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57 Cards in this Set
- Front
- Back
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Gas perm lenses only have |
Spherical correction fitting |
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Fitting method |
Apical ( steeper ) Corneal alignment ( flatter ) On k fit |
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Fit assessment terms |
Clearance (tear pooling) signifies steepness Bearing (lack of tear pooling) signifies flatness Aligned ( even tears on the cornea ) |
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Positive tear layer is caused by |
Clearance centrally |
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Apical fit |
Small diameter |
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Bow tie |
Bearing |
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In order to determine the type of fitting method used on a gas perm , look at the size of HVID |
Large D = corneal alignment Medium D = corneal alignment Small D = apical, interpalpebral |
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For starting point |
Transpose and drop cyl |
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Spherical corneas have no |
Astigmatism |
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Bearing |
Lens is bearing centrally , it is flat centrally pushing the tear fil periphery Lens is bearing peripherally there will be no tear film |
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Clarence |
Lens is fitting steeper Lens has Clarence centrally the lents fits STK centrally and tears will pool on center |
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Alignment |
Lens fits perfect and even |
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FTK |
- tear later |
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STK |
+ tear layer |
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Determining diameter |
2.0 - 2.5 mm smaller the visible iris diameter OR 1.2 - 1.5 mm from the limbus edge to edge Normal rigid lens is 8 - 10.5 mm |
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Determining diameter |
Large diameter will be fit flatter then K Small diameter corneas are fit stepper then k |
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Steps and rules for fitting gas perm lenses |
1. Transpose rx to minus cyl 2. Compensate for VD 3. Determine CRA 4. Determine flattest corneal meridian 5. Determine BC 6. Apply FAP OR SAM |
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CRA |
Difference between K and rx If there is more then 1D use 25% rule |
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Flattest corneal meridian |
FLATTEST K |
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Determining BC |
look at HVID FAP OR SAM |
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Tear film |
Fitting steeper create a plus tear lens Fitting flatter creates a minus tear lens |
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Topo |
Warm colors are vertical Warm colors represent steep Cool colors represent flat |
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Flat fit |
Show more tears in the periphery of lens |
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Dumbbell pattern |
Either horizontally left vertically signifies a large amount of astigmatism |
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Corneal diameter |
10.5 or smaller > 8.0 mm |
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Corneal diameter |
11.0 mm - 11.5 > 8.5 |
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Corneal diameter |
12 mm or larger > 9.0 |
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HVID STK |
11.8 and Below |
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STK |
You add |
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FTK |
You subtract |
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When determining the BC of a gas perm lens we need to use the |
Radioscope |
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If you fit 0.75 FTK |
It will create a negative tear layer r |
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If a lens is bearing centrally |
The lens is FTK |
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If you intend to fit a small diameter because the patient has a small palpébral fissure you need to fit a |
Apical fit |
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Gas perm was STK and too steep |
Make diameter smaller to loosen the fit |
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A lens that is bearing will create |
A negative tear layer |
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Rigid gas perm lenses generally work best w thinner lens |
True |
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Corneal alignment fit is FTK |
0.25 D - 1.00 D |
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Corneal alignment fit is then K, so you just coMpensate for the tear layer with |
FLATTer , FAP Creating a negative tear layer , we compensate for the negative power by FAP |
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Normal diameter for a gas perm lens is __ larger than the ___ |
2 mm , Pupil |
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Apical |
7.5 - 8.0 D |
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Corneal alignment |
8.5 - 9.5 d |
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A lens that is bearing centrally |
Create a negative tear layer |
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More then 4 compensate Ex : +5.00 CAP |
+ 5.50 |
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If you choose to fit with a small diameter , this will |
Steepen the lens , causing Clarence |
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Apical fit |
stk and has smaller diameter |
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Corneal alignment |
FTK , larger diameter |
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To compensate for the tear layer that is created from a centrally bearing lens , you must |
Apply the FAP rule |
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Bearing centrally |
ftk - tear layer |
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Lens too tight |
Increase the D |
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Lens too tight |
Increase the D |
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Lens too lose |
Decrease the D |
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VD |
Does not have be compensated if under 4 Diopters. If it is over 4 diopters we do, CAP |
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CRA |
If this number is 1 diopter or less, it is usually tolorated by the patient. You can also use the 25% rule. Take the CRA amount, and mutiply it times 4. If that number is less than the spherical amount you are fine. If the number is more than the spherical power, you must make the lens flatter to compensate for the residual astigmatism. |
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Cornea alignmef Z |
the lens diameter will be large and it will be flatter than K, with Clarence peripherally. |
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12 mm or larger get a diameter of 9.0mm. |
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