Keratometry Flashcards
Why is keratometry done?
• To determine:
– Central corneal radii (mm) and power (D)
– Principle meridians of cornea
- To Differentiate between regular and irregular astigmatism (when axis are not at 90 degrees to each other).
- To Observe the quality of reflected mires (a mire is an image reflected off the cornea - if this image was very distorted then for example you would be able to work out that the patient has dry eye - this process checks the integrity of tear film)
- To Measure NIBUT (noninvasive break-up time)
- To determine the of BOZR (back optic zone radius) of a contact lens (for RGPs and SCLs)
Why would you choose to carry out a non-invasive tear break up test?
If you think px has e.g. dry eye then you know the tear film is disrupted. Therefore if you put fluorescein in the eye it would further disrupt the tear film giving you an inaccurate tear break up time. Thus you may choose to carry out a non-invasive tear break up test.
What is the definition of keratometry?
The measurement of the principle radii of the anterior surface of the cornea (tear film)
How does keratometry work (i.e. what’s the principle behind it)?
The cornea (tears) acts as a mirror since light is reflected from it!
– Therefore, we measure radius of curvature of this ‘mirror’ (i.e. tear layer)
– Keratometry uses first Purkinje as the image reference
– Size of the image depends on the size of the object (called mires), the radius of curvature (of the cornea) and the distance between mires and cornea
– The image is actually formed just within the cornea
What’s the maths behind the initial concept of keratometry?
What was the problem with this maths?
How have we adapted this?
Refer to image for the maths
We need to measure h’. In theory, if h’ could be read off against a scale on the eye piece graticule, the equation could then be used to find r, however, Because eye is never stationary, it is impossible to read measuring scale on the graticule.
We work out the height of h’ via projecting two images off the eye and seeing how much prism (i.e. how much you need to moth the images by - a measure of distance) you need to make both the images touch. This concept is called image doubling
How does Image doubling (prismatic deviation) work?
And how do keratometers use this concept?
How it works is that there is a disk (called a Scheiner disc) it has two pinholes in it through which light rays from the mire travel through and are reflected off the cornea, producing two images at different points. (The images are produced at different points due to the distance between the pinholes (and curvature of the cornea)).
- Some keratometers have fixed mire separation with variable image doubling (i.e. you are moving the prism until the images touch)
- Others have fixed doubling and variable mires (i.e. where there is a fixed prism and you are moving the mires until the images touch)
Can the keratometer that relies on varying prisms measure both meridians simultaneously?
Yes
Can the keratomter that relies on varying mires measure both meridians simultaneously?
NO
Can the one position keratometer measure both meridians simultaneously?
Yes - that’s how it gets its name - you only need to measure once and it does both meridians. It relies on the concept of varying prism.
What is the one position keratometer often referred to as?
Bausch & Lomb
How do you use the one position keratometer?
• Focus the eye piece
– According to examiner’s refractive error
– Similar to slit lamp, turn from positive to negative
– Main source of error if done incorrectly
– Should be done in the dark with white sheet of paper instead of px’s eye (i.e. Position white paper where eye should be (against head rest)
Now you are ready to set up the px (this can be done in two ways:
Method 1:
• Line up the instrument
– Adjust height of instrument and/or px’s chair
– Use marker for outer canthus
– Examiner shines light (pen torch) through eye piece
- Examiner will see light reflecting from cornea
- Px will see reflection of their own eye
- Instruct px to look at this
Method 2:
• Or use the foresight – Line up small cone on side of the instrument with the outer canthus.
You then want to focus the initial image:
Focus eye piece graticule (cross) First
- Focus image blurred and double (basically you wanna focus the circles if they are blurred you do this by moving keratometer towards or away from px, until focussing image is sharp and single)
- Move keratometer sideways, up or down so surrounding eyepiece graticule is now inside the bottom right circle.
Then align the crosses and minuses so the signs are on top of eachother.
[You can see the end point of all focussing in the image attached]
Take note though that the px will not always have corneal astigmatism at 90 and 180 degrees therefore you will have to rotate the machine until it looks like the image attached otherwise image will look kind of slanted.
Problem solving
Why may you see less than three mires in the keratometer?
Why may you see poor quality mires [look at image]?
Vertical palpebral aperture too small for measurement- basically instruct px to open their eyes wide
Basically due to an Unstable tear film;
NIBUT can be observed where mires distort/break up during test
– Epithelial distortion
– Corneal scar
– Irregular astigmatism
• Keratoconus, corneal graft, refractive surgery
In the two position keratometer if the the Px’s principles aren’t at 90 and 180 degrees what do you see through the keratometer and as a result what do you have to do?
How do you use the two position keratometer?
[Remember that the distance is fixed (similar to B&L keratometer), and the external dials are used to move the mires (B&L ext dials move the prisms)]
How do we read the two types of scales available on a keratometer?
