Properties of CL Materials (ideal vs realistic) Flashcards
1
Q
What is the ultimate desire of lens wearer?
A
To feel as they are wearing no lenses at all (but maintaining perfect vision)
2
Q
What are examples of motivation for a px to get CLs?
A
- Functional benefits:
o CLs can give more natural vision
o Unimpeded vision for sports
o Glasses not slipping down or fogging up - Emotional benefits:
o ↑confidence (school children etc)
o Enhancing natural appearance/youthful look
3
Q
What are the 4 key objectives of an ideal CL?
A
- 4 key objectives:
o Comfort
o Vision
o Health – maintain health of eye and don’t damage the eye
o Convenience - Material should provide ALL these features, ALL the time, to ALL CL wearers
- Some CL properties may conflict with other objectives &/or ocular needs
o e.g. SiHi CL material provides high O2 transmissibility for corneal health, but its hydrophobicity may interfere with lens wettability – a key factor in vision & comfort - Challenge is to optimise desired features while minimising the issues they bring
4
Q
What are the 13 important CL properties?
A
- Density
- Refractive index
- Optical transmission
- Dimensional stability
- Surface properties
- Water content
- Mechanical properties
- Oxygen permeability
- Ocular compatibility
- Toxicity and chemical stability
- Quality control of material
- Method and ease of sterilisation
- Ability to be tinted
5
Q
Describe density in CLs?
A
- Defined as mass/ volume
- A lens of high density may be disadvantageous because it would be heavier and may not centre properly
- Conversely in the cases of prism ballast lenses this may be beneficial
6
Q
Describe refractive index (RI) in CLs?
A
- Required refracting power of a lens is directly related to its RI & determines thickness & curvature of the lens
- Important in CLs as permeability is proportional to thickness of material
- Therefore:
o Thicker lens = ↓Dk
o ↑RI = thinner lens = ↑Dk
7
Q
Describe optical transmission in CLs?
A
- Ratio of light which passes through a material as a %
- Generally optical transmission should be 90%+
- Most CLs exceed this therefore this is of limited concern clinically, as it is unlikely to be a factor when choosing a suitable CL
8
Q
Describe dimensional stability of CLs?
A
- High water content lenses are susceptible to dimensional changes during wear (they dehydrate more than low water content lenses)
- Ability to rehydrate is important to dimensional stability
- If a lens absorbs water rapidly, it will return to a water saturated state after each blink (by absorption of tear fluid)
- Therefore, rapid rehydration is advantageous in maintaining dimensional stability
9
Q
Describe surface properties (wettability) of CLs?
A
- Wettability
o Adhesive (how much they want to stick to the material) = cohesive (how much water molecules want to stick to other water molecules) forces acting on a lens surface
Want these forces to be balanced
o CLs that can sustain complete wetting allow a thick coverage of tear film, a smooth recovery of tear layer after eye closure and good VA
o If adhesive and cohesive forces are balanced, will get nice spreading with smaller wetting angle <90° (hydrophilic)
o If much stronger cohesive forces vs adhesive forces, then water will form beads and the wetting angle will be >90° (hydrophobic)
10
Q
Describe water content of CLs?
A
- Water content can be affected by factors in the environment:
o Temperature
o pH
o Tonicity of the liquid its in – water either being drawn into the lens or drawn out of the lens - Has important clinical implications ↑water content, the more comfortable the CL but there is a ↓in stability of material
11
Q
Describe mechanical properties of CLs?
A
- Tensile strength:
o Important consideration w/ respect to resistance to damage during lens handling & long-term durability - Tear strength:
o Resistance of material of tear propagation from a notch or defect - Modulus of rigidity (‘stiffness’ of material):
o Relevant for SiHi lenses as addition of silicone results in lower amount of water
o Generally, means SiHi lenses have higher modulus than other lenses
Too low a modulus -> difficult to handle & give insufficient movement on blink (poor tear exchange)
Too high a modulus -> can lead to mechanically induced pathology such as SEALS and CLIPC
12
Q
Describe oxygen permeability of CLs?
A
- ↑water content leads to ↑oxygen permeability (greater supply of oxygen to the cornea)
- Want oxygen permeability to be as high as possible
13
Q
Describe ocular compatibility of CLs?
A
- CL are biocompatible w/ the ocular surface – are non-toxic & well tolerated (body doesn’t react) by ocular tissue
- Most obvious clinical manifestation ocular incompatibility is spoliation
- Clinical manifestations include protein, make up, microbial spoliation, calcium
o Stick to ocular surface and change CL’s properties – can shorten break up time, can cause inflammatory responses (protein deposits)
o Less of a problem now as CLs are used for a relatively short period of time before replacement (e.g. dailies) -> if people wear lenses for long time, these things can build up
14
Q
Describe toxicity & chemical stability of CLs?
A
- Lens can cause toxicity as it traps immobile tear pool behind lens and metabolic products can build up their and damage the anterior corneal layers
- Toxicity induced by an immobile CL causes rapid accumulation of metabolic by products in anterior corneal layers
- May result in limbal hyperaemia, peripheral corneal infiltrates & keratic precipitates
- Further surface damage may be result of solution toxicity causing various patterns of punctate epithelial keratopathy
15
Q
Describe quality control of material?
A
- Important for CL since they are medical devices and must custom fit
- Should be inspected during manufacturing process AND pre fitting (i.e. examine under magnification for anomalies before fitting completion)
