list 5 reasons for carrying out keratometry on a px
- the determination of:
- central corneal radii (mm) and power (D)
- principle meridians of cornea - differentiate between regular and irregular astigmatism
- observe quality of reflected mires
- measure non-invasive tear break up time
- measurement of BOZR of contact lenses
what does a difference in the principle meridians of the cornea cause
corneal astigmatism
what is the difference between regular astigmatism and irregular astigmatism
regular = either 90 or 180 is the principle meridians irregular = oblique astigmatism not 90 or 180
how is a reflection of the mires formed
the keratometer sends an object of the mires to the tear film which reflects it back
how do you observe non-invasive tear break up time with a keratometer
the mire which is usually a circle, isn’t a circle anymore but is distorted when the tears break up
if you ask the patient to blink and the tears don’t become better = a problem with the cornea e.g. keratoconous
as the tear film follows the corneal curvature exactly
what is the definition of keratometry
the measurement of the principle radii of the anterior surface of the cornea (tear film)
i.e. the two principle meridians
what is the principle of keratometry
the cornea (tears) acts as a mirror since light is reflected from it
- therefore we measure the radius of curvature of this ‘mirror’
- uses first purkinje image (as it hits the anterior cornea i.e. the part we are measuring)
- size of the image depends on: the size of the object (mires), the radius of curvature (of the cornea) and the distance between the mires and the cornea
- the image is actually formed just within the cornea
how does the size of the image reflect back from the tears on the cornea, represent the curvature of the cornea
because the keratometer sends a certain size of the object (mires) to the eye, of a certain distance, and you can measure the height of the reflection coming back = curvature of the cornea
what does the size of the image reflected back from the keratometer mires depend on
the size of the object (mires), the radius of curvature (of the cornea) and the distance between the mires and the cornea which are all fixed
the tears on the cornea reflects this image, giving a image with a virtual height
what is the approximate keratometry equation
r = 2(h’/h)d
r - curvature
h - object height
h’ - image height
d - distance between object and image
what is the theoretical way of using the keratometry equation to find r and why is this just a theory
if h’ could be read off against a scale on the eye piece graticule
but because a subject’s eye is never stationary, it is impossible to take a measurement using a measuring scale on the graticule due to image doubling which occurs
what is the easier more practical way of measuring r of the cornea using a keratometer
to judge when the two images are just touching, even if there is slight movement
i.e. easier to judge the height of a certain virtual object by making two of them (objects) and then making them touch
what component in the keratometer sends the object of the mires into the eye
scheiner disc
what two different way does a keratometer use the principle of doubling (prismatic deviation)
- some keratometers have a fixed mire separation with variable image doubling (the prism can move the bottom mire up and the other mire is fixed, to make the two come close together, so the mires don’t move, but image doubling does move)
- others have fixed doubling and variable mires (prisms are fixed, but the object/mires can be moved further away from the eye)
name the 2 methods used to measure the principle meridians of the cornea in keratometry
- variable image doubling
2. variable mires
explain how a keratoemter uses variable image doubling to measure the principle meridians of the cornea
- one image creates a doubling effect in the vertical and horizontal meridians
- the mire separation is fixed
- the image doubling is variable
- there are two prisms which are moved by external controls
- the scheiner disc has apertures for the two perpendicular meridians
what is the advantage of the variable image doubling method to keratometry
both meridians can be measured at the same time
explain how to set up the keratometer for the examiner
- should be according to the examiner’s refractive error
- turn from positive to negative
- main source of error in measurements is if the refractive error is done incorrectly
- should be done in the dark with a white sheet of paper instead of px’s eye (as black mires on black pupil makes it quite hard to see the mires)
explain how to set up the keratometer for the patient
line up the instrument
- adjust the height of the 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
- OR use the foresight: line up small cone on the side of the instrument with the outer canthus
explain how you will correctly focus the image of the mires on a variable image doubling keratometer
- focus the eyepiece graticule (cross) to make sure your at the right distance
- initially the image of the mires will be blurred and double
- move the keratometer towards or away from the px until the focussing image (mires) are sharp and single
- move the keratometer sideways, up or down so that the bottom right hand circle is surrounding the eye piece graticule (cross)
what needs to be done next, after the variable image doubling keratometer has a correctly focussed image of the mires i.e everything sharp and circles are single
align the plus and minus signs:
- adjust the external controls to overlap the plus and minus signs
line up the two plusses until you see a single plus and the same with the minuses
what must you do to a variable image doubling keratometer when the principle meridians are not 180 and 90 degrees i.e. when corneal astigmatism is not oblique
rotate the instrument so everything is straight and read off the axis
what can be the cause if less than three mires are seen in a variable image doubling keratometer
vertical palpebral aperture is too small for measurement: instruct px to open their eye wide
what can be the 4 possible cause if there is a poor quality of the mires in a variable image doubling keratometer
- unstable tearfilm: non-invasive TBUT can be observed where the mires distort/break up during the test
- epithelial distortion
- corneal scar
- irregular astigmatism: keratoconous, corneal graft, sometimes refractive surgery (as the flaps are more peripheral to where we are looking)
