RGP Lenses Flashcards
What parameters are needed for ordering RGP lenses? (7)
- Prescription & BVD
- Keratometry
- HVID (lens is 2mm less)
- Lid position
- Pupil diameter (dim and light)
- Tear film assessment
What are the indicators of fitting RGP lenses? (5)
- High cylinders
- Irregular astigmatism like keratoconus
- Lasts longer than soft CLs (cost efficient)
- Vision quality is better
- Creates a tear film and helps to mask corneal astigmatism
What are the contraindications of RGP lenses? (4)
- Interaction from the lids cause discomfort
- Takes longer to adapt than soft lenses
- More susceptible to particles behind the lens (dust)
- Not good for sports as the lens can call out due to loss in elasticity
How do you choose an RGP lens parameters? (3)
- BOZR to match flattest K reading (highest value)
- Total lens diameter = HVID - 2
- Spherical component of rx corrected with BVD accounted for
When could you order a tri-curve RGP lens?
When corneal astigmatism is approximately 1.50D - the BOZR would be the flattest K reading (highest value) - 0.05-0.10mm
If the pupil is large (7mm) then what diameter RGP lens would need fitting and how will this affect the BOZR?
You want the BOZD to be 1-2mm larger than the pupil but increasing the diameter would mean the lens fits steeper. You need to flatten the BOZR to ensure optimum fitting.
How does the thickness of a +4.00D lens differ from a -4.00D lens? How will this affect the fitting?
Thick positive lenses = centre of gravity at corneal apex, therefore lens will ride low - FIT LENS STEEPER
Thick negative lenses = centre of gravity is behind, therefore, if too thick then lens will be lifted high due to the lids - FIT FLATTER
Describe the dynamic fit (white light) assessment of an RGP lens and what needs to be recorded (6)
- Centration in primary gaze (horizontal and vertical decentration in mm)
- Movement on blink (mm)
- Type of movement (Smooth/jerky/apical rotation)
- Speed of movement (Fast/moderate/slow)
- Direction of movement
- Stability / Lid attachment
Describe the static fit (Fluorescein and cobalt light) assessment of an RGP lens and what needs to be recorded (3)
- Central (Pooling = steep ; Touch = flat)
- Mid-peripheral
- Edge (0.5mm = Ideal ; thin = steep fitting as less edge lift ; thick = flat fitting as excessive edge clearance)
What is the lower of the liquid lens if:
1) Flat fitting lens
2) Steep fitting lens
1) Negative
2) Positive
How would you change an RGP lens if it is too:
1) Flat
2) Steep
1) Tear lens = negative ; ↓ BOZR and ↑ BVP or ↑ TD
2) Tear lens = positive ; ↑ BOZR and ↓ BVP or ↓ TD
When making changes to fitting of an RGP, how much should you change:
1) TD by
2) BC by
1) 0.4mm
2) 0.1mm
What are the advantages (4) and disadvantages (3) of aspheric RGP lens designs?
Adv:
- Follows the contour of the cornea
- Reduced edge clearance = more comfortable as less lid interaction
- Less pupil dependent so reduces glare and flare
- If diameter is changed you don’t also need to change BOZR
Disadv:
- Poor centration
- Difficult to manufacture
- Cannot modify
What are the advantages of tri-curve RGP lens designs? (2)
- More control in terms of being able to change parameters
- Minimises lens flexure = more comfort
What are the advantages (5) and disadvantages (2) of a spherical RGP lens design?
Adv:
- Peripheral curves can be altered to optimise fit
- Good circulation of tears beneath lens
- Supports tear meniscus
- Maintains good lid-lens relationship to promote normal blink pattern and comfort
- Easier to remove lenses
Disadv:
- Does not follow contour of cornea
- More pupil dependent
Advantage (1) and disadvantages (2) of PMMA as a RGP lens material
- Optically good and durable
- Does not allow enough oxygen through
- Not ocular health friendly
Advantages (5) and disadvantages (3) of silicone acrylate as a RGP lens material
- Good range of materials available
- Good scratch resistance
- Good vision with limited lens flexure
- Good dimensional stability
- Low to medium Dk available
- 3 and 9 o’clock staining
- Attracts proteins from tears
- Some materials are fragile with a breakage problem
What does the fluorine atom in fluorosilicone acrylate RGP lenses provide and what are the advantages (6( and disadvantages (2) of this material?
Replaces hydrogen atoms = improves surface wettability, deposit resistance and tear film stability
- Very high Dks available
- Few deposit problems
- Better wettability
- Suitable for EW
- Fragile if too thin
- Requires careful manufacture
- Corneal adhesions in some cases
- Dimensional stability depends on material and manufacturer
What does a CLs aftercare H+S consist of? (9)
- Type of lens worn
- History of any previous lenses worn
- Reason for visit
- Any concerns (vision? comfort - end of day?)
- Wearing time (Today; Days/week; Hrs/Day)
- Care system - ask for demo
- Ensure px has back-up spexs
- Are the lenses meeting the needs of the px?
- Pain/Irritation/Dryness?
What needs to be recorded in a CLs aftercare examination?
1) Centration in primary gaze (mm)
2) Corneal coverage (complete or partial)
3) Upgaze blink
4) Horizontal lag
5) Push-up test
Upgaze blink values for:
1) Optimal fit
2) Tight fit
3) Loose fit
1) 0.2 - 0.6mm
2) Little to no movement
3) > 0.6mm
Horizontal lag values for:
1) Optimal fit
2) Tight fit
3) Loose fit
1) 0.2mm - 0.4mm
2) Little to no movement
3) > 0.4mm
How is the push-up test recorded in CLs assessments?
0% = Extremely loose 50% = Desired 30-70% = Acceptable 100% = Extremely tight
Why is a tight fit bad (3)
- Inflammation
- Neovascularisation
- Microbial Keratitis
