Topography Flashcards
What is a typically cornea radius of curvature
7.5mm
Is 8.6 a flat or steep radius of curvature for cornea
Flat
Does a flat corean have more or less power
Less
Does a steep cornea have more or less power
More
An approximate relationship for the calculation of optica power of the cornea by considering one surface, the anterior
Keratometric equation
What do we need to accurately measure total corneal power
Anteiror corneal curvature, posterior corneal curvature, and CCT
Corneal curvature and optical power
P=nā-n/R
What does keratometry measure
Only measures the front surface
Make assumptions that the back surface looks exactly like the anterior surface
Large curvature means a _______ radius of curvature
Small
What is the image on the corneal surface
Upright
Minified
Virtual
Focal distance
F=R/2
What does the keratometry actually measure
The tear film
Keratometry limitation
- Only works in center
- Normal cornea
- significant assumptions
- assumes rotational symmetry
- assumes some symmetry
- assumes normal astig
- corneal abnormalities may be challenging
- cannot measure aspherity
- tear film may limit the measurement accuracy
Mires: flatter at the center steeper at the periphery
Non-equidistant mires
Denser mire spacing
Steeper cornea
Sparser mire spacing
Flatter cornea
Mires: WTR astigmatism
Oblong like a football lying down
Mires: ATR astigmatism
Oblong like football standing on end
Mires: flatter at the center and the periphery
Asphericity
Keratoconus mires
Steeper in the bottom, flatter in the top
Denser mires
Smaller magnification
Increased curvature
More power
Sparser mirers
Larger magnification
Reduced curvature
Less power
Things about topography
- provides multitude of other data such as CL fitting
- applicable to ant corneal abnormality
- curvature and optical power on any meridian we select. Not limited to just two
- easy-to-use, requires just proper centration
Keratometry
- measurements only at central 3mm
- curvature and optical power only for 2 meridians
- manual alignment
If given the power of the power of the cornea (42.2D), how do you find the radius of curvature?
337.5/42.2=8mm
Is there more or less power in the periphery of the cornea?
Less power, because its steeper
Simulated keratometry
Simulated, not real
Based on keratometric assumptions
