1. Gas Permeable fitting techniques
Week 10
Gas Permeable fitting techniques

2. Gas perm fitting techniques
Steps and rules for fitting gas perm lenses
Transpose Rx to minus cylinder.
Compensate for vertex distance, if needed.
Determine calculated residual astigmatism.
Determine flattest corneal meridian.
Determine base curve.
Compensate for tear film. ( FAP or SAM )

3. Gas perm fitting techniques
transposition
example
CL’s ONLY come in - cyl.
ALWAYS transpose from + cyl to - cyl or the power may be wrong in the end!
Algebraically add the +cyl to the spherical power. Change the + cyl to - cyl. Change axis by 90 degrees.
x90 =
x180
x30=
x 120
x180=
x90

4. Gas perm fitting techniques
Compensate for vertex distance
Examples ( Chart in text page 162)
Manifest refraction is usually done at 12 mm from the cornea.
CL’s are on the cornea, so the power will need to be compensated for this adjustment.
There are many conversion charts for this.
Only needs to be converted if the spherical power or spherical equivalent is +/ D or more.
Acronym= CAP ( closer, add plus.)
D at 12 mm= D
+6.75 D at 12 mm=
+7.50 D
+8.50 D at 12 mm=
+9.50 D

5. Gas perm fitting techniques
Determine calculated residual astigmatism
examples
A patient may have an Rx with D cyl but their K readings may show 2.00 D of corneal cyl. A difference of .50D.
Most patients will tolerate 1 D or less of uncorrected residual astigmatism.
If there is more than 1 D, s steeper, larger optic zone diameter should be used or a toric gas perm lens should be fit.
25% rule VS patient VA
K’s 39.75/41.75 x90 =2D cyl
x90=3D cyl
= 1 D of CRA
K’s 45.75/42.00 x90 =3.75D
x180 =5D
=1.75D of CRA
K’s44.00/44.75 x90 =.75D
x 90 =1D
0.25D of CRA

6. Gas perm fitting techniques
Determine flattest corneal meridian
examples
Most corneas have 2 principal meridians. (90 & 180)
Which is the flattest or smaller number of the two “K” readings on the right eye?
That is the “K” used as a starting point when you decide to fit either flatter than K, steeper than K, or on K.
47.00/46.25 x 90
Flattest K? ________
39.87/41.12 x90
Flattest K?________
44.00/44.37 X180
This number is the starting point for your next step!

7. Gas perm fitting techniques
Determining base curve
examples
Not all gas perm lenses are fit on ”K”.
You need to look at fissure, amount of corneal astigmatism, and HVID.
Keep in mind, when you fit steeper (apical) or flatter (alignment) fits, you will have to compensate for the tear lens, FAP & SAM.
x 90
47.00/45.00 x 90= flattest
Apical fit =steeper than
Amount of corneal astigmatism =2.D
Look at fitting BC chart.
You will fit apical BC at
If you fit alignment, BC will be
Remember, once you determine from the fitting chart how much flatter/steeper to fit than flattest “K”,steeper get steeper (larger), flatter get flatter(smaller)

8. Gas perm fitting techniques
Compensate for the tear film
examples
When ordering a gas perm CL, there is only a spherical power.
The tear film that forms under the lens makes up for the cylinder not ground into the CL.
Fitting steeper creates a plus tear lens.
Fitting flatter creates a minus tear lens.
Whatever amount flatter or steeper you fit the lens, is the amount you need to add to the spherical power.
On “K” needs no compensation.
x90, /42.25 x90
Alignment fit (FAP)
Fitting chart says fit .25 flatter than “K”.
flattest “K” is
42.00 is .25 D flatter than
This will create a – 0.25 D tear lens.
We need to compensate for that by add to our spherical power.
-3.50 plus +.25 =-3.25 D

9. Gas perm fitting techniques
If you forget this step, The overall power of the lens will be wrong.
Let’s see why !
42.00/43.0 x90
x180
Let’s fit alignment
is flattest “K”
BC fitting chart with corneal cyl=0.50D flatter than “K”
42.00 & 0.50 D FTK= 41.50, this will create a minus tear layer.
Power is if we add the – 0.50 created by the tear lens the final power will be the equivalent of -2.50, not the we want.
This is why we do FAP.
-2.00, (FAP)from the 0.50 D FTK=
-1.50 add the tear lens and you get the intended power of -2.00D

10. Gas perm fitting techniques
Optical zone diameter
Optical zone is based on total diameter.
On average, the peripheral curves, blends and finishing edges total about 1.2 mm of the CL diameter.
Remember, gas perm lenses are only 8-9 mm . I.2 mm of edging can interfere with the wearer’s VA if they have large pupils.
Patient interview can help tremendously . What type of job, what meds are they on, ect…
Why do we need to know this?

11. Gas perm fitting techniques
Optic zone diameter
If your patient has a smaller diameter lens and a large pupil, make sure you tell the manufacturer to make that edging smaller so it will not interfere with the patient’s VA. This can also be avoided by making the overall diameter larger if possible.

12. Gas perm fitting techniques
These 6 steps are the basic steps. There are many tricks and rules that make CL fitting such a specialized area that will take years to perfect.
Let’s try out what we’ve learned today. Take your time, use the steps and the charts!

13. STUDY,STUDY,STUDY,test next week
Test on gas perm material but you still need to know the overall basics. me with your questions! I’m here to help you succeed!