Corneal Topography Flashcards
What is the corneal cap?
find that centrally
also known as optic cap or apical zone
-spherical zone
What is the size of the corneal cap ?
4mm in diamter
Where is the corneal cap decentered ?
- 2- 0.6mm nasally
0. 2mm superiorly
What is the mid region of the cornea ?
paracentral region (or mid peripheral region) of greatest flattening -it spans from 4-8mm from the centre of the cornea
What is the peripheral region of the cornea?
8-11mm from the centre
-the greatest asphericity in the nasal and superior nasal areas
What are the 3 regions of the cornea?
corneal cap
paracentral/mid peripheral region
peripheral region
Why is knowledge of corneal curvature important ?
an aid in determining the intial cl to be placed on the eye esp in fittings of rigid cl fittings
What can an indication of rapid changes of curvature ?
can be indicative of a compromised cornea and can aid in the diagnosis of Keratconus
What is the measurement of the radius of curvature of the cornea based on ?
the fact that the front surface of the cornea acts as a convex mirror
What is useful to measure in determining the central radius of curvature of cornea ?
measure peripheral radius values - esp in complicated conditions e.g post refractive surgery
How can conventional keratometers be adapted ?
using a peripheral fixation point
Why can keratometers not be used to determine corneal curvatures accurately ?
- if the surface being measured does not have a constant radius of curvature
- not radially symmetrical
So, what has been developed in order to determine corneal curvatures ?
-dedicated instruments using other technologies have been developed to measure overall corneal topographer
What does the keratometre measure ?
measures the curvature along a fixed chord length (2-3mm) lies in the optical spherical zone of the cornea- which is much smaller than the actual corneal diameter (11-12mm)
-should not assume that we can determine the peripheral curvature from this since the cornea flattens from the mid periphery zone outwards
What is a different method to visualise corneal shapes ?
corneal topography
What is the keratometry measurement ?
-Measurement of the anterior curvature
of the cornea in 2 principal meridians
-3mm
What is the keratoscopy / corneal topography measurement ?
-Study of light reflected from the anterior
surface of the cornea ( in 360 degrees) using corneal topography
-8-10mm
What is a limitation of the corneal topography map instrument ?
- it does not cover the whole of the cornea- what does it mean of an average corneal RGP lens - which is 9.5mm in diameter- this instrument does not provide a complete corneal shape analysis over the area which the lens will cover (so the lens is 9.5/10mm in diameter, in the pic the chord is smaller than that)- also in the actual image of the eye- some of these rings are incomplete
- these are missing data points
What are the missing data points due to ?
they are due to a shadow caused by the nose of patient- most software algorithms will fit in any gaps- so the curvature map is able to show a completed circle- so a completed map over the full length of the chord- you can recognise these data point by the small black dots that are in the coloured map imaged.
What are other artefacts caused by ?
the eyelids or the shadow created by the eyebrows, irregular tear film layers, or ruptures (break up to time) which is too short or corneal scars
What is corneal topography ?
also known as keratoscopy
What is the aim of corneal topography / keratoscopy ?
To accurately describe the shape of the corneal surface in all meridians
What is the technique used in corneal topography /keratoscopy ?
- similar to keratometry
- determines the size of the image of a target reflected in the corneal surface
- a series of circular concentric targets are used - placedo disc image - this arrangement allows both central and peripheral curvatures to be measured
What is video keratoscopy ?
-capture the image on a computer - use a sophisticated image processing software to provide immediate analysis of the reflected image
What does this technique shows in results?
-that the cornea is aspheric
- P-value between 0.75 and 0.85 for healthy cornea
(although variable)
-the rate of flattening is asymmetrical at its centre-
- kind of like a shape of an egg
-the corneal cap is almost spherical and flattens towards periphery
What is the first kind of assessment for a topography map ?
-placido disc is the
qualitative assessment
-a series of concentric black and white rings on a flat circular disc with a central sight hole - WHICH Is called a placido disc.
Where is the placido disc placed ?
- infront of the cornea
- and the reflections are observed
What does it mean if the reflected signs are closer to each other ?
the corneal curvature is steeper
What does it mean if the reflected signs are wider apart from each other ?
indicate flatter corneal curvatures
What is the qualitative assessment for astigmatism (placido disc) ?
rings become oval
-these are stretched over the horizontal axis - WTR or stretched over the vertical axis - ATR
What are the quantitive assessments of these corneal topographers ?
-utilise the same principipal of projecting a grid on the cornea but a camera will pick up the reflected rings from the placido discs from the cornea which is done by the law of reflection which they send back into the camera and it is then analysed and turned into numbers
What is the most frequently used computational approach ?
slope of surface method
What is the slope of surface method ?
to express the radius of curvature from these ring Myers
What does the slope of surface method do ?
-devices that use this technology measure slope directly as a function of distance from a central reference axis and derive a curvature form these results
What do these distance based instruments do ?
only estimating the average shape of the cornea since the algorithms are based on radially symmetrical surface.
- these measurements tend to result in an under estimation of radius fo curvatures in areas that may be steeper than the central cornea and over estimate in areas that are flatter
- the algorithms are now modified and now based on radius of curvature in an attempt to provide a better estimate of the shape of the cornea
What is the cone topographer?
cone comes close to eye
limited to px with deep set eyes
-image is captured with a video camera
-a computational approach is adopted to analyse the data and derive a description of cornea shape
Why is the choice of computational method important in cone topographer ?
- this will largely dictate the accuracy and validity of the keratascope
What is the Aladdin byometer?
large cone topographer / aslo an autorefractor (also measures axial length)
- maps are full of data
- look complicated
What are power curvature maps?
graphical visualisation of the corneal shape
What do the cooler colours show ?
-slower rate of change
What do the hotter colours show?
-faster /sharper rates of change- steeper meridian
What is absolute scaling option ?
absolute - the values that these colours represent are fixed values for dioptric step, range and with reference colour set to average fro normal populations (usually 43D and 43.5D) - they are fixed between eyes and patients
What is relative scaling option ?
adjusted to the patient and the eye
- the average point if green
- step sizes are the same as the absolute scale
- (reference colour set to mean curvature of cornea being measured
What is auto-scaling option ?
done by the software the maximise the use fo the scale
- step size may be small even to the point that the differences in colours are insignifcant
e. g in a eye with a range of 1D between the steepest AND flattest point each colour step represents 1/26 colours= 0.04D - in a eye with a range of 7D of corneal astigmatism (range of 7D between the steepest and flattest point ) each of those colours represent 7/26 colours= 1/4 of D - so the maps will look exactly the same but depending on the value of each colour will actually tell you what the difference is
What is a axial curvature map?
- it is also known as a sagittal, colour or power map
- there is point of val;eu- wherever the cross is located in the centre - that will give you the radius in D and mm on the right hand side
- the simulated k value - the are also in Dioptres and MM
- look at slide show 19;57
What is the advantage of the axial map ?
smooth out the data and highlight the general direction of astigmatism or other corneal observations
What are elevation maps ?
these show variations. from a reference shape set by the software
What are elevation maps useful for ?
assessing the regularity of astigmatism and location of it
- kerataconus
- used for planning corneal surgeries
- the reference points is alway the centre of the cornea
What do red colours show in elevation maps ?
show steeper areas or areas with more saggital height vs the reference shape
What do cooler/blue colours show in elevation maps ?
shows a flattening or decreased saggital height vs the reference shape
what does the elevation map show in the slide show 18?
shows the variation from a best fit sphere, the central cornea rises above it and the mid periphery drops below it
When are we using topography ?
- Fitting (specialty) contact lenses and assessing the fit, as well as problem shooting
- Assessing corneal pathologies, and progression over time and see how treatment is going
- Planning refractive surgery such as LASIK
- Planning cataract surgery and IOL implants
What happens when you fit a RGP lens on the eye e.g?
add fluroscene to the tears to visualise the tear film behind the CL
- then assess the fluroscene pattern
- where the cornea is steeper than the cl more tears will pool behind the lens to fill the gap between the cornea and RGP lens- this is the reason why a spherical RGP can correct up to 2.5D corneal astigmatism because it will u-p the gap in the steepest meridian with the px tears
- in the flattest meridian- it will touch the cornea- and that shown in the black areas along that meridian in the far periphery
Why is it good to see these fluroscene patterns before you fit the CL in ?
-by using simulated fluroscene patterns- you will be able to fit these lenses more successfully at first attempt- will expect what you will see to find with these lenses before you actually fit them
What is another example of corneal astigmatism using NIH deck topographer ?
this bow tie pattern shoes steeper in horizontal
-while flatter in vertical - ATR (
What is found in keratacconus?
in a meridian map - typical steepend area in the inferior nasal area of cornea
- looks like an uneven bow tie
- when keratacconus gets worse - the steepening disappears and shows into a circle in the inferior nasal area - hot colours represent classic inferio-nasal steepening
Why does the cone looks larger in the axial map compared to the curvature map for keratacconus (slide 22)
due to the averaging values the effect of steeping becomes more visible .
What is another way you can diagnose keratacconus ?
-shows a scissor reflex during retinoscopy
What is an advantage of the topographer ?
you can track changes over time
What is another clinical example - orthokeratology ?
where a px wears a rigid lens overnight so they sleep with that lens and dont need specs during the day
- this is achieved by flattening the centre of the cornea buy re-distrubutiing the corneal epithelial cells towards the mid periphery
- this temporarily changes the shape of the cornea and as a result the shape of the eye
What can softwares show?
oxygen transmissibility of soft lenses
- an intact tear film and good oxygen supply to cornea are good for comfortable lens wear
- the oximap of topographer displace the oxygen transmissibility of soft lenses in different colours
