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Contact lens > Orthokeratology > Flashcards

Orthokeratology Flashcards

1
Q

Ok cls - 3 back surface designs

A

Simple
Blended
Sigmoidal

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2
Q

Toric OK cls indications

A

Peripheral corneal astigmatism
Significant peripheral elevation differences >/= 15um in principal meridians
Incomplete ring of mid-periph steepening post OK
Unknown cause of ok decentration

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3
Q

Corneal rheology Theory

A

Central cornea pushed in
Mid periphery pulled out
BOZR pushes in (+ force)
Reverse curve pulls cornea out (- force)

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4
Q

What other corneal changes occur in OK

A

Bonding of stroma
Central thinning epithelium,
Mid periphery thickening (mainly stromal)
Changes in corneal saggital height
Refractive changes die to corneal shape change

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5
Q

Effects of OK

A 
Corneal curvature changes 
Corneal thickness
Mainly epithelium 
Central thinning 
Mid-periph thickening 
Minor change to sag height
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6
Q

Munnerlyn’s formula

A

Estimates amount of tissue displaced

S = td^2 x d/3

S= change in sag height um
td = diameter tx zone mm
D = desired dioptricc change
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7
Q

According to mountford wats the mean max sag height change in OK

A

20UM

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8
Q

End point of OK

A 
When cornea becomes spherical e=0
Unaided VA 6/6 or better
Sl.hyperopia 0.5d on removal
Regular topos - bulls eye
Min regression 10-12hrs post removal
Stable rx over 2-3mths
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9
Q

How is OK and e value of cornea related

A

Myopia reduction will depend on the e value
- if e=0 - generally not much reduction - OK not going to work

Diff in e = 0.21 x Rx change

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10
Q

If e value and rx don’t match

A

Myopia due to axial instead of corneal

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11
Q

Px selection for OK cls

A 
-0.50 - -4.00D
<1.50 DC wtr
K approx 42D
Relatively high e values >/=0.5
Steeper periphery corneas - e between 0 and +1
<6mm pupil in dim 
HVID >11m
Normal anterior eye
Sporting person
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12
Q

Bad OK pxs

A

Prev failure with GP cl
Disease of cornea, conj, adnexa - e.g. dry eye
AC inflammtion
Systemic conditions exacerbated by cls - e.g. diabetes
Older pxs cornea less likely to respond well
Unrealistic expectations
Low sph + high cyl
Peripheral astigmatism
V steep/flat ks
Spherical Cornea e=0
- e /rx don’t match

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13
Q

Trial lens process

A 
Select initial cl design 
Use topical anaesthesia if needed (not often) - stop excess tearing 
Assess cl after 5-10min
- ask px keep eye closed 
White light
Blue light
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14
Q

Bulls eye pattern myopia OK

A 
Central bearing 3-4.5mm
Wide/deep tear reservoir paracentral 
Mid periph pooling
Good lateral centration /pupil coverage
Min movement on blink
Active tear exchange
No/small bubbles in tear reservoir
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15
Q

Normal pooling - OK

A

Central pooling

Mid periph bearing

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16
Q

Overnight wear advantages

A 
Less need for adaptations
Less discomfort 
Maximise OK effects 
Easier for px
High dk/t are avail
17
Q

OK lens assessment

A 
Morning after
Is cl bound? 
Educate about loosening adherent cl 
Measure aided VA
O/R
SLE
Unaided VA
Ret/refraction
Topos
Subtractive map - monitor changes
18
Q

Cl adherence

A

Common after overnight gp Cl wear
Aq thinning/increased tear viscosity
Px education vital
- detecting bound cl
- method of release - blinking/lubricants
- removing bound cl can cause epithelial trauma/pain

19
Q

Central islands

A

Indicates BC not flat enough
Inadequate central compression corneal epithelium
Excessively tight mid periph bearing
Refit

20
Q

Smiley face

A 
Superior decentration 
Cl fit too flat 
TD too small
Refit 
- steepen reverse curve 
- steepen alignment curve
21
Q

Frowny face

A

Inferior decentration
TD too small
Superior is steeper

22
Q

Lateral decentration

A 
TD too small
Excesssive lid force 
Eccentric apex 
Corneal asymmetry in nasal/temporal meridian
Refit
23
Q

Superior decentration

A

BOZR too flat
TD too small
High minus

24
Q

Inferior decentration

A

BOZR too steep
TD too small
Loose lids
Apex too low

25
Q

How does pupil size affect OK

A

Larger pupil. Better
Larger tx zone
Small pupils don’t allow rays of myopic defocus in periphery

26
Q

Ocular factors assoc with slow OK progression

A

Larger pupil diam
Deeper AC
Steeper/more prolate topo

27
Q

Why is low myope not best for ok

A

Insufficient mid-periph steepening
- inadequate myopic defocus in periph

Consider alternative tx/custom design

28
Q

Sph OK reduces myopia by

A

32-63%

29
Q

Toric OK reduce elongation by

A

52%

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