Orthokeratology Flashcards
Ok cls - 3 back surface designs
Simple
Blended
Sigmoidal
Toric OK cls indications
Peripheral corneal astigmatism
Significant peripheral elevation differences >/= 15um in principal meridians
Incomplete ring of mid-periph steepening post OK
Unknown cause of ok decentration
Corneal rheology Theory
Central cornea pushed in
Mid periphery pulled out
BOZR pushes in (+ force)
Reverse curve pulls cornea out (- force)
What other corneal changes occur in OK
Bonding of stroma
Central thinning epithelium,
Mid periphery thickening (mainly stromal)
Changes in corneal saggital height
Refractive changes die to corneal shape change
Effects of OK
Corneal curvature changes Corneal thickness Mainly epithelium Central thinning Mid-periph thickening Minor change to sag height
Munnerlyn’s formula
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
According to mountford wats the mean max sag height change in OK
20UM
End point of OK
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
How is OK and e value of cornea related
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
If e value and rx don’t match
Myopia due to axial instead of corneal
Px selection for OK cls
-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
Bad OK pxs
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
Trial lens process
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
Bulls eye pattern myopia OK
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
Normal pooling - OK
Central pooling
Mid periph bearing
Overnight wear advantages
Less need for adaptations Less discomfort Maximise OK effects Easier for px High dk/t are avail
OK lens assessment
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
Cl adherence
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
Central islands
Indicates BC not flat enough
Inadequate central compression corneal epithelium
Excessively tight mid periph bearing
Refit
Smiley face
Superior decentration Cl fit too flat TD too small Refit - steepen reverse curve - steepen alignment curve
Frowny face
Inferior decentration
TD too small
Superior is steeper
Lateral decentration
TD too small Excesssive lid force Eccentric apex Corneal asymmetry in nasal/temporal meridian Refit
Superior decentration
BOZR too flat
TD too small
High minus
Inferior decentration
BOZR too steep
TD too small
Loose lids
Apex too low
