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