RGP Fitting Flashcards
1
Q
What are the types of RGP lenses?
A
- Corneal (7-12mm) – same or less than diameter of cornea – discussed in this lecture
- Corneal-scleral (12.1 - 15mm)
- Mini-scleral (15.1 - 18mm)
- Scleral (>18mm)
2
Q
What is important about corneal RGP fitting?
A
- Less margin for error in preliminary measurements though as requires precise fitting
o RGP lenses are like shoes – most people only have 1 size that fits them
o Soft CLs are like socks – most people fit a few sizes
3
Q
What are the advantages of RGP lenses?
A
- Robust:
a. Can be re-polished – if not surface treated
b. Longer lens life – i.e. years – depends on px’s compliance & wear time - Easy to keep clean:
a. Soaking solutions stronger
b. Fewer solution problems – constituents (e.g. preservatives) don’t soak into lens
i. If px on soft CLs are struggling with reactions to solutions (can try switch them to RGP) - Easy to handle – relatively – once px is used to them – will avoid inside out problem
- Stable vision:
a. Can avoid need to fit toric lenses (if corneal astigmatism) – tear lens - Good oxygen supply
a. Good oxygen permeability & transmissibility - Dehydration is less problematic – compared to soft lens materials
- Customised designs – tint, design, Rx (high), can change curves, prosthetics
- Good tear exchange – aids metabolism, oxygen
4
Q
What are the disadvantages of RGP lenses?
A
- Physical comfort:
a. Edge sensation present to begin with – when you blink
b. Need to build up tolerance px will get used to it
c. Poor for intermittent wear - Greater movement of lens:
a. May become dislodged will know if px cannot feel it at all (as it is sat on conj instead) - FB entrapment
5
Q
When should you pick an RGP lens?
A
- Moderate corneal astigmatism – can avoid using toric lens & give more stable vision in spherical RGP than toric soft lens
- High complex Rxs – better quality of vision with RGP than soft lens
- History of allergies – not as many allergies to solutions w/ RGPs
- Problem w/ SCLs:
o Damage
o Deposits
o Solutions – e.g. allergy
o SCL induced papillary conjunctivitis - Poor compliance:
o Less likely to have problems w/ RGP – be v careful w/ any px w/ poor compliance - High visual demands – get better quality of vision w/ RGP
- Financial reasons – tend to be more cost effective if looked after well
6
Q
When should you avoid an RGP lens?
A
- Contact sports:
o Rugby, judo, karate, boxing etc
Lens can be smashed - Water sports:
o May get washed out - Dusty environments – more chance of FB trapped under the lens
o Must wear eye protection
7
Q
What are the parameters for RGP lenses?
A
- BOZD: back optic zone diameter
o 1-1.5mm smaller than TD. Large enough to cover pupil in different light conditions. - FOZD: 0.5mm larger than the BOZD
- Centre thickness: too thin a lens leads to flexure which can lead to breakage & residual astigmatism
o If lens too thick, harder for oxygen to get in - Edge lift: lens doesn’t touch down right at very edge
o To prevent mechanical pressure on cornea, allow tear exchange & enable lens removal
If lens fully touching at edges then lens can seal itself on & can get adhesion - Dk: oxygen permeability needs to be maximised depending on wear time & condition of ocular surface
8
Q
Describe the tear lens in RGP lenses?
A
- Think about this once picked a lens & looked at fit & thinking about Rx
- There is layer of tears between eye and contact lens
- An approx. rule of thumb, for a rigid lens the tear lens power increases by about +0.25D for each 0.05mm that BOZR of the lens is steeper than corneal radius (created a +ve tear lens)
- Correspondingly, on any cornea the back vertex power (BVP) of rigid CL needs to be changed by -0.25D for each 0.05mm that BOZR is made steeper, to compensate for the extra +ve power of the liquid lens (created a -ve tear lens)
- If the lens BOZR is made flatter by 0.05mm, BVP needs to be changed by +0.25D
- An aligned & perfectly fitting CL has ‘zero’ value power is just that of the CL (correct BVP)
- If lens is not absolutely spot on then this can change total correction
o If lens is flat -> creates a -ve tear lens
o If lens is steep -> creates a +ve tear lens
9
Q
What is the British Standard for RGP lenses?
A
- Minimum specification is BOZR / TD / BVP
o Then the lens type & material
NB Soft CLs use same specification
10
Q
Describe spherical vs aspheric lenses?
A
- Spherical – spherical back optic zone w/ spherical peripheral zones
o Manufacturing is easier - Aspheric – elliptical lens design, gradual flattening from centre out
o Manufacturing requires computer-controlled lathes
o Sometimes slightly better comfort as can be slightly thinner
11
Q
Describe the steps in selecting the 1st RGP lens?
A
- Find out about eye:
a. Shape, size, pupils & refractive error – keratometry, HVID, pupil size, Rx
b. Health (slit lamp, history & symptoms) - Choose appropriate lens type
- Work out shape you want
a. Find nearest available - Once know what shape are going to order, work out what power need
a. BVP calculation (same as TriA calculations)
- Check Manufacturer’s Advice:
o If at all possible
o Choose 1st lens according to manual – will guide you on selecting BOZR & lens power
o If not available, use Generic formulae as estimate for 1st choice trial lens
NB Designed for SPHERICAL lenses, not aspheric
12
Q
Describe selecting the RGP material?
A
- Corneal Oxygen Requirements:
o Dk (Oxygen requirements) – maximise O2 transmissibility if px going to wear them 16 hours a day 7 times a week
o Thin lens – more oxygen to lens
o Thick lens – less oxygen, easier to handle – could be good for part time - Centre thickness of lens (esp. high plus):
o Consider edge form too – can be like knife if manufacturer trying to maximise oxygen permeability
o Maximum wearing time required
o Affects how much oxygen can get in - Wettability:
o Ability of tears to form a stable layer on surface of material - Flexure:
o Can cause residual astigmatism on toric corneas
Especially true of thinner lenses (<0.15mm) on a toric cornea - Compliance:
o If likely to be poor go for simple to care for materials – choose one less prone to deposits
13
Q
What are the common RGP materials?
A
- Silicone acrylate – more prone to lens flexure
- Fluorosilicone acrylate – better wettability, fewer deposits – BUT can be brittle if too thin
- Fluorosiloxanyl styrene – least common
These materials are available as wide variety of trade names & w/ different tints & oxygen permeability levels
Common trade names include Boston and Optimum
14
Q
What do you need to consider when fitting RGP lenses?
A
- Need to consider:
o Pupil size
Make sure optical portion > scotopic pupil (pupil in dim light – largest)
o Environment:
Atmosphere
Dusty? Dry?: - Px must wear eye protection
Activity – e.g. contact sports
15
Q
Describe BOZR and the calculation in RGP fitting?
A
- If difference in K reading is ≤1DC:
o Fit on flattest K (round to 0.05mm) - Positive lenses tend to drop a little (because of central thickness), consider fitting 0.05mm steeper to stabilise – if 1st choice is little flat
- If difference in K readings is >1DC:
o Can still be worth trying a spherical RGP BOZR
o Using the GENERIC formula:
BOZR = FlatK – ( (FlatK - SteepK)/3 )
o Round this to nearest 0.05mm – will give you a lens slightly steeper than flattest K
o Can use this formula for any level of toricity:
If toricity minimal, this will round to flat K
If v small cyl, flat K is reasonable place to start - ^ THIS METHOD IS FROM “THE CONTACT LENS MANUAL”
Every 0.1 difference in K readings is a difference of about 0.50D
16
Q
Describe BOZD in RGP fitting?
A
- Often predetermined by laboratory – not usually included in CL specification
- Can consider a smaller BOZD in a toric cornea to minimise area of mismatch but only if pupils will allow
- Small BOZD: <7.30mm -> need a steeper BOZR
- Medium BOZD: 7.30 - 7.90mm
- Large BOZD: >7.90mm -> need a flatter BOZR If px has particularly large pupils in low level light
- Standard BOZD will be fine for most people
17
Q
Describe total diameter in RGP fitting?
A
- Need to know HVID & VVID (aka palpebral aperture (PA))
- Consider pupil size:
o BOZD > (scotopic) pupil
By 1-2mm (roughly)
o TD < HVID
By ~2mm (roughly)
o TD > BOZD
By ~2mm (roughly) - Consider availability:
o Most lenses are between 9.20 to 9.80mm in diameter
Example for scotopic pupil of 6mm & HVID of 11mm: want BOZD of 7-8mm, TD of 9mm based on HVID -> BUT TD needs to be larger than BOZD so ~9.5mm should work
18
Q
How do you measure the pupil diameter for RGP fitting?
A
- Measure with ruler
- Need maximum (dim light) and minimum (bright light)
- Can use Burton lamp to help with this
o If no Burton lamp then have room as dim as possible for maximum that still allows you to see pupil margin - Can use slit lamp for minimum – good if hard to see pupil margin
19
Q
How do you prepare an RGP lens?
A
- Lens will arrive dry
- Inspect lens before insertion -> look for any damage
- Can focimeter lens to check power
- Lens must be cleaned:
o Cleaner – approx. 20secs in palm of hand
o Rinse – lots of rinsing w/ saline on both sides, rinse again on both sides – cleaner is toxic to ocular surface
o Conditioner – a drop of this on lens just before insertion
o Or Multi-Purpose Solution (MPS) just like for soft CLs, use that solution for everything double check the bottles (if it is a MPS or a lens cleaner)
20
Q
Describe centration assessment of RGP?
A
- Use white light for this
- Lens position after the blink – lid attachment vs intrapalpebral
- Lens centration after the blink – central, temporal, nasal, inferior, superior
- Lens centration in the interblink period
- Some decentration is acceptable as long as pupil remains covered by optic zone
o Ask px at aftercare how vision is & whether getting any disruption
21
Q
Describe coverage assessment of RGP?
A
- Lens should be centred to cornea in primary position of gaze & should not cross limbus on excursion of gaze & upgaze
- Should be noted as ‘no crossing of limbus in all positions of gaze’ or ‘crosses limbus inferiorly on upgaze’ etc
- RGP lens should not go onto conjunctiva – should lways remain on cornea
- Should not cross the pupil margin in any direction of gaze
- If lens is moving onto conj likely to be a flat fit – px will get unstable vision due to lens movement & other complications as result
- N.B. assessed with white light
22
Q
Describe movement assessment of RGP?
A
- Lens movement of lens after blink
- Ideally by 1-2mm – remember diameter of cornea when assessing this
- RGP lens movements look large – can easily say it is moving too much when it is not
- Can retract lids to look for movement w/ just gravity
o If lens is sitting inferior – can nudge it back to look at movement & see what happens
A well-fitted lens should drop gently over apex of cornea
A flat-fitting lens will slide around apex with a more rapid movement, can be a “swan dive” type movement
A tight lens will move much more slowly & remain over apex rather that drop towards the lower limbus, can “rock” side to side as it does move - Tight RGP may move more than soft CL
23
Q
Describe the NaFl pattern analysis of spherical RGP?
A
- Brighter glow, thicker the tear film
- Dark regions indicate minimal tear-film thickness
- Fluorescence visible if tear lens has minimum thickness of about 10-20µm
- “Pooling” (v bright NaFl) means that tear lens is quite thick
- No glow at all shows CL is touching
- Look at 3 areas – with lids retracted & lens centred if necessary:
o Centre
o Mid-periphery
o Periphery
24
Q
Describe the NaFl pattern analysis of toric RGP?
A
- Brighter the glow, thicker the tear film
- Dark regions indicate minimal tear-film thickness
- Classic “dumbbell” or “infinity symbol” patten when a spherical lens is fitted on a significantly toric conrea
o This can vary depending on fit & amount of astigmatism - Region of “pooling” and regions of touching
25
Describe an aligned fit RGP?
Fluorescein in middle - thin layer of tears in centre of lens – should be visible fluorescein but won’t be as bright as at edge
Darker band around central area (touch)
Brighter ring of fluorescein – edge lift – allows tears in
Tear profile – thinner in middle compared to at edge
Light fluorescence in centre – good thickness of tear lens underneath
26
Describe a flat fitting RGP?
Tear profile – touch in middle (no NaFl)
Dark central region on CL – touch
Lot of edge lift – wide green band extending out to edge -> won’t see clear normal zones – lens just sitting touching centre
No fluorescence right at centre – shows there is touch & no tear lens under
This is also a toric lens – can see due to dark band & fluorescein above & below
27
Describe a steep fitting RGP?
Lot of pooling in centre – lifted off in that area so really thick pool of tears
Suckered on at edges – v little edge lift
V bright fluorescein in middle
V thin band fluorescein at edge
May show a “rocking” side to side movement
V bright area – thick tear lens
Air bubble(s) can happen in a particularly steep fit – formed in v wide tear lens
28
Describe the mid-peripheral pattern in an aligned fit, flat fit and steep fit RGP?
* Alignment – should be mid-peripheral band of 360° touch
Pink lines-> indicate points of the band
* Flat fit – no clear mid-peripheral zone is visible due to central touch
No clear zone -> touch in middle & then edge ring -> no mid-peripheral band
* Steep fit – v wide mid-peripheral band of touch due to fluorescein pooling in centre small pool in middle & thinner edge circle
29
Describe the peripheral pattern in an aligned fit, flat fit and steep fit RGP?
* Also known as edge clearance or edge lift
* Should be approx. 0.5-1mm for 360° - measure w/ 1mm wide beam on slit lamp
Aligned fit: Light NaFl in middle, then dark band, then edge lift important for tear exchange
V wide edge clearance – dark circle in middle, no mid-peripheral band, lots of edge lift
* Wide edge clearance = flat fit
Steep fit – v narrow edge clearance – would also have pool in middle
* Narrow edge clearance = steep fit
30
Describe an aligned fitting RGP?
* Good centration/constant pupil coverage – in all directions of gaze
* No limbal crossing – at no point does RGP go onto conjunctiva MUST stay on cornea
* Adequate movement on blink, movement straight up and down – 1mm to 2mm
* Central – trace of fluorescein indicating minimal clearance
* Mid-periphery – band of touch – no NaFl (no tear lens)
* Peripheral – adequate band of bright fluorescein should be brighter than what is seen in centre
* CL manual:
o Alignment w/ standard corneal lens:
Alignment or merest hint of apical clearance >central 7.00mm
Mid-peripheral alignment >~1.50mm
Edge clearance ~0.4mm wide
o Alignment w/ aspheric lens:
Alignment or light touch >central 5.00mm
Mid-peripheral alignment
Narrow edge clearance just under 0.2mm wide
The brighter the NaFl, the thicker the tear lens
31
Describe a steep fitting RGP?
* No limbal crossing
* Minimal movement on blink – even a v tight RGP usually shows some degree of movement
o Or slow or limited recovery when lens is moved
* Central – bright area of pooling (thick tear lens), can include air bubbles (if v tight)
* Mid-peripheral – wider band of touch (from edge of pool to peripheral region)
* Peripheral – narrow band of bright fluorescein
* May show a “rocking” side to side movement
* NB: good initial comfort due to lack of movement – be wary of new RGP wearer who isn’t aware of the lens or not getting any reflex tearing – may mean lens on cornea
* CL Manual:
o NaFl pattern gives central pooling
o An air bubble is sometimes present w/ excessive central clearance
o Heavy bearing is seen at transition as an area of dark blue touch beyond central pooling
o Smaller area of central pooling, greater the degree of steepness
o Periphery gives only a thin annulus of NaFl around lens edge
32
Describe a flat fitting RGP?
* Poor centration, can be in any direction depending on lids
o Common to see inferior decentration
* Limbal crossing
* Excessive movement on blink, often fast
* Central – area of touch (dark), the flatter the fit the smaller the area of touch
* Mid-Peripheral – not a clear band of touch as can merge with the central zone
* Peripheral – wide band of bright fluorescein, can include air bubbles (if v flat – due to pooling)
* CL Manual:
o Fitting pattern gives a dense central area of dark blue touch surrounded by NaFl to edge of lens
o Area of touch is small w/ an indistinct as opposed to a sharply demarcated border
o NaFl encroaches beneath periphery of central portion where alignment would be expected w/ a correct fit
33
Describe a lid attached fitting RGP?
* Upper edge of the lens remains tucked under the top lid during blink so lens appears to sit slightly high
* This is acceptable especially in those who require a large TD or have a lower superior lid position (it is more important to avoid inferior decentration) -> pupil must be covered
* Can actually be more comfortable when first fitting RGPs as there is no interaction between the top lid and edge of the lens
* This is acceptable as long as the pupil remains covered
* Fluorescein pattern assessment must still be done with the lens centre
* Leave the lens lid attached for white light assessment but then centre it for assessing NaFl pattern
34
What are the general tips for adjusting fits in RGP?
* INCREASE TD by 0.5mm -> FLATTEN BOZR by 0.05mm
* REDUCE TD by 0.5mm -> STEEPEN BOZR by 0.05mm
* Steep fit PLUS tear lens -> MINUS ORx
* Flat fit MINUS tear lens -> PLUS ORx
* 0.05mm STEEPENING of BOZR -> +0.25DS tear lens induced
* 0.05mm FLATTENING of BOZR -> -0.25DS tear lens induced
