Week 4 - GP Lenses, fitting, verifying GP lenses

Question 1

What are 6 advantages to PMMA (first material rigid lenses were made from)? (durability/resistance/lifespan/modifications/vision/optimum people)?

Answer 1

• Very durable
• Resistance to scratches and breakage
• Long lifespan
• Can be repolished and modified
• High visual performance
• For corneas under 3.00D of cylinder best contact lens option for optical clarity

Question 2

What are 7 advantages to RGP lenses? (oxygen/options/handling/cleaning/cost/vision2)

Answer 2

• O2 permeability
• Large diameter options gives better stability
• Easy handling for insertion and removal
• Cleans and polishes easily
• Cost effective due to long lifespan
• Clear visual experience for low astigmatism
• High astigmatism (cyl) can be fit successfully

Question 3

What patient’s are good for RGP’s? 7 (cornea 2/prev wearers problems 4/ conditions)

Answer 3

-Corneal Irregularities (keratoconus, PMG)
-Corneal Astigmatism
-Patients experiencing spectacle blur
-Patients experiencing Flare due to PMMA’s small OZ
-Soft contact lens drop outs (ie. GPC, dry
eye, corneal edema, allergy to solutions)
-PMMA dropouts - refit instantly
-dry eye pxs

Question 4

When fitting an RGP lens what do we want to consider in regards to the apical clearance?

Answer 4

maintain a slight apical clearance

Question 5

The optical zone diameter is dependent on what 3 things?

Answer 5

pupil size
lens diameter
palpebral fissure size

Question 6

What is the minimum and maxiumum center thickness measurements for an RGP lens?

Answer 6

no less than 0.10mm and no more than 0.13mm

Question 7

What occurs if the lens is too thin?

Answer 7

lens flexure (distortion/warping which affects the pxs vision/comfortability)

Question 8

What 2 measurements are the corneal diameter dependent on?

Answer 8

HVID
PA

Question 9

What two conditions should you measure the pupil for the optical zone diameter?

Answer 9

in light and dim conditions as sizes will vary

Question 10

How should we expect thick lenses to appear/fit? 4

Answer 10

-ride low
-mold the cornea
-less tear exchange
-may appear to fit loose on assessment

Question 11

How do thin lenses appear?

Answer 11

fit tight

Question 12

How should we consider fitting based on the thick/thinness of the potential lens?

Answer 12

thin lens = looser fit initially
thick lens = tighter fit initially

Question 13

What determines the peripheral curve?

Answer 13

width of the optic zone

Question 14

What is a blending curve used for?

Answer 14

Remove sharp junctions of the peripheral and
secondary curves

Question 14

How do we make a lens looser when regarding the periphery curve?

Answer 14

widen the periphery
curves or flatten them

Question 15

What is the secondary curve also known as?

It is often flatter than the base curve and sits directly on the cornea.

It aligns the lens with the flatter part of the cornea next to the apical zone.

Answer 15

Immediate curve - it is between the base curve and the periphery curve

Question 15

The edges should have a smooth finish.

What are the 3 zones of the edge?

Answer 15

• Anterior Zone
• Posterior Zone
• Edge Apex

Question 16

If the lens is -5.00D or higher what is added to the edge?

Answer 16

bevel - added to the anterior surface to contour to the lens edge

Question 17

Where are the 3 zones of the edge of an RGP touching?

Answer 17

The anterior zone touches the lid during blinking.

The posterior curve flares the edge away from the cornea.

The apex is the junction between the two. you want it to be well rounded.

Question 18

What do we use to measure the power of a RGP?

Answer 18

Lensmeter

Question 19

What do we use to measure the base curve of the RGP?

Answer 19

Radiuscope

Question 20

What do we use to measure the diameter of the RGP? 2

Answer 20

V-gauge
measuring magnifer

Question 21

What do we use to measure the center thickness of the RGP?

Answer 21

Thickness guage

Question 22

What is one of the most important parameters to verifying in an RGP? Why?

Answer 22

Base Curve Radium (BCR)
affects lens-to-cornea relationship

Question 23

What state do we want to make sure the lens is in before measuring with a radiuscope?

Answer 23

needs to be clean and dry to provide clear images

Question 24

What is a measuring magnifer used to see? 3

Answer 24

• The optic zone diameter
• Quality of the blend in the peripheral curve widths
• Any lens surface defects present

Question 25

What can a rough edge feel like to the patient? 2

Answer 25

• foreign body sensation
  -irritation

Question 26

How do we detect a rough edge? 4

Answer 26

• palm method - pass lens over the palm to detect any defects
• Projection magnifier
• Slit lamp biomicroscope
• Measuring magnifier

Question 27

Using the lensmeter and the radiuscope we can determine the design of the gas permeable lens. T/F

Answer 27

TRUE

Question 28

If the lines are not clear on the radiuscope what does it mean?

Answer 28

Lens is warped we can not rely on the data. Refit and find orginal BC

Question 29

How should you record a toric lens power? Example prescription: -5.00 -1.00 x 180

Answer 29

-5.00/-6.00

Question 30

What general problems could be caused by the lens thickness? 4

Answer 30

Flexure
oxygen permeability
lens position
patient comfort

Question 31

what does < 2 mean?

Answer 31

less than 2

Question 32

What does > 2 mean?

Answer 32

greater than 2

Question 33

How do we find Total Astigmatism?

Answer 33

Total Astigmatism = Corneal Astigmatism + Residual Astigmatism

Question 34

What are the 5 edge types?

Answer 34

a) Posterior
b) Central
c) Anterior
d) Blunt
e) Sharp

Question 35

How do we differentiate between back toric & bi-toric? 5 steps

Answer 35

1) Measure the BC of the toric lens
2) Calculate the amount of cyl in the BC
3)Measure the Rx of the lens
4)Calculate the amount of cyl in Rx
5)Find the difference between BC Cyl & Rx Cyl

Question 36

After the following steps have been done:
1) Measure the BC of the toric lens
2) Calculate the amount of cyl in the BC
3)Measure the Rx of the lens
4)Calculate the amount of cyl in Rx
5)Find the difference between BC Cyl & Rx Cyl

What kind of lens design has a cyl less than or equal to 0.75D ?

What kind of lens design has a cyl more than 0.75D?

Answer 36

Less than or equal to 0.75 D = Back Toric

More than 0.75 D = Bi-Toric

Question 37

What is residual astigmatism?

Answer 37

The difference between the corneal and refractive astigmatism
Or known as any astigmatism left ‘uncorrected’ by the spectacle prescription

Question 38

When reading a radiuscope does the steeper or flatter curve come into focus first?

Answer 38

Steeper