CLM - Soft and RGP Materials - Week 4

Question 1

List 9 requirements of a contact lens material.

Answer 1

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

Question 2

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

Answer 2

More fragile

Question 3

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

Answer 3

Thermoplastic, zero O2 permeability

Question 4

Is PMMA low or high cost? Is it wettable?

Answer 4

Low cost, fairly wettable

Question 5

What is PMMA used for?

Answer 5

Only used for trial lenses

Question 6

What is an advantage of silicon RGP lenses?

Answer 6

Highest Dk

Question 7

List 4 limitations of silicon RGP lenses.

Answer 7

Very hydrophobic
Increased deposition
More expensive
Non-water permeable

Question 8

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

Answer 8

Poor wetting
Deposits
Flexure problems
Not used now

Question 9

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

Answer 9

Next generation of silicon polymers for RGP lenses

Side chains are fluorinated which improves wetting properties and reduces protein deposition

Question 10

List two advantages and two disadvantages of FSA.

Answer 10

Advantages
Improved wettability
Oxygen transmissibility

Disadvantages
Susceptible to lipid deposits
Sensititive to harsher cleaning agents

Question 11

List three advantages of second generation FSA.

Answer 11

Improved wetting
Deposit resistance
Improved durability

Question 12

Why is corneal hypoxia effectively avoided in most GP materials?

Answer 12

Because most have a Dk/t over 87

Question 13

What UV wavelengths can FSA absorb until?

Answer 13

Most absorb to near 400nm

Question 14

What is the purpose of a Wratten filter?

Answer 14

Increases contrast for UV absorbing lenses to visualise fluorescein patterns

Question 15

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

Answer 15

PMMA _ hydroxyl groups

Simplest is HEMA

Question 16

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

Answer 16

The ratio between hydrophilic and hydrophobic groups

Question 17

How is water content and Dk related?

Answer 17

Water dissolves O2 for Dk effect

Question 18

What is deposit resistance related to (2)?

Answer 18

Pore size

Surface charge

Question 19

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

Answer 19

HEMA

Question 20

Do HEMA contact lenses have high or low Dk?

Answer 20

Low

Question 21

Are HEMA contact lenses ionic or non-ionic?

Answer 21

Non-ionic

Question 22

Are HEMA contact lenses still used?

Answer 22

Rarely

Question 23

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

Answer 23

No

Minus lenses are thicker in the periphery and Dk can be <10

Question 24

What can occur with HEMA contact lens wear?

Answer 24

Limbal hyperaemia

Question 25

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

Answer 25

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

Question 26

List three advantages of higher water content hydrogel materials.

Answer 26

Increased Dk Longer WT Increased thickness for improved handling

Question 27

List 5 limitations of HWC lenses.

Answer 27

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

Question 28

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

Answer 28

Ionic - mainly protein deposition | Non-ionic - lipid deposition

Question 29

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

Answer 29

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

Question 30

Which dehydrate quicker, hydrogels or silicon hydrogels?

Answer 30

Hydrogels

Question 31

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

Answer 31

Silicon hydrogels

Question 32

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

Answer 32

Low water content, but high Dk due to silicon content

Question 33

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

Answer 33

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

Question 34

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

Answer 34

Significantly higher than hydrogels, making them stiffer

Question 35

List 3 advantages of SiH contact lenses.

Answer 35

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

Question 36

List 2 limitations of SiH contact lenses.

Answer 36

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

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

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

Answer 39

No coatings, higher Dk for water content

Question 40

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

Answer 40

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

Question 41

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

Answer 41

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

Question 42

Compare the Dk to water content for SiH and hydrogels.

Answer 42

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