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31 Cards in this Set
- Front
- Back
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Steeper K's require ___________ CL BC
Flatter K's require ____________ CL BC |
Steeper
Flatter |
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Steep BC with have a sm/lg mm amount and a sm/lg D in comparison to a flat BC |
smaller mm, bigger D |
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Flat BC with have a sm/lg mm amount and a sm/lg D in comparison to a steep BC |
larger mm, smaller D |
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Calculation for BC |
BC = 337.5 / K |
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Soft CL BC usually range from ____ to ____ |
8.2 to 9.0 mm
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What are you looking for when evaluating the fit of a CL |
Centration [overlap]` Movement Corneal coverage Lag [not sagittal depth] |
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What is conjunctival drag |
when the conjunctiva moves when the lens moves, so no real independent lens movement
slightly too steep or tight |
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Hydrogels |
*HEMA (2-hydroxyethyl methacrylate) based Hydrogels *Principal disadvantage - it relies upon water to transport oxygen across material *Max Dk of around 80 *Easily fabricated into CL, cheap, highly flexible, stable to pH and temp changes; increase in stability with cross-linking *incorporates hydroxyl group to make hydrophilic *Oxygen transmission through hydroxyl / water molecules |
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Refractive Index comparison with H2O in Hydrogel Materials |
decrease in refractive index with an increase in H2O content
Water content changes with temp, dehydration, pH [on the eye the lens dehydrates and water content goes down] |
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Silicone Hydrogels |
Silicone provides extremely high oxygen permeability, while Hydrogel facilitates flexibility, wettability and fluid transport [aids in lens movement]
Exceptional oxygen transmission and durability; Si material is hydrophobic
Bi-phasic or 2 channel molecular structure: one channel for oxygen transmission [siloxy phase] and a second channel for hydrogel [water and ion transmission] |
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Group I Hydrogel |
low water (<50%) non-ionic |
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Group II Hydrogel |
high water (>50%) non-ionic |
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Group III Hydrogel |
low water (<50%) ionic |
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Group IV Hydrogel |
high water (>50%) ionic |
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Higher water content |
More transmissible to oxygen Dehydrates more on the eye Deposits more |
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Lower water content |
Less transmissible to oxygen Dehydrates less on the eye, so may be better for dry eye complaints Deposits less |
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Ionic |
Lens surface has charge Attracts large protein molecules like lysosyme Deposits more |
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Non-ionic |
Lens surface not charged May deposit less |
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Dk |
oxygen permeability (material specific) |
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Dk/L or Dk/t |
transmissibility = net volume of oxygen passing through a specific lens (lens specific)
High plus lens - much lower in center High minus lens - much lower at edges |
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HEMA-based hydrogels Oxygen transmissibility related to water content |
Dk increases logarithmically with increased water content |
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HEMA-based hydrogels High water content lens transmits more or less oxygen |
More
But often must be made thicker because material is more fragile
Lens also dehydrates on eye, so may transmit less than when fully hydrated |
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HEMA-based hydrogels Low water contact lens transmits more or less oxygen |
Less
But can be made very thin and dehydrates less on eye |
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Silicone Hydrogels More rigid material - higher or lower elastic modulus |
higher
Young's modulus of elasticity (E) E = Force / change in length Increase in modulus - material more stiff, harder to bend Decrease in modulus - material more pliable
Stiffness of a lens depends on lens design [lens thickness] |
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Silicone Hydrogel High/Low water content High/Low Dk |
Low water content High Dk |
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Methods of Lens Stabilization |
Toric back surface Prism ballasting Thin zones |
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Prism ballasting |
"watermelon seed" principle - the lens is thicker at the base and the upper lid squeezes it down
lens can be further stabilized by eccentric lenticulation or periballasting
PureVision Toric, Air Optix Astig, ProClear Toric |
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Thin Zones |
Double slab off
lens is thinned at apex and base
can be unstable in low minus lenses
comfortable due to thinner lens, but more likely to rotate
AdvanceAstig, OASYS for Astig, Focus Daily Toric |
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Typical Torc Perameters availible |
Cyl powers are usually -0.75, -1.25, -1.75 (some also -2.25 and -2.75)
For higher cal, usually need to order "custom" extended perameters (increase $$$)
Usually trial lenses are "free" for doctor, but require an order eventually |
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LARS |
Adjustment for Lens Rotation Left Add Right Subtract Corrects for what optics you need in the lens; do nothing for changing the fit of the lens |
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What if vision is not good? What do you check? |
Check SpecRx Lens not being stable enough Lens is defective [Rx, axis, prism, optics, BC] Due to dry eye or poor tear film Pt may be too "picky" to wear soft torics - try RGP |
