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23 Cards in this Set
- Front
- Back
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HEMA (Hydroxyethylmethacrylate)
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Backbone material in most SCLs.
Makes the lens more hydrophillic and attracts polar compounds and water due to negative charge. |
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PVP (Polyvinylprolidone)
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Lens wetting agent.
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Glycerylmethacrylate
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Increases lens softness and comfort.
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Methylmethacrylate
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Gives Optical and Dimensional stability.
Also present in hard lenses for stability. |
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Phosphoryline Choline
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Increases SCLs wetness.
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Silicone
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increases oxygen in the lens.
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SCL Characteristics: Group 1
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Low Water non-ionic.
Attracts the least amoutn of deposits, but wets less. Less Oxygen permeable, but made thinner to increase oxygen permeability. Stays cleaner longer. |
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SCL Characteristics: Group 2
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High Water non-ionic
Doesn't attract much debris. May be good for allowing oxygen. Thicker. |
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SCL Characteristics: Group 3
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Low Water Ionic
Attracts depris, less oxygen unless made thinner. Used for Disposable lenses. Not recommend if High debris in tears (Construction Worker or Depositors). |
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SCL Characteristics: Group 4
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High Water Ionic
Not good for patients with High Chol. or proteins in tears. Good for high myope/hyperope with thicker lenses. Good Planned replacement or disposable lens. DO NOT MAKE NON-DISPOSABLE LENS from this material. |
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Silicone Hydrogels
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Low Water content, High Oxygen transmission.
Better Dk value, but higher modulus (especially in 1st developed) so less comfortable, but better physiologically. Attract more lipids, but less protein. |
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Mertz Study
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Study on the transmissibility of SCLs.
Gave Dk/t concept. |
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Definition of Dk/t
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Permeability of material as determined by its components/thickness = TRANSMISSIVITY
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Optic Zone for SCL & HCL
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SCL: anterior surface.
Ant. optic zone cannot be made smaller than 8.0mm (glare occurs). HCL: posterior surface. |
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Problem with Optic Zone in High Minus Lenses?
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As the edges get thicker in increasingly high minus lenses, the optic zone get smaller. In large pupils, this induces flare. This is a problem at night.
Can't make OZ bigger because it would increase edge thickness. Can't help patient with this problem, advisable not to inform patient or they will notice it. |
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Two Methods to Tighten CL fit
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Increase the SAG!
1. Keep Base Curve (BC) the same and increase the diameter. This increases the Sag and tightens the lens. 2. Keep the Radius the same and steepen the base curve. The Sag increases and the lens tightens. |
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Two Methods to Loosen CL fit
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Decrease the SAG!
1. Decrease the diameter (14.5 to 14.0). Loosens/Decreases Sag. 2. Flatten the BC (8.2 to 8.6) (45D to 43D). |
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Quiz Question: How does BC and D related in terms of CLs?
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BC is measured in mm and D which are reversely related.
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TEST QUESTION: What is loosest and tightest of the following:
D: 13.8 BC: 8.3, 8.6, 8.9 D: 14.5 BC: 8.6, 8.9, 9.2 |
Loosest - D: 13.8 BC: 8.9
Tightest - D: 14.5 BC: 8.6 |
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SCL Residual Astigmatism
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CRA = TRA
Calculated Residual Astigmatism = Total Refractive Astigmatism |
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HCL Residual Astigmatism
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CRA = TRA - (-corneal astigmatism)
Calculated Residual Astigmatism = Total Refractive Astigmatism |
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RGPs Residual Astig. Calculation.
Given: k -1.75x180 RX: -2.50 -1.00 x180 |
CRA: -1.00x180 - (-1.75x180) = +0.75x180 = -0.75x090
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SCL Residual Astig Calculation
Given: k -1.25x180 (FM 43.00) RX -3.00 -1.00x180 |
Patient will likely need toric.
CRA=TRA. Look at the refraction cylinder to determine toric or not. Remember the 4:1 rule. |
