SCL Fitting 3 Flashcards
What properties affect the comfort of RGPs?
- Oxygen permeability
- Oxygen transmissibility
- Wettability
- Modulus
- Water content
Assuming that the lenses fit well, what are sorts of things are you expecting to find during a contact lens assessment if px complains of comfort?
- Incomplete blink
- Corneal staining inferior due to dryness – Conjunctival staining from tight fit
- Reduced TBUT (< 10sec)
- MGD (less lipids means more evaporation)
- Reduced Schirmer’s test (< 15mm after 5 min)
- Reduced central tear prism height (< 0.2mm) – Normal central 0.2 to 0.4mm – Normal periphery 0.1 to 0.2mm
- Reduced vision
What is Schirmer’s test?
Basically where you put a little bit of paper touching the eye and see how much of it gets wet in a set period of time - it is an indication of tear volume
Which stain, lissamine green or rose bengal, do px prefer?
Lissamine green as it is not associated with stinging or pain
What do rose bengal and lissamine green stain?
Dead and degenerate cells
What does circum-limbal (i.e. where the edges of a CL would be) staining indicate?
Lens is too tight or edge is too sharp
What is the TBUT like in a px with dry eye?
It is reduced
When are RGPs a great option for a px with dry eye?
If the dry eye is NOT a result of insuffiicient tear volume.
-RGPs are insufficient if dry eye is caused by reduced tear prism
What is tear prism?
How is it measured?
Height of tears pooled at the bottom of the eye.
Make a spot of 1mm/0.5mm and see how much of that spot the tear height gets to
Judging from the name of a material how can we infer that it is a hydrogel material?
Hydrogel materials end with the suffix ‘filcon’
[RGP materials end with the suffix ‘focon’]
What is the relationship between ionic hydrogels and protein deposition?
Ionic hydrogels tend to have a negatively charged surface which makes it sensitive to chnages in pH and osmolarity and therefire more likely to attract tear proteins.
What are non-ionic hydrogels?
Ionic hydrogels that have been treated to remove the ionic charge
What are FDA groupings?
Groups that are allocated according to water content and whether the material is ionic or not
What are the disadvantages of having a higher water content?
Dehydration - lens dries out
Mechanically weaker - lower modulus
More likely to attract Deposits
What are the disadvantages of an ionic SCL material?
More likely to attract deposits
What is the relationship between water content and Dk?
Increased H2O will increase Dk
[– More oxygen→healthier]
What are the advanatges and disadvantages of having a low modulus?
Advantages:
- Easy to fit
- Increased initial comfort
- Reduced mechanical induced ocular complications
Disadvantages:
- Difficult to handle
What is a disadvanatge of a high modulus?
Material is stiffer and so more uncomfortable.
Linked to a foreign body sensation
True or False- Lenses in FDA group 4 tend to attract more protein deposits than any other group
True
What does the Fifth FDA group consist of?
Silicone hydrogels
What are the advanatges of Silicone hydrogels and why were they created?
They were created for continuous wear. They have a high oxygen transmissability.
Advantages of Silicone hydrogels are that because of its increased oxygen transmissability hypoxia is less likely to occur and as a result the following symptoms are less likely to occur:
- – Hyperemia
- – Limbal injection
- – Vascularization
- – Corneal edema
- – Myopic creep (-0.25 to -0.50D more myopia)
Silicone hyrdogels arguably provide better comfort than hydrogels
Why do silicone hyrdogels arguably provide better comfort than hydrogel lenses?
Lower water content of silicone hydrogel materials cause slower dehydration
– SiH materials are treated to enhance wettability and comfort to make up for the hydrophobic nature of the silicone component
Why is oxygen flux a better parameter to judge by than Dk/t?
Dk/t tells us about the materials oxygen transmissibility but not necessarily how much oxygen reaches the cornea udring a set period of time ( like oxygen flux does).
True or False- There is a strong corrolation between oxygen transmissibilty ans corneal oedema
True - – Low-Dk hydrogel lenses cause approx 7% to 15% corneal swelling overnight (as a pose to the 3-4% of corneal swelling that occurs naturally overnight).
True or False- The amount of overnight swelling induced by SiH lenses is close to normal values
True - SiH materials induce significantly less overnight corneal swelling (ranging from 2% to 5%) • These values are similar to overnight (closed-eye) corneal swelling without contact lens wear
What is the relationship between silicone content and water content of silicone hydrogel lenses (first generation)?
Increased Si content results in decreased H2O content
How does water content affect modulus?
Decreased H2O content results in higher modulus
True or False- Second generation SiH lenses are formulated in such a way that they have a highh water content and low modulus
True
What are disadvanatges of Silicone hydrogels?
Mechanical irritation from a stiffer silicone hydrogel material – Increased incidence of Superior epithelial arcuate lesions (SEALs) and mucin balls
• Increased lipid based deposits – May slide around on the lens surface: affecting vision and comfort dynamically – Conventional SCL materials more protein based
How do flexibility and vision affect one another?
- Comfort can best be achieved from a flexible CL that drapes easily over the cornea and has minimal interaction with the eyelids during blinking
- But a high degree of flexibility can be a disadvantage when trying to achieve optimum vision – The optical performance of a lens will be significantly improved if it can mask corneal astigmatism – An increase in stiffness or rigidity will achieve this but at the expense of initial comfort
Describe the differences between RGPs and SCLs
What are mucin balls?
How long do they take to resolve?
Mucin balls are speherical and translucent bodies sandwiched between a contact lens and the cornea - they can be observed minutes after you insert a lens and can become embedded into the cornea.
They take 24 hours to resolve
Are mucin balls associated with decreased comfort?
No
Deposits on the lens can could worsened vision, if vision is still bad/foggy after having removed the lens what may we suspect?
The possibility that the px is experiencing corneal oedema
