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OD3 - Clin. Optometry Sem 1 and Preacademic Period > CL - RGP fitting 2 - Week -2 > Flashcards

CL - RGP fitting 2 - Week -2 Flashcards

1
Q

What are the 2 goals for BOZR when fitting spherical RGPs on toric corneas?

A

Minimise contact/bearing on flatter meridian

Achieve adequate centration

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2
Q

Describe the fit of an RGP on a toric cornea if BOZR = flattest K (3)

A

Excessive edge clearance on steeper meridian
Contact along flatter meridian
Usually poor centration

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3
Q

Describe the fit of an RGP on a toric cornea if BOZR = steepest K (4)

A

Excessive central pooling
Harsh bearing along peripheral regions of flatter meridian
Dimple veiling (centrally) + 3&9 staining likely
Centration often good

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4
Q

What is the optimal BOZR for a toric cornea?

A

BOZR 2/3 towards flatter K

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5
Q

What does the fluorescein stain of a good RGP fit on a toric cornea look like?

A

“Dumb-bell” or “H” pattern

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6
Q

List 5 indications for toric BOZR

A

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

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7
Q

What’s the difference between tricurve and quadcurve?

A

Tricurve has a central spherical curve with 2 spherical peripheral curves whereas quadcurve has 3 spherical peripheral curves

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8
Q

What does the number of peripheral curves in an RGP lens design affect?

A

Increased peripheral curves –> smoother transitions

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9
Q

If the RGP lens requires flattening, would you increase or decrease the number of peripheral curves in the design?

A

increase

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10
Q

What is the typical BOZR for an aspheric RGP lens?

A

Trick question, there is no true BOZR for an aspheric lens

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11
Q

List 3 advantages of aspheric RGP lenses

A

Less flare
Less corneal distortion
Less edge clearance (therefore more comfort and less 3+9 stain)

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12
Q

What is a conoid rgp design?

A

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)

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13
Q

Is a conoid design fitted with apical or edge clearance?

A

apical

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14
Q

How does a conoid design affect corneal distortion?

A

increases it

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15
Q

When are PMMA materials used?

A

only for trial lenses

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16
Q

What is the main advantage to using fluorosilacrylate (FSA) material for gas-permeable lenses? Explain

A

High Dk. This high Dk is facilitated by fluorine and silicone

17
Q

What are the 5 ideal features of a gas-permeable lens material?

A 
Good rigidity and stability
Excellent wettability
Good VA
High Dk
High deposit resistance
18
Q

Name 4 examples of good gas-permeable materials [important]

A 
Boston XO (B&L)
Boston XO2 (B&L)
Tyro97 (menicon)
Menicon Z (stock lenses)
19
Q

What factors should be considered when setting a lens diameter for a gas-permeable lens? (5)

A

Width of palpebral aperture
Lid geometry + position of lid margins wrt cornea
Lid tension
Lid movement during blinking (complete/incomplete)
Corneal diameter (HVID)

20
Q

How may a gas-permeable lens with a negative power and large total diameter fit on a patient?

A

may ride high

21
Q

How can we use the patient’s anatomy to help centre the RGP?

A

superior lid attachment fit helps lens centre. I.e. upper eyelid hooking over the lens can give you a better fit.

22
Q

What is the range of lens diameters used for RGPs?

A

7.5-11mm

23
Q

Name the two different types of fits for RGP lenses and describe how the lens differs in size and steepness

A
  1. Lid attachment fit: larger flatter lens, aim for central alignment
  2. Intrapalpebral: smaller steeper lens, apical clearance
24
Q

What type of lens steepness will give you greater lid interaction?

A

flatter lens gives more lid interaction

25
Q

What is the golden rule of thumb for changes in BOZR with BOZD/TD for lens fitting?

A

For every 0.5mm increase in BOZD/TD, must be 0.05mm increase in BOZR to maintain the same fitting (NaFl) pattern

26
Q

How does an increase in BOZD affect lens fit?

A

Increase apical clearance, tightens fit

27
Q

How does a decrease in BOZD affect lens fit?

A

Decrease apical clearance, loosens fit

28
Q

What can be done to the FOZD to reduce lens thickenss?

A

can decrease FOZD to help decrease lens thickness

29
Q

Should the FOZD be larger or smaller than BOZD? Why?

A

FOZD should be slightly larger than BOZD to minimise visual disturbance (reduce flare)

30
Q

What happens if FOZD = BOZD?

A

increased chance of lens breakage as junctions coincide

31
Q

What is AEC? What is AEL?

A

Axial Edge Clearance: the separation between the edge of the lens and the cornea
Axial Edge Lift: a mathematical property of the lens

32
Q

Is AEC > or < AEL generally?

A

Generally, AEC < AEL

33
Q

What is the AEC usually for an RGP lens?

A

~0.08mm

34
Q

How can we alter visible edge clearance? [important] (4)

A

Alter radius of peripheral curves
Alter width of peripheral curves
Change number of peripheral curves
Alter BOZD

35
Q

What is the centre thickness (CT) for negative RGP lenses? What about positive? Describe relationship between centre thickness and power of the lens.

A

Negative: 0.10-0.20mm
Positive: >/= 0.25mm

(NB: as get more negative, decrease CT, as you get more positive, increase CT)

36
Q

What should the BOZR be for an RGP lens?

A

Should match the back surface to cornea over as large an area as possible

37
Q

What is the ideal peripheral curve for an RGP lens?

A

Flat enough to prevent edge indentation on movement to flatter periphery
Steep enough not to irritate lids and to prevent movement onto limbus

38
Q

What is the ideal BOZR for an RGP lens?

A

Large enough to minimise flare

small enough to allow good alignment

39
Q

What is the ideal diameter for an RGP lens?

A

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

OD3 - Clin. Optometry Sem 1 and Preacademic Period (25 decks)

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