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OP2202 > 14. Contact lens material properties > Flashcards

14. Contact lens material properties Flashcards

1
Q

What are polymers?

A

Long chain molecules of monomers

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2
Q

Polymers are carbon based. However, they are also attached to other elements, what is the benefit of this?

A

To improve material properties

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3
Q

What properties do long chained polymers give?

A

Toughness and elasticity

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4
Q

How is stability and stiffness achieved in polymers?

A

More cross-linking

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5
Q

Describe the function of some polymers such as: hydrogels, hydroxyl and silicones

A

Hydrogel: Attract and bind to water
Hydroxyl: make polymers more hydrophilic
Silicones: Can attach to increase oxygen permeability

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6
Q

CL materials are classified under which system and by how many part codes

A

ISO system
6 part code

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7
Q

What is Dk range?

A

Numerical code identifying oxygen permeability in ISO units

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8
Q

Which lens are more stiffer, hydrogel or silicone?

A

Silicone lenses

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9
Q

Describe water gradient lenses?

A

They are made with silicone core, with a low water content hence high oxygen circulation.
This lenses have hydrogel on both sides of the lens hence making them very soft.

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10
Q

Manufactures rely on in vitro data but what are the 3 disadvantages?

A
  1. Tests are often too simple
  2. Procedure is not standardized
  3. Tests do not reflect clinical reality
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11
Q

What is the most important feature in soft lenses manufacture?

A

Water content

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12
Q

WC influences most …?

A

Hydrogel material properties

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13
Q

2 disadvantages of high WC lenses: 1 in terms of manufacture and one in terms of patient ?

A
  1. They are difficult to manufacture
  2. High dehydration tendency
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14
Q

Relation between WC and Dk

A

Higher the WC, lower the Dk

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15
Q

What kind of stains do high WC lenses cause?

A

Smile stain

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16
Q

What is meant by tensile strength?

A

Resistance to deformation while under tension

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17
Q

What is meant by tear strength?

A

Resistance to tear propagation from a notch or defect

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18
Q

What is meant by modulus of rigidity?

A

Extent material will deform- compression

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19
Q

Hydrogel vs silicone lens, which one is easy to tear?

A

Hydrogel lenses

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20
Q

Mechanical properties of a lens are determined by which 2 factors?

A
  1. Polymer
  2. Water content
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21
Q

Low modulus in RGP lens can result in?

A

Parameter changes and reduced ability to correct corneal toricity

22
Q

In hydrogels high water content leads to which 2 problems in mechanical properties?

A
  1. Tear resistance
  2. Durability
23
Q

Modules of a lens influences what 2 things?

A
  1. Comfort
  2. Vision
24
Q

How does low modules affect RGP lenses?

A

Increased comfort but increased flexibility causes lens flexure

25
Q

How does low module affect soft lens in 4 ways

A
  1. lens drapes over cornea
  2. Greater initial comfort
  3. Low risk of mechanical complications
  4. Handling issues
26
Q

Stiff lenses lead to?

A

Papillary conjunctivitis

27
Q

Refractive index of a lens is determined by?

A

Polymer composition

28
Q

Relation between refractive index and WC

A

Refractive index reduces with increasing WC

29
Q

What is the ideal optical transmittance of a CL?

A

Must be high with little scatter

30
Q

What does wettability as a surface property of a lens mean?

A

Adhesion of liquids to solid

31
Q

What does coefficient of friction mean and it is also referred to as?

A

Means resistance to movement
also referred to as lens lubricity

32
Q

Surface properties of a lens are dependent on what 3 factors?

A
  1. Polymer properties
  2. WC
  3. Hydration
33
Q

Surface properties of a lens are dependent on what 3 factors?

A
  1. Polymer properties
  2. WC
  3. Hydration
34
Q

Surface properties are extremally important for?

A

Comfort

35
Q

Good surface wettability is import for what 5 reasons?

A
  1. Stable tear film
  2. Stable vision
  3. Minimizing lens deposit
  4. Normal eyelid physiology
  5. Comfort
36
Q

Low coefficient friction is essential for what 3 reasons?

A
  1. Normal ocular surface physiology
  2. Normal eyelid physiology
  3. Comfort
37
Q

Density of a lens is dependent on what 2 factors?

A
  1. Polymer composition
  2. WC
38
Q

Relationship between density of hydrogels and WC

A

As density decreases, WC increases

39
Q

Dimensional stability meaning?

A

It is the ability to maintain original dimensions under various conditions

40
Q

Dimensional stability is affected by which 3 factors?

A
  1. Temperature
  2. pH
  3. Tonicity
41
Q

Hydrogels loss water during wear. How does this affect dimensional stability in 2 ways?

A
  1. Can result in reduced BC and TD
  2. Lens tightens during wear
42
Q

What are the 2 classes of UV protection and how are they classed?

A

Class I and class II: Given classes depending on how much UVA and UVB they block

43
Q

What is meant by oxygen permeability?

A

Dk
D: ease of which oxygen moves through a material
k: solubility of oxygen in a material

44
Q

Relation between oxygen and water content in a lens

A

Higher the WC in a lens, more oxygen can pass through the lens.

45
Q

How does oxygen reach the cornea in hydrogel lenses?

A

Passing through the water

46
Q

Relation between WC and lens permeability

A

Both related exponentially

47
Q

Oxygen is more soluble in water or silicone?

A

Silicone

48
Q

3 disadvantages of lenses with increasing WC

A
  1. Difficult to manufacture
  2. Tear easy
  3. Dehydrated easy
49
Q

Which lenses offer greater oxygen performance: silicone or hydrogel?

A

Silicones

50
Q

Stiff lenses cause what type of staining?

A

SEAL stain

OP2202 (16 decks)

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