Orhto K Flashcards
How old is ortho k
1960’s
What is the limitations of an ortho K lens
Comfort? Stability on cornea up to 2D convinience? cost ethical?
what’s topography
measurement of corneal shape/ contour
keratomotry only does central 3mm
analysis of keratoscopy reflections
if mires are thin and close together- STEP CORNEA
if mires are flat and far apart- FLAT CORNEA
what do generated colour maps show:
WARM- red colours show STEEPENING
COOLER- blue colours show FLATTENING
u should expect centre to be red and periphery to be blue.
what is ortho k
overnight lens wear- reverse geometry- stable fit
WHY would u want ortho k
reversibility- overnight- recovery after not wearing it. Thickness recovered after a day, curvature recovered after a week, rx recovered after a week
freedom- no lost lenses- can do sports/swimmin, improved vdu use, easy to handle
comfort- CL use dry eyes not an issue, i.e environment, vdi use. Leaves tear film un disrupted throughout day
control- how much rx u want to correct?
how does ortho k lenses work
flatten central cornea
refocuses light on correct part of retina
central thinning+ mid peripheral thickening- we re-distribute the cells/tissue.
Owen found flattening of anterior and posterior corneal surfaces
more on how ortho k works
calculations of posterior curvature and shape based on anterior topography and corneal thickness.
posterior cornea doesn’t flatten- instead oblates- peripheral corneal thickness changes
How many curves on an ortho k lens
what are they yo
4 curves clearance curve- periphery curve perifere curve- alignment curve reverse curve- fitting curve base curve radius
whats the central base zone
treatment zone flattens 0.2 flattening-1D over flatten by 0.5D to last for daytime approx 6mm use Munnerlyns formula- Ablation depth=(Rx D²)/8(n-1) refractive index of corns is 1.377
whats the reverse zone/fitting curve
closest to base zone
v steep- to bring lens in LANDING ZONE- cl is parallel to eye
we adjust here for cent ration
0.6mmm
space for the replaced epithelial tissue which is re-distributed
what’s the alignment zone
parallel to cornea
responsible for cent ration
increases effect of OrhtoK lens
channel for tear film
peripheral zone-
edge
for tear pump- tear exchange
0.4mm width and 11mm radius
overall diameter
LARGE DIAMETER-
10.50-11.00mm
aids centration and also makes lens comfortable
who can u do ortho k on
low myopes-
-1 to -4.50ds
with the rule can be corrected unto -1.50
and against the rule can be correct dot -1.00
how do u asses ortho k fittings
with fl
if a px comes into ur practice how do u fit ortho k lenses
FITTING SET- Initial trial can dispense from stock can of fl assessment expensive/time consuming infection? don't need topog
EMPIRICAL- custom made-lower chair time can lose control of parameters simple and cost effective toruble shooting including need good topog and accurate rx
which corneal torridity are ok to use ortho k on
central astigmatism
if limbus to limbus then WTR is ok
however AVOID ATR
however now there is newer designs
back surface torics
improved fit
correction for larger amounts of cyls
wot about hyperopia and ortho k
more difficult
only to +2
differnet design
what about presbyopia and ortho k
multifocal cornea?- effect of pupil size, quality of vision problems
try mono vision
how does orhto k control myopia progression
peripheral retina responsible for emmetropisation
rays hitting peripheral retina control axial growth
need to be optimal rx correction tho as image shell needs to be at the back of the eye
what are some ortho k studies that show myopia progression being limited
COOKI LORIC CRAYON CANDY SMART
