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Methods Block 2 > Ketatometry > Flashcards

Ketatometry Flashcards

1
Q

Keratometry

A
  • measure the cornea

- uses radius of curvature and curvature

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2
Q

Eye Structure

A
  • Gullstrand Eye
  • two lens optical system
  • Cornea and crystalline lens
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3
Q

Cornea Thickness, refractive index, and radius of curvature

A

Thickness: 0.5mm
Refractive index: 1.376
Radius: 7.7mm anterior
6.8mm posterior

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4
Q

Aqueous humor thickness and refractive index

A

Thickness: 3.1mm

Refractive index: 1.336

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5
Q

Crystalline lens thickness, refractive index, radius of curvature

A 
Thickness: 3.6mm
Refractive index: 1.386 cortex
                             1.406 nucleus
Radius: 10mm anterior
             6mm posterior
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6
Q

Vitreous body thickness and refractive index

A

Thickness: 17.2mm

Refractive index: 1.336

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7
Q

Optical power of the cornea

A
  • ANTERIOR power is what we use because posterior is super hard to measure and the power is really close even when you take thickness into account
  • +43.83D
  • super powerful plus lens
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8
Q

Curvature vs. Radius of curvature

A

Curvature:

  • amount of curvyness
  • Dipoters
  • Larger curvature=smaller radius of curvature

Radius of curvature

  • length from apex to center of curvature
  • m
  • Larger radius of curvature=smaller curvature
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9
Q

Normal Base curve

A

-8ish mm

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10
Q

Cornea

A
  • Largest contributor to ocular optical power
  • mainly attributed to anterior surface of curvature
  • is a strong lens, but acts as a mirror
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11
Q

Refractive status

A

Emmetropes:60D
Myotropes: 64D
Hyperopes: 56D

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12
Q

Steep and flat corneal curvature

A

Steep: higher optical power
Flat: Lower optical power (larger radius of curvature)

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13
Q

Astigmatism

A
  • distinct orientations of more and less optical power

- principal meridians

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14
Q

Sturm Conoid

A
  • sagital (horizontal) and tangential (vertical) focus
  • circle of least confusion: potential for point image instead of lines image usually seen
  • directly related to different curvatures along the two principal meridians
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15
Q

Toric and spherical surfaces

A

Toric: different radius of curvature along the two principal meridians
Spherical: large or small, the meridians have the same radius of curvature

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16
Q

With the rule

A
  • Steeper(more power) on vertical meridian

- flater on horizontal meridian

17
Q

Against the rule

A
  • steeper(more power) on horizontal meridian
  • flatter on vertical meridian
  • notice in older
18
Q

Normal

A
  • meridians differ by 90 degrees
  • vertical is roughly 90
  • horizontal is roughly 180
19
Q

Cornea imaging

A
  • convex mirror like
  • image is smaller, erect, and virtual
  • larger curvature=smaller image
  • based on image magnification, we can determine curvature, and thus, power
20
Q

Keratometric technique

A
  • project image on retina
  • line it up
  • compare magnifications
  • measure radius of curvature
  • uses doubling technique to stabilize image even with patient moving
21
Q

Types of Keratometers

A
  1. Javal-Schoitz: line up pyramid with box
  2. Zeiss- line plus sides inside eachother
  3. Baush and lomb-like up plus and minus signs on circles
22
Q

Keratometry limitations

A
  1. Data from small optical zone (3mm)
  2. Assumes small angles/ paraxial approximation
  3. Assumes mostly spherical cornea
  4. Assumes symmetry with visual axis, corneal apex, and axis of instrument
  5. Assumes normal astigmatism
  6. ignores corneal abnormalities
  7. cannot measure asphericity
  8. tear film changes things
23
Q

Benefits of topography

A
  1. provides multitude of other data
  2. can detect corneal abnormality
  3. measures curvature and power on any meridian
  4. EASY
  5. used in follow ups and planning refractive surgery
  6. can monitor changes in corneal surface
  7. helps with CLfit

-Still subject to tear film stability alterations

24
Q

Placido topography

A
  • most commonly used
  • concentric contrasting circles
  • records mires
25
Q

Mire spacing

A

Normal spacing: normal cornea
Sparser: thinner lines, more space between lines, flatter cornea
Denser: thicker lines, less space between lines, steeper cornea

26
Q

Curvature map

A

Red=steep(smaller magnification)
Blue=flat (larger magnification)
green=good

-healthy cornea will have a lot of blue

27
Q

Keratometric equation

A

R*P=337.5

-this number is 1-refractive index of the cornea

28
Q

Rough color map numbers

A

Red= 5mm and 58.79D
Green=7-8mm and 45D
Blue= 8-9mm and 39D

29
Q

Keratoconus

A

-highly asymmetric corneal surface

30
Q

Elevation map

A

-measured in micro meters

31
Q

Scaling

A

-MAKE SURE YOU PAY ATTENTION TO SCALING!!

32
Q

Dependent factors of topography

A
  • dependent on pupil size

- influenced by tear film

33
Q

Corneal Asphericity

A
  • not exactly sphere
  • all eyes have asphericity
  • helps compensate spherical aberration

Methods Block 2 (7 decks)

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