RGP Fitting 3 Flashcards
How do we define an aspheric lens?
A lens where either one or both surfaces are non-spherical.
What are the advantages of aspheric lenses?
They produce a visual advantage of best form lenses and a cosmetic advantage of thinner, flatter and lighter lenses.
How do elliptical aspheric lenses need to be fitted?
They generally need a flatter fitting than spherical lenses– To provide alignment across the corneal surface fit them 0.05mm flatter than flattest K
How can polynomial and variable eccentricity aspheric lenses be fitted?
These CAN be fitted in the same way as spherical lenses. - On the flattest K
Describe the structure and fit of a fully aspheric lens.
It has a completely aspheric back surface (which theoretically fits perfectly along the cornea). [They may be uncomfortable to some patients as lens edge presses into cornea]
Describe the structure and fit of a mainly aspheric/part sphere.
It has a mainly aspheric back surface with a spherical peripheral curve. This provides it with: – Good edge clearance – and as a result Prevents edge pressing into cornea – Allowing Assist in tear exchange [However, edge lift is sometimes insufficient]
What are polynomial aspheric lenses also known as?
“Spherical optic/ aspheric periphery lenses”
Describe the structure of polynomial aspheric lenses and how they may be fitted.
They have a central spherical portion (BOZ) with an aspheric peripheral zone area. These are typically fitted 0.05 mm flatter than the flattest k.
When altering total diameter of an aspheric lens what do we need to remember?
That nothing else needs to be tweaked. No change should be made to the radius or the power.
What are the advantages of aspheric lenses?
Advantages AS • More comfortable as Achieves true alignment – because Fit is more closely to corneal topography • Pressure more evenly distributed over cornea • Less lid sensation • Useful in higher astigmatism • More gradual FP (centre to periphery) • They are easier to fit – as Fewer parameters to consider and changing a parameter such as total diameter doesn’t require change to other parameters such as BOZR or power
What are the disadvantages of aspheric lenses?
• In general these lenses need to be fitted flatter – in order to provide an acceptable amount of movement • Larger lenses are needed to help centration which in turn reduces amount of oxygen getting to the peripheral cornea. • The lens edge may press into peripheral cornea • Some aspheric lenses e.g. Bi-aspheric lenses can induce residual astigmatism – And aberrations • There are less Variations available in design -which makes it harder to customise for px that are not entirely getting on with their lenses. – Manufacturer’s info should be read
What are advantages of multicurve lenses over aspheric lenses?
More control over individual lens parameters, e.g: – BOZD – Flatter/steeper periphery – Adjust edge clearance [Essentially you can give the patient a customised fit]
What are advantages of aspheric lenses over multicurve lenses?
They tend to have a more successful first fit ( as you are only really thinking about three parameters; BOZR, TD and BVP) They tend to have a uniform fluorescein pattern - as there are no blended junctions thus more comfortable for the px
What would you do if you had a px with a multi-curve lens for which everything is perfect, however, the edge clearance ONLY is too large?
Consider refitting with an aspheric lens. (because edge clearance is thinner in an aspheric as a pose to a multi-curve).
When should a lens be lenticulated?
When the BVP is greater than -6.00D or +4.00D
How can we change centre and edge thicknesses of a lens?
By changing the front optic zone diameter (FOZD)
How does reducing the Front Optic Zone Diameter affect thickness in a positive lens?
Reducing the FOZD in a positive lens reduces centre thickness
How does reducing the Front Optic Zone Diameter affect thickness in a negative lens?
Reducing the FOZD in a negative lens reduces periphery thickness.
What type of edge carriers are the following?
A- plano
B- Positive
C- Negative
What is high riding and which lenses display this quality?
Negative lenses - high riding is when the lens sits too high this happens because negative lenses are thicker in the periphery and so show natural lid attachment occurs holding the lens up
[Plus lenses aren’t thick enough in the periphery for the lid to hold onto it]
What type of edge carrier would you suggest for negative lenses?
You would suggest a positive edge carrier for negative lenses ( to reduce edge thickness).
Why is a negative edge carrier used?
To increase thickness in the periphery ( thus allowing it to be thick enough in the periphery for lid addtachment so the lens can be lifted up).
Why is a positive edge carrier used?
To reduce thickness in the periphery
What type of edge carrier would you suggest for positive lenses?
A negative edge carrier for positive lenses ( to improve lens centration - as it will lift the lens up)
What are the possible pay-offs of selecting the right edge carrier for the lens?
Better vision
Better comfort
Easier adaptation to the lens by the px
A more stable lens position
What does a plano edge carrier do?
Miantains lens thickness
Do positive lenses naturally high or low ride?
Posiitve lenses naturally low ride ( thus it is important to consider a negative edge carrier to lift the lens up - thus improving centration of the lens).
How does centre of gravity affect position of a lens?
The more anterior the centre of gravity the lower it rides e.g. with positive lenses the centre of gravity is inside the lens ( i.e. more anterior than with minus lenses) thus positive lenses ride lower.
What is the relationship between steepness and centre of gravity?
In steeper lenses the centre of gravity is further back (more posterior) and as a result there is better centration as it doesn’t fall down as much.
What is the relationship between diameter and centre of gravity?
The centre of gravity is further back in larger diameter lenses - which means lens is less likely to fall and so we achieve better centration.
Why is the edge profile important?
It is important for patient comfort.
In general to edge profiles need to be adjusted?
No - unless patient complains about ‘feeling the edge of the lens’
What should an edge profile always be?
It must always be rounded, smooth and well finished.
What are the different type of edge profiles?
A. Posterior
B. Central
C. Anterior
D. Blunt
E. Sharp
Why may a tint be allocated to contact lenses?
For the sake of identification. i.e. gRey for the Right Eye and bLue for the Left Eye.
This is called a handling tint.
[In some people with very light iris colour it may enhance eye colour]
