Properties of Contact Lens materials Flashcards
quality or state of the material that allows the oxygen to move through it
dk - Oxygen Permeability
High oxygen content = ______
Higher the dk
dk values:
Low dk = less than 30
Medium dk = 30- 60
High dk = higher than 60
states how much oxygen goes through the lens
Dk/t - Oxygen transmissibility
the Thinner the lens = the _____ amount of oxygen can pass through the lens
Higher
Dk/t for daily wear:
25
Dk/t for overnight wear:
87 and above
- this is a measurement of the cornea’s oxygen thirst following lens wear
- allows practitioners to compare different performances of lenses
Equivalent Oxygen Performance - EOP
describes oxygen flux through a contact lens as if the eye were responding to various amounts of atmospheric oxygen
generally conducted using lens materials of various thickness
Equivalent Oxygen Performance - EOP
- most valid indicator of real-life clinical performance
- it is the measure of the percent increase in the corneal thickness with great precision
Corneal Swelling
Four measurement used to evaluate tensile properties:
- Tensile Strength
- Modulus elasticity
- Coefficient of elongation
- Tear Strength
- measure how much force can be applied to the material before it breaks
- the greater the force, the better the strength and the more durable the material
Tensile strength
refers to the flexibility of the material
materials that has low ______ are more flexible
Modulus elasticity
- refers to how far a lens material can be stretched before it will break
- value is stated by percentage based on the size of the lens sample tested
Coefficient of Elongation
measurement refers to how much force has to be applied to a lens material before it will tear
Tear strength
the higher the _____ lenses generally provide more oxygen to the cornea
Water Content
High water content materials become _____ while on the eye
Dehydrated
High water content tend to ______ than low water content lenses
lose more water
a physical property of which defines how light rays are affected as they pass through the material
Refractive index
as water content increases the refractive index _______
decreases
Materials with low index of refraction will require ______ lens design to achieve the desired optical effect on power
Thicker lens design
term which contact lens interact with the corneal tissue, therefore it is important to evaluate the physiological balance of the material with the ocular environment
Biocompatibility
tests to evaluate biocompatibility:
- 21 day Rabbit test
- Resistance to microbial growth
compatibility of the lens materials with lens care systems
Biocompatibility
it is assessed in the laboratory periodically by measuring:
- water content
- sagittal value
- power thickness
- base curve
to make sure that the lens parameters are not changing
Dimensional Stability
this ensures that the material can maintain consistent lens performance
Dimensional Stability
since contact lenses are placed in the tear film, which is primarily made up of water,
this property must be always assessed prior to placement on the eye
Hydrolytic Stability
term in which only materials that are stable in water can be used for contact lenses
Hydrolytic stability
the more rigid the lens material is, the ______ they are to handle
easier
is the characteristic of lens performance that is important to RGP materials.
Wettability
With RGP lenses, wettability is tested in the laboratory by measuring the ______
wetting angle
The higher the wetting angle, the _____ wettable the surface is
less wettable
The wetting angle of a hydrophilic material is considered _____
ZERO
