RGP material

Little information about rigid contact lens and its materials

1. RGP MATERIALS Swastika Adhikari Intern Optometrist Vittala International Institute of Ophthalmology

2. Introduction Contact lenses are the small, light weight optical devices which worn on the eye that corrects the refractive error for better vision. Other purposes: Cosmetic and Therapeutic [Encyclopaedia] Several materials have been used in fabrication of contact lenses where first material used was glass. Contact lens materials should be ideal.

3. Ideal Contact Lens Material The ideal contact lens would: Provide sufficient oxygen for normal corneal metabolism. Be physiologically inert. Be very wettable on the eye. Resist lens deposit formation. Maintain stable dimensions. Be durable. Be transparent with minimal light loss. Be optically regular so its optics are predictable. Have physical properties which allow the creation and retention of high quality surfaces. Require minimal maintenance by wearer. Be easy to fabricate lenses from.

4. RGP Material 1. Cellulose Acetate Butyrate PMMA 2. Butyl Styrene Styrene 3. Siloxane Acrylates (SAS) 4. Fluoro-Siloxane Acrylates 5.Perfluoroethers Non-Gas Permeable Rigid Gas Permeable

5. RIGID GAS PERMEABLE (RGP) POLYMERS A. Non-Gas Permeable:- 1. Poly Methyl methacrylate: Used in contact lenses late 30s (1936-1938) Readily machined and polished so, optically clear. Fairly wettable when clean Easy to care 0.2% - 0.5% water when hydrated fully Almost zero O2 permeability Produces spectacle blur and in the long-term, polymegethism and/or corneal exhaustion syndrome might occur.

6. B. Rigid Gas Permeable Materials 1. Cellulose Acetate Butyrate: Introduced by Eastman, mid-1930s More flexible than PMMA Water resistant Can be molded or lathed Hydroxyl groups result in 2% water content Material stability lower than PMMA Dk range 4 - 8 Incompatible with preservative Benzalkonium Chloride

7. 2. Butyl Styrene ( t- Butyl Styrene): Dk of 25, while low, was quite competitive at the time of release. The refractive index of 1.533 is the highest of any RGP material. The specific gravity (SG) of 0.95 is the lowest of any RGP. The combination of high n and low SG offers the thinnest, lightest lens possible. This made the material ideal for high Rxs. Material was not particularly successful in the marketplace.

8. The surface was prone to scratch and some solutions were reported to reduce lens wettabiliity. Limited availability Novel material as combination of high RI and low specific gravity. [desirable properties]

9. 3. Siloxane Acrylates (SAS) Properties: The era of successful RGP materials was heralded by the introduction of Polycon , the original siloxane acrylate material, in the late 1970s. A PMMA backbone gives the material its dominant physical properties, especially rigidity. Si-O-Si bond is flexible and extensible. This results in significant increases in oxygen permeability but a reduction in material rigidity. Dks in low to medium. A wetting agent may be incorporated to enhance lens wettability, usually methacrylic acid (MA). Material chemistry results in a net negative charge on the lens surface, especially if methacrylic acid is used as the wetting agent.

10. Advantages: SAs have higher oxygen permeabilities than all the lens materials. At the time of their introduction this resulted in improved corneal physiology. The lower rigidity of SAs allows lenses to conform more closely to the shapes of the corneas. This reduces the likelihood of the lenses being displaced from the cornea during normal use. By offering improved physiology, and to a lesser extent lower rigidity, larger diameter lenses could be fitted. This allowed the optic zone diameter to be increased, thereby overcoming some of the problems of smaller optic zones, especially in low light levels.

11. Disadvantage: Surface charge makes them more deposit prone. R elatively soft surfaces, hence they scratch more readily. R elatively brittle and are cracked or broken more easily. Low rigidity allows the lens to conform to corneal shape, reducing the completeness of correction of corneal astigmatism by a simple spherical lens. Flexure problems with SA materials of relatively low rigidity are possibly related to the siloxane content of the material. M ay be influenced by age, their environment, lens care products and the stresses placed on them by storage cases or astigmatic eyes. Unknown factors during lens fabrication may also affect lens parameters.

12. EXAMPLES: Boston ll, lV Alberta ll, lll Menicon O2 Optacryl 60, Ext Paraperm O2, EW Polycon ll, HDK Persecon CE

13. 4. Fluoro-Siloxane Acrylates (FSAs) Properties: The element fluorine (F) is added to basic Silicone acrylate to enhance O2 permeability. A lower surface charge results some materials may wet a little better. Some materials may resist deposits more. However the allusion to Teflon -like (poly(tetrafluoroethylene) properties cannot be justified. Dks of 40 - 100 or more are achievable. Dks are high enough for extended wear. M ore flexible than Silicone Acrylates. Surfaces are relatively easily scratched.

14. EXAMPLES Equalens Fluorex FluoroPerm Quantum ll Alberta N-FL

15. 5.Perfluoroethers The perfluoroether lens material is novel and in a distinct material category. It should not be confused with FSAs. A perfluoroether consists of: Fluorine. Oxygen. Carbon. Hydrogen. The fluorofocon A material consists of: Perfluoroether. PVP (poly(vinyl pyrrolidine)). MMA (methyl methacrylate).

16. Advantages: High Dk, potentially sufficient to support extended wear. No surface charge, thus reducing the likelihood of lens spoilage. High flexibility results in conformity to the corneal shape in situ . This results in stable vision and possibly greater comfort.

17. Disadvantages: Low refractive index High specific gravity Low yields/high cost Average wettability Greater flexibility on eye

18. RGP MATERIALS AVAILABLE Dk RGP materials PMMA low Airlens II, Alberta <40 Alberta N, Boston lV, Fluorex 100, 200, 400, FluoroPerm 30, Optacryl K, Paraperm O2 Paraperm O2, O2+, EW Polycon ll Low to mod (40-60) Boston 7, Equalens, Fluorex 600,800 Mod to High (>60) Equalens II, FluoroPerm 92, Optacryl Z

19. References The Text book of contact lens- Jaypee The IACLE Contact Lens Course MODULE 2 RGP contact lens materials- An Article

20. Competence, like truth, Beauty and a contact lens, is in the eye of the beholder. -J.peter