RGP Fitting Flashcards
Types of RGP lens
- corneal
- corneal/scleral
- mini scleral
- scleral
Advantages of corneal RGP fitting
- robust
- easy to clean
- easy to handle
- stable vision
- good oxygen supply
- dehydration lens problematic
- customised design
- good tear exchange
Corneal RGP disadvantages
- physical comfort
- greater movement
- fb entrapment
When to choose an RGP
- moderate corneal astigmatism
- complex rx
- history of allergies
- problem with SCL
- financial
- poor compliance
- high visual demands
When to avoid an RGP
- contact sports
- water sports
- dusty environments
RGP parameters
- BOZR 1/1.5mm smaller TD
- FOZD 0.5mm larger than BOZR
- centre thickness - too thin causes flexage
- edge lift prevents mechanical pressure on cornea
RGP tear lens
- power increases by +0.25DS for each 0.05mm that BOZR is steeper
- BVP needs to be changed by -0.25DS for each 0.05mm to compensate
- opposite is also true
Spherical vs aspherical
Spherical - spherical back optic zone with spherical peripheral zones
Aspheric - elliptical lens design gradual flattening from centre out
Steps in selecting a first lens
- Find corneal shape/size/pupil/rx/health
- Choose separate lens type
- Work out shape you want
- Once shape selected, work out power using BVP calculation
Steps in Selecting RGP material
- Oxygen requirement - Dk - thin lens - increased O2 - thick lens - less O2
- Centre thickness
- Wettability
- Flexture can cause residual astigmatism
- Compliance, if poor go for easy care materials
Common RGP materials
- silicone acrylate
- flourosilicone acrylate - better wettability
- flourosilioxanyl sterene
BOZR considerations and calculation
-Difference in k readings is less than or equal to 1.00DC, then it to flattest k
- BOZR = flat k - (flat k - steep k) / 3
- round to nearest 0.05mm
What is an alignment fit
- good centration/constant pupil coverage
- no limbal crossing
- adequate movement on blink
- central trace of flouroscein
- midperipheral adequate band of bright flouroscein
- peripheral adequate band of bright flouroscein
What is a steep fit
- good centration
- no limbal crossing
- minimal movement on blink
- central bright area
- midperipheral wider band of touch
- peripheral adequate band of bright flourascein
Lid attached fit
Upper edge of lens tucked under top lid during blink
- common in large TD
- can actually be mo comfortable and less lid interaction
- acceptable as long as pupil is covered
