Week 1.11 RGP Contact Lenses Flashcards
Some general notes about RGP
RGPs have been around lot longer than SiHi
Less commonly fitted compared to SCL
Creates a very loyal px base as generally they are more durable and long lasting and less risk of infection
Custom made lens. Any design/material too
How many curves does a contact lens have
Tricurve
What are the measurements needed to fit a RGP lens
- history to ensure RGP wear acceptable/tolerable
- health check of eye
- keratometry readings
- pupil measurements
- HVID
- VPA - vertical palpebral aperture - gap between top and bottom lid
What do we need to know for our selection of an initial trial lens
- total diameter
- BOXR - need to know for curvature of the lens
- BVP
- Lens design
- lens material
- cost?
Are RGP cheaper than SCL?
RGP is cheaper than SCL however initial cost is there aswell
For RGP lens what we want the HVID total diameter to be
HVID - 2mm
TD - 9.8mm is perfect
For spherical corneas do we want a larger or smaller fit RGP? And why?
For spherical corneas, the larger the RGP the more stable the fit - less movement seen and VA stable
For toric corneas do we want a larger or smaller fit RGP? And why?
For toric corneas, a smaller fit may make the fit/vision more stable - doesn’t rock on cornea or spin with blinking as much
How do we know what the BOZR of the lens should be
We start by fitting BOZR that is most similar to largest or flattest K
Tear lens and RGPs
- can take advantage of the tears underneath the lens
- water has a weaker refractive index than the lens but higher than air
- if we use flattest k we can use tear film to correct astigmatism in some cases
However cannot always fit RGP on flattest K because at some point, toxicity of cornea may be too much
What is the rule of thumb for keratometry readings to work out the amount of astigmatism regarding keratometry readings NOT rx
0.10mm difference in keratometry readings equates to +/-0.50DC
0.05mm difference will therefore be 0.25DC
What would be the approximate BOZR for spherical RGP lenses based on k readings
If the astigmatism by keratometer is:
Spherical to 0.74D - fit on flattest k reading
0.50 to 1.00D - fit on flattest k reading to 0.05 steeper than flattest k reading
1.00 to 2.50D - fit near flattest k reading (0.05 to 0.10 steeper at most)
Over 2.50 - toroidal back optic zone is recommended
What BOZR would we need if 7.8 @180 and 7.6@90?
7.8 and 7.6 there’s a 0.2mm difference so 1.00DC
7.8 is flattest K so you want 0.05 steeper than 7.8 so 7.75
Value gets smaller so steeper/tighter/smaller
For values over 2.50 astigmatism shown by keratometer readings…
… we no longer use spherical RGP lenses as the lens will still rock too much even if we try steepening the flattest K
If u put the lens with flattest k and doesn’t fit with how u expected, improve the fit using the two simple RGP rule:
1) flattening (increasing)/ steepening (decreasing) the BPZE by 0.05mm is equivalent to a change in -/+0.25D respectively
2) flattening/steepening the BPZR by 0.05mm requires an increase/decrease of 0.5mm to BOZD
BOZD
The diameter of the initial central curvatures that contains corrective power
Important that it is larger than max pupil size (measured in dark light)
- usually approx 0.7mm larger than pupil diameter
What about peripheral curves
- often predetermined by manufacturer
- first curve is 0.7mm flatter than BOZR
- final curve is approx 10.50mm
- If you make it flatter u get less corneal irritation but might irritate the lids
- too little peripheral clearance causes poor tear exchange
- too much will make your lens unstable
When do we need to correct for back vertex distance
- greater than -/+4.00D
If spec rx is -3/00/0.75x180 what’s the BVP and do we need to do anything to correct the astigmatism
BVP is -3.00DS
Table indicates flattest k, fluid lens will correct the -0.75DC
- If spec rx is -4.00/-1.25x90
What’s BVP and flattest k
BVP is -4.00
Table indicates fitting 0.05mm steeper BOZR than flattest k, fluid will correct the -1.25DC
What is the suffix given to all RGPs
-focon
What’s the suffix given to soft lenses
-filcon
Is all astigmatism corneal?
No some is lenticular astigmatism so u need to know px rx as well as keratometry readings.
7.70@180
7.00@90
Rx: -3.00/-1.00x180
What’s the amount of astigmatism
In the cornea:
- 7.70 and 7.00 that’s a 0.70 difference so 0.50x7=3.50D
So most steep should be at 90 (because astigmatism here is at 90) which is as 7.00@90
