Corneal Optics Flashcards
What part of the cornea protects the stroma?
Epithelium
What part of the cornea is responsible for the transparency
Stroma
What controls the water content of the stroma?
Endothelium
What can affect the transparency of the cornea?
Increased water
Cornea has a ________ optical power
Fixed
Cornea has about _______ of the total power of the eye (relaxed)
2/3
What is the calculated power of the cornea?
42D
What is the refractive index of the cornea
1.377 (mostly from the stroma)
Refractive index of _________ dominates the other corneal layers
Stroma
What is the corneal diameter
12mm
What is the CCT?
0.5mm
What is CCT measured with
- pachymetry
- scheimpflug
- Ct
What are the most important aspects of the cornea
- power
- curvature
- diameter
- thickness
What is the front power of the cornea?
49.08D
What is the power of the back surface of the cornea?
-7.057D
What is the total corneal power?
49.08 + -7.057 = 42.023D
Is the cornea a spherical surface?
No but we still calculate the power like we do for a spherical lens
Equivalent power of the cornea
Fc= F1 + F2 - (d/n)F1F2
=43.05D
What is the back vertex power of the cornea?
Fav=F2+ (F1/(1-d/n*F1)
=43.83D
How do you fix spherical abberations?
By making the lens aspherical
Asphere compared to sphere
Asphere has a flatter curvature compared to spherical
Q=0
Spherical
Q=-n
Prolate
Q=+n
Oblate
What are the different apshericities
- conicoid
- ellipsoid
- eccentricity
- shape factor
Cornea is regarded as a _______, which is ellipsoidal
Conicoid
The anterior corneal asphericity (Q) values are usually ________
Negative
What does the negative Q value of the anterior corneal asphericty indicate?
That the cornea flattens away from the vertex
What is the significance of the corneal flattening away from the vertex or center?
- To reduce spherical aberrations
2. To make smooth join with the main globe of the eye
Reducing spherical aberrations with asphericity in the cornea
The amount of corneal asphericity is sufficient enough to eliminate it. Q required to eliminate SA at the anterior surface is -0.528
What is the predominant higher order aberration?
Spherical aberrations
What can change the asphericity of the cornea?
Refractive surgery
At what eccentricity is the spherical aberration 0?
0.73
Corneal diameter is from where to where?
Limbus to limbus
Why is it important to have accurate measurement of CD?
- contact lens diameter sufficiency
- right size anterior chamber IOLs
- diagnose diseases
What instruments can you use to measure corneal diameters?
- autorefractor
- corneal topographer
- hand help mm ruler or slit lame graticule
What is the horizontal diameter of the cornea?
11mm in adults (10 in children)
What is the vertical diameter of the cornea?
12mm in adults (11mm in children)
Corneal diameter and age
Decreases slightly with age due to age related narrowing of anterior chamber increasing the chances of glaucoma
Gender and corneal diameter
Males have a significantly wider horizontal corneal diameter than females
Race and corneal diameter
Asians CD is smaller than caucasians
Anterior corneal curvature (ACC) exhibits ______
Toricity
What are the two principle meridians of the cornea?
Tangential
Sagittal
Tangential meridian of the cornea
Lies along the radius line from the vertex
- vertical
- more curvature here
Sagittal meridian of the cornea
- perpendicular to the tangential meridian
- horizontal
- less curvature
Are the curvatures the same in both meridians of the cornea?
No
Cornea is significantly _______ in both radius of curvature and asphericity
Asymmetric
If the refractive index, pupil diameter and asphericity are considered constant, the spherical aberration will do what?
- decrease if the corneal surface flattens
- increase as the cornea becomes steeper
More aberration
Steeper
Hyperope tend to be ______ than the myopic eye
Flatter
Each alteration in curvature equal to _______ in radius corresponds to a variation of 0.25 D
400th of a mm
Studies show _______ after the corneal refractive surgery using excimer lasers
Increased corneal asphericity
Cornea too curved leads to this condition
Keratoconus
Cornea too flat leads to this condition
Cornea plana
The average cornea curvature has a smaller radius in the ______ meridian compared to the other meridian
Vertical
The average cornea curvature has a smaller radius in the vertical meridian compared to the horizontal meridian, which contributes to the ___________ in adults
With the rule astigmatism
Astigmatism occurs when the vertical meridian of cornea is steepest
With the rule
Astigmatism occurs when the horizontal meridian of the cornea is steepest
Against the rule
Corneal curvature and age
Steeper with increasing age due to changes in elasticity of the cornea
Gender and corneal curvature
Females have significantly steeper average corneal curvature than males
Race and corneal curvature
Caucasian, hispanics, asians, and African americans found no significant differences between the races studied
What does CCT influence?
Measured IOP and consequently the classification and management of glaucoma
A thin cornea leads to _______ of IOP
Underestimation
A thick cornea leads to _____ of IOP
Overestimation
What relies on the accurate measurement of CCT
Detection and management of CL complications and certain surgical procedures such as astigmatic keratotomy, LASIK, and PRK
Age and CCT
Reduction of CCT with age due to decreased keratocytes with age
Gender and CCT
Difference between genders was 4.7 microns
Race and CCT
African americans and other African decent have thinner CCT than other races
For a conicoid surface, the radius of curvature at off-axis potions depends upon
- the distance from the vertex
- on the meridian at that point
Studies show that the mean posterior vertex radius is ______ and a mean asphericity of Q=___
- 8mm
- 0.42
Why is the posterior corneal curvature (PCC) of lesser significance than the anterior surface?
Because of the small refractive index difference across the posterior corneal boundaries, but it is not of negligible significance
