CL - RGP fitting 2 - Week -2 Flashcards
What are the 2 goals for BOZR when fitting spherical RGPs on toric corneas?
Minimise contact/bearing on flatter meridian
Achieve adequate centration
Describe the fit of an RGP on a toric cornea if BOZR = flattest K (3)
Excessive edge clearance on steeper meridian
Contact along flatter meridian
Usually poor centration
Describe the fit of an RGP on a toric cornea if BOZR = steepest K (4)
Excessive central pooling
Harsh bearing along peripheral regions of flatter meridian
Dimple veiling (centrally) + 3&9 staining likely
Centration often good
What is the optimal BOZR for a toric cornea?
BOZR 2/3 towards flatter K
What does the fluorescein stain of a good RGP fit on a toric cornea look like?
“Dumb-bell” or “H” pattern
List 5 indications for toric BOZR
Poor CL centration
Harsh bearing along flatter meridian
3+9 o’clock staining
Unsatisfactory vision (lens flexure, poor centration)
Lens warpage
Corneal cylinder > 2.50D for spherical GP
What’s the difference between tricurve and quadcurve?
Tricurve has a central spherical curve with 2 spherical peripheral curves whereas quadcurve has 3 spherical peripheral curves
What does the number of peripheral curves in an RGP lens design affect?
Increased peripheral curves –> smoother transitions
If the RGP lens requires flattening, would you increase or decrease the number of peripheral curves in the design?
increase
What is the typical BOZR for an aspheric RGP lens?
Trick question, there is no true BOZR for an aspheric lens
List 3 advantages of aspheric RGP lenses
Less flare
Less corneal distortion
Less edge clearance (therefore more comfort and less 3+9 stain)
What is a conoid rgp design?
Where the periphery of the lens is tangential to the central curve (i.e. where the edge of the lens is not a curve but flat)
Is a conoid design fitted with apical or edge clearance?
apical
How does a conoid design affect corneal distortion?
increases it
When are PMMA materials used?
only for trial lenses
What is the main advantage to using fluorosilacrylate (FSA) material for gas-permeable lenses? Explain
High Dk. This high Dk is facilitated by fluorine and silicone
What are the 5 ideal features of a gas-permeable lens material?
Good rigidity and stability Excellent wettability Good VA High Dk High deposit resistance
Name 4 examples of good gas-permeable materials [important]
Boston XO (B&L) Boston XO2 (B&L) Tyro97 (menicon) Menicon Z (stock lenses)
What factors should be considered when setting a lens diameter for a gas-permeable lens? (5)
Width of palpebral aperture
Lid geometry + position of lid margins wrt cornea
Lid tension
Lid movement during blinking (complete/incomplete)
Corneal diameter (HVID)
How may a gas-permeable lens with a negative power and large total diameter fit on a patient?
may ride high
How can we use the patient’s anatomy to help centre the RGP?
superior lid attachment fit helps lens centre. I.e. upper eyelid hooking over the lens can give you a better fit.
What is the range of lens diameters used for RGPs?
7.5-11mm
Name the two different types of fits for RGP lenses and describe how the lens differs in size and steepness
- Lid attachment fit: larger flatter lens, aim for central alignment
- Intrapalpebral: smaller steeper lens, apical clearance
What type of lens steepness will give you greater lid interaction?
flatter lens gives more lid interaction
