CLM - Soft and RGP Materials - Week 4 Flashcards
List 9 requirements of a contact lens material.
Optically transparent
Non-toxic
Chemically inert
Oxygen permeable
Wettable
Inexpensive/abundant
Easy to manufacture
Tough
Low density
If a contact lens is surface coated, is it more or less fragile?
More fragile
What is PMMA, is it thermoplastic or thermosetting, and what is its O2 permeability?
Thermoplastic, zero O2 permeability
Is PMMA low or high cost? Is it wettable?
Low cost, fairly wettable
What is PMMA used for?
Only used for trial lenses
What is an advantage of silicon RGP lenses?
Highest Dk
List 4 limitations of silicon RGP lenses.
Very hydrophobic
Increased deposition
More expensive
Non-water permeable
List three disadvantages of silicone acrylates for RGP lenses. Are they still used?
Poor wetting
Deposits
Flexure problems
Not used now
What is FSA? hat property does it have and why? What does this reduce?
Next generation of silicon polymers for RGP lenses
Side chains are fluorinated which improves wetting properties and reduces protein deposition
List two advantages and two disadvantages of FSA.
Advantages
Improved wettability
Oxygen transmissibility
Disadvantages
Susceptible to lipid deposits
Sensititive to harsher cleaning agents
List three advantages of second generation FSA.
Improved wetting
Deposit resistance
Improved durability
Why is corneal hypoxia effectively avoided in most GP materials?
Because most have a Dk/t over 87
What UV wavelengths can FSA absorb until?
Most absorb to near 400nm
What is the purpose of a Wratten filter?
Increases contrast for UV absorbing lenses to visualise fluorescein patterns
What is the simplest soft contact lens material and how is it made?
PMMA _ hydroxyl groups
Simplest is HEMA
What does the water content of soft contact lenses depend on?
The ratio between hydrophilic and hydrophobic groups
How is water content and Dk related?
Water dissolves O2 for Dk effect
What is deposit resistance related to (2)?
Pore size
Surface charge
What material were the majority of early contact lenses made of?
HEMA
Do HEMA contact lenses have high or low Dk?
Low
Are HEMA contact lenses ionic or non-ionic?
Non-ionic
Are HEMA contact lenses still used?
Rarely
Is the Dk for HEMA contact lenses evenly distributed? What can occur in minus lenses? Explain why.
No
Minus lenses are thicker in the periphery and Dk can be <10
What can occur with HEMA contact lens wear?
Limbal hyperaemia
What are higher ater content hydrogel materials composed of? Explain how they have such high water content.
HEMA + NVP
NVP has amide groups which are highly polar - two sites for water binding
List three advantages of higher water content hydrogel materials.
Increased Dk
Longer WT
Increased thickness for improved handling
List 5 limitations of HWC lenses.
Reduced life span in conventional lenses
Variable degrees of dehydration between materials
Larger material pore size
Absorbs tear film components
Possible dry eye
What component of the tear film do ionic and non-ionic HWC contact lenses absorb?
Ionic - mainly protein deposition
Non-ionic - lipid deposition
In which group (I, II or IV) of HWC contact lenses are dry eye symptoms more common? Explain why.
Most common in group I
Group II > group IV
Polar head groups associated with tear film lipids may be attracted to HWC materials
Which dehydrate quicker, hydrogels or silicon hydrogels?
Hydrogels
What is the only true high DK soft contact lens material?
Silicon hydrogels
What is the water content and Dk value of SiH contact lenses?
Low water content, but high Dk due to silicon content
Are SiH contact lenses inherently hydrophobic or -philic? What does this mean for wettability (2) and lipid deposition?
Inherently hydrophobic - has poor wetting
Increased lipid deposition
Surface is typically treated to retain wetting
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
List 3 advantages of SiH contact lenses.
Minimal limbal hyperaemia, neovascularisation or oedema
May reduce dry eye symptoms
Adequate water content ensures rapid recovery from lens adherence after overnight ear
List 2 limitations of SiH contact lenses.
Extended ear complications similar to hydrogels
Higher incidence of mecanical complications (mucin balls)
List 3 conditions that can still be caused by SiH contact lenses despite its high quality profile.
Giant papillary conjunctivitis
Contact lens induced acute red eye
Contact lens induced peripheral ulcer
How are 1st generation SiH contact lenses treated and why?
Plasma treated to buffer the silicone from the ocular surface and tears
How are 2nd generation SiH contact lenses treated and why?
No coatings, higher Dk for water content
How are 3rd generation SiH contact lenses treated and why? What is special about this generation?
No coatings, they are constructed from a unique backbone that is inherently rewettable
It breaks traditional water-Dk modulus relationships
What is a new concept for contact lens currently in development?
The idea of a water gradient, where the lens core is LWC and the surface is HWC
Compare the Dk to water content for SiH and hydrogels.
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
