Week 4 - GP Lenses, fitting, verifying GP lenses Flashcards
What are 6 advantages to PMMA (first material rigid lenses were made from)? (durability/resistance/lifespan/modifications/vision/optimum people)?
- 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
What are 7 advantages to RGP lenses? (oxygen/options/handling/cleaning/cost/vision2)
- 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
What patient’s are good for RGP’s? 7 (cornea 2/prev wearers problems 4/ conditions)
-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
When fitting an RGP lens what do we want to consider in regards to the apical clearance?
maintain a slight apical clearance
The optical zone diameter is dependent on what 3 things?
pupil size
lens diameter
palpebral fissure size
What is the minimum and maxiumum center thickness measurements for an RGP lens?
no less than 0.10mm and no more than 0.13mm
What occurs if the lens is too thin?
lens flexure (distortion/warping which affects the pxs vision/comfortability)
What 2 measurements are the corneal diameter dependent on?
HVID
PA
What two conditions should you measure the pupil for the optical zone diameter?
in light and dim conditions as sizes will vary
How should we expect thick lenses to appear/fit? 4
-ride low
-mold the cornea
-less tear exchange
-may appear to fit loose on assessment
How do thin lenses appear?
fit tight
How should we consider fitting based on the thick/thinness of the potential lens?
thin lens = looser fit initially
thick lens = tighter fit initially
What determines the peripheral curve?
width of the optic zone
What is a blending curve used for?
Remove sharp junctions of the peripheral and
secondary curves
How do we make a lens looser when regarding the periphery curve?
widen the periphery
curves or flatten them
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.
Immediate curve - it is between the base curve and the periphery curve
The edges should have a smooth finish.
What are the 3 zones of the edge?
- Anterior Zone
- Posterior Zone
- Edge Apex
If the lens is -5.00D or higher what is added to the edge?
bevel - added to the anterior surface to contour to the lens edge
Where are the 3 zones of the edge of an RGP touching?
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.
What do we use to measure the power of a RGP?
Lensmeter
What do we use to measure the base curve of the RGP?
Radiuscope
What do we use to measure the diameter of the RGP? 2
V-gauge
measuring magnifer
What do we use to measure the center thickness of the RGP?
Thickness guage
What is one of the most important parameters to verifying in an RGP? Why?
Base Curve Radium (BCR)
affects lens-to-cornea relationship
What state do we want to make sure the lens is in before measuring with a radiuscope?
needs to be clean and dry to provide clear images
What is a measuring magnifer used to see? 3
- The optic zone diameter
- Quality of the blend in the peripheral curve widths
- Any lens surface defects present
What can a rough edge feel like to the patient? 2
- foreign body sensation
-irritation
How do we detect a rough edge? 4
- palm method - pass lens over the palm to detect any defects
- Projection magnifier
- Slit lamp biomicroscope
- Measuring magnifier
Using the lensmeter and the radiuscope we can determine the design of the gas permeable lens. T/F
TRUE
If the lines are not clear on the radiuscope what does it mean?
Lens is warped we can not rely on the data. Refit and find orginal BC
How should you record a toric lens power? Example prescription: -5.00 -1.00 x 180
-5.00/-6.00
What general problems could be caused by the lens thickness? 4
Flexure
oxygen permeability
lens position
patient comfort
what does < 2 mean?
less than 2
What does > 2 mean?
greater than 2
How do we find Total Astigmatism?
Total Astigmatism = Corneal Astigmatism + Residual Astigmatism
What are the 5 edge types?
a) Posterior
b) Central
c) Anterior
d) Blunt
e) Sharp
How do we differentiate between back toric & bi-toric? 5 steps
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
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?
Less than or equal to 0.75 D = Back Toric
More than 0.75 D = Bi-Toric
What is residual astigmatism?
The difference between the corneal and refractive astigmatism
Or known as any astigmatism left ‘uncorrected’ by the spectacle prescription
When reading a radiuscope does the steeper or flatter curve come into focus first?
Steeper
