Properties of contact lenses Flashcards
What are properties both RGP lenses and SCL have in common?
Oxygen permeability
Oxygen transmissibility
Wettability
Modulus
UV-absorption
[Note UV-absorption doesn’t change the way the material behaves just can be a feature of both lenses]
What are specific properties of RGPs?
Stability/hardness
Flexure
Refractive index
Specific gravity
What are specific properties of SCL?
Water content
What is oxygen permeability (Dk)?
Amount of oxygen passing through a contact lens material over a set amount of time and pressure difference
What is DK and what does each letter mean?
DK is a term used to refer to oxygen permeability.
D = diffusion coefficient: dissolved molecules move within the material
k = solubility coefficient: number of oxygen molecules dissolved in the material
What improves the rate of DK?
↑ temp = ↑ Dk
What are the two units DK may be recorded in?
Traditionally recorded in FATT units, however, more modernly recorded in ISO units.
[ISO units are 75% of the FATT units]
In the assessment, you will be asked for a contact lens name. You don’t need to know each and every name of each type of contact lens but one or two would be good.
Name some examples of RGP lenses
Menicon Z
Boston EO
Fluroperm
Boston II
Boston IV
The higher the DK, the better or worse the lens?
The higher the DK, the better the lens.
What are group codes in relation to DK of RGP lenses?
So depending on DK of RGP lens they can be sorted into groups 1-6. 1 being the group with the smallest DK value (1-15) and 6 being the group containing the highest DK value (175-200).
What does DK/t refer to?
Oxygen TRANSMISSIBILITY
What is oxygen transmissibility?
O2 transmissibility (Dk/t): amount of oxygen passing through a contact lens of specified thickness over a set amount of time and pressure differenc.
D = diffusion coefficient
k = solubility coefficient
t = thickness of lens (or sample of average lens thickness -3.00D is used in measurements)
What happens to DK as thickness of a contact lens increases?
↑ thickness = ↓ Dk
In which cases is it (more) important to consider DK/t (oxygen transmisissibility)?
In toric lenses ( remember one meridian shall be thicker than the other)
and
High positive prescriptions (as these lenses are thicker in the centre).
What can low DK/t (oxygen transmissibility) result in?
Corneal chnages such as:
– Microcysts
– Polymegethism
– Oedema
– Blebs
What is polymegathism?
Polymegethism is a greater than normal variation in size of the corneal endothelial cells.
Which lenses tend to be thicker RGP or SCL?
RGP contact lenses (thus they tend to have a lower DK/t value).
What did the Harvitt and Bonanno study (1999)conclude about appropriate DK/t values?
A DK/t value of 35 is needed to prevent swelling.
What is the effect of corneal swelling (minimal but overtime) upon vision?
Causes the patient to become more myopic
In 1984 what did Holden and Mertz conclude was the DK/t needed of lenses for daily wear?
24
What is oxygen flux?
volume of oxygen passing through a specified area of a contact lens over a set amount of time
What are conditions that affect oxygen flux?
Altitude - in different areas of the world there is less oxygen availability (e.g mexico) - partial pressure of oxygen is lower (mmhg).
Whether the eye is closed or open.
If you don’t wear a contact lens what percentage of oxygen flux do you have?
100%
Is oxygen flux easy or difficult to measure?
Very difficult to measure thus we tend to talk about EOP (Equivalent oxygen percentage) instead.



