Unit 3 Flashcards
What are Rigid Gas Permeable lenses made from?
PMMA was originally used but virtually no oxygen permeability
More silicon based
- fluorine silicon/acrylate
- F-S/A
Less of an issue with deposits
Less of a problem with dryness
- although still can be uncomfortable
Good oxygen permeability
What is the purpose of contact lenses?
Correct visual defects
- myopia
- short-sightedness
- hyperopia
- long-sightedness
Astigmatism
What are the advantages of using contact lenses?
Personal appearance
- not wearing glasses
Convenience
Activity
Increase peripheral vision
Therapeutic necessity
Future - drug delivery?
What are the contraindications in the use of contact lenses?
History of allergy
History of ocular infection/inflammation
Insufficient tear production
Diabetes
Hypothyroid
Pregnancy
Exposure to dust/chemicals
Inability to manipulate lenses
Other medication
- systemic
- topical
What are the different types of contact lenses?
The ideal lens
Hard lenses
RGP lenses
Soft lenses
What is the ideal contact lens?
Allows good visual acuity
Good oxygen permeability
Good wettability
Does not allow deposits to form
Not prone to scratching/breaking
Safe
- not detrimental to eye health
Comfortable and easy to use
Range of options
- cost
- duration
What are the properties of hard contact lenses?
Used since late 1940s
Polymerised products of acrylic acid and methacrylic acid (PMMA)
Have small diameter
Not very permeable to oxygen
Very rarely used now
- superseded by RGP
What are the properties of rigid gas permeable (RGP) lenses?
Small
- smaller than diameter of cornea
Good optical properties
Safe
Durable
Reduced myopia progression in young people
Uncomfortable
Poorer wetting than soft lenses
Sometimes called “hard” lenses
- misleading
- some degree of flexibility and oxygen permeability
What are the properties of soft (hydrogel) lenses?
Flexible plastic lens
Conforms to shape of cornea
Absorbs and binds water
- hydroxyl and lactam groups
Increased gas permeation
Better wetting than RGP
87% worldwide market
What are the disadvantages of soft (hydrogel) lenses?
Increased water means more fragile and more adherence of environmental contaminants
What were soft (hydrogel) lenses made from?
Original was HEMA
- 2-hydroxyethyl methacrylate
Forms hydrophilic gel network
- alone or mix with other monomers to hold very high percentage of water
- very stable/durable and good wettability
- comfort
Not oxygen-permeable enough for extended wear
Prone to deposit formation
What are soft (hydrogel) lenses made from?
Silicone hydrogels
- most commonly used materials
- high oxygen permeability even with low water % so good for extended wear
- durable
More expensive
Need hydrophilic co-polymer
- increase wettability
What are the properties of PVA hydrogel lenses?
Low cost
High wettability
Biocompatible
Low oxygen permeability
What are the properties of hyaluronic acid lenses?
Used to modify HEMA- and silicon-based hydrogels
- lubrication
Expensive
What are the advantages of soft (hydrogel) lenses?
Comfort
Easier to insert
Reduced spectacle blur vs RGP
Less likely to trap particles
Less likely to dislodge
- larger size
Disposable options available
