Corneal Optics Flashcards
What is the corneal power and how much of the total power does it account for?
+42 diopters
-about 2/3 of the total power of the eye
What is the refractive index of the cornea?
1.377
Each corneal layer has its own refractive index, but which layer dominates it?
The stroma
What is the diameter of the cornea?
12mm
What is the corneal thickness?
O.5mm
Is the cornea a spherical surface?
Nope
What aberration is associated with spherical lenses?
Spherical aberration
-marginal rays focus at a different spot than the central rays
What does an aspheric lens do?
It makes the curvature flatter to nullify the spherical aberration
The cornea is regarded as what shape?
Conicoid (ellipsoidal)
The anterior corneal asphericity values are usually what? What does that indicate?
- usually negative
- the cornea flattens away from the vertex
What are the two significant things that come from the corneal flattening away from the vertex?
- To reduce spherical aberration
2. To make it smooth and join with the globe of the eye
Normal eccentricity is what?
0.73
What are the three corneal optical properties that effect vision?
- Corneal diameter
- Anterior corneal curvature
- Central corneal thickness
How is the corneal diameter measured?
Limbus to limbus
Accurate measurement of corneal diameter helps what three things?
- To make sure contact lens diameter is sufficient to cover the entire cornea
- To produce the right size anterior chamber IOLs
- To diagnose diseases
What is the average horizontal diameter? Vertical?
Horizontal: 11mm
Vertical: 12mm
What does corneal diameter do with age? Why?
Decreases with age due to narrowing of the anterior chamber (glaucoma)
Males have slightly _____________ corneal diameter than females?
Slightly wider horizontal corneal diameter
Asian CD is ___________ than whites?
Asian CD is smaller
What is toricity?
Curvature is different in different meridians
Is the cornea symmetric in regards to curvature and asphericity?
Nah it’s asymmetric
Corneal shape geometry leads to differences in what?
Differences in spherical aberration
If the corneal surface flattens, what will happen to the spherical aberration?
It will decrease
If the cornea becomes steeper, what will happen to the spherical aberrations?
They will increase
Are myopes or hyperopes flatter?
Hyperopes
Each alteration in curvature equal to 400th of a mm in radius corresponds to a variation in how many diopters?
0.25D
Corneal refractive surgery showed an increase in what?
Corneal asphericity
A cornea that is too curved is a condition called what?
Keratoconus
A cornea that is too flat leads to a condition called what?
Cornea plana
In an average cornea, which meridian has a smaller radius?
Vertical meridian has a smaller radius (steeper)
Corneal curvature does what with age?
Becomes steeper with age due to changes in elasticity of the cornea
Which gender has steeper corneal curvature?
Females
Is there a difference in the corneal curvature between races?
Nope
A thin corneal thickness leads to what?
Underestimation of IOP
A thick corneal thickness leads to what?
Overestimation of IOP
What happens to the corneal thickness with age?
-decrease thickness with age
What race has a thinner corneal thickness than the others?
African americans
For a conicoid surface, the radius of curvature at off-axis points depends on what two things?
- The distance from the vertex
2. On the meridian at that point
Which surface has the most significance on the corneal curvature?
Anterior surface corneal curvature
The mean posterior vertex radius is what? With a mean asphericity of what?
- 8mm
- 0.42
