corneal surface.
 Corneal cylinders of upto 3D can escape
detection by use of photokeratoscopy.

21.  Scanning slit
topography system.
 40 scanning slit
beams(20 from left
and 20 from right)
to scan the cornea
 Each of the 40 slit
images triangulates
one slice of ocular
surface.
 Total duration of
examination 1.5sec

22. Orbscan II :the placido disc added in orbscan 1

23.  Computer calculates
a hypothetical
sphere that matches
as close as possible
to to actual corneal
shape being
measured.
 Areas above sphere
shown in warm
colours and areas
below in blue colour

24.  The
Pentacam(Oculus
Inc) obtains
images of anterior
segment by
rotating
Schiempflug
camera which is a
digital charged
coupling device.

25.  Takes 50 meridonial sections through centre
of cornea which allows the system to realign
the central thinnest point of each section
before it reconstructs the corneal image.
Thus it eliminates any eye movement
occurring during the examination.
 Enables measurement of corneas with severe
irregularities such as keratoconus that may
not be amenable to placido imaging
 Enables calculation of pachymetry from
limbus to limbus

26.  It projects a calibrated grid onto fluroscein
stained tear film, takes photographs and uses
computer assisted algorithms to analyse the
pictures.
 The accuracy of the system is 0.3D for
diameter of 7mm.

27.  Uses the technique of lightwave interfernce.
 The interference fringes can cover the entire
anterior ocular surface not just cornea.
 Not in widespread clinical use.

29.  GREEN: near
normal power
 BLUE and its
shade: lower than
normal power
 RED and its shade:
higher than normal
power

30.  NORMALISED SCALE(VARIABLE SCALE):
 Different colour scales assigned to each map
 Instrument identifies actual minimal and
maximal keratometric diopteric value of
particular cornea
 Gives more detailed description

31.  ABSOLUTE SCALE(FIXED SCALE)
 Preset colour scale with same diopteric steps
 Minimum and maximum assigned to same
colours
 Larger increment in steps(0.5D) so may miss
subtle changes

32.  AXIAL MAP(SAGGITAL
MAP): original and most
commonly used map.
 Measures radius of
curvature for a
comparable sphere with
centre of rotation on
the axis of
videokeratoscope.
 Localised changes in
curvature and
peripheral changes are
poorly represented.

33.  LOCAL TANGENTIAL
CURVATURE
MAP(INSTANTANEOUS
MAP): it displays
tangential radius of
curvature or tangential
power which is
calculated by referring
to a neighbouring point
and not to axis of
videokeratoscope.
 Reflects local changes
and peripheral data
better than axial map

34.  REFRACTIVE MAP: displays refractive power
of cornea.
 ELEVATION MAP: displays corneal elevation
relative to a reference plane.
 DIFFERENCE MAP: displays changes in
certain values between 2 maps.
 RELATIVE MAP: displays some values by
comparing them to an arbitrary standard..

35.  Simulated keratometry(Sim K) :maximum
power of surface along any axis and the
power orthogonal to that axis(Sim K1 and Sim
K2)
 Surface Regularity Index(SRI): measure
regularity in central 4.5mm
 Zero for smooth surface, increases with
increasing astigmatism
 Surface Asymmetry Index(SAI):weighted
summation of differences in corneal power
between corresponding points 180 degrees
apart

36.  Asphericity: described quantitavely by the Q
value
 Q=0 for sphere
 Q<0 for prolate surface
 Q>0 for oblate surface
 Normal cornea has Q value of -0.26

38.  Anterior Float(Elevation Best Sphere):
 Anterior best fit sphere is calculated to best
match the anterior corneal surface.
 Elevation BFS map subtracts the calculated
BFS against the eye surface.
 Posterior Float(Elevation Best Sphere Map):
describes the back surface of the cornea in
the same manner.
 Keratometric(mean Power) Map:displays
refractive power of anterior surface of cornea
 Thickness (Pachymetry )map: displays
corneal thickness
 Warm colour indicates thinner cornea

39.  Keratometric
reading
 White to white
distance in mm
 Thinnest point of
cornea
 Angle kappa
readings
 Irregularity within
central 3mm and
5mm

40.  Normal cornea flattens progressively from
centre to periphery, nasal area flattening
more than temporal area
 Approximate distribution of keratographic
pattern varies as
 Round(23%)
 Oval(21%)
 Symmetric bow tie typical for regular
astigmatism(18%)
 Asymmetric bow tie(32%)
 Irregular(7%)

42.  Typically there is inferior area of steepening.
 Area of increased power surrounded by
concentric areas of decreased power
 An inferior superior power asymmetry
 A skewing of the steepest radial axis above
and below the horizontal meridian

43.  Keratoconus Predictability Index(KPI):
derived from 8 topographic indices.
KPI of >0.23 is indicative of keratoconus
KISA % index:
product of central K reading, I-S value,
astigmatism measured by Sim K value and
Skewed radial axis index(SRAX).
60%-or more is suspect
100% diagnostic

44.  RABINOWITZ DIAGNOSTIC CRITERIA :
 Consists of 3 topography derived indices
1.Keratometry value of 47.2 D or greater in the
central cornea
2. Inferior –Superior asymmetry value –
1.4D-1.9D is suggestive and >1.9D is
diagnostic
3. Posterior float elevation >40 micro m
suggestive of posterior ectasia

46.  Classical butterfly
appearance
 Against the rule
astigmatism in
early stage
 Thinning is
concentric to the
limbus generally
between 4 to 8 O
clock position

47.  Flattening over areas of peripheral
thinning(mostly superior and inferior cornea)
 High against the rule or oblique astigmatism
is a common feature

48.  Topographic
pattern with a
polygonal shape is
most common
pattern
 Depending on
number of
incisions made
square , hexagons
or octagons can be
seen

49.  Characterised by topograhic changes in
cornea following contact lens wear as a
result of mechanical pressure exerted by lens
 Usually 4 different form which occur alone or
with one another
i.Peripheral steepening
ii. Central flattening
iii. Furrow depression
iv. Central moulding

51.  To guide removal of tight sutures after
corneal surgery
 Guide contact lens fitting
 Evaluate effect of keratographic procedure