Optics: Corneal Optics

Question 1

The cornea is a…

Answer 1

Highly transparent structure of meniscus form

Question 2

Cornea has a ________ optical power

Answer 2

Fixed

Question 3

The cornea has about ________ of the total power of the relaxed eye

Answer 3

2/3rd (about 42D)

Question 4

Cornea refractive index

Answer 4

1.337

Question 5

Refractive index of the _________ dominates the other corneal layers

Answer 5

Stroma

Question 6

Diameter of cornea

Answer 6

12mm

Question 7

Central corneal thickness

Answer 7

0.5mm

Question 8

Cornea is _____ a spherical surface

Answer 8

NOT

Question 9

Cornea is regarded as an ___________

Answer 9

Ellipsoidal

Question 10

Anterior corneal asphericity (Q) values are usually _______________ which indicates that the cornea ______________________the vertex

Answer 10

Negative, flattens away from

Question 11

Significance of corneal flattening away from the vertex or center:

Answer 11

-to reduce spherical aberration
-to make smooth join with the main globe of eye

Question 12

Eccentricity value for the average normal cornea:

Answer 12

Q = -0.26

Question 13

Corneal diameter is measured from…

Answer 13

Limbus to limbus

Question 14

Accurate measure of corneal diameter helps:

Answer 14

-ensure contact lens diameter is sufficient to maintain full coverage of the cornea
-to produce right size anterior chamber IOLs
-to diagnose diseases

Question 15

Factors that affect corneal diameter:

Answer 15

-age, CD decreases slightly with age
-gender, males have significantly wider horizontal CD
-race, Asian CD is smaller than Caucasian

Question 16

Anterior corneal curvature (ACC) exhibits _________

Answer 16

Toricity

Question 17

Two principal meridians:

Answer 17

-tangential meridian
-saggital meridian

Question 18

Tangential meridian

Answer 18

Lies along the radius line from the vertex

Question 19

Sagittal meridian

Answer 19

Perpendicular tot he tangential meridian

Question 20

Cornea is significantly _____________ in both radius of curvature and asphericity

Answer 20

Asymmetric

Question 21

If refractive index, pupil diameter, and asphericity are considered constant, the spherical aberration will ________ if the corneal surface flattens, and ________ as the corneal surface becomes steeper

Answer 21

Decrease, increase

Question 22

Hyperopic eyes tend to be __________than myopic eyes

Answer 22

Flatter

Question 23

Each alteration in corneal curvature equal to _______ of a millimeter in radius corresponds to a variation of _____ diopters +

Answer 23

400th, 0.25D

Question 24

Cornea too curved leads to a condition called _____________

Answer 24

Keratoconus

Question 25

Cornea too flat leads to a condition called ________

Answer 25

Cornea plana

Question 26

When the vertical meridian of the cornea is steepest ______________ astigmatism occurs

Answer 26

With the rule

Question 27

When the horizontal meridian of the cornea is steepest __________________ astigmatism occurs

Answer 27

Against the rule

Question 28

Instruments used to accurately measure the anterior corneal curvature (ACC)

Answer 28

Keratometer, IOL master, corneal topographer

Question 29

What affects ACC?

Answer 29

-age, corneal curvature becomes steeper w age due to elasticity changes in cornea
-gender, females have steeper corneal curvatures due to shorter axial length

Question 30

Thin cornea can lead to ___________________of IOP and thick cornea can lead to _______________of IOP

Answer 30

Underestimation, overestimation

Question 31

Factors affecting central corneal thickness (CCT)

Answer 31

-age, CCT reduces with age due to decrease in keratocytes
-race, African Americans and those of African descent have thinner CCT than other races

Question 32

For a Conoid surface, the radius of curvature depends on:

Answer 32

-distance from the vertex on the meridian at that point

Question 33

Posterior corneal curvature is of less significance because of the…

Answer 33

Small refractive index difference across the posterior corneal boundary

Question 34

Posterior vertex Q value

Answer 34

-0.42