Flashcards in Midterm I Deck (143)
In order to steepen a GP lens, what must be done (with numbers)?
1. increase BCR by 0.50D
2. increase OZD by 0.3mm
What type of lenticular should be used for lower plus lenses or tighter lids?
What is the upper lid position in normal lid position? 1. Lower lid? 2. What is the required GP lens fit? 3
1. 1/3 to 1/6 over cornea (10:00 and 2:00)
2. at limbus
2. lid attachment fit
What is the distance from the extension of the case curve edge to the surface of the lens?
radial edge lift (REL) (less than AEL)
If a GP lens has too much fluorescein at the edges (excessive edge width), what are the options for changing the lenses (with numbers)?
1. steepen SCR/PCR by at least 1.0mm
2. decrease SCW/PCW by at least 0.2mm
What are the key relationship changes to BCR and OZD to maintain the same fitting relationship (with numbers)?
1. flatten BCR (dec) 0.25D for every 0.4 to 0.5mm increase in OZD
2. steepen BCR (inc) 0.25D for every 0.4 to 0.5mm decrease in OZD
What is the measurement that represents the true curvature data?
What are sagittal depth calculations used for clinically?
1. maintaining or changing the fitting relationship
2. comparing two soft contact lenses
3. calculating center thickness of a CL
What are the results of using plasma of GP lenses?
1. cleaning contamination from substrates
2. improved lubricity
3. decrease in wetting angle measurements (good thing), thereby improveing on-eye wettability
4. increased bond strength
How would you decrease the edge lift of a lens?
steepen the peripheral curve radii and/or decrease the peripheral width
When were rigid gas permeable lenses first used?
In order to flatten a GP lens, what must be done (with numbers)?
1. decrease BCR by 0.50D
2. decrease OZD by 0.3mm
What is the typical edge thickness for unpolished lenses?
0.10 to 0.12mm
What are the best strategies to examine the fluorescein pattern of a GP lens?
1. use 10x or less mag
2. cobalt blue filter
3. wratten yellow filter
4. wide beam on slit lamp
Do myopes accommodate and converge more or less in spectacles?
What are the advantages to a lid attachment fit?
1. less movement
2. more comfortable
What type of prism is not able to go into contact lenses?
What is the typical center thickness for plano lenses?
0.20 to 0.21mm
What are the benefits of PMMA's?
1. readily machined
2. low cost
3. fairly wettable
4. easy to care for
What are the lens types that have a Dk of around 100?
1. Optimum Extra
2. Boston XO
3. Fluoroperm 151
4. HDS 100
What is the typical center thickness for +3.00 and up lenses?
0.30 to 0.60mm
What is selected to provide the appropriate power of a GP lens?
front/power curve (anterior optic zone)
What is the average range of horizontal visible iris diameter (HVID)?
10 to 13mm
What is the range of lenses that use a plano shape?
-2.00 to -3.00
What is the lenticular curve of a GP lens viewed with?
What do the peripheral curve designs provide?
1. fit cornea better
2. encourages tear exchange behind lens
3. aids in lens removal
4. assists in centration
What should the SCW be for a tetra-curve design? 1. ICW? 2. PCW? 3
What are the different types of monomers used in GPs?
1. methacrylic acid (MA)
2. methyl methacrylate (MMA)
3. hydroxyethyl methacrylate (HEMA)
4. glycerol methacrylate (GMA)
5. vinyl pyrrolidone (VP)
6. vinyl alcohol (VA)
7. phosphorylcholine (PC)
What is the distance from the edge of lens to extension of base curve radius (parallel to optic axis)?
axial edge lift (AEL)