Keratometry Flashcards
gullstrand eye
2 lens optical system
what is the rough average of all the RIs of the components of the eye?
about 1.33
What is the optical power of the first surface of the cornea?
+48.83D
What does keratometry measure?
the anterior surface of the cornea
-radius of the different meridians
what is the radius of curvature of the cornea?
7.7mm
radius of curvature
- units: m
- larger the radius, smaller the curvature
- same algebraic sign as curvature
curvature
- diopters
- larger the curvature, smaller the radius
- same algebraic sign as radius
What is the largest contributor to ocular optical power?
cornea, mainly attributed to the anterior surface of curvature
What is there power of an emmetropic eye?
60D
astigmatism
- no point imaging
- distinct meridians with more and less power
- not spherical, shaped like a watermelon
sturm conoid
- circle of least confusion
- directly related to 2 different radius of curvatures along two different principal meridians
With the rule astigmatism
steeper vertical
against the rule astigmatism
steeper on horizontal
can shift with age
is the cornea a mirror?
yes, it also reflects
What is the light actually reflecting off of on the cornea?
the tear film
image from the cornea as a mirror
virtual, upright, demagnified
What kind of mirror is the cornea?
convex mirror
the smaller then radius of curvature…
the larger the curvature, the smaller the image
different types of keratometers
- the fixed doubling technique
- javal-schiotz keratometer
- ziess
- baush + Lomb
@180: radius of curvature 7.8mm, Power 43.25 D,
@90: radius of curvature 7.8mm, power 43.25 D
the cornea is spherical because the powers are equal at different meridians
@180: radius of curvature 7.8mm, Power 43.25 D,
@90: radius of curvature 7.6mm, power 44.50 D
WTR astig, steeper meridian is vertical
@180: radius of curvature 7.8mm, Power 43.25 D,
@90: radius of curvature 8.2mm, power 41.15 D
ATR astig, steeper meridian in the horizontal
keratometry limitations
- primary data taken from 4 points on the center 3mm
- assumes a lot. paraxial app only, low order aberrations only
- assumes rotationally symmetric cornea (spherocylindrical)
- assumes some symmetry, such as corneal apex, visual axis, and the axis of the instrument to coincide
- assumes normal astig
- keratocones may be challenging
- cannot measure asphericity
- tear film
keratometry vs topography
topography is present and future
topography
- multitude of info such as CL
- applicable to any corneal abnormality
- curvature and optical power on any meridian we select. not limited to just two
- easy to use
keratometry in comparison to topography
- measurement only at 3mm
- curvature and optical power only for two meridians
- manual alignment
topography features
- still subject to tear film stability
- Dx tool for corneal abnormalities
- learning curve-a multitude of info needs to assessed and evaluated
- high precision measurement of distribution of corneal curvature
why is topo an indispensable tool for dr
- indispensable in planning and f/u in laser vision correction
- helps select a CL
- can monitor and document changes in corneal surface
- can measure peripheral as well!
placido topo
- lasted till early 90s
- projects concentric rings on the cornea and records digitally the mires
- auromated computer based data processing
if the mires are closer together…
steeper cornea
if the mires are spread out
flatter cornea
if the mires look elongated
astig
if the mires look wavy in weird spots
irregular astig probably
curvature map green
good
keratometric equation
RxP=337.5
corneal asphericity reading
good for fitting CL
elevation/altimetric map
essential for surgery
measured in microns
simulated abberometry
dependent on pupil size
color at the edge of healthy cornea
blue
corneal asphericity
- not exactly sphere
- helps compensate spherical aberration
what would cause a flatter central cornea?
abnormalities or surgery
