wk4: CL/M - Soft and RGP Materials Flashcards
List 9 requirements of contact lens materials
optically transparent
non-toxic
chemically inert
oxygen permeable
wettable
inexpensive/abundant
easy to manufacture
tough
low density
Why do we want contact lenses to be tough?
so they can last for however long they are scheduled for
What contact lenses don’t require cleaning/disinfection?
daily disposables
List 5 important properties of rigid lenses
oxygen permeability
wetting of material
deposit resistant
durable, stable, minimal flexure
may be surface coated - more fragile
How does the wetting of a rigid lens material vary with silicone content
inversely influenced
Is PMMA thermosetting or thermoplastic?
thermoplastic (i.e. moulded with heat and pressure)
Describe the quality of the optical clarity provided by PMMA material
excellent
In regards to PMMA:
- how expensive?
- what percentage water content?
- how much O2 permeability?
cheap
0.2-0.5% water content
zero O2 permeability
Which lens material has the highest Dk?
Silicone lenses
What are the limitations of silicone lenses? (4)
very hydrophobic
therefore increased deposition and poor wetting
expensive
non-water permeable
Are silicone RGPs used today?
No. Because of their limitations
How commonly are silicone acrylates used today as lens materials?
they aren’t
List 2 limitations of silicone acrylates
poor wetting deposits
flexure problems
What is the purpose of fluoro-silicone methacrylates (FSA) lenses? (2)
aimed to improve the wettability and oxygen transmissibilty
What benefits do the fluorinated side chains provide to FSA lenses? (2)
Improve wetting properties
Reduce protein deposition
List 2 downsides to 1st gen FSA lenses
More susceptible to lipid deposits
More sensitive to harsher cleaning agents
How do 2nd gen FSA materials compared to 1st gen FSA materials? (4)
improved wetting and deposit resistance
improved material durability
higher Dk
improved potential for extended wear
State the range of Dk values you can get for 2nd gen FSA materials?
40-100+ (mid-high dk)
What is the Dk/t for 2nd gen FSA materials? How does this relate to extended wear?
Dk/t >87, meaning these materials havepotential for extended wear
Name 5 examples of 2nd gen FSA materials
boston xo
menicon z
fluorex 500
fluoroperm
paragon hds 100
How does the silicone content of 2nd gen FSA materials compare to other FSA materials? Why?
Lower silicone content (5-7%) with the use of other monomers for oxygen permeability
Most modern FSA lenses absorb near to what wavelength?
near to 400nm (so UVA and UVB)
What is the benefit of a wratten filter for UV absorbing lenses?
increases contrast
What does the water content of a soft contact lens material depend on?
% of hydrophilic to hydrophobic groups in polymer
What is PMMA material used for?
ONLY USED FOR TRIAL LENSES
How is water conent and Dk related?
water dissolves O2 for Dk effect
Describe the Dk of hydrogel material. What values of Dk would you expect?
related to water content for majority of materials (Dk 8.3-38.9)
What is deposit resistance of hydrogel materials related to? (2)
pore size
surface charge
What is the preffered option for hydrogel lens properties? (out of the 4 groups in US FDA material classification)
Group II: HWC >50% and non ionic (HWC = high water content)
Is HEMA ionic or non ionic?
non-ionic
In regards to HEMA:
- water content?
- Dk level?
38% water content (low/moderate)
Low Dk
How commonly is HEMA used today?
rarely
Why might peripheral corneal hypoxia occur with HEMA lens materials? What does this cause?
due to lower water content and lower oxygen supply. Causes VEGF signalling causing blood vessels and hyperaemia at limbus
List 2 advantages of higher water content hydrogel material (other than higher water content lol)
increased Dk and improved physiology, longer WT
Increased thickness for improved handling
Why do you need to take care to remove protein when using HWC materials?
because HWC tends to absorb more tear film and tends to deposit more protein
List 6 limitations of HWC materials
Reduced life span
Variable degree of dehydration
Slightly higher costs
Larger material pore size
Absorption of tear film components
Dry eye?
What type of deposition does HWC group II and IV result in? (2)
Group II: mainly lipid deposition
Group IV: mainly protein deposition
Is HWC group II ionic or non-ionic? What about group IV?
II: non-ionic
IV: ionic
Name 2 characteristics of GMA combined with HEME or MMA
low dehydration, low deposition
What type of deposition does HEMA and Methacrylic acid have?
protein deposition
Which hydrogel material is sensitive to pH?
HEMA and Methacrylic acid (MA)
What is HEMA and MMA known for?
sucessfl as an opaque tinted material
What is phosphorylcholine (PC)? Name a hydrogel material containing PC
synthetic analogue of a natural phospholipid that mimics cell surfaces.
HEMA + PC
Which FDA group and ionic/non-ionic are the following materials?
HEMA
GMA with HEMA or MMA
HEMA + MA
HEMA + MMA
HEMA + PC
PVA
HEMA: group 1 (non-ionic)
GMA with HEMA/MMA: group I and II (non-ionic)
HEMA + MA: group IV (ionic, -vely charged)
HEMA +MMA: group II (non-ionic)
HEMA + PC: group II (non-ionic)
PVA: group II (non-ionic)
How much deposition do PVA materials get?
they resist deposition
Which FDA group of lens material does data suggest is better for dry eye?
Group II material
Which lens materials are considered the only true high-Dk SCL materials? (1)
Silicone hydrogels
What are the hydrogel monomers in silicone hydrogels important for?
allowing CL movement
List 3 characteristics (and 2 sub-characteristics) of silicone hydrogels
High dk (oxygen permeability)
Inherently hydrophobic (poor wetting, increased lens deposition)
Higher elastic modulus than hydrogels without silicone (therefore stiffer)
List 3 advantages of silicone-hydrogels
Minimal limbal hyperaemia, corneal neovasc, or oedema
May also reduce symptoms of dry eye
Water content of 20-40% ensures water transmissability for rapid recovery from any lens adherence after overnight wear
List 3 disadvantages of silicone-hydrogels
Other EW complications such as GPC, CLARE, CLPUC
Higher incidence of mechanical complications
Highlights that factors other than O2 are still important
Briefly describe the 3 generations of silicone hydrogels
1st gen: plasma-treated, high modulus
2nd gen: no coatings, higher Dk for water content
3rd gen: break traditional water-Dk-modulus relationships
What was used on 2nd generation silicone hydrogels as an internal wetting agent? Why?
PVP (provides a hydrophilic layer on the surface of the material)
What is modulus?
= stiffness vs water content
Do SiH contact lenses have a higher or lower elastic modulus compared to hydrogel? Are they more or less stiff?
Significantly higher than hydrogels, making them stiffer
