Week 4 - Properties of contact lens materials Flashcards
What is the challenge to designing contact lenses?
• Ultimate desire to feel as if not wearing lens at all
• Challenging goal is to meet the needs and expectations of contact lens wearers, while avoiding everyday discomfort issues that contribute to lens discontinuation
What is the motivation for wanting to wear contact lenses, by patients?
• Functional benefits
- More natural vision
- Unimpeded vision for sport
- Glasses not slipping down
• Emotional benefits
- Increased confidence (school children etc.)
- Enhancing natural appearance / vouthful look
What are the four key objectives for an ideal contact lens?
• Comfort
• Vision
• Health
• Convenience
Material should provide all these features, all the time, to all contact lens wearers
So why are CL properties important?
• Some contact lens properties may conflict with objectives/ocular needs
- E.g. SiHi contact lens material provides high oxygen transmissibility for corneal health, but its hydrophobicity may interfere with wetability; factor for vision/ comfort
what are the 13 contact lens factors which are important?
- 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
- Density: what is it?
• Mass/volume
• A lens of high density may be disadvantageous because it could be heavier, and not centre properly
• Conversely in the case prism ballast lenses, may be beneficial
- Refractive index: what is it?
• The required refracting power of a lens is directly related to its RI and determines the thickness and curvature of the lens
• This is important in contact lenses as permeability is proportional to the thickness of the material
• Therefore, a
- Thicker lens = Lower Dk
- Increased RI = Thinner lens = Increased Dk
- Optical transmission:
• This is the ratio of light which passes through a material as a %
• Generally, Optical transmission should be 90%+
• Most contact lenses exceed this therefore this is of limited concerned clinically
- Dimensional stability:
• High water content lenses are susceptible to dimensional changes during wear,due to fluctuations in content
- Therefore the ability to rehydrate is important to dimensional stability
• If a lens absorbs water, each blink will return lens to water saturated state after each blink
- therefore rapid rehydration is advantageous in maintaining dimensional stability
- Surface properties:
Wettability
• Adhesive = Cohesive forces acting on a lens surface.
• Contact lenses that can sustain complete wetting allow a thick coverage of the tear film, a smooth recovery of the tear layer after eye closure and good visual acuity.
- Water content:
• Water content can be affected by temperature, PH and the salt condition of the cell namely:
- Hypertonic condition
- Isotonic condition
- Hypotonic condition
• It can have important clinical implications;
- The higher the water content, the more comfortable the lens, but also lower the stability of the material
- Mechanical properties:
• Tensile strength:
- Important consideration with respect to resistance to damage during lens handling and long term durability
• Tear strength:
- Resistance of material of tear propagation from a notch or defect
- Mechanical properties: Modulus of rigidity?
• Stiffness of material
• For SiHi lenses as the addition of silicone results in lower amount of water
• This means Sili lenses have higher modulus than other lenses
• Clinically this is relevant;
- Too low a modulus would be difficult to handle and give insufficient movement on blink (poor tear exchange)
- Too high a modulus can lead to mechanically induced pathology such as SEALS and CLIPC
What is SEAL and CLIPC?
• SEAL: Superior epithelial arcuate lesion
• CLIPC: CL induced papillary conjunctivitis
- Oxygen Permeability:
• Increased water content, also increases oxygen permeability
