Optics: Corneal Optics Flashcards
The cornea is a…
Highly transparent structure of meniscus form
Cornea has a ________ optical power
Fixed
The cornea has about ________ of the total power of the relaxed eye
2/3rd (about 42D)
Cornea refractive index
1.337
Refractive index of the _________ dominates the other corneal layers
Stroma
Diameter of cornea
12mm
Central corneal thickness
0.5mm
Cornea is _____ a spherical surface
NOT
Cornea is regarded as an ___________
Ellipsoidal
Anterior corneal asphericity (Q) values are usually _______________ which indicates that the cornea ______________________the vertex
Negative, flattens away from
Significance of corneal flattening away from the vertex or center:
-to reduce spherical aberration
-to make smooth join with the main globe of eye
Eccentricity value for the average normal cornea:
Q = -0.26
Corneal diameter is measured from…
Limbus to limbus
Accurate measure of corneal diameter helps:
-ensure contact lens diameter is sufficient to maintain full coverage of the cornea
-to produce right size anterior chamber IOLs
-to diagnose diseases
Factors that affect corneal diameter:
-age, CD decreases slightly with age
-gender, males have significantly wider horizontal CD
-race, Asian CD is smaller than Caucasian
Anterior corneal curvature (ACC) exhibits _________
Toricity
Two principal meridians:
-tangential meridian
-saggital meridian
Tangential meridian
Lies along the radius line from the vertex
Sagittal meridian
Perpendicular tot he tangential meridian
Cornea is significantly _____________ in both radius of curvature and asphericity
Asymmetric
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
Decrease, increase
Hyperopic eyes tend to be __________than myopic eyes
Flatter
Each alteration in corneal curvature equal to _______ of a millimeter in radius corresponds to a variation of _____ diopters +
400th, 0.25D
Cornea too curved leads to a condition called _____________
Keratoconus
Cornea too flat leads to a condition called ________
Cornea plana
When the vertical meridian of the cornea is steepest ______________ astigmatism occurs
With the rule
When the horizontal meridian of the cornea is steepest __________________ astigmatism occurs
Against the rule
Instruments used to accurately measure the anterior corneal curvature (ACC)
Keratometer, IOL master, corneal topographer
What affects ACC?
-age, corneal curvature becomes steeper w age due to elasticity changes in cornea
-gender, females have steeper corneal curvatures due to shorter axial length
Thin cornea can lead to ___________________of IOP and thick cornea can lead to _______________of IOP
Underestimation, overestimation
Factors affecting central corneal thickness (CCT)
-age, CCT reduces with age due to decrease in keratocytes
-race, African Americans and those of African descent have thinner CCT than other races
For a Conoid surface, the radius of curvature depends on:
-distance from the vertex on the meridian at that point
Posterior corneal curvature is of less significance because of the…
Small refractive index difference across the posterior corneal boundary
Posterior vertex Q value
-0.42
