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What percent of all cl wearers wear GP's |
10% |
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what type of lenses offer better visual acuity - soft cl or GP? |
GPs |
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the incidence of keratitis is much lower with use of soft cls or GPs? |
GPs |
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what curve of the gp is known as the fitting curve |
base curve |
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what curve of the gp contains the refractive error correction? |
base curve |
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the size of the optic zone is dependent on what two variables? |
base curve & pupil size (needs to cover pupil) |
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a steeper base curve needs a smaller or larger optic zone? |
smaller |
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what is the name of the curve immediately outside the base curve? |
secondary curve |
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are intermediate curves found in all gps? |
no, only those with more than 3 curves |
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the peripheral curve is sometimes referred to as the _____ curve |
tertiary |
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what curve of the gp is responsible for the alignment of lens to cornea? |
base curve |
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what curve of the gp is responsible for the transition from the central cornea to the flatter corneal periphery? |
secondary curve |
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what curve of the gp is responsible for controlling the edge lift? |
peripheral curve |
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what curve of the gp is responsible for allowing tear exchange? |
peripheral curve |
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if you order a "standard" tricurve lends, what radius of each curve will be sent? |
secondary curve = BC + 1.5mm
peripheral curve = BC + 3.0mm |
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what is the standard PC radius that is made when a "flat tricurve" is ordered? |
3.5 mm flatter than the BC (BC + 3.5mm) |
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when describing edge lift, which term is more commonly used - axial edge or radial edge? |
axial edge lift - which is the Z value |
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Z, axial edge lift, is a measurement of what? |
vertical distance from the lens edge to the extension of the base curve |
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E, radial edge lift, is a measurement of what? |
the extension of the lens egde perpendicular to the extension of the base curve |
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what type of fitting method are you using when you fit according to the Rx and K readings? |
empirical fitting |
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what type of cl is best for:
-7.50 -1.00x180 |
soft sph
soft toric
soft aspheric
GP sph |
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what type of cl is best for:
+6.50 -4.00x090 |
GP bitoric (bc cyl >-2D) |
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what type of cl is best for:
+1.00 -4.50x180 |
GP bitoric |
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what type of cl is best for:
-1.50 -0.25x090 |
soft sph
GP sph |
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under what conditions are soft torics indicated? |
> -.75D
-0.75D diff btw corneal & refractive astig
-Rule of 4s - if cyl is > 25% of the sph, use soft torics
-significant amount of CRA |
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under what conditions are GP torics indicated? |
-when cyl >> sph power
-low sph (near plano)
-corneal toricity > refractive astig |
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if base curve is steepened, what happens to the sag depth? |
increases |
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if the base curve is flattened, what happens to the sag depth? |
decreases |
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what happens to the sag depth when OAD is decreased? |
decreases |
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for every 0.5mm decrease in OAD, what must we do to the base curve in order to maintain the same fitting relationship? |
steepen it by 0.25D |
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if OAD is increased, what happens to the sag depth? |
increases |
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for every 0.5mm increase in OAD, what must we do to the base curve in order to maintain the same fitting relationship? |
decrease (flatten) it by 0.25D |
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if OZD is increased, what happens to the sag depth? |
increases |
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what affect does a change in the peripheral curve have on the sag depth? |
none |
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if OZD is decreased, will the lens fit flatter or steeper? |
flatter - so bc must be steepened in order to maintain the same fitting relationship |
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what are the four steps in designing a GP lens? |
1.choose appropriate diameter
2.select proper base curve
3. evaluate lens fit
4. select proper power |
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what four things are considered when choosing an appropriate lens diameter? |
1. lid position & palpebral aperature size
2. corneal curvature (steeper corneas need a small diameter)
3. power of lens (thick lenses need small diameter to keep it light)
4. type of lens |
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what size diameter is considered large? |
9.4mm and greater |
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what size diameter is considered small? |
9.0mm and less |
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what type of fitting style is used when the lids cover the superior limbus? |
superior-central or lid-attached fitting styles |
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What diameter is best when fitting a pt whose lids cover the superior limbus? |
large diameter (greater than 9.4) |
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what affect does the pupil size have on the optic zone diameter of a lid attached lens? |
none |
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what affect does the pupil size have on the optic zone diameter of an interpalpebral fitting lens? |
it determines the optic zone size (pupil <5mm = 7.5 bc, pupil >7 = 8 bc) |
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what size optic zones are used with lid attached fitting lenses? |
7.8 - 8.5 (larger than avg) |
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how is the bc determined in a lid attached fitting lens? |
flat K or slightly flatter |
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how is the bc determined in an interpalpebral fitting lens? |
steeper than flat K, based on corneal cyl |
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how much does the OAD have to be changed in order to have a significant effect? |
0.3mm |
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steeper base curves - tend to decenter... |
inferiorly |
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flatter base curves tend to decenter |
superiorly |
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extremely flat base curves tend to decenter... |
inferiorly (fall) |
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what two methods can be used to evaluate lens fit? |
slit lamp
burton lamp |
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what are the big disadvantages of the burton lamp? |
(hand held UV lamp held in front of pt)
- magnification can't be varied
- cornea can't be inspected |
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the ideal gp lens mvmt |
1.0 - 2.0mm |
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what is the importance of good gp lens mvmt? |
-allow for good tear exchange
-flush out metabolic waste
-maintain normal corneal metabolism |
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how thick does the tear film have to be in order to fluoresce? |
20 microns |
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ideal edge width |
.4mm |
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how do we correct an insufficient edge lift? |
flatten PC |
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if egde lift is too great, the lens may decenter.... |
superiorly |
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how do we correct an excessive edge lift? |
steepen bc
steepen pc
decrease z value |
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troubleshooting an inferiorly displaced lid attached lens...
OAD, BC, PC, CT |
inc OAD
flatten BC & PC
dec CT
consider lenticular |
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troubleshooting an inferiorly displaced interpalpebral lens...OAD, BC, PC, CT |
dec OAD
steepen BC & PC
dec CT
consider lenticular |
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troubleshoot a superiorly decentered lens |
usually only a prob if pupils are large
- steepen BC
- increase CT
consider thin, sspheric |
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troubleshoot a lateraly decentered lens |
increase OAD
steeper BC
consider aspheric back surface
(more common on ATR corneas) |
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troubleshoot a lens with flexure.. |
inc CT
use lower DK material
fit lens flatter |
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initial poor vision in a gp lens could be due to... |
lens not wetting
flexure |
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poor vision that has acquired after a time of wearing GPs could be due to... |
lens warpage
scratched lens
lens deposits
power change due to aggressive cleaning |
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how much must we change the OAD/OZD to be clinically significant? |
0.3mm |
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how much must we change the BC to be clinically significant? |
0.50 D (.1mm) |
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how much must we change the CT to be clinically significant? |
0.03mm |
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how much must we change the PC radius to be clinically significant? What about the PC width? |
radius - 1mm
width - .2mm |
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the desired edge or center thickness of CL is that of a ____D lens |
-3.00D |
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powers greater than _____D should have a plus lenticular design |
-5.00 |
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what powered lenses should have a minus lenticular design? |
-1 and less
all plus lenses |
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T/F giving a pt colored lenses to differentiate btw OD and OS will change the pts eye color |
FALSE |
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what colors can be used for OD lenses to differentiate them from OS? |
Green
Gray
(Red in Ortho K only) |
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what colors can be used for OS lenses to differentiate them from OD? |
Blue
(yellow in Ortho K only) |
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what colors are used for the OD and OS ortho K lenses? |
red OD
yellow OS |
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