Week 1 - RGP fitting 1 Flashcards

1
Q

What are the 4 types if RGP lenses?

A

• Corneal (7-12mm) - our focus
• Corneal-scleral (12.1-15mm)
• Mini-Scleral (15.1-18mm)
• Scleral (>18mm)

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2
Q

what are the advantages to RGP’s? (1-3)

A
  1. Robust
    - Can be re-polished (if not surface treated
    Longer lens life (i.e. years)
  2. Easy to clean
    - Soaking solutions stronger
    - Fewer Solution problems (constituents don’t soak into lens)
  3. Easy to handle (relatively)
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3
Q

what are the advantages to RGP’s? (4-8)

A
  1. Stable vision
    Can avoid need to fit toric lenses (tear lens)
  2. Good oxygen supply
  3. Dehydration less problematic (compared to soft lens materials)
  4. Customised designs
  5. Good tear exchange
    - Aids metabolism
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4
Q

Disadvantages of RGP lenses?

A
  1. Physical comfort
    - Edge sensation present to begin with
    - Need to build up tolerance
    - Poor for intermittent wear
  2. Greater movement of lens
    - May become dislodged
  3. FB entrapment
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5
Q

When do you chose an RGP over SCL?

A

• Moderate corneal astigmatism
• High complex prescriptions
• History of allergies
• Problem with SCLs
• Financial reasons
• Poor compliance
• High Visual demands

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6
Q

When should you avoid using RGP lenses?

A

• Contact sports
- Rugby, judo, karate, boxing etc
• Water Sports
- May get washed out
• Dusty environments

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7
Q

What is the definition of BOZR and BOZD??

A

• BOZR : Back optic zone radius
• BOZD : Back optic zone diameter

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8
Q

What are the parameters for BOZD and FOZD, centre thickness and edge lift?

A

• BOZD: 1-1.5mm smaller than TD. Large enough to cover pupil in different light conditions.
• FOZD: 0.5mm larger than the BOZD
• tc: Centre thickness: Too thin a lens leads to flexure which can lead to breakage and residual astigmatism
Edge lift: to prevent mechanical pressure on the cornea, allow tear exchange and enable lens removal

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9
Q

What are the 3 types of tear lenses?

A

• Flat fit creates negative tear lens
• Alignment fit creates neutral tear film
• Steep fit creates positive tear lens

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10
Q

What are the tear lens rules of thumb?

A

• Rigid lens: Tear lens power increases about +0.25D for each 0.05mm that the BOZR of lens is steeper than corneal radius
• For the BVD of the rigid contact lens needs to be changed by -0.25D for each 0.05mm that the BOZR is made steeper, to compensate for the extra positive power of the liquid lens
- If the lens BOZR is made flatter by 0.05mm, the BVD needs to be changed by +0.25 D

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11
Q

British standards - what is the minimum specification?

A

• BOZR/ TD / BVP
e.g. 7.55/9.20/-5.25
Then lens type and material

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12
Q

What is spherical vs aspherical

A

• Spherical - spherical back optic zone with spherical peripheral zones
• Aspheric - elliptical lens design, gradual flattening from the centre out

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13
Q

What steps are needed for selecting a 1st RGP lens?

A
  1. Find out about the eye
    - Shape, size, pupils and refractive error
    - Health (slit lamp, history & symptoms)
  2. Choose the appropriate lens type
  3. Work out the shape you want
    - Find the nearest available
  4. Once you know what shape you are going to order, work out the power you need - BVP calculations from Tri A
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14
Q

What steps need to be considered before selecting an RGP material? (1-2)

A
  1. Corneal Oxygen Requirements
    - Dk (Oxygen transmissibility)
    - Thin lens - more O, to lens
    - Thick lens - easier to handle
  2. Centre thickness of lens (esp. high Plus)
    - Consider edge form too - can be like a knife if manufacturer trying to maximise oxygen permeability
    - Maximum wearing time required
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15
Q

What steps need to be considered before selecting an RGP material? (3-5)

A
  1. Wettability - ability of the tears to form a stable layer on the surface of the material
  2. Flexure
    • Can cause residual astigmatism on toric corneas
    • Especially true of thinner lenses (less than 0.15mm)
  3. Compliance
    If likely to be poor go for simple to care for materials
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16
Q

What are common RGP Materials?

A

• Silicone acrylate - more prone to lens flexure
• Fluorosilicone acrylate - better wettability, fewer deposits. But can be brittle if too thin.
• Fluorosiloxanyl styrene

Common trade names are Boston and Optimum

17
Q

What is considered before Fitting the lens?

A

• Pupil size
Make sure optical portion >scotopic pupil

• Environment
- Atmosphere
- Dusty? Dry?
- Activity (sports etc)

18
Q

So how do we know if the RGP lens is suitable to fit depending on factors?

A

• Check manufacturers advice
- if possible, chose first lens according to manual

19
Q

How do we fit the BOZR if different in K is ≤ 1DC?

A

• If difference in K readings is ≤ 1DC
- Fit on flattest K (round to 0.05mm)
• Positive lenses tend to drop a little, (central thickness) consider fitting 0.05mm steeper to stabilise

20
Q

How do we fit the BOZR if different in K is > 1DC?

A

• Can still be worth trying a spherical RGP BOZR:
• Using the GENERIC Formula
BOZR = FlatK - (FlatK - SteepK)/3
• Round to nearest 0.05mm, this will give you a lens slightly steeper than flattest K
• You can use this formula for any level of toricity - if toricity minimal, this will round to Flat K, so you don’t need to remember lots of formulae!

21
Q

What are the different sources if deciding a BOZR?

A

• Contact lens manual
• Contact kens practice- depends on BOZD, Flattest K - 0.05 for standard BOZR

22
Q

How is BOZD determined?

A

• Often predetermined by the laboratory
• But you need to know if the standard lens won’t work due to pupil size
• Can also consider a smaller BOZD in a toric cornea to minimize the area of mismatch

Small: <7.30mm (need a steeper BOZR)
Medium: 7.30-7.90mm
Large: >7.90mm (need a flatter BOZR)

23
Q

How is TD (total diameter) decided?

A

Consider pupil size
• BOZD > (scotopic) pupil by 1- 2 mm (roughly)
• TD < HVID (Horizontal Visible Iris Diameter) By ~2mm (roughly)
• TD > BOZD By ~2mm (roughly)
• Remember these are STARTING points
• Consider availability
• Most lenses are between 9.20 to 9.80mm in diameter

24
Q

How are the pupil diameters measured?

A

• Measured with a ruler
• Need maximum (dim light) and minimum (bright light)
• Can use Burton lamp to help with this
• If no Burton lamp then have the room as dim as possible for maximum that still allows you to see the pupil margin
• Can use slit lamp for minimum