RGP fitting Flashcards
Types of RGP lenses
- corneal
- corneal/scleral
- mini scleral
- scleral
Corneal RGP Advantages + Disadvantages
ADVANTAGES:
- robust
- easy to clean + handle
- stable vision
- good O2 supply
- dehydration lens problematic
- customised design
- good tear exchange
DISADVANTAGES:
- physical comfort
- greater movement
- FB entrapment
When to choose an RGP
- moderate corneal astigmatism
- complex rx
- hx of allergies
- problems with SCL
- financial
- poor compliance
- high visual demands
When to avoid an RGP
- contact sports
- water sports
- dusty environment
RGP parameters
- BOZR 1/1.5mm smaller TD
- FOZR 0.5mm larger than BOZR
- centre thickness - too thin causes fledge
- 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 zone
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
- O2 requirement - Dk - thin lens - increased O2 - thick lens = less O2
- centre thickness
- wettability
- flexure 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 + calculations
- difference in k readings is less than / equal to 1.00DC - then 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 NaFl
- mid peripheral adequate band of bright NaFl
- peripheral adequate band of bright NaFl
What is a steep fit
- good centration/ constant pupil coverage
- no limbal crossing
- minimal movement on blink
- central NaFl pooling / bright area
- mid peripheral wider band of touch
- peripheral narrower than 0.5mm edge lift
CORRECTION
- flatter BOZR
- decrease BOZD
- thicker lens
- reduce TD to increase mobility
What is a flat fit
- poor centration
- limbal crossing
- increased movement on blink
- central area of dark
- mid peripheral clearance
- peripheral wider band of edge lift
CORRECTION
- steepen BOZR
- increase TD to stabilise lens
- use thinner lens to reduce eye movement
- BOZD increased by 0.05mm - BOZR requires to be flattened by 0.05mm
Lid attached fit
Upper edge of lens tucked under top lid during blink
- common in larger TD
- can actually be more comfortable as less lid interaction
- acceptable as long as pupil is covered
