Intro to RGP Flashcards
Materials: PMMA
- Around since 1930’s, hard lenses
- Offers excellent optics, durability and is easily manufactured
- Allows NO oxygen to the cornea, causes corneal edema and changes corneal integrity
Materials: Silicone Acrylate
- 1980’s
- Allows gas permeability
- Attracts deposits
- Has poor wettability
Materials: Fluorosilicone Acrylate
- 1990’s
- Allows gas permeability
- Increased wettability
- Increased stability
- Deposit resistance
Materials: Methylmethacrylate
- Durable
- Resilience
- Optical quality
What are most lenses fabricated with today?
Fluorosilicone acrylates
Why plasma treat a RGP lens?
Increases the hydrophilicity of the surface, which allows for:
-a reduced wetting angle -increase in patient comfort
-enhances the adaptability to the (hard) lens
Oxygen Permeability
- Dk value: O2 permeability of RGP material
- Dk/t value: actual O2 transmission of CL material, depending on thickness of lens
The Dk/t of a finished lens will always be greater than the Dk value of the material
When are RGP lenses used?
- Critical vision needs
- Astig correction
- Irregular cornea
- Soft lens failure
- Post refractive therapy
- Presbyopia
- Myopia control
Good candidate for RGP
- Highly motivated
- Wants precise VA
- Wants low cost and low maitenence
- Corneal astig= refractive astig
- With the rule (easier fit)
Contradiction to RGP
- Involvement in contact sports
- Individuals only wanting to do part time wearing
- Against the rule/oblique
- Large pupils
Diameter
Approx. 2mm less than HVID
- Steep cornea usually needs smaller diameter
- Flat cornea usually needs larger diamter
Base curve
Found with K readings (flatter curve)
- Spherical and low cyl are fit on K or slightly steeper
- As corneal cyl increases base curve will steepen
Optic Zone Diameter
- Must be large enough to provide coverage in low illumination
- OZD is total diameter - width of peripheral curves
1mm change in diameter must be offset by 0.01mm change in curvature
Calculating lens power
- Minus cyl form
- Only sph component
- Vertex
- Use sph. equiv. for mild RA
Residual Astigmatism
When patient have a different amount of cyl in their spec RX versus cornea.
Most common cause is from crystalline lens
RA= RC-CC
