Ketatometry Flashcards
Keratometry
- measure the cornea
- uses radius of curvature and curvature
Eye Structure
- Gullstrand Eye
- two lens optical system
- Cornea and crystalline lens
Cornea Thickness, refractive index, and radius of curvature
Thickness: 0.5mm
Refractive index: 1.376
Radius: 7.7mm anterior
6.8mm posterior
Aqueous humor thickness and refractive index
Thickness: 3.1mm
Refractive index: 1.336
Crystalline lens thickness, refractive index, radius of curvature
Thickness: 3.6mm
Refractive index: 1.386 cortex
1.406 nucleus
Radius: 10mm anterior
6mm posteriorVitreous body thickness and refractive index
Thickness: 17.2mm
Refractive index: 1.336
Optical power of the cornea
- ANTERIOR power is what we use because posterior is super hard to measure and the power is really close even when you take thickness into account
- +43.83D
- super powerful plus lens
Curvature vs. Radius of curvature
Curvature:
- amount of curvyness
- Dipoters
- Larger curvature=smaller radius of curvature
Radius of curvature
- length from apex to center of curvature
- m
- Larger radius of curvature=smaller curvature
Normal Base curve
-8ish mm
Cornea
- Largest contributor to ocular optical power
- mainly attributed to anterior surface of curvature
- is a strong lens, but acts as a mirror
Refractive status
Emmetropes:60D
Myotropes: 64D
Hyperopes: 56D
Steep and flat corneal curvature
Steep: higher optical power
Flat: Lower optical power (larger radius of curvature)
Astigmatism
- distinct orientations of more and less optical power
- principal meridians
Sturm Conoid
- sagital (horizontal) and tangential (vertical) focus
- circle of least confusion: potential for point image instead of lines image usually seen
- directly related to different curvatures along the two principal meridians
Toric and spherical surfaces
Toric: different radius of curvature along the two principal meridians
Spherical: large or small, the meridians have the same radius of curvature
With the rule
- Steeper(more power) on vertical meridian
- flater on horizontal meridian
Against the rule
- steeper(more power) on horizontal meridian
- flatter on vertical meridian
- notice in older
Normal
- meridians differ by 90 degrees
- vertical is roughly 90
- horizontal is roughly 180
Cornea imaging
- convex mirror like
- image is smaller, erect, and virtual
- larger curvature=smaller image
- based on image magnification, we can determine curvature, and thus, power
Keratometric technique
- project image on retina
- line it up
- compare magnifications
- measure radius of curvature
- uses doubling technique to stabilize image even with patient moving
Types of Keratometers
- Javal-Schoitz: line up pyramid with box
- Zeiss- line plus sides inside eachother
- Baush and lomb-like up plus and minus signs on circles
Keratometry limitations
- Data from small optical zone (3mm)
- Assumes small angles/ paraxial approximation
- Assumes mostly spherical cornea
- Assumes symmetry with visual axis, corneal apex, and axis of instrument
- Assumes normal astigmatism
- ignores corneal abnormalities
- cannot measure asphericity
- tear film changes things
Benefits of topography
- provides multitude of other data
- can detect corneal abnormality
- measures curvature and power on any meridian
- EASY
- used in follow ups and planning refractive surgery
- can monitor changes in corneal surface
- helps with CLfit
-Still subject to tear film stability alterations
Placido topography
- most commonly used
- concentric contrasting circles
- records mires
Mire spacing
Normal spacing: normal cornea
Sparser: thinner lines, more space between lines, flatter cornea
Denser: thicker lines, less space between lines, steeper cornea
Curvature map
Red=steep(smaller magnification)
Blue=flat (larger magnification)
green=good
-healthy cornea will have a lot of blue
Keratometric equation
R*P=337.5
-this number is 1-refractive index of the cornea
Rough color map numbers
Red= 5mm and 58.79D
Green=7-8mm and 45D
Blue= 8-9mm and 39D
Keratoconus
-highly asymmetric corneal surface
Elevation map
-measured in micro meters
Scaling
-MAKE SURE YOU PAY ATTENTION TO SCALING!!
Dependent factors of topography
- dependent on pupil size
- influenced by tear film
Corneal Asphericity
- not exactly sphere
- all eyes have asphericity
- helps compensate spherical aberration
