14. Contact lens material properties

Question 1

What are polymers?

Answer 1

Long chain molecules of monomers

Question 2

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

Answer 2

To improve material properties

Question 3

What properties do long chained polymers give?

Answer 3

Toughness and elasticity

Question 4

How is stability and stiffness achieved in polymers?

Answer 4

More cross-linking

Question 5

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

Answer 5

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

Question 6

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

Answer 6

ISO system
6 part code

Question 7

What is Dk range?

Answer 7

Numerical code identifying oxygen permeability in ISO units

Question 8

Which lens are more stiffer, hydrogel or silicone?

Answer 8

Silicone lenses

Question 9

Describe water gradient lenses?

Answer 9

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.

Question 10

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

Answer 10

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

Question 11

What is the most important feature in soft lenses manufacture?

Answer 11

Water content

Question 12

WC influences most …?

Answer 12

Hydrogel material properties

Question 13

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

Answer 13

1. They are difficult to manufacture
2. High dehydration tendency

Question 14

Relation between WC and Dk

Answer 14

Higher the WC, lower the Dk

Question 15

What kind of stains do high WC lenses cause?

Answer 15

Smile stain

Question 16

What is meant by tensile strength?

Answer 16

Resistance to deformation while under tension

Question 17

What is meant by tear strength?

Answer 17

Resistance to tear propagation from a notch or defect

Question 18

What is meant by modulus of rigidity?

Answer 18

Extent material will deform- compression

Question 19

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

Answer 19

Hydrogel lenses

Question 20

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

Answer 20

1. Polymer
2. Water content

Question 21

Low modulus in RGP lens can result in?

Answer 21

Parameter changes and reduced ability to correct corneal toricity

Question 22

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

Answer 22

1. Tear resistance
2. Durability

Question 23

Modules of a lens influences what 2 things?

Answer 23

1. Comfort
2. Vision

Question 24

How does low modules affect RGP lenses?

Answer 24

Increased comfort but increased flexibility causes lens flexure

Question 25

How does low module affect soft lens in 4 ways

Answer 25

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

Question 26

Stiff lenses lead to?

Answer 26

Papillary conjunctivitis

Question 27

Refractive index of a lens is determined by?

Answer 27

Polymer composition

Question 28

Relation between refractive index and WC

Answer 28

Refractive index reduces with increasing WC

Question 29

What is the ideal optical transmittance of a CL?

Answer 29

Must be high with little scatter

Question 30

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

Answer 30

Adhesion of liquids to solid

Question 31

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

Answer 31

Means resistance to movement also referred to as lens lubricity

Question 32

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

Answer 32

1. Polymer properties 2. WC 3. Hydration

Question 33

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

Answer 33

1. Polymer properties 2. WC 3. Hydration

Question 34

Surface properties are extremally important for?

Answer 34

Comfort

Question 35

Good surface wettability is import for what 5 reasons?

Answer 35

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

Question 36

Low coefficient friction is essential for what 3 reasons?

Answer 36

1. Normal ocular surface physiology 2. Normal eyelid physiology 3. Comfort

Question 37

Density of a lens is dependent on what 2 factors?

Answer 37

1. Polymer composition 2. WC

Question 38

Relationship between density of hydrogels and WC

Answer 38

As density decreases, WC increases

Question 39

Dimensional stability meaning?

Answer 39

It is the ability to maintain original dimensions under various conditions

Question 40

Dimensional stability is affected by which 3 factors?

Answer 40

1. Temperature 2. pH 3. Tonicity

Question 41

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

Answer 41

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

Question 42

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

Answer 42

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

Question 43

What is meant by oxygen permeability?

Answer 43

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

Question 44

Relation between oxygen and water content in a lens

Answer 44

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

Question 45

How does oxygen reach the cornea in hydrogel lenses?

Answer 45

Passing through the water

Question 46

Relation between WC and lens permeability

Answer 46

Both related exponentially

Question 47

Oxygen is more soluble in water or silicone?

Answer 47

Silicone

Question 48

3 disadvantages of lenses with increasing WC

Answer 48

1. Difficult to manufacture 2. Tear easy 3. Dehydrated easy

Question 49

Which lenses offer greater oxygen performance: silicone or hydrogel?

Answer 49

Silicones

Question 50

Stiff lenses cause what type of staining?

Answer 50

SEAL stain