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OD2 (S1&2) > CLM - Soft and RGP Materials - Week 4 > Flashcards

CLM - Soft and RGP Materials - Week 4 Flashcards

1
Q

List 9 requirements of a contact lens material.

A

Optically transparent
Non-toxic
Chemically inert
Oxygen permeable
Wettable
Inexpensive/abundant
Easy to manufacture
Tough
Low density

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2
Q

If a contact lens is surface coated, is it more or less fragile?

A

More fragile

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3
Q

What is PMMA, is it thermoplastic or thermosetting, and what is its O2 permeability?

A

Thermoplastic, zero O2 permeability

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4
Q

Is PMMA low or high cost? Is it wettable?

A

Low cost, fairly wettable

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5
Q

What is PMMA used for?

A

Only used for trial lenses

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6
Q

What is an advantage of silicon RGP lenses?

A

Highest Dk

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7
Q

List 4 limitations of silicon RGP lenses.

A

Very hydrophobic
Increased deposition
More expensive
Non-water permeable

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8
Q

List three disadvantages of silicone acrylates for RGP lenses. Are they still used?

A

Poor wetting
Deposits
Flexure problems
Not used now

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9
Q

What is FSA? hat property does it have and why? What does this reduce?

A

Next generation of silicon polymers for RGP lenses
Side chains are fluorinated which improves wetting properties and reduces protein deposition

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10
Q

List two advantages and two disadvantages of FSA.

A

Advantages
Improved wettability
Oxygen transmissibility

Disadvantages
Susceptible to lipid deposits
Sensititive to harsher cleaning agents

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11
Q

List three advantages of second generation FSA.

A

Improved wetting
Deposit resistance
Improved durability

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12
Q

Why is corneal hypoxia effectively avoided in most GP materials?

A

Because most have a Dk/t over 87

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13
Q

What UV wavelengths can FSA absorb until?

A

Most absorb to near 400nm

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14
Q

What is the purpose of a Wratten filter?

A

Increases contrast for UV absorbing lenses to visualise fluorescein patterns

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15
Q

What is the simplest soft contact lens material and how is it made?

A

PMMA _ hydroxyl groups
Simplest is HEMA

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16
Q

What does the water content of soft contact lenses depend on?

A

The ratio between hydrophilic and hydrophobic groups

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17
Q

How is water content and Dk related?

A

Water dissolves O2 for Dk effect

18
Q

What is deposit resistance related to (2)?

A

Pore size
Surface charge

19
Q

What material were the majority of early contact lenses made of?

A

HEMA

20
Q

Do HEMA contact lenses have high or low Dk?

A

Low

21
Q

Are HEMA contact lenses ionic or non-ionic?

A

Non-ionic

22
Q

Are HEMA contact lenses still used?

A

Rarely

23
Q

Is the Dk for HEMA contact lenses evenly distributed? What can occur in minus lenses? Explain why.

A

No
Minus lenses are thicker in the periphery and Dk can be <10

24
Q

What can occur with HEMA contact lens wear?

A

Limbal hyperaemia

25
Q

What are higher ater content hydrogel materials composed of? Explain how they have such high water content.

A

HEMA + NVP
NVP has amide groups which are highly polar - two sites for water binding

26
Q

List three advantages of higher water content hydrogel materials.

A

Increased Dk
Longer WT
Increased thickness for improved handling

27
Q

List 5 limitations of HWC lenses.

A

Reduced life span in conventional lenses
Variable degrees of dehydration between materials
Larger material pore size
Absorbs tear film components
Possible dry eye

28
Q

What component of the tear film do ionic and non-ionic HWC contact lenses absorb?

A

Ionic - mainly protein deposition
Non-ionic - lipid deposition

29
Q

In which group (I, II or IV) of HWC contact lenses are dry eye symptoms more common? Explain why.

A

Most common in group I
Group II > group IV
Polar head groups associated with tear film lipids may be attracted to HWC materials

30
Q

Which dehydrate quicker, hydrogels or silicon hydrogels?

A

Hydrogels

31
Q

What is the only true high DK soft contact lens material?

A

Silicon hydrogels

32
Q

What is the water content and Dk value of SiH contact lenses?

A

Low water content, but high Dk due to silicon content

33
Q

Are SiH contact lenses inherently hydrophobic or -philic? What does this mean for wettability (2) and lipid deposition?

A

Inherently hydrophobic - has poor wetting
Increased lipid deposition
Surface is typically treated to retain wetting

34
Q

Do SiH contact lenses have a higher or lower elastic modulus compared to hydrogel? Are they more or less stiff?

A

Significantly higher than hydrogels, making them stiffer

35
Q

List 3 advantages of SiH contact lenses.

A

Minimal limbal hyperaemia, neovascularisation or oedema
May reduce dry eye symptoms
Adequate water content ensures rapid recovery from lens adherence after overnight ear

36
Q

List 2 limitations of SiH contact lenses.

A

Extended ear complications similar to hydrogels
Higher incidence of mecanical complications (mucin balls)

37
Q

List 3 conditions that can still be caused by SiH contact lenses despite its high quality profile.

A

Giant papillary conjunctivitis
Contact lens induced acute red eye
Contact lens induced peripheral ulcer

38
Q

How are 1st generation SiH contact lenses treated and why?

A

Plasma treated to buffer the silicone from the ocular surface and tears

39
Q

How are 2nd generation SiH contact lenses treated and why?

A

No coatings, higher Dk for water content

40
Q

How are 3rd generation SiH contact lenses treated and why? What is special about this generation?

A

No coatings, they are constructed from a unique backbone that is inherently rewettable
It breaks traditional water-Dk modulus relationships

41
Q

What is a new concept for contact lens currently in development?

A

The idea of a water gradient, where the lens core is LWC and the surface is HWC

42
Q

Compare the Dk to water content for SiH and hydrogels.

A

Dk starts high and slowly decreases for SiH before rising again - graph is like a negative quadratic curve
For hydrogels, it begins low, and slowly increases, more rapid increase at higher ater content

OD2 (S1&2) (174 decks)

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