Unit 3 Flashcards by Megan Blomer

Diagnostic instruments

-Dry eye testing
-Corneal sensation
-Specular microscopy
-Slit lamp assesment
-Burton lamp
-Corneal topography
-Corneal tomography

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Diagnostic images

Anterior segment OCT
-Images of cross sections of diseased cornea and sclera lens

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Radius of curvature

-Describes the curve of a surface by referring to the length of the curves radius
-Measured in mm

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Short radius means..

Small number
Steep CURVE

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Long radius means..

Large number
Flat curve

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Relationship of Corneal power

P=(n-1) / r
P= corneal power
n= 1.3375
r= radius of curvature (m)

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FOZR

Front optic zone radius

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BOZR

Back optic zone radius
-determins the tear lens between the back surface of the RGP and cornea

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BOZD

Back optic zone diameter
-Should be 1.5mm large than pupil size

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total diameter

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OAD

overall diameter

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BVP

back vertex power

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Centre thickness

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RGP tear lens - On K (apical alignment)

-Zero powered tear lens
balanced tears on end and middle

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RGP tear lens - steeper than K (apical clearance)

Plus powered tear lens
flatter tears at the end, bigger tears in the middle

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RGP tear lens -flatter than K (apical bearing)

Minus powered tear lens
larger tears at the end , thinner tears in middle

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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

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SAM rule

Steeper Add Minus

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FAP rule

Flatter add plus

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General rule

For every 0.05mm the rgp base curve is changed, the tear lens increased or decreased by 0.25D

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If we steepen a lens we create a plus powered tears lens so what do we do…

Add minus power to final BVP

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If we flatten a lens we are creating a minus powered tear lens so we…

Add plus power to final BVP

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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