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methods BLOCK 2 > Keratometry > Flashcards

Keratometry Flashcards

1
Q

gullstrand eye

A

2 lens optical system

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2
Q

what is the rough average of all the RIs of the components of the eye?

A

about 1.33

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3
Q

What is the optical power of the first surface of the cornea?

A

+48.83D

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4
Q

What does keratometry measure?

A

the anterior surface of the cornea

-radius of the different meridians

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5
Q

what is the radius of curvature of the cornea?

A

7.7mm

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6
Q

radius of curvature

A
  • units: m
  • larger the radius, smaller the curvature
  • same algebraic sign as curvature
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7
Q

curvature

A
  • diopters
  • larger the curvature, smaller the radius
  • same algebraic sign as radius
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8
Q

What is the largest contributor to ocular optical power?

A

cornea, mainly attributed to the anterior surface of curvature

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9
Q

What is there power of an emmetropic eye?

A

60D

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10
Q

astigmatism

A
  • no point imaging
  • distinct meridians with more and less power
  • not spherical, shaped like a watermelon
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11
Q

sturm conoid

A
  • circle of least confusion

- directly related to 2 different radius of curvatures along two different principal meridians

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12
Q

With the rule astigmatism

A

steeper vertical

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13
Q

against the rule astigmatism

A

steeper on horizontal

can shift with age

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14
Q

is the cornea a mirror?

A

yes, it also reflects

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15
Q

What is the light actually reflecting off of on the cornea?

A

the tear film

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16
Q

image from the cornea as a mirror

A

virtual, upright, demagnified

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17
Q

What kind of mirror is the cornea?

A

convex mirror

18
Q

the smaller then radius of curvature…

A

the larger the curvature, the smaller the image

19
Q

different types of keratometers

A
  • the fixed doubling technique
  • javal-schiotz keratometer
  • ziess
  • baush + Lomb
20
Q

@180: radius of curvature 7.8mm, Power 43.25 D,

@90: radius of curvature 7.8mm, power 43.25 D

A

the cornea is spherical because the powers are equal at different meridians

21
Q

@180: radius of curvature 7.8mm, Power 43.25 D,

@90: radius of curvature 7.6mm, power 44.50 D

A

WTR astig, steeper meridian is vertical

22
Q

@180: radius of curvature 7.8mm, Power 43.25 D,

@90: radius of curvature 8.2mm, power 41.15 D

A

ATR astig, steeper meridian in the horizontal

23
Q

keratometry limitations

A
  • primary data taken from 4 points on the center 3mm
  • assumes a lot. paraxial app only, low order aberrations only
  • assumes rotationally symmetric cornea (spherocylindrical)
  • assumes some symmetry, such as corneal apex, visual axis, and the axis of the instrument to coincide
  • assumes normal astig
  • keratocones may be challenging
  • cannot measure asphericity
  • tear film
24
Q

keratometry vs topography

A

topography is present and future

25
Q

topography

A
  • multitude of info such as CL
  • applicable to any corneal abnormality
  • curvature and optical power on any meridian we select. not limited to just two
  • easy to use
26
Q

keratometry in comparison to topography

A
  • measurement only at 3mm
  • curvature and optical power only for two meridians
  • manual alignment
27
Q

topography features

A
  • still subject to tear film stability
  • Dx tool for corneal abnormalities
  • learning curve-a multitude of info needs to assessed and evaluated
  • high precision measurement of distribution of corneal curvature
28
Q

why is topo an indispensable tool for dr

A
  • indispensable in planning and f/u in laser vision correction
  • helps select a CL
  • can monitor and document changes in corneal surface
  • can measure peripheral as well!
29
Q

placido topo

A
  • lasted till early 90s
  • projects concentric rings on the cornea and records digitally the mires
  • auromated computer based data processing
30
Q

if the mires are closer together…

A

steeper cornea

31
Q

if the mires are spread out

A

flatter cornea

32
Q

if the mires look elongated

A

astig

33
Q

if the mires look wavy in weird spots

A

irregular astig probably

34
Q

curvature map green

A

good

35
Q

keratometric equation

A

RxP=337.5

36
Q

corneal asphericity reading

A

good for fitting CL

37
Q

elevation/altimetric map

A

essential for surgery

measured in microns

38
Q

simulated abberometry

A

dependent on pupil size

39
Q

color at the edge of healthy cornea

A

blue

40
Q

corneal asphericity

A
  • not exactly sphere

- helps compensate spherical aberration

41
Q

what would cause a flatter central cornea?

A

abnormalities or surgery

methods BLOCK 2 (8 decks)

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