Week 4 - Adapting to Rigid Lenses Flashcards
Major Reasons for Discomfort with RGP’s
Why do the edge of an RGP cause discomfort? 4 (prescription2/design/general)
- Poorly contoured edges or poorly blended peripherial
curves. - Thick edges
- High Minus needs a lenticular cut to center lower
- high plus, aphakic lens to help keep lens high (thicker lens will make it drop down)
Major Reasons for Discomfort with RGP’s
A lens that is stored dry causes discomfort. T/F
TRUE
Major Reasons for Discomfort with RGP’s
The smaller the lens means their is less lid attachment so the px notices the lens more.
What happens everytime the px blinks?
the eyelid has to glide over the lens edge which is uncomfortable
Major Reasons for Discomfort with RGP’s
What 2 situations with an RGP lens cause mechanical abrasions? 2
-BC too flat
-deposits on lens surface
Major Reasons for Discomfort with RGP’s
If the edge is too steep in relation to the eyelid how will the lens be decentered?
Lens is decentered inferiorly (down)
Major Reasons for Discomfort with RGP’s
If the edge is too flat in relation to the eyelid how will the lens be decentred?
Lens is decentered superiorly (up)
Major Reasons for Discomfort with RGP’s
The patient will notice the lens in. Is there something wrong with the lens?
No there is not a fitting issue, the feeling of excessive lens movement will go away with adaption
Major Reasons for Discomfort with RGP’s
If the optic zone size is too small, what can it cause and what do we do to fix it?
-Can cause flare
-we need to make the optic zone larger
Major Reasons for Discomfort with RGP’s
If the lens is decentered, what can it cause and what do we do to fix it?
-Can cause flare
-fix issue as to why it is decentered
Major Reasons for Discomfort with RGP’s
Mild Photophobia is normal.
What is normally wrong with the lens for it to cause photophobia?
usually due to a flat fitting lens
Major Reasons for Discomfort with RGP’s
The patient will tear when getting use to RGP lenses. This should go away as the patient adapts.
What can the px experience when they are tearing up? 2
- the lens can become dislodged
- variable vision
Major Reasons for Discomfort with RGP’s
If the patient is tearing up excessively what should we check for?3
What can we change to help? 2
- The edges (thick edges can cause tearing)
- Check the lid impact (Can cause frothing/ greasy film over lens)
- Inspect periphery of lens
- Change to higher Dk material
- Change fit
What kind of lens design has 1 curve (BC) on the Radiuscope and 1 power on the lensmeter?
Example: 7.03(48.00) -4.00
Spherical lens
What kind of lens design has 1 curve (BC) on the radiuscope and 2 powers on the lensmeter?
Example: 7.67(44.00) -2.00/-4.00
Front Toric
What kind of lens design has 2 curves (BC) on the radiuscope and 2 powers on the lensmeter?
Use Example
7.85/7.50 (43.00/45.00)
-2.00/-3.00
Bi -Toric as the residual astigmatism is MORE THAN 0.75D
43.00-45.00=2.00
(-2.00)(-3.00) difference = -1.00
2.00-1.00=1.00
Residual astigmatism = -1.00
What kind of lens design has 2 curve (BC) on the radiuscope and 2 powers on the lensmeter?
Use Example
7.85/7.67 (43.00/44.00)
-2.00/-3.25
Back Toric as the residual astigmatism is EQUAL TO OR LESS THAN 0.75D
43.00-44.00=1.00D
(-2.00)(-3.25) = difference = -1.25
1-1.25=0.25
Residual astigmatism= -0.25
If the RGP lens is steeper than the K how will this look on the cornea?
there will be apical clearance
What kind of tear lens is created when we fit steeper than the K?
what do we do to the CL RX to neutralize the tear lens?
plus tear lens
add minus to the CL RX
If the RGP lens is fit on the K how will this look on the cornea?
there will be apical alignment
What kind of tear lens is created when we fit on the K?
what do we do to the CL RX to neutralize the tear lens?
plano tear lens
no power added to the CL RX
If the RGP lens is fit flatter than the K how will this look on the cornea?
there will be apical bearing
What kind of tear lens is created when we fit flatter than the K?
what do we do to the CL RX to neutralize the tear lens?
Minus tear lens
add plus to the CL RX
What is a tear lens?
the tears that get trapped under a RGP lens during tear exchange and have their own power.
What does SAM stand for?
Steeper Add Minus
What does FAP stand for?
Flatter Add Plus
What is the SAM rule?
For every 0.25D that the optical zone curvature is greater (steeper than the K) than the corneal curvature add -0.25D to the contact lens power.
0.25D = add -0.25
0.50D = add -0.50
0.75D = add -0.75
What is the FAP rule?
For every 0.25D that the optical zone curvature is less (flatter than K) than the corneal curvature add +0.25D to the contact lens power.
0.25D = add +0.25
0.50D = add +0.50
0.75D = add +0.75
Sphere RGP design
more or less than 4D (cornea is sphere)
If the cyl is less than -0.75 and the cornea is spherical (no corneal astigmatism) you can drop the cyl. T/F
TRUE
Sphere RGP design
more than than 4D (cornea is sphere)
We need to vertex the powers if they are 4.00D or more. T/F
TRUE
Sphere RGP design
more than than 4D (cornea is sphere)
What order do we do the steps for the short method.
1) Drop cyl
2) Vertex
3) SAM/FAP
4) power to order
RGPS can be ordered to the 1/8th dioptre. T/F
recite the 1/8th dioptres.
TRUE
0.12 0.25 0.37 0.50 0.62 0.75 0.87 1.00
What is this formulae?
AT = AC + AR
Total Astigmatism = Corneal Astigmatism + Residual Astigmatism
What is the formulae to find Residual astigmatism?
Total astigmatism (from gls rx) - Corneal astigmatism (from K readings) = Residual Astigmatism (amount left uncorrected by gls)
RESIDUAL ASTIGMATISM IS ALWAYS MINUS
When do we need to find the residual astigmatism before figuring out the CL design?
when the K readings are different i.e the cornea is not a sphere - this will be most of the time
What does Coc stand for?
Calculated over correction
How should we set out our answers when designing a lens?
Final Cl Power = BC: xxx PWR: xxx(sph/difference between the meridians with residual/Coc cyl NOT vertexed cyl, prism goes here if needed for FT design) DIA: xxx Design of the lens
When do we use the Bernie Stuart Diameter?
ONLY when told to used a bernie stuart nomogram
