RGP Fitting

Question 1

What are the types of RGP lenses?

Answer 1

• 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)

Question 2

What is important about corneal RGP fitting?

Answer 2

• 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

Question 3

What are the advantages of RGP lenses?

Answer 3

1. Robust:
   a. Can be re-polished – if not surface treated
   b. Longer lens life – i.e. years – depends on px’s compliance & wear time
2. 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)
3. Easy to handle – relatively – once px is used to them – will avoid inside out problem
4. Stable vision:
   a. Can avoid need to fit toric lenses (if corneal astigmatism) – tear lens
5. Good oxygen supply
   a. Good oxygen permeability & transmissibility
6. Dehydration is less problematic – compared to soft lens materials
7. Customised designs – tint, design, Rx (high), can change curves, prosthetics
8. Good tear exchange – aids metabolism, oxygen

Question 4

What are the disadvantages of RGP lenses?

Answer 4

1. 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
2. Greater movement of lens:
   a. May become dislodged  will know if px cannot feel it at all (as it is sat on conj instead)
3. FB entrapment

Question 5

When should you pick an RGP lens?

Answer 5

• 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

Question 6

When should you avoid an RGP lens?

Answer 6

• 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

Question 7

What are the parameters for RGP lenses?

Answer 7

• 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

Question 8

Describe the tear lens in RGP lenses?

Answer 8

• 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

Question 9

What is the British Standard for RGP lenses?

Answer 9

• Minimum specification is BOZR / TD / BVP
  o Then the lens type & material
   NB Soft CLs use same specification

Question 10

Describe spherical vs aspheric lenses?

Answer 10

• 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

Question 11

Describe the steps in selecting the 1st RGP lens?

Answer 11

1. Find out about eye:
   a. Shape, size, pupils & refractive error – keratometry, HVID, pupil size, Rx
   b. Health (slit lamp, history & symptoms)
2. Choose appropriate lens type
3. Work out shape you want
   a. Find nearest available
4. 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

Question 12

Describe selecting the RGP material?

Answer 12

1. 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
2. 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
3. Wettability:
   o Ability of tears to form a stable layer on surface of material
4. Flexure:
   o Can cause residual astigmatism on toric corneas
    Especially true of thinner lenses (<0.15mm) on a toric cornea
5. Compliance:
   o If likely to be poor go for simple to care for materials – choose one less prone to deposits

Question 13

What are the common RGP materials?

Answer 13

• 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

Question 14

What do you need to consider when fitting RGP lenses?

Answer 14

• 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

Question 15

Describe BOZR and the calculation in RGP fitting?

Answer 15

• 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

Question 16

Describe BOZD in RGP fitting?

Answer 16

• 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

Question 17

Describe total diameter in RGP fitting?

Answer 17

• 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

Question 18

How do you measure the pupil diameter for RGP fitting?

Answer 18

• 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

Question 19

How do you prepare an RGP lens?

Answer 19

• 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)

Question 20

Describe centration assessment of RGP?

Answer 20

• 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

Question 21

Describe coverage assessment of RGP?

Answer 21

• 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

Question 22

Describe movement assessment of RGP?

Answer 22

• 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

Question 23

Describe the NaFl pattern analysis of spherical RGP?

Answer 23

• 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

Question 24

Describe the NaFl pattern analysis of toric RGP?

Answer 24

• 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

Question 25

Describe an aligned fit RGP?

Answer 25

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

Question 26

Describe a flat fitting RGP?

Answer 26

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

Question 27

Describe a steep fitting RGP?

Answer 27

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

Question 28

Describe the mid-peripheral pattern in an aligned fit, flat fit and steep fit RGP?

Answer 28

• 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

Question 29

Describe the peripheral pattern in an aligned fit, flat fit and steep fit RGP?

Answer 29

• 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

Question 30

Describe an aligned fitting RGP?

Answer 30

• 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

Question 31

Describe a steep fitting RGP?

Answer 31

• 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

Question 32

Describe a flat fitting RGP?

Answer 32

• 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

Question 33

Describe a lid attached fitting RGP?

Answer 33

• 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

Question 34

What are the general tips for adjusting fits in RGP?

Answer 34

• 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