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Contact Lenses > Keratometry > Flashcards

Keratometry Flashcards

1
Q

Reasons for doing keratometry

A
  • Determination of
    – Central corneal radii (mm) and power (D) i.e measure the curvature of the cornea
    – Principle meridians of cornea
  • Differentiate between regular and irregular astigmatism
  • Observe quality of reflected mires
  • Measure NIBUT (non- invasive break up time)
  • Measurement of BOZR contact lenses
  • Distortion
  • Tear film anomaly e.g problem with oily layer of tear film
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2
Q

Where do we measure the curvature of the cornea i.e central corneal radii

A

Done in the two principle meridians

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3
Q

What happens in irregular astigmatism

A

Not at 90 degrees on top

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4
Q

What are the different types of keratometers

A
  • Some assume that there is regular astigmatism
  • And other keratometers involve measuring each meridian at one time
  • If it measures H and V meridians automatically , it assumes the axis is 90 degrees on top of each other
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5
Q

What is a mire:

A
  • REFLECTION OF OBJECT SENT TO THE EYE
  • Which is converted to radius of corneal curvature
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6
Q

What can the quality of light source sent to eye tell you about

A

· If there is problem with tear film i.e dry eye
· Because the tear film reflects the light source that you send to the eye
· If tear film is uneven, the image of the mire that you originally send is not comparable to the image sent to the eye in the first place = they look different

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7
Q

What does TBUT involve and what is it:

A
  • Fluorescence
  • How long does breaking up of tear film take
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8
Q

Why is it not good sometimes to assess quality of tear film:

A
  • Cause px has disruptive tear film already from dry eyes
  • If you add something to the tear film, it will disrupt it even more in a px with dry eyes
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9
Q

What happens in NIBUT:

A

· Send light to the eye in concentric circles
· Reflection of it = count how many seconds before concentric circles break up

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10
Q

What can you observe by observing quality of mire:

A
  • You could observe certain eye conditions like keratoconus
  • And other conditions that affect the shape of the cornea or even the quality of the tears.
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11
Q

What is keratometry:

A
  • The measurement of the principle radii of the anterior surface of the cornea (tear film)
  • IT’S THE TEAR FILM THAT REFLECTS, NOT THE CORNEA, SO KERATOMETRY IS A MEASUREMENT OF THE TEAR FILM
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12
Q

How does keratometry work

A
  • Cornea ( tears ) acts as a mirror since the light sent to the eye is reflected from it
  • Therefore, we measure radius of curvature of this ‘mirror’ (i.e. tear layer)
  • Can calculate that because it knows the distance at which you send the light to the eye
  • Can measure radius of curvature because it can measure the size of the reflection coming back
  • Uses first Purkinje image = tear film
  • 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
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13
Q

Where are the mire’s reflected from:

A

The tear film

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14
Q

What are we measuring really in keratometry:

A

Curvature of tear film

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15
Q

What is the tear film:

A

A very thin layer across the whole of the anterior surface of the cornea.

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16
Q

How is the curvature of cornea calculated - keratometry equation:

A
  • Cornea acts as spherical mirror of curvature r, then object of height h will be imaged with a height h’
  • Based on Newton’s magnification relationship, the approximate keratometry equation is r = 2(h’/h)d
  • Can send known object of a known height ( h ) to the eye and this is reflected back
  • As know distance from machine and front surface of cornea
  • Can measure height of reflection i.e image h’
  • And with formula can find out curvature of cornea
  • 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
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17
Q

What is the problem with using the equation and reading h’ off scale on eye piece graticule to find curvature of cornea:

A

As the eye is never stationary and has tiny movements it is impossible to read measuring scale on the graticule (cross or circle in picture) i.e measure the height of the reflection

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18
Q

What is the other method of measuring curvature of cornea instead of measuring distance of reflection coming back i.e h’:

A

Image doubling i.e prismatic deviation

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19
Q

What is image doubling:

A
  • Creating two reflections i.e two identical images coming back/projections
  • The keratometer splits the image into two
  • So we see two mires
  • We are trying to move those images closer together until they are just touching
  • Use that to measure radius of cornea
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20
Q

How does image doubling i.e prismatic deviation work:

A
  • Instead of measuring the distance of refection coming back……
  • If you have two identical images coming back, you can convert that height into the distance, that you need to move the images to just make them touch
  • i.e if you send a mire to the eye and you’re using a scheiner disc to create two of those reflections i.e two identical images coming back
  • What you can do is, as you know the distance between the two pinholes in the scheiner disc i.e it is fixed
  • You can calculate for every radius available for the cornea, how far the two images are separated
  • The machine allows you to move some prisms inside the instrument that move the two mires to just make them touch
  • The machine can calculate for every bit you need to move that prism until the mires come together
  • And that relates to curvature of cornea
  • So depending on how steep curvature of cornea is, can move the prism to make the two reflections just touch
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21
Q

Variable doubling keratometer:

A
  • Can change the movement of the prism
  • Shine the mire into the eye, two reflections come back
  • Using dials to move the two mires closer together until touching
  • Then read off radius of curvature of cornea in that direction
  • FIXED MIRE SEPARATION
    MOVING THE PRISM
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22
Q

Fixed doubling keratometer:

A
  • Two directions separate
  • Has fixed prisms
  • The machine sends out two images to eye and you have to move images from the outside to make them match
  • FIXED PRISM
    MOVING MIRES MANUALLY FROM OUTSIDE
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23
Q

What are the two methods of keratometry:

A
  1. Variable prisms
    1. Variable mires
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24
Q

Variable imaging doubling – TWO POSITION

A

· It’s one mires (whitish circle )
· And within the instrument, we are using prisms to double the image horizontally and vertically.
· So we have the original mires coming back from cornea.
· And we are seeing one split in the vertical meridian and one split on the horizontal axis.
· So if we are measuring the curvature over the horizontal meridian, one of the principal meridians.
· Then we’re trying to make these two lines just touch, just overlapping each other.
· And simultaneously we can also do this in the vertical meridian where we are overlapping the two pluses.

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25
Q

Variable mires – ONE POSITION

A
  • Have two different mires sent to eye
  • We’re moving the mires mechanically over this axis to make sure that the two closest together are just touching.
  • So the actual mires is only one red block and one green staircase.
  • And then with the prism, we are creating two different ones = two staircases and two blocks.
  • And we are physically moving these mires closer together or further apart until the staircase and the block, are just touching.
  • This can do one position at one time
  • The schemer disc will create one green and one red
  • And then another green and a red one
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26
Q

Summary of one position keratometer:

A
  • Only need to do it once in one position as both meridians are measured at the same time
  • The keratometer that shows the circle with the pluses and minuses
  • Bausch and lomb
  • Mire separation is fixed
  • Image doubling is variable
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27
Q

What does the one position keratometer do:

A
  • The instrument will send one circle to the eye, but you will see three.
  • And the Scheiner disc will then create these two perpendicular meridians i.e these extra mires, so there is one in the middle and one above and one to the side.
  • Have to move two prisms moved by external controls – one in the vertical direction where pluses overlap and one in the horizontal direction where minuses overlap
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28
Q

Focussing the eye piece:

A

– 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 in front of headrest where px’s eye is instead of using px’s eye
- This allows you to see targets clearly

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29
Q

What condition do we focus eyepiece and keratometer:

A

· In the dark = this relaxes accommodation
· But want some light so use white sheet of paper and torchlight

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30
Q

Setting up the patient:

A

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 to line up instrument
* Examiner will see light reflecting from cornea
* Px will see reflection of their own eye
* Instruct px to look at this light coming out on other side of keratometer
OR
* Use the foresight
○ So line up small cone on the side of instrument with the outer canthus
○ And look on the side of it.
○ You should be approximately in the centre of the cornea
- Will see image of mire

31
Q

What do you see once you’ve lined up the instrument

A
  • A full on light coming and shining on the eye.
  • Then you know that you are in the right position.
32
Q

What are the 3 mires:

A
  1. Original one
  2. Mire created with the prism for the vertical meridian
  3. Mire created with the prism for the horizontal meridian
33
Q

What will look blurry and sharp originally:

A
  • Mires = blurry
  • Graticule black cross = has to look sharp
34
Q

What does it mean if some circles look blurry:

A

Too far or too close to px

35
Q

What to focus:

A
  • First focus black cross – eyepiece in graticule
  • Then focus circles = images
36
Q

How can you focus the image and mires:

A
  • Move keratometer towards or away from px, until focussing image is sharp and single,
  • Move keratometer sideways, up or down so surrounding eyepiece graticule
  • And every time the patient blinks or breathes it goes slightly out of focus again.
  • So you have to keep your hand on the joystick to make constantly small movements in and out to make sure those mires are sharp and single.
37
Q

Why do you move the keratometer sideways, up or down:

A

To make sure you surround the eye piece graticule i.e black cross in the middle of the bottom right mire

38
Q

What do we do once weve made mires clear and your gratitcule is clear:

A

Use the external controls to overlap the plus and the minus signs.

39
Q

How can you overlap the plus and minus signs:

A

· Adjust the external horizontal and vertical controls
· Have to move these so the crosses are on top of each other and the minuses are on top of each other
· So with one of the horizontal control overlap the two minuses, so it becomes a single minus.
· Then with the other vertical control overlap the pluses so it becomes a single plus

40
Q

What do you do once you’ve overlapped plus and minus:

A

Read off the radius of the cornea in the vertical meridian as well as in the horizontal meridian

41
Q

Wat does it mean when images are titled:

A
  • Meridians are not 90 or 180
  • Move instrument until images are aligned again and straight
42
Q

What happens if its regular astigmatism:

A

It will be 90 degrees apart.

43
Q

What is not possible in the one position keratometer:

A

It is impossible to measure two axes that are not 90 degrees on top of each other.

44
Q

What is one position keratometer not good for:

A

For irregular astigmatism.

45
Q

What happens every time a px blinks:

A
  • They get a fresh layer of tear film over cornea
  • Eyelids come up
  • Things floating in tear film = distorted
  • Make them blink a couple times and then open = adjust it
46
Q

What happens when a patient has very dry eyes:

A

The mires become a little bit distorted because the tears are not able to reflect the mires in all the way 360 degrees around

47
Q

Why is there sometimes less than 3 mires seen - what is solution for this:

A
  • Not enough space to fit 3 mires in area
  • Ptosis
  • VPA too small for measurement
  • So ask px to open eyes wider
48
Q

What causes poor quality mires:

A
  • 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
49
Q

How can you tell mires have a very poor quality:

A

They are not round or circular

50
Q

How do you make the mires round again in tear break up time:

A

Every time px blinks that will exchange those tears for a fresh tears.

51
Q

How can you assess NIBUT in keratometry:

A

You can count the seconds it takes before the circles don’t look like circles anymore

52
Q

What is different about two position keratometer:

A

· Can only measure one meridian at a time
· So you can pick up irregular astigmatism which you cant do in the one position keratometer.

53
Q

What are you doing in two position keratometer:

A
  • Moving the blocks and the staircase physically towards each other or further away.
  • When the staircase and the blocks are not sharp or single,you have to move the keratometer slightly in or out.
54
Q

What does it mean if principal meridians are not at 180 degrees directly:

A

· If principal meridians are not at 180 degrees directly, everything is tilted
· You are trying to turn the instrument until black line from green block and red block line up

55
Q

What is advantage of two position keratometer:

A
  • It has an internal fixation light or a mirror for the patient to fixate ( a little red dot )
  • It’s quite bright so will help with aligning the instrument.
56
Q

What is two position keratometer also called:

A

Javal-Schiotz

57
Q

What is seen through a two position keratometer:

A

Actual mires…
· Staircase and the block.
· It’s doubled….so we have two staircases and two blocks.
· And we are focusing on the ones that are closest together.

58
Q

Method of two position keratometer:

A
  1. Move keratometer slightly in or out to make the staircase and blocks sharp and single
  2. Line up staircase and block until they just touch - focusing on the ones that are closest together
  3. The endpoint is when the furthest point in the green staircase is just touching the red block.
  4. The black line needs to be a continuous line between staircase and block.
  5. There’s also a graticule that you will need to focus the eye piece on. That is a black line that is going on a 45 degree angle.
  6. And the endpoint is that line is also going through the point where the staircase and block are lined up.
  7. So rotate mires until black lines going through mires are aligned and continuous
  8. If the two overlap, it tends to become like a lighter colour, which means you’ve gone too far so you need to reduce the radius.
59
Q

Summary of steps for getting image in keratomery:

A
  • Focus eye piece graticule ( black long line )
  • Focus doubled mires until sharp
  • Move mires in and out with external dials until inner ones ( green and red just touch )
  • Align all through graticule i.e black long line in centre of view
60
Q

What do you write down the radius to

A
  • The closest 0.01mm
  • For contact lens competency, it’s to the closest 0.05mm
61
Q

What do you write down the power to:

A
  • The nearest 0.25D
  • For contact lens competency, it’s to the closest 0.25D
62
Q

How do you read off the scales in one position keratometer:

A

You do that while looking through the eyepiece, but the patient can sit back.

63
Q

How do you read off the scales in two position keratometer:

A
  • The patient can also sit back
  • But you read that off the outside the eyepiece so you don’t have to look through the eyepiece.
64
Q

Summarise process of keratometry:

A

· You setup and align the instruments
· You focus your eyepiece
· You try to take your first measurements
· And then once you are there, you ask the patient to blink a few times, open their eyes wide and try not to blink.
· Slightly adjust the horizontal, slightly adjust the vertical and you’re done.
· Then the patient can sit back.
· You will write your measurements down.
· Don’t move dials.
· Ask the patient to come back forward and readjust very, very slightly because you’re not expected to find a big difference and you do that three times in total.
· And then you do the same thing on the other eye.

65
Q

What do you note down:

A
  • Radius (mm) and power (D) in both meridians
  • Angle of each meridian
  • Any distortion of mire images
66
Q

How do you work out corneal astigmatism:

A

0.05mm difference between 2 radii = 0.25D corneal astigmatism

67
Q

Why do we work out corneal astigmatism:

A

· Aid the selection of the lens type
· E.G. what kind of corneal lens, what kind of soft lens, do we need a spherical one do we need a toric one, do we need a front surface or a back surface, rigid corneal lens?

68
Q

Why are we measuring VA with spectacles at the start:

A

Want to make sure they see same if not better with the contact lenses

69
Q

What is the BVD when you don’t know it:

A

12mm

70
Q

How can the BOZR of an RGP be measured:

A
  • With the aid of an attachment to the keratometer
    • Lens holder with a front surface silvered mirror
    • Drop of water is placed between holder and RGP contact lens
  • So can drop an RGP lens from back and measure back surface so using back surface of lens to fit on cornea
71
Q

How can the BOZR of an SCL be measured:

A

‘Wet cell’ is used to measure BOZR of a SCL

72
Q

What is used to measure BOZR:

A

· Conversion scales to measure back surface of kens
· Cause keratometers are calibrated for convex surfaces

73
Q

Limitations of keratometry:

A

· The true refractive index of the cornea (1.376) is not used; instead an index of 1.3375 is assumed…
* So there is a difference between the true refractive index of cornea and the 1.3375 assumed refractive index of cornea
* So the conversion between power to radius is a bit incorrect = measurement error on basis that incorrect index of cornea is assumed
* This allows the instrument to read the total corneal power, or approx 90% of the front surface power
- Reflection/ measurement is from an area 3- 4mm of the central cornea
* So cant make any assumptions about periphery or mid periphery of cornea
* Because reflection is only coming from central cornea i.e only curvature of central cornea is assessed
* Contact lenses are much bigger than 3-4mm
* So corneal shape limited when using keratometer, only gives 2 principle meridians in central 3 – 4 mm of cornea
- Instrument inaccuracy means that 0.05mm changes in K are not clinically significant
- K readings may be dependent on the instrument used, due to calibration differences
- CL fitting can only be assessed with the use of diagnostic lenses
- Scale different between two instruments
- Between and within instrument variability

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