Tear fluid squeeze pressure - force holding cl in place - centres cl by opposin gravity/eyelid force

GP CLs Flashcards by Areez Nasar

Adherence of a cl increased by:

increased OZD/TD
Decreased BOZR(Steep BC)
Decrease peripheral/IM curve radii
decrease thickness

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Cl adherence decreased by:

decreased OZD/TD
Increased BC
Increased thickness
increase peripheral/IM curve radii

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Capillary attraction

F = 1/t

Attraction (F) inversely proportional to tear layer thickness (t)

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What is the TFSP

Tear fluid squeeze pressure

force holding cl in place
centres cl by opposin gravity/eyelid force

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ELF

Holds cl between blinks - -ve pressure counteracts gravity

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CoG shifts backwards when:

Increased minus power
Increased TD
Decreased bc
Decreased centre thickness

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CoG shifts forwards

More positive power
Decreased TD
Increased bc
Increased centre thickness

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Techniques to promote adaptation

Viscous wetting solution
Eye closure 5-10s
Downward gaze in dim room
Refrain from primary gaze

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White light static assessment

Cl Position post settling
- central/high/low riding/decentred obliquely

Centration - with/without lids

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White light dynamic assessment

Post blink - recentration
Lateral gaze - cross limbus?
Speed/lag

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Importance of static assessment

Static only way to confidently make valid comparisons between designs

Dynamic fits affected by:
Gaze direction
Blink force
PAS
lid tension

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Static assessment

Centration
Central pooling 
Mid peripheral bearing 
Edge lift on peripheral curve 
Tear exchange

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Steep fit

Central pooling
Mid peripheral bearing 
Narrow edge clearance 
Typically unstable - decentres inferiorly 
Little/no movement

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Flat fit

Central bearing 
Mid peripheral pooling 
Excessive edge clearance 
Excess movement on blink 
Discomfort / inconsistent vision

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How to correct a steep fit

Decrease TD
Increase BC
Decrease BOZD
Flatten BPRs

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How to correct flat fit

Increase TD
Decrease BC
Increase BOZD
steepen BPRs

What does the final GP rx consist of

BVP trial cl
Tear lens power
Over rx

Conditions only fitted with GP

Keratoconus
Traumatised corneas
Post graft

Benefits of GP

Customisable 
Less contamination risk
Wider range 
Lifestyle issues - eg sports 
Better VA
Less deposit build up
Less surface dehydration

GP parameters

TD = HVID -2mm
BC = 0.1 steeper than flattest K or flattest K for high astigmatism (2.50+) (toric BC)
BOZD = scotopic pupil +1mm (min)

FAP

Flat add plus

Every 0.1mm flatter BC -> O/R with +0.50

SAM

Steep add minus

For every 0.1mm steeper BC -> O/R with -0.50

Px selection for simultaneous GP Mfs

Reading position straight ahead
Can read any position - not just down
Early mod presbyopes
IM task demands - computer, music sheets, sewing etc

Px selection for translating GP Mfs

Reading position down
Need excellent DV
High adds
Large/small pupils - not pupil dependent
Lower lid within 1mm of limbus
Fair PAS - allow 2mm translation
Sclera flatter than cornea - allow translation

Advantages of translating GP Mfs

Excellent dv/nv Normal stereo Good contrast Customisable - wide ranges/high adds/choice of materials

Disadvantage to translating GP Mfs

Need stabilisation - prismballast/truncation Very mobile - not as comfortable Bifocal - gaze dependent - IM portion absent/poor

Advantages of simultaneous GP Mfs

``` Minimal gaze dependence Comfort = to SV GP normal stereo No jump -prismatic effect Easier fit than alternating ```

Disadvantages of simultaneous GP Mfs

``` Difficult to get high adds - may need over specs/monovision Compromised VA Some contrast loss Spectacle blur in higher adds Not avail in BST design ```

Fix GP Mf low riding

Avoid steep fit Increase TD Change stabilisation method? - negative carrier for high plus Avoid high WTR astigmatism >2.50

Fix high riding GP Mf

``` Decrease TD - if narrow PAS avoid flat fit Avoid WTR astigmatism Avoid excess lid attachment Diff cl design - prism ballast ```

Fix laterally decentred GP Mf

Increase TD | Avoid mod ATR astigmatism >1.50

GP MF - poor DV

- O/R - giving least amount minus Reduce add in Dom eye Change Dom eye to SV Larger zone over which add works - if large pupils

GP Mf - poor NV

Increase add in non-Dom eye O/R - if add >0.75 change to FST non Dom eye Smaller zone over which add works - small pupils

Gp Mf rotation

Nasal rotation <30deg accepted / >30deg compensate | Any Temporal rotation compensate

Assessing GP MF FIT

Speed of return: <1s accept / >1s adjust fit (BC) | Seg position: to sit between 20-30% of pupil - if >1/3mm into pupil - adjust (decreasing TD)