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
What is the golden rule of thumb for changes in BOZR with BOZD/TD for lens fitting?
For every 0.5mm increase in BOZD/TD, must be 0.05mm increase in BOZR to maintain the same fitting (NaFl) pattern
How does an increase in BOZD affect lens fit?
Increase apical clearance, tightens fit
How does a decrease in BOZD affect lens fit?
Decrease apical clearance, loosens fit
What can be done to the FOZD to reduce lens thickenss?
can decrease FOZD to help decrease lens thickness
Should the FOZD be larger or smaller than BOZD? Why?
FOZD should be slightly larger than BOZD to minimise visual disturbance (reduce flare)
What happens if FOZD = BOZD?
increased chance of lens breakage as junctions coincide
What is AEC? What is AEL?
Axial Edge Clearance: the separation between the edge of the lens and the cornea
Axial Edge Lift: a mathematical property of the lens
Is AEC > or < AEL generally?
Generally, AEC < AEL
What is the AEC usually for an RGP lens?
~0.08mm
How can we alter visible edge clearance? [important] (4)
Alter radius of peripheral curves
Alter width of peripheral curves
Change number of peripheral curves
Alter BOZD
What is the centre thickness (CT) for negative RGP lenses? What about positive? Describe relationship between centre thickness and power of the lens.
Negative: 0.10-0.20mm
Positive: >/= 0.25mm
(NB: as get more negative, decrease CT, as you get more positive, increase CT)
What should the BOZR be for an RGP lens?
Should match the back surface to cornea over as large an area as possible
What is the ideal peripheral curve for an RGP lens?
Flat enough to prevent edge indentation on movement to flatter periphery
Steep enough not to irritate lids and to prevent movement onto limbus
What is the ideal BOZR for an RGP lens?
Large enough to minimise flare
small enough to allow good alignment
What is the ideal diameter for an RGP lens?
Large enough to allow adequate width for peripheral curves, minimise lid interaction and assist good centration
Small enough to allow tear exchange, prevent limbal bumping and minimise lens mass
