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OV2006 contact lenses > Keratometry > Flashcards

Keratometry Flashcards

1
Q

Why do keratometry ?

A

do to determine shape of cornea- so we know the curvature of the CL that will fit well to this shape

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2
Q

What can keratometry be used to determine ?

A

-central corneal radius in mm and converted into power (D) of the 2 principal meridians of the cornea
-differentiate between regular and irregular astigmatism
-observe quality of reflected mires
-measure NIBUT
Measure of BOZR CL

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3
Q

What can we use keratometry to differentiate ?

A

between regular and irregular astigmatism

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4
Q

What is the regular astigmatism?

A

principal meridians are 90 degrees to each other

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5
Q

What can we use keartometry to observe?

A

the quality of the reflected mires

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6
Q

What is the idea of keratometry ?

A

send certain light source to the eye which is reflected back by the tears which you can determine the shape of the cornea over a certain range

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7
Q

What is the light source you send to the eye?

A

mire

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8
Q

What can you do after observing the quality of the reflected mire ?

A

can then observe any issues e.g eye problems such as keratanoa and conditions which affect shape of cornea or quality of tears

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9
Q

What is NIBUT?

A

measure how long how the tears take to evaporate

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10
Q

What is BOZR CL?

A

back optic zone of radius

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11
Q

What is the definition of keratometry ?

A

the measurement of the principal radii of the anterior surface of the cornea (tear film)- the mires are reflected from the tear film - actually measuring curvature of tear film-

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12
Q

Where is the tear film ?

A

thin layer across the whole anterior surface of the cornea

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13
Q

What is the principal of keratometry ?

A
  • the cornea/tears act as a mirror since light is reflected from it
  • measuring the radius of curvature of this mirror
  • uses 1st purkinje image
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14
Q

What does the size of the image produced in keratometry depend on?

A

size of object which is (mires), the radius of the curvature of the cornea and the distance between mires and cornea

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15
Q

Where is the image produced by keratometry actually formed?

A

lies just within the cornea

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16
Q

What does the cornea act as?

A

spherical mirror of curvature of R

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17
Q

Where will the object of height be ?

A

-the object of height will be imaged with a heigh h’

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18
Q

What happens based on Newtons Magnification realtionship ?

A

the approximate keratometry equation is r = 2(h’/h)d

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19
Q

What happens in theory ?

A

if h’ could be read off against a scale on the eye piece graticule, the equation could then be used to find r (radius of cornea)

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20
Q

Why is it impossible to take a measurement using a scale on the GRATicule?(Cross or circle)

A

because eye of px is never stationary.

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21
Q

What does the keratometry do instead?

A

It is much easier to judge when 2 images are just touching, even if there is slight movement

  • much easier to see even if there is little movement
  • so it splits the image into 2
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22
Q

When looking at eye what is the mag when looking through microscope ?

A

-high mag

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23
Q

What happens when looking though the microscope to the the purkinje image ?

A

every time px breathes or blinks the purkinje image starts moving

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24
Q

What does the keratometer do to the image when viewing ?

A

splits into 2

-called principal of doubling/prismatic deviation

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25
Q

How does the Doubling/prismatic deviation work ?

A

sending mire to eye- with prism it creates doubling system- see 2 mires-trying to move these images closer to each other until they are just touching - that’s how we measure the radius of the cornea

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26
Q

What are the 2 principals of doubling ?

A
  1. some keratometers- have fixed mire separation
    - mires are fixed DISTANCE -made into 2 images and just the prism that moves
  2. and others have fixed prism- but the mires are moving around - closer to each other
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27
Q

What are the 2 methods of doubling called?

A
  1. Variable image doubling

2. Variable mires

28
Q

How does variable image doubling occur in the machines?

A

-one mire- white circle
within the instrument- using prism top double the image horizontally and vertically
-original mire from cornea- then see one split in the vertical meridian (image) and one in horizontal meridian

29
Q

What happens if we are now measuring the radius of curvature over the horizontal and vertical meridian - in variable image doubling system?

A

one of the principal meridians

  • we are trying to make the lines touch - overlap each other- (minus overlap)
  • simultaneously do this in the vertical meridian where plus should overlap .
30
Q

How does variable mires occur?

A
  • have 2 different mires sending to the eye - moving mires mechanically over this axis to make sure the 2 closest together are just touching
  • the actual mires is one red block and one green staircase
  • with prism it creates 2 staircases and 2 blocks - we are physically moving the mires closer together or further apart until the staircase and block are touching
31
Q

What are the advantages of the 2 methods?

A

even if the px breathes or blinks , you can still estimate when the 2 are aligned or just touching

32
Q

What is the one position keratometer?

A
  • that shows the circle with the pluses and the minuses
  • referred to as bausch and lomb
  • measure both meridians at the same time.
  • the mire separation is fixed
  • the image doubling is variable
33
Q

What is the measurement of 2 meridians?

A

there are 2 prisms which we need to move with the external controls

  • instrument send 1 circle to the eye but you will see 3 - the shine of this will create on perpendicular meridians these extra mires
  • then we have one in the middle and one to the side
  • we are moving prisms by overlapping + and -
34
Q

What do we do before we start sending mires to px eye?

A

-start by focusing eyepiece
-done in the dark
performing in the dark- relaxes accomodation
-turn from most positive to most negative and should be the practitioners refractive error
-can use the lab book at the place of where the eye of px would be - so can see target clearer
-see black circle or line which should be in focus

35
Q

How do we set up the px in the keratometer?

A
  • Line up the instrument- px will come forward
  • adjust height of instrument and chair
  • use marker for outer canthus
  • shine light through the eye piece so px can see light coming out the other side of keratometer so they can look into that
  • high mag
  • Examiner will see light reflecting from cornea
  • Px will see reflection of their own eye
  • Instruct px to look at this
36
Q

WHAT is the main source of error in keratometry ?

A

when eyepiece is not focused

37
Q

How else can we line up the px?

A

line up the small cone on side of instrument with outer canthus
-then will see reflection from cornea
and possibly can see image of the mire- won’t be that clear

38
Q

What is the cross ?

A

-is the graticule you see through the eye piece
-make sure its completely sharp
-dont have shadow
-

39
Q

What will you see through the keratometer ?

A

will see 3 mires

one in the vertical meridian. and one in the horizontal meridian and one is the original- will be slightly blurring

40
Q

How can we get the correctly focused image ?

A
  • Move keratometer towards or away from px, until focusing image is sharp and single,
  • keep hand on the joy stick as px moves or breathes it can get out of focus- to make sure mires are sharp and single
  • Move keratometer sideways, up or down so surrounding eyepiece graticule (black cross) in the middle of the bottom right mire (which is the original mire) and all of this is sitting approx in the centre of cornea
  • might expect to see it higher up
  • once you done all of that - mires are clear-can use external controls to overlap the plus and the minus sign
41
Q

What happens after your graticule and mires are sharp ?

A

can use external controls to overlap minus and plus signs

-that is important

42
Q

What are horizontal controls?

A

-overlapping the minuses - with one of the controls- horizontal- trying to overalap the minus as it becomes a single minus

43
Q

What are vertical controls ?

A

-overlap the pluses to make one plus

44
Q

What happens when the mires look rotated?

A

When principal meridians are not 180 and 90 degrees, the instrument should be rotated

45
Q

What to do in the e.g in slideshow ?

A
  1. move instrument in or out
  2. haven’t found vertical or horizon radius- need to turn vertical dial to overlap - and horizontal dial to overlap +
  3. change horizontal dial to change to single +
  4. turn vertical dial to overlap the minus

(if ever do this in practice first do the horizontal then vertical)

46
Q

Why is blinking important when doing keratometry on px eyes?

A

to keep the tear film clean and fresh and has equal thickness all over the cornea.

47
Q

What happens to keratomtery results when px has dry eyes ?

A
  • the mires become little bit distorted

- becasue the tears are not able to reflect the mires all the way 360 degrees around

48
Q

What if px can only see 2 mires?

A

problem with a vertical plaperbral aperture- see 2 horizontal but not on top

  • instruct px to open eyes wide and hold eyelid (careful not to push on eyelid to not change the shape of cornea)
  • Ptosis- tricky to take measurement - have to hold eyelid
49
Q

Why can mires have poor quality/not perfectly circular?

A
  • unstable tear film
  • Epithelial distortion
  • corneal scar
  • irregular astigmatism- keratoconus, corneal graft , refractive surgery
50
Q

How is unstable tear film shown ?

A
  • tbut - BLACK BIT SHOWS NO TEARS PRESENT COVERING THE CORNEA- they have been broken up- which will distort the mires
  • every time px blinks they will exchange those tears
  • mires are not round
51
Q

What is the 2nd keratometer ?

A

two position keratometer
Javal -Schiotz
-measure one meridian at a time
-e.g staircase and blocks are measured at horizontal axis
-if want to measure vertical meridian have to tilt it
-move towards or further away
-There is an internal fixation light or mirror for patient to fixate
-Decide on correct distance from eye
-Mires should be sharp and single

52
Q

Why is it good to measure one meridian at a time in the Javal-Schiotz?

A

-can pick up irregular astigmatism which you cant do with bausch

53
Q

What is Java schiotz mires?

A

staircase and block -which is doubled

  • going to look at the ones that are closest together
  • the ones we line up together
  • green staircase is just touching the red block- tiny gap
  • black line needs to be continous
  • also a graticule that needs to be focused - at 45 degree angle
54
Q

Why is the black line helpful in Java schiotz mires?

A

shows that axis is not at 180degrees

  • need to move the instrument- as soon as you turn the axis - you see the line can be continuous - got your end point and axis
  • if overlap - see lighter colour- means you turned too far
55
Q

How do you read the scales in 1 position keratometer?

A
  • looking through the eyepiece while px sits back
56
Q

How do you read the scales in 2 position keratometer?

A
  • px can sit back

- look from outside of instrument

57
Q

What do we write the radius closest to for CL competency in Java schiotz ?

A

0.05mm

58
Q

What do we write the power closest to for CL competency in Java schiotz?

A

0.25D

59
Q

What do we write the radius and power closest to for in 1 position keratometer?

A
  1. 01mm

0. 25D

60
Q

how do we take down measurements?

A

Once the keratometer is aligned correctly, record

  • Radius (mm) [and power (D)] in both meridians (reminds you of power of cornea)
  • Angle of each meridian
  • Any distortion of mire images
  • Repeat 3 times
  • Good measurements include stable fixation
  • Blink few times, open eye wide, try not to move, and keep jaws closed §Repeat fellow eye
61
Q

How can we use rule of thumb to calculate corneal astigmatism ?

A

Example notation:
H 8.26 along 05 (+41.75) clear mires
V 7.74 along 95 (+44.25) clear mires

If using power - can subtract them 2
-But as we are using radius we calculate the difference between them 2
8.25-7.75= 0.5mm difference between horizontal and vertical meridian
=2.50D corneal astig (0.05 / 0.5= 10
10x0.25D=2.50D- RULE OF THUMB)

62
Q

What is the idea of calculationg corneal astigmatism?

A

to aid the selection of the lens type

-what kind of corneal/soft lens- spherical,toric , front surface back surface

63
Q

How do we do notations in the lab book for CL?

A

-Name, date, spectacle Rx from front of lab book §i.e. last EE (< 1 yr)
-VA with spectacles and BVD measured
-Look up conversion to ocular Rx
-Measure Ks H and V
-This will help you understand the fluorescein pattern expected :
all refractive astigmatism is caused by CA

64
Q

what are some of the limitations of the keratometer ?

A
  1. The true refractive index of the cornea (1.376) is not used; instead an index of 1.3375 is assumed- sets off difference- measurement error- due to incorrect index assumed
  2. This allows the instrument to read the total corneal power, or approx 90% of the front surface power
    c) Reflection/ measurement is from an area 3-4mm of the central cornea- little bit of difference- cant make assumptions of mid or periphery of cornea - corneal shape is limited
    - Depends on actual instrument and corneal size
65
Q

What are some extra limitations of keratometer ?

A
  • 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- scale may be different
  • CL fitting can only be assessed with the use of diagnostic lenses
66
Q

What is the biggest limitation of the keratometer ?

A
  • Only curvature of central cornea is assessed- have no idea what is happening with the shape of the cornea towards the periphery ???

OV2006 contact lenses (11 decks)

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