Term
| What are two categories of contact lenses? |
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Definition
1. hydrogels-absorb water 2. silicone elastomers-don't absorb water |
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Term
| Which type of lens would be used therapeutically for keratoconus? for a bandage? |
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Definition
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Term
| Why are soft contact lenses less likely to trap a foreign body? |
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Definition
Soft contact lenses are larger and cover more of the eye. |
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Term
| Name four disadvantages of soft contact lenses to RGPs. |
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Definition
1. lower VA for astigmatism patients since it must be ground into the lens 2. greater risk of contamination and harder to clean than RGPs 3. more expensive in the long run 4. dryness at the end of the day is a big cause of contact lens dropout |
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Term
| What are six contraindications for soft lense use? |
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Definition
1) high cylinder than can be corrected 2) unacceptable quality of vision 3) inability to obtain lens centration 4) active anterior segment disease (e.g. diabetes) 5) patient unable or unwilling to care for lenses 6) unfavorable environment (e.g. chemical exposure) |
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Term
| How did manufacturers make soft lenses such as MMA into the more hydrophilic PMMA? |
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Definition
| remove the H from the hydroxy group and replace it with an alcohol |
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Term
| Why does the fairly low Dk of polyhema (9.5) not reduce oxygen permeability so much? |
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Definition
| PolyHema can be made very thin. |
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Term
| How do you make a hydrogel lens more permeable to oxygen? |
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Definition
increase the water content |
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Term
| Name one soft lens that contains no HEMA at all? |
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Definition
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Term
| At what percentage of water do you actually get a lower Dk for silicone elastomer hydrogels than for hydrogels? |
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Definition
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Term
| What is one of the disadvantages of Silicone elastomer soft lenses not being very wettable? How do the silicone elastomer companies remedy this? |
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Definition
| Being less wettable makes silicone elastomer lenses less comfortable. CIBA and B&L uses a plasma treatment to smooth over the surface. Vistakon mixes hydrophilic chemicals into the lens matrix. |
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Term
| Why is Dk/t more clinically relevant than Dk? |
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Definition
1. Dk/t tells the transmissibility of a specific lens 2. takes into account the lens power and design Dk on the other hand is just the permeability of a lens material, which increases with increasing water for hydrogels, but decreases with increasing water for silicone elastomers. |
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Term
| What is the units of t in a Dk/t calculation? What are most Dk/t values we work with between? |
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Definition
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Term
| What is the stiffness of a hydrogel material referred to as? |
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Definition
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Term
| What FDA group are most medium content water lenses? |
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Definition
| Group IV, hence they're ionic and may attract deposits faster |
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Term
| What classification of lenses according to our classification scheme has the lowest modulus? |
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Definition
| High water content, least stiffness, most flexible, lowest modulus |
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Term
| What is one of the problems with spincasting of hydrogels? |
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Definition
| you may not get the same back surface power from one lens to another |
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Term
| A faster spin speed in spincasting soft lenses will produce a ____ power of lens. |
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Definition
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