Corneal Optics

Question 1

What is the corneal power and how much of the total power does it account for?

Answer 1

+42 diopters

-about 2/3 of the total power of the eye

Question 2

What is the refractive index of the cornea?

Answer 2

1.377

Question 3

Each corneal layer has its own refractive index, but which layer dominates it?

Answer 3

The stroma

Question 4

What is the diameter of the cornea?

Answer 4

12mm

Question 5

What is the corneal thickness?

Answer 5

O.5mm

Question 6

Is the cornea a spherical surface?

Answer 6

Nope

Question 7

What aberration is associated with spherical lenses?

Answer 7

Spherical aberration

-marginal rays focus at a different spot than the central rays

Question 8

What does an aspheric lens do?

Answer 8

It makes the curvature flatter to nullify the spherical aberration

Question 9

The cornea is regarded as what shape?

Answer 9

Conicoid (ellipsoidal)

Question 10

The anterior corneal asphericity values are usually what? What does that indicate?

Answer 10

• usually negative

- the cornea flattens away from the vertex

Question 11

What are the two significant things that come from the corneal flattening away from the vertex?

Answer 11

1. To reduce spherical aberration

2. To make it smooth and join with the globe of the eye

Question 12

Normal eccentricity is what?

Answer 12

0.73

Question 13

What are the three corneal optical properties that effect vision?

Answer 13

1. Corneal diameter
2. Anterior corneal curvature
3. Central corneal thickness

Question 14

How is the corneal diameter measured?

Answer 14

Limbus to limbus

Question 15

Accurate measurement of corneal diameter helps what three things?

Answer 15

1. To make sure contact lens diameter is sufficient to cover the entire cornea
2. To produce the right size anterior chamber IOLs
3. To diagnose diseases

Question 16

What is the average horizontal diameter? Vertical?

Answer 16

Horizontal: 11mm
Vertical: 12mm

Question 17

What does corneal diameter do with age? Why?

Answer 17

Decreases with age due to narrowing of the anterior chamber (glaucoma)

Question 18

Males have slightly _____________ corneal diameter than females?

Answer 18

Slightly wider horizontal corneal diameter

Question 19

Asian CD is ___________ than whites?

Answer 19

Asian CD is smaller

Question 20

What is toricity?

Answer 20

Curvature is different in different meridians

Question 21

Is the cornea symmetric in regards to curvature and asphericity?

Answer 21

Nah it’s asymmetric

Question 22

Corneal shape geometry leads to differences in what?

Answer 22

Differences in spherical aberration

Question 23

If the corneal surface flattens, what will happen to the spherical aberration?

Answer 23

It will decrease

Question 24

If the cornea becomes steeper, what will happen to the spherical aberrations?

Answer 24

They will increase

Question 25

Are myopes or hyperopes flatter?

Answer 25

Hyperopes

Question 26

Each alteration in curvature equal to 400th of a mm in radius corresponds to a variation in how many diopters?

Answer 26

0.25D

Question 27

Corneal refractive surgery showed an increase in what?

Answer 27

Corneal asphericity

Question 28

A cornea that is too curved is a condition called what?

Answer 28

Keratoconus

Question 29

A cornea that is too flat leads to a condition called what?

Answer 29

Cornea plana

Question 30

In an average cornea, which meridian has a smaller radius?

Answer 30

Vertical meridian has a smaller radius (steeper)

Question 31

Corneal curvature does what with age?

Answer 31

Becomes steeper with age due to changes in elasticity of the cornea

Question 32

Which gender has steeper corneal curvature?

Answer 32

Females

Question 33

Is there a difference in the corneal curvature between races?

Answer 33

Nope

Question 34

A thin corneal thickness leads to what?

Answer 34

Underestimation of IOP

Question 35

A thick corneal thickness leads to what?

Answer 35

Overestimation of IOP

Question 36

What happens to the corneal thickness with age?

Answer 36

-decrease thickness with age

Question 37

What race has a thinner corneal thickness than the others?

Answer 37

African americans

Question 38

For a conicoid surface, the radius of curvature at off-axis points depends on what two things?

Answer 38

1. The distance from the vertex | 2. On the meridian at that point

Question 39

Which surface has the most significance on the corneal curvature?

Answer 39

Anterior surface corneal curvature

Question 40

The mean posterior vertex radius is what? With a mean asphericity of what?

Answer 40

5. 8mm | - 0.42