Unit 3 Flashcards
Diagnostic instruments
-Dry eye testing
-Corneal sensation
-Specular microscopy
-Slit lamp assesment
-Burton lamp
-Corneal topography
-Corneal tomography
Diagnostic images
Anterior segment OCT
-Images of cross sections of diseased cornea and sclera lens
Radius of curvature
-Describes the curve of a surface by referring to the length of the curves radius
-Measured in mm
Short radius means..
Small number
Steep CURVE
Long radius means..
Large number
Flat curve
Relationship of Corneal power
P=(n-1) / r
P= corneal power
n= 1.3375
r= radius of curvature (m)
FOZR
Front optic zone radius
BOZR
Back optic zone radius
-determins the tear lens between the back surface of the RGP and cornea
BOZD
Back optic zone diameter
-Should be 1.5mm large than pupil size
TD
total diameter
OAD
overall diameter
BVP
back vertex power
CT
Centre thickness
RGP tear lens - On K (apical alignment)
-Zero powered tear lens
balanced tears on end and middle
RGP tear lens - steeper than K (apical clearance)
Plus powered tear lens
flatter tears at the end, bigger tears in the middle
RGP tear lens -flatter than K (apical bearing)
Minus powered tear lens
larger tears at the end , thinner tears in middle
Ideal GP fit
-centered over pupil
-Visual axis remains BOZD
-TD at least 1.4mm less than HVID (promotes tear exchange)
-Movement 1 to 1.5mm with each blind which allows tear exchange and removes debris in tear lens
-Edge clearance less than 1mm width
-Lid attachment vs interpalpebral fit
SAM rule
Steeper Add Minus
FAP rule
Flatter add plus
General rule
For every 0.05mm the rgp base curve is changed, the tear lens increased or decreased by 0.25D
If we steepen a lens we create a plus powered tears lens so what do we do…
Add minus power to final BVP
If we flatten a lens we are creating a minus powered tear lens so we…
Add plus power to final BVP
When the flat k is on the verticle meridian what type of astigmatism is this
ATR astigmatism
Characteristic of steeper lens
tight fit
less movementc
characteristic of flatter lens
looser fit
more movement
what does radiuscope measurment?
BOZR of RGP
when you increase lens diameter what happens?
-sag increases
-fit is tighter/steeper
when you decrease lens diameter what happens
-sag decreases
-fit is looser/flatter
when a lens is to flat with fluroscein how does it look?
pooling on the outside / peripheral
when a lens is too steep how does it look with fluroscien?
pooling on the inside centre / apical
how does a proper contact lens fit look with fluroscein?
pupil is visible , little bit of fluroscien along the peripheral
when do you vertex a power
higher than +/- 4.00D
when do you preform spherical equivalence?
if less than -0.75 astigmatism
Spherical abberation
Longitudinal spread of focus
positive spherical abberation
rays further from the principal axis are refracted more than central rays
ex; distance centre multifocal contact lenses, orthokeratology lens
negative spherical abberationt
-rays further from the principal axis are refracted less than central rays
ex; near centre multifocal contact lens
true or false; aspehric lenses reduce spherical abberations?
true
myopia management
-reduce perihperal defocus with plus power periphery in lenses
-multifocal CLs
-OrthoK
key concept in myopia management
slow axial length elongation
Shape Factor
Cornea is usually aspheric
SF= 1 - e2
Proloate cornea
flattens towards periphery
oblate cornea
steeper towards peripehry
Contact lens magnification formula
CLM: image corrected w/ contact lenses / image corrected with spectacles
Myopes accomodation
accomodative demand is higher with contact lenses compared to gls
hyperopes accomodation
accomodative demand is lower with contact lenses compared to gls
how can prismatic effect occur?
decentered lenses
how can induced astigmatism occur?
flexure = bending of lens on a toric cornea
-eyelids can induce astigmatism for the reason when you blink the contact lens can flex which creates more asitg
halo effect
bright circles or rings around a light source
most noticeable in a dim or dark area
large pupil with a smaller optic zone
coma abberation
higher order lens abberation
off axis points appear distorted with a tail
Flexure on a lens results in..
the lens will bend in the steeper meridian
the change in shape of the lens causes astigmatism
what should the edge clearnece be for alignemnt RGP fit?
less than 1mm width
normal amount of Shape factor
0.13 to 0.35
borderline amount of shape factor
0.02 to 0.12 and 0.36 0.46
abnormal amount of shape factor
0.00 to 0.01 and 0.47 to 1.0
