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Optics II block 5 > Corneal Optics > Flashcards

Corneal Optics Flashcards

1
Q

What part of the cornea protects the stroma?

A

Epithelium

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2
Q

What part of the cornea is responsible for the transparency

A

Stroma

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3
Q

What controls the water content of the stroma?

A

Endothelium

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4
Q

What can affect the transparency of the cornea?

A

Increased water

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5
Q

Cornea has a ________ optical power

A

Fixed

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6
Q

Cornea has about _______ of the total power of the eye (relaxed)

A

2/3

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7
Q

What is the calculated power of the cornea?

A

42D

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8
Q

What is the refractive index of the cornea

A

1.377 (mostly from the stroma)

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9
Q

Refractive index of _________ dominates the other corneal layers

A

Stroma

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10
Q

What is the corneal diameter

A

12mm

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11
Q

What is the CCT?

A

0.5mm

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12
Q

What is CCT measured with

A
  • pachymetry
  • scheimpflug
  • Ct
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13
Q

What are the most important aspects of the cornea

A
  • power
  • curvature
  • diameter
  • thickness
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14
Q

What is the front power of the cornea?

A

49.08D

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15
Q

What is the power of the back surface of the cornea?

A

-7.057D

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16
Q

What is the total corneal power?

A

49.08 + -7.057 = 42.023D

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17
Q

Is the cornea a spherical surface?

A

No but we still calculate the power like we do for a spherical lens

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18
Q

Equivalent power of the cornea

A

Fc= F1 + F2 - (d/n)F1F2

=43.05D

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19
Q

What is the back vertex power of the cornea?

A

Fav=F2+ (F1/(1-d/n*F1)

=43.83D

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20
Q

How do you fix spherical abberations?

A

By making the lens aspherical

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21
Q

Asphere compared to sphere

A

Asphere has a flatter curvature compared to spherical

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22
Q

Q=0

A

Spherical

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23
Q

Q=-n

A

Prolate

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24
Q

Q=+n

A

Oblate

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25
Q

What are the different apshericities

A
  • conicoid
  • ellipsoid
  • eccentricity
  • shape factor
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26
Q

Cornea is regarded as a _______, which is ellipsoidal

A

Conicoid

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27
Q

The anterior corneal asphericity (Q) values are usually ________

A

Negative

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28
Q

What does the negative Q value of the anterior corneal asphericty indicate?

A

That the cornea flattens away from the vertex

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29
Q

What is the significance of the corneal flattening away from the vertex or center?

A
  1. To reduce spherical aberrations

2. To make smooth join with the main globe of the eye

30
Q

Reducing spherical aberrations with asphericity in the cornea

A

The amount of corneal asphericity is sufficient enough to eliminate it. Q required to eliminate SA at the anterior surface is -0.528

31
Q

What is the predominant higher order aberration?

A

Spherical aberrations

32
Q

What can change the asphericity of the cornea?

A

Refractive surgery

33
Q

At what eccentricity is the spherical aberration 0?

A

0.73

34
Q

Corneal diameter is from where to where?

A

Limbus to limbus

35
Q

Why is it important to have accurate measurement of CD?

A
  • contact lens diameter sufficiency
  • right size anterior chamber IOLs
  • diagnose diseases
36
Q

What instruments can you use to measure corneal diameters?

A
  • autorefractor
  • corneal topographer
  • hand help mm ruler or slit lame graticule
37
Q

What is the horizontal diameter of the cornea?

A

11mm in adults (10 in children)

38
Q

What is the vertical diameter of the cornea?

A

12mm in adults (11mm in children)

39
Q

Corneal diameter and age

A

Decreases slightly with age due to age related narrowing of anterior chamber increasing the chances of glaucoma

40
Q

Gender and corneal diameter

A

Males have a significantly wider horizontal corneal diameter than females

41
Q

Race and corneal diameter

A

Asians CD is smaller than caucasians

42
Q

Anterior corneal curvature (ACC) exhibits ______

A

Toricity

43
Q

What are the two principle meridians of the cornea?

A

Tangential

Sagittal

44
Q

Tangential meridian of the cornea

A

Lies along the radius line from the vertex

  • vertical
  • more curvature here
45
Q

Sagittal meridian of the cornea

A
  • perpendicular to the tangential meridian
  • horizontal
  • less curvature
46
Q

Are the curvatures the same in both meridians of the cornea?

A

No

47
Q

Cornea is significantly _______ in both radius of curvature and asphericity

A

Asymmetric

48
Q

If the refractive index, pupil diameter and asphericity are considered constant, the spherical aberration will do what?

A
  • decrease if the corneal surface flattens

- increase as the cornea becomes steeper

49
Q

More aberration

A

Steeper

50
Q

Hyperope tend to be ______ than the myopic eye

A

Flatter

51
Q

Each alteration in curvature equal to _______ in radius corresponds to a variation of 0.25 D

A

400th of a mm

52
Q

Studies show _______ after the corneal refractive surgery using excimer lasers

A

Increased corneal asphericity

53
Q

Cornea too curved leads to this condition

A

Keratoconus

54
Q

Cornea too flat leads to this condition

A

Cornea plana

55
Q

The average cornea curvature has a smaller radius in the ______ meridian compared to the other meridian

A

Vertical

56
Q

The average cornea curvature has a smaller radius in the vertical meridian compared to the horizontal meridian, which contributes to the ___________ in adults

A

With the rule astigmatism

57
Q

Astigmatism occurs when the vertical meridian of cornea is steepest

A

With the rule

58
Q

Astigmatism occurs when the horizontal meridian of the cornea is steepest

A

Against the rule

59
Q

Corneal curvature and age

A

Steeper with increasing age due to changes in elasticity of the cornea

60
Q

Gender and corneal curvature

A

Females have significantly steeper average corneal curvature than males

61
Q

Race and corneal curvature

A

Caucasian, hispanics, asians, and African americans found no significant differences between the races studied

62
Q

What does CCT influence?

A

Measured IOP and consequently the classification and management of glaucoma

63
Q

A thin cornea leads to _______ of IOP

A

Underestimation

64
Q

A thick cornea leads to _____ of IOP

A

Overestimation

65
Q

What relies on the accurate measurement of CCT

A

Detection and management of CL complications and certain surgical procedures such as astigmatic keratotomy, LASIK, and PRK

66
Q

Age and CCT

A

Reduction of CCT with age due to decreased keratocytes with age

67
Q

Gender and CCT

A

Difference between genders was 4.7 microns

68
Q

Race and CCT

A

African americans and other African decent have thinner CCT than other races

69
Q

For a conicoid surface, the radius of curvature at off-axis potions depends upon

A
  • the distance from the vertex

- on the meridian at that point

70
Q

Studies show that the mean posterior vertex radius is ______ and a mean asphericity of Q=___

A
  1. 8mm

- 0.42

71
Q

Why is the posterior corneal curvature (PCC) of lesser significance than the anterior surface?

A

Because of the small refractive index difference across the posterior corneal boundaries, but it is not of negligible significance

Optics II block 5 (11 decks)

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