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.
What does EOP refer to?
Level of oxygen at surface of cornea under the CL
What is the EOP in an eye not wearing a contact lens?
20.9% in an open eye and 8% in a closed eye (at sea level altitude).
What is the EOP required in people that wear daily contact lenses?
>10% (>24 Dk/t)
What is the EOP required in people that have extended wear of contact lenses (extended wear refers to people sleeping in them)?
>18% (87 Dk/t) (to prevent natural 4% corneal swelling that happens overnight)
What is the difference between extended wear contactv lenses and continous wear contact lenses?
With extended wear- after 6 nights and seven days you have to soak those lenses (even in monthly prescribed extended wear lenses).
[If you had weekly extended wear lenses you would wear them for a week day and night then get rid of them. If you had monthly extended wear lenses you would soak them in solution over night after 6 nights and seven days of wear before popping them back in. Over the course of a month in extended wear lenses you would have soaked them four times.]
Continuous wear lenses - put the lenses in at the start of the month and take the lenses out at the end of the month - this includes sleeping in them.
What is wetability?
The ability of a drop of liquid to adhere to a surface.
Why is wetability a property worth considering in CLs?
Essentially you don’t want tears sitting in droplets on the CL surface (i.e. CL acting hydrophobic) you want a nice spread of tears over the CL surface (i.e. contact lens acting hydrophilic) . That nice spread of tears across the CL would be good wetability.
[↓cohesive forces within liquid = ↑ attraction between fluid and surface]
• ↑ wettability = ↑ spread of liquid over a surface = improved vision and comfort.
What does contact angle refer to and how does it relate to wetability?
How tears sit on a contact lens. If tears sit as literally droplets then the contact angle is high.
If the tears spread nicely over the CL then the contact angle is deemed low.
↓ angle = ↑ wettability
What is modulus?
How well the material resists deformation by pulling or stretching
What does a high modulus mean?
A higher modulus is stiffer and has better resistance to deformation
What is the downside to a high modulus of the contact lens?
The patient may complain of a foreign body sensation, due to the stiffer lens material.
What are the principle determinants of comfort?
Material modulus, lens design and surface properties such as lubricity and wettability are the principal determinants of comfort
Can you deform an RGP lens?
Only very slightly it tends to stay as the same structure.
Can you deform a SCL?
Yes you can stretch and rip it.
When we talk about stability and hardness of contact lenses what factors are we taking into account?
Resistance of the material to penetration – Beware that some tests relate to rigidity and others to scratch resistance
- Breakage in production process (how likely this is to happen)
- Distortion while wearing (astigmatic corneas)
- Flexure resistance
What is flexure resistance?
It refers to number of flexing cycles before fracture of the CL
– Ideally we want CL to return to original form after deformation
– As Incomplete recovery (i.e. the CL not going back to it’s original shape) causes warpage/distortion in image.
[Image attached shows how it is tested]
Why are we concerned with on- eye flexure (i.e. the CL flexing on the eye)?
It means the patient will experience a residual cylinder and poor fluctuating vision (because with every blink the lens would be flexing).
What type of lenses are more succeptible to on-eye flexure?
On-eye flexure is associated with increased material flexibility due to high Dk lenses
In terms of an optometrist fitting CLs, how can they fit to avoid the likelyhood of on eye flexure?
Reduce flexure by fitting flatter
Reduce flexure by fitting very thin lenses
or fit toric lenses where required.
PMMA
A very old material - first material contact lenses (RGPs) were made out of.
High refractive index - 1.49
What is the benefit of a lens having a high refractive index?
A higher refractive index allows for a thinner lens.
What is the order of materials used for contact lens production?
First and foremost PMMA (1.49) was used.
Then silicone acrylates.
Then Flurosilicon acrylate.
Why do we care about specific gravity?
It could be used to control RGP centration on the cornea.
So naturally the healthier the lens the more it will be pulled downwards thus it may sit too low. To change this (it is normaly a last resort as first you try to do things like chnage fiting or diamtere of lens) you can chnage the specific gravity of the lens.
Alternating the specific gravity can change the mass of the CL by up to 20%
How can we deduce things from the name of a material e.g. Folcon III 2?
What are the advantages of PMMA?
-Great stability
– No deposits (doesn’t attract lipids or proteins from your own tears)
– Inert and a-toxic
– Remains transparent
– Low weight (so will sit nicely where it should be sitting)
What are the disadvanatges of PMMA?
– Zero Dk!!! Thus causing hypoxia, oedema and central corneal clouding.
Why is silicone added to a CL material?
To increase Dk
What are the advantages of silicon acrylate as pose to PMMA as a material for CLs?
Silicone acrylate has an increased Dk.
Improved modulus
More stabile as its flexible.
What are examples of silicone acrylate lenses?
Boston IV
Paraperm EW
What are the disadvantages of silicone acrylate for a contact lens material?
It is has reduced permeability (silicone is hydrophobic)
It is brittle
It crazes ( cracks appear).
The higher the silicone acrylate content the lower the wetability.
What is the effect of fluorine in flurosilicone acrylates?
It increases Dk.
What are examples of lenses made from fluorosilicone acrylate?
Boston ES
Fluoroperm 60
Boston XO
What is the advanatge of Fluorosilicone acrylate over silicone acrylates as a material for contact lenses?
Increased oxygen permeability
– Lenses are less hydrophobic
– Less protein deposits
What are the disadvanatges of Fluorosilicone acrylates when compared to silicone acrylates, as a material for contact lenses?
– They are less stable (mainly because its a softer material)
– Increased breakage ( again because its a softer material)
– Higher modular weight
– More prone to lipid deposits ( not ideal for patients with lots of lipids in their tears)
What is surface (plasma) treatment ?
So basically you can treat the surface of a contact lens to change its properties.
You can do the following:
O2 bombardment
- This Chemically alters the lens surface
- It Enhances post manufacture cleaning (so that soak the patient does)
- This Improves surface wetting (making the lens more comfortable)
How long does surface plasma treatment last for on a CL?
This is variable - for some patients it lasts six months, for others it only lasts a month.
Thats why when we prescribe surface treated RGPs we may have a planned replacement depending on the patient.
How do you choose which RGP to use?
First choice: pre-fixed by lens design
- Recommended by manufacturer – Website
- Own/supervisor’s favourite – Good performance/price
- Patient always worn it
- Advert/article in journal
What is the point of the water content in SCL?
O2 transported through H2O
• Increased H2O will increase Dk
[but definitely keep in mind that the thickness of the lens makes almost,if not just as much difference to the Dk/t}
Historically, poor oxygen transmission of initial low water content hydrogel materials cause problems in those who:
- Over-wore their lenses incl overnight
- Wore relatively thick lenses due to a high spherical prescription or be a toric design.
What were signs of this?
limbal hyperaemia
stromal striae
neovascularisation
epithelial microcysts
Development of SCLs
Initially what was the disadvantage of high water content SLCs and what was attempted to be done to counteract this?
During the day the water would just evaporate from the lens (especially in thin lenses) causing dehydration of the cornea below. This led to corneal staining.
They tried to counteract this by increasing thickness of the lens. This meant the Dk value was still high but the Dk/t value dropped to around 20 ( regulation states a Dk/t value of 24 is needed for daily wear - so basically this was a massive flop).
[Other disadvanatges included it being mechanically weaker and attracting more lens deposits due to water content}
What’s the water content like in disposable content lenses and what does this mean for the lens?
They have a mid to higher water content.
This means they have more deposits and a shorter lifespan.
Why is water content of SCLs important?
The water content of SCLs is important for oxygen to pass through the material to the cornea
What is good about silicone as a material for CLs?
It is incredibly good at transporting oxygen
What can we deduce from the name of a material used to make contact lenses e.g. Nelfilcon A II 2?
What is the effect of ionic hydrogels?
Ionic hydrogels tend to have a negatively charged surface which makes them sensitive to changes in pH and osmolarity and more likely to attract tear proteins (which are positively charged) - not necessarily a problem but in some patients works well and in others it doesn’t.
What are non-ionic hydrogels?
Ionic hydrogels that have been treated to remove the negative charge.
How can we group SCLs?
By FDA group.
What do you roughly need to know about silicone hydrogels for the assessment?
- Introduced in 1999 (Europe) and 2001 (USA)
- Development focused on meeting (or surpassing) the oxygen transmissibility requirements for overnight wear (Extended Wear and Continuous Wear)
- Si component transports O2 (not water)
- Superior health benefits of these materials for continuous wear – And similar advantages with daily wear of SiH
How do SiH eliminate occurance of hypoxia?
–The cornea depends on oxygen to maintain its thickness and transparency and avoid hypoxia.
– CLs reduce the amount of oxygen reaching the cornea, creating a hypoxic environment
• There is a strong negative correlation between corneal swelling and oxygen transmissibility
– Low-Dk hydrogel lenses cause 7-15% corneal swelling overnight
– Significantly less overnight corneal swelling with SiH materials (2-5%)
Silicone Hydrogels (SiH) lenses provide more O2 to cornea compared to conventional hydrogel materials.
What are the advantages of SiH lenses?
Less hypoxia causes reduced:
– Hyperemia
– Limbal injection
– Vascularization
– Corneal edema
– Myopic creep (0.25 to 0.50D more myopia)
• Better comfort??
– Lower water content of silicone hydrogel materials cause slower dehydration (great for certain types of dry eye patients)
– SiH materials are treated to enhance wettability and comfort to make up for the hydrophobic nature of the silicone component
What are disadvnatges of SiH lenses?
Maximum oxygen permeability, but not always maximum comfort
– (Comfort is a very subjective term)
• Increased modulus
– (Rememeber: High modulus =stiff material= discomfort)
– Low modulus= poor handling= reduced durability
• Possible foreign body sensation – due to a stiffer material than conventional hydrogels.
[Not a disadvantage but worth bearing in mind - you cant sleep in ALL SiH lenses]
If a patient has lots of LIPID deposits - do you fit them with a hydrogel or silicone contact lens?
A conventional Hydrogel lens
If a patient has lots of PROTEIN deposits - do you fit them with a hydrogel or silicone contact lens?
Silicone hydrogel lens
