RGP Fitting 1 Flashcards
What are our three aims of RGP fitting and how are they achieved?
Good Visual performance - achieved by correct alignment with the cornea
Minimal ocular response -achieved by ensuring we have good tear interchange which is helped by sufficent lens movement
Optimised comfort - achieved by weight distribution
What is tear interchange when we talk about contact lenses and why is it important?
Tear interchange is the conceot of tears behind the contact lens being replaced every time you blink with fresh ones.
It is important as if it does not happen essentially you have stagnant tears - which makes you more prone to infections
Where does the main discomfort come from RGP lenses?
The lens movement
[Lens movment is necessary though for tear interchange]
What we trying to achieve in an RGP fitting?
- Reasonable centration
- Adequate movement
- Alignment or minimal clearance centrally
- Acceptable edge clearance
Do we want the apex of the cornea ( centre of the cornea) touching the RGP?
What should you see when you instil fluorescin in an eye wearing an RGP lens?
No you do not want them touching as it may cause slight rubbing which could cause swelling of the cornea.
When you instill fluorescin you want to see a green rim around the lens suggesting you have a good lens lift (edge lift) and you want to see a little flourescin in the centre of the cornea (indicating there are tears there and that the apex of the cornea and the RGP are not touching).
Why is it important to consider forces when fitting RGPs and what are these forces to consider?
Essentially RGPs are smaller than the cornea (8-10mm Vs 12 mm) thus they are basically hanging off the cornea as a result its important to consider how forces may affect their positioning.
The forces to consider are:
- Eye lid force
- Frictional forces
- Capillary attraction
- Centre of gravity
- Specific gravity of the CL material
What is the vertical palpebral aperture distance?
The distance from the upper eyelid to the lower lash line.
In the picture it is the measurement marked 3.
How do frictional forces affect positioning of an RGP lens?
When do we really need to be mindful of this?
Essentially in the image imagine the sanding block is the contact lens and the wooden desk is the cornea.
The moving force would be gravity pulling the lens down.
Normal force is atmospheric pressure keeping the lens where it is.
The frictional force moving the lens up is the viscosity of the tears, when tears are more aqueous they are less viscous ( the ratio of aqueous component to mucin is higher) , thus the contact lens gets moved up.
This is particularly evident on first time fitting where the px’s eyes will water up. When tears pool up all at once i.e. when you start crying all of a sudden viscosity of tears is low.
[Lens can also have trouble sticking to the eye if it is too watery]
What is capillary action and how does it affect the lens?
It is the force that allows a porous material to soak up a liquid.
(If you have a narrow space between the tearfilm and lens then the lens will be sucked onto the cornea).
Essentially for a contact lens, it is the force that ensures it stays stuck to the cornea.
Does the centre of gravity force affect both plus and minus lenses the same and if not why?
[RGP]
NO
Plus lenses have a thicker centre and so the centre of gravity sits much lower back away from the cornea (sits in the lens actually). Thus the lens wants to move more away and down from the cornea.
In myopes the centre of gravity sits further towards the cornea. This allows the lens to stay closer to the cornea. [This is an advnatage if px is a myope as lenses tend to be more stable on the cornea]
How does the centre of gravity force affect steep lenses as a pose to flat/flatter lenses?
Steeper lenses stay closer to the cornea as their centre of gravity is closer to t he cornea.
Whereas flatter lenses have a centre of gravity further away from the cornea and as a result are more liekly to move away and down.
How does specific gravity of the contact lens material affect its fitting?
Heavier material will mean the lens drops more
What is the horixonatl visible iris diameter?
Basically the literal diameter of your iris.
What are the expected horizontal visible iris diameters?
11.6 – between 10.2- 13.0mm
Why would you want to know the vertical palpebral aperture?
Needed to work out the total diameter for the CL we will prescribe
Needed to explain/predict the behaviour of a CL
How may a narrow palpebral aperture affect contact lens fitting?
A narrow palpebral aperture means there would be difficullty inserting.
What is associated with a large palperbral aperture and as a result wil be seen under flourescin?
A large palpebral aperture is associated with incomplete blinking. As a result you may see corneal staining - corneal dessication
What is the average value for vertical palpebral aperture?
9.0-10.5mm
[It is always recorded to one decimal]
In what conditions do we measure pupil size?
We measure pupil size in ambient lighting (bright light- 100% cubicle light or burton lamp white light)
and
low illumination (Reading light or Burton lamp blue light)
Decribe the shape of a Spherical RGP?
At the centre it should have the same radii as the cornea (we get this from keratometry). As it goes towards the dges of the cornea the radii of the lens should flatten (mimmicking a cornea).
Describe how the structure of a C3 RGP contact lens would be.
So you would have a BOZR zone
A first back peripheral radius and a second back peripheral radius.
When fitting a spherical RGP lens what is the rule of thumb for the back optic zone radius?
We always fit on the flattest K reading meridian.
In the case of corneal astigmatism we do the following:
(the table in the photo - the reason we adjust is that in corneal asigmatism if we are fitting according to the flattest meridian it is too flat for the other meridian thus the lens experiences a rocking action, to minimise this rocking action we make the adjustments in the table).
What can you deduce about the fit of this RGP lens and why?
It is perfect.
There is no pooling in the centre.
There are no centre air bubbles nor air bubbles in the periphery.
The edge flourescin ring around the edges is complete - there are no gaps its a full circle.
The fluorescein ring all around the edges is of even thickness.
If you saw this staining what can you deduce about the fitting of this RGP?
It is ill-fitting , we know this as you can see a pool of green (i.e. where the fluorescin has pooled up in the pupil area. This is circled in the image in red.
This is beacause the RGP hasn’t been fitted on the flattest meridian and so the lens is steeper than the cornea leaving that space for flourescein to pool up.
How would you change the fitting of this lens to make it fit better?
You would want the lens to be flatter.
The smallest change to make to the diameter that would be clinically significant would be increasing it by 0.1mm.
[If there was an air bubble in the centre you would want to order the lens flatter by at least 0.3 mm]
What can you deduce about the fitting of this RGP lens and why?
The RGP lens is too flat for this cornea.
You can deduce this as the edge thickness is uneven - and even has a gap.
How can flat or steepness of an RGP affect prescription?
Depending on how the tear film takes up the space between the lens and the cornea, it can affect overall prescription.
FP –> Fluorescein Pattern
Can you change the total diameter itself and not have to worry about any other parameters when fitting an RGP lens?
No you have to think about how chnaging the TD will affect flourescein pattern and as a result order either a steeper or flatter lens.
Increasing TD by 1 step (0.5mm) means the fluorescein pattern becomes 1 step (0.05mm) steeper. So if you want to keep the same fluorescein pattern, you need to make the BOZR 1 step (0.05mm) flatter.
When making adjustments to RGP fittings what rules do you need to remember?
Summary slide of the lecture:
