Orthokeratology Flashcards
who can fit ortho k CLs
all qualified optoms
what type of material did early ortho k lenses use how was it fitted when was it used unto how much myopia could it correct what was the disadvantage to it
- Used conventional hard lens designs
- Fitted progressively (to push the cornea down, so wasn’t comfortable) flatter over a period of months
- Daytime wear
- Small amounts of myopia only, i.e. 1.00 or 2.00 dioptres.
- Unstable fit
what is modern ortho k lenses termed as
what do modern materials allow for
what type of lenses are they and wha does that allow for
what is attempted with the first pair
- Today’s orthokeratology can be termed “Accelerated Orthokeratology”
- Modern lens materials allow overnight lens wear
- Reverse geometry lenses= stable fit
- Full correction attempted with the first pair
what is a reverse geometry lens
instead of conventional lens which is fitted steep in the centre and flatter in the periphery, ortho k lenses are flat in the centre and steep in the periphery
list 5 reasons you will want to fit ortho k lenses
- Reversible
- Safe
- Convenient
- Not affected by marginal dry eye
- Practice building
how are ortho k lenses reversible
how long does it take for corneal thickness to recover
how does it take for corneal curvature to recover
how long does it take for refractive correction and binocular uncorrected visual acuity to recover
- Overnight orthokeratology: Refractive & corneal recovery after discontinuation of reverse geometry lenses
- Corneal thickness recovered fully after one night of no lens wear
- Corneal curvature recovered after 1 week
- Refractive correction and binocular uncorrected visual acuity recovered fully after 2 weeks
what 4 facts about ortho k lenses makes it safe to wear over night
- RGP wear is safest form of CL wear
- Overnight RGP wear shows very low rates of infection (no bacteria sticking to lens compared to SCL)
- Ortho K is notcontinuous wear
- Patient wear lenses for 6 to 8 hours instead of 24+
what is the incidence of getting microbial keratitis with ortho k lenses equivalent to
daily wear soft lenses
list 4 reasons why ortho k lenses are convenient
- No lost lenses during the day
- No problems with swimming, saunas hot-tubs etc!
- Lenses are cleaned in the morning when the patient may be a bit more awake
- Easy to handle
how often is ortho k lenses recommended to replace and why
every 6 months
to avoid bacteria and scratches on lens
why are ortho k lenses good for patients who have marginal dry eye
Orthokeratology lenses are worn overnight so these environmental factors are not an issue.
e.g. VDU use, central heating, air con
give 3 reasons why fitting ortho k lenses is good for practice building
- Orthokeratology is a “specialist” contact lens
- Patients don’t look to get contact lens supply from the internet or supermarkets
- Patients fitted with orthokeratology lenses talk to their friends, family colleagues about it
how many curves are there in an ortho k lens and name each curve
4 curves:
- peripheral curve
- alignment curve
- fitting curve
- base curve
which curve is the treating zone
the base curve
what is the function of he central zone of the ortho k lens
what equation applies to this
what is a regression factor
what is the approx diameter of this zone
which formula is used to work out this curve
- this is where you will work out how much to change the prescription by
- Kflat–Rx = change you want to get
- regression factor = normally +0.50/+0.75, you want them to have a +0.75D rx to compensate for the regression by the end of the day as the cornea changes
- Diameter approx = 6mm
- Munnerlyn’s formula = what ophthalmologists use for refractive surgery. the higher the rx the smaller the treatment zone, the lower the rx the smaller the treatment zone
which curve is the fitting zone of the ortho k lens how is it fitting what does this cause the CL to be what is the approx width why will you adjust this area for what does it provide
- Reverse Curve
- Very steep radius
- Brings contact lens parallel to the cornea by sicking the epithelium as its used as an area where the actual tissue is being moved and can accumulate
- Approx 0.6 mm width
- Adjust here to improve centration to bring treatment zone back onto the cornea
- Provides space for the replaced epithelial tissue (redistribution)
where is the alignment zone what is it responsible for what is the approx width what does it provide why will you adjust this area for
- Parallel to the cornea
- responsible for contraption
- Approx 1.0 mm width
- Provides channel for tear film forces
- Adjust here to improve centration
how does the alignment zone provide centration and a channel for the tear film forces
it gives a negative force that will suck the actual cells from the centre out towards the reverse curve
so provides centration and also a channel for the tear film
where is the bearing surface of an ortho k lens
which diameter are they fitting with and why
- The bearing surface of the OK lens is on a less sensitive part of the cornea
- 10-11mm TD
- because it is more comfortable to fit as theres more contact with the periphery wheres the sensitivity of the cornea is less
- i.e. it fits just outside the limbus
what is the function of the peripheral zone which curve is found here what is the approx width what is it's radius what may this give
- only there to help get the lens out
- peripheral curve
- approx width = 0.4mm
- radius = 11.00mm
- this may give edge clearance, but this may not be a problem
what is the overall diameter of an ortho k lens
what does this TD aid
what advantage does it also have
- Average 10.50 –11.00mm
- Aids centration
- Also makes lens comfortable
what will you need to get good centration and correction with a toric ortho k lens
what does this allow for
- need alignment fit 360º in the periphery
- allows lens to centre and forces to be distributed evenly across treatment zone
which type of corneal toricity does a spherical/conventional ortho k lens work fine for
and which type does it not work so well for and why
- works well for central astigmatism
i. e. central cyl and spherical periphery - will get a successful fit and as centre of the lens i.e. treatment zone must be spherical
- won’t work well for limbus to limbus astigmatism WTR or ATR
- it won’t work well in one meridian as the sag won’t fit and it won’t get enough pressure build up to get it to work
e.g. if px had 2.50D cyl and was fitted with a spherical/conventional ortho k treatment zone, px will end up with 1.25D cyl in centre
what type of lens is now available for limbal to limbal astigmatism
- Back surface peripheral torics
what affect can a toric ortho k lens have on a 3D cyl and in what time frame
can go from 3DC to 0.50DC after 3 months
when is a toric ortho k usually used
and what does it depend on
- usually used when cyl is over -1.50DC
- rx with
how does a ortho k lens work
- Orthokeratology flattens central cornea
- By squeeze film force from tears by using a -ve force outside of the treatment zone by sucking out those cells
- The lens should never make contact with the cornea
- The cornea then focuses correctly
what 3 theories are there about the, changes are actually taking place in the cornea with orthokeratology
1)
- central epithelial thinning
- mid peripheral corneal thickening
- Orthokeratology may redistribute corneal tissue ratherthan bend cornea
i. e. a movement of cells of the epithelium instead of bending found from the mid peripheral thickening
2)
- Orthokeratology causes central corneal epithelial thinning
- Mid peripheral stromal thickening
3)
- Flattening of ant / posterior corneal surface, during adaptive stages of ortho k
so no definitive answer
why does mid peripheral stroma thickening (1 theory about ortho k) not cause scarring and what can it mean if stromal thickening becomes permanent as a result of ortho k
- it cannot be reversed
- but does not cause corneal scarring because its a 5-10um tear layer, but no movement
- if stromal thickening is permanent, this can be good for myopia control (but don’t know for sure yet)
what 5 things/types of people does ortho k lenses work for
- Refractive errors
- Corneal Health
- Systemic diseases
- Lifestyle
- Patient expectations
which types of refractive errors does ortho k lenses work for in terms of:
- myopia
- WTR cyl
- ATG cyl
- low to moderate myopes, i.e. -1.00DS to -5.00DS
- With-the-rule cyls can be corrected up to -2.50DC
- Against-the-rule cyls can be corrected up to -1.00DC
list 4 corneal health exclusions for ortho k lenses i.e. you shouldn’t fit for
- Corneal irregularities, e.g. keratoconus, scarring, previous refractive surgery etc
- Clinical dry eye
- Active anterior segment disease
- Abnormal topography
–Corneal eccentricity is important
Normal eccentricity is 0.5
why can you not fit someone who has clinical dry eye with ortho k lenses
as a certain amount of tars is needed with ortho k lenses to move the lens around
which systemic condition should you not fit ortho k lenses and why
which 3 systemic medications should you not fit ortho k lenses with
- diabetics - as they have reduced corneal sensitivity and corneal healing
- do not fit with people taking
- Roaccutane
- Thyroxine
- IOP medications
which lifestyle is best to fit ortho k lenses on
which is not the best lifestyle to fit ortho k lenses
- Best patients are those who require the convenience of no lens wear in the day and sleep at least 6 hours at night!
- Patients who have irregular sleep patterns may not be the easiest to fit.
- However there are normally ways round shift patterns etc.
- No pyjama folders - fussy people
what patient expectations is ortho k lenses best for
- Patients should expect good vision all day binocularly.
- It is not unusual to have one eye that sees better than the other.
- The refraction may not be plano post treatment.
as it gives a bit of +0.50/+0.75 in the morning to compensate for the regression. vision in the middle of the day will be good
which 2 fitting options do you have for fitting someone with ortho k lenses
- fitting set/inventory
- empirical
how is a ortho k lens fitted with a fitting set/inventory
- Initial trial fit
- dispense from inv’ stock.
- No topographer
- NaFl assessment.
- Expensive and time consuming
- vCJD
how can fitting a px with an ortho k using a fitting set/inventory be expensive and time consuming
as have to throw lenses away from the dispensing stock after
or if not have to sterilise every lens and have a record of every px thats used it
how is a ortho k lens fitted empirically
- No initial trial (better as no trial lenses)
- Custom made lenses..
- more successful
- Lower chair time.
- Lose control of parameters/ You get what you’re given!
- Trouble shooting often included from suppliers
what is the disadvantage to fitting ortho k lenses empirically
lose control od parameters/you et what your given
why is the 6 disadvantages of using flourescein when fitting using the fitting set/inventory way
the flourescein evaluation
- doesnt make sense
- makes ortho k lens fitting complicated
- fitting empirically is better using software
- your assessing on a open eye which is not how the patient uses their ortho k lens (e.g. they will have closed eyes and sleeping)
- lens is dynamic at first insert and lacrimose
- ability to assess thin teat layer is impossible e.g.
what is a better way to assess a ortho k fit than flourescein
topography
what are the 3 things that are all you need to fit an ortho k lens empirically
- Subjective Rx
- Topography
- Ø Cornea
with the 1st aftercare visit:
- when should it take place
- should the lenses be worn
- what 5 things need to be done/checked
- day after the 1st night of using, early in the morning
- lenses should NOT be worn
- Px’s impression
- Slit lamp evaluation is vital
- VA
- Residual refraction
- Topography
with the 1st aftercare visit:
- what should you expect in your subjective refraction
- who will you see slower change in
- what temporary symptoms may a patient experience
- approx 2D reduction - so give residual rx CLs to use for a few days
- slower change with astigmats
- possible reflections and visual fluctuation
with the 2nd aftercare visit:
- when should it be carried out
- which time of the day is ideal
- should the lenses be worn
- what 5 things should you check
- what stage should the px be at by now
- after 5-7 nights
- early morning appointment ideal
- lenses NOT to be worn
- Overall impression
- VA unaided
- Slit lamp evaluation
- Topography
- Subjective refraction
- treatment starts to slow down and px comes closer to end point
with the 3rd aftercare visit:
- when should it be carried out
- what time of day should it be carried out and why
- should the lenses be worn
- what 5 things should you check
- what stage should the px be at by now
- after 10-14 nights
- late afternoon appointment ideal - to measure regression
- lenses NOT to be worn
- Overall impression
- VA unaided
- Slit lamp evaluation
- Topography
- Subjective refraction
- px should have their final result by now
what 4 things shows a good result of the ortho k lens
- Comfortably wear their lenses overnight
- Happy with their vision
- Minimal/no staining on slit lamp
- Topography shows well centered treatment zone
what 2 possible findings can u get rom a slit lamp investigation of things that can go wrong with an ortho k lens and what should you do about it
- The lens can stick on the cornea and rip off the corneal epithelium when removing it
- stop lens wear until the epithelium has healed
- this can happen if treating too high rx or the lens fitting is not 100% right
- instruct the px to put some re-wetting drops in at night and in the morning before removing the lens
- 3 and 9 o’clock staining
- in the mid periphery
- means the lens is to flat
- send a topography map off to the manufacturers to tweak the parameters
what is a good topography outcome of an ortho k result called
list 2 signs on the map that represent this
what are these results good for
- bullseye
- Well centred area of flattening
- Mid peripheral ring of steepening
- May take time to develop good ring
- good for myopia control
list 4 topography outcomes with does not represent a good ortho k fit
- decentration - smiley face
- decentration - frowny face
- decentration - lateral
- central steepening
what causes a decentration smiley face topography result from an ortho k and what needs to be done to resolve this
- high riding lens/decentering superiorly
- WTR astigmatism
- Increase the sagittal depth
- Steepen RC e.g 0.1
- Choose lower eccentricity e.g 0.2
- Steepen AC
what causes a decentration frowney face topography result from an ortho k and what needs to be done to resolve this
- low riding lens/decentering inferiorly
- Decrease saggital depth
- Choose higher eccentricity e.g. 0.2
- Flatten AC?
- Flatten RC?
what causes a lateral decentration topography result from an ortho k and what needs to be done to resolve this
- Diameter too small
- Eccentricity nasal > eccentricity temporal
- Choose larger diameter
- If not poss then try steepening RC by e.g. 0.1
- Or flatten RC by 0.1 and AC by 0.05
lateral not seen as often as lenses are now bigger
what causes a central steepening topography result from an ortho k and what needs to be done to resolve this
- Sagittal depth too high
- Steep alignment curve?
- Decrease Sagittal Depth
- Choose higherEccentricity by e.g. 0.2
- Flatten Alignment Curve?
- Flatten Reverse Curve?
what is an ortho k lens a realistic alternative to
refractive surgery
