ICLC_Mod%206.3.final.ppt
- 1. CONTACT LENS CHARACTERISTICS AND OXYGEN TRANSMISSION
- 2. The IACLE Contact Lens Course (all formats) is the sole property of the International Association of Contact Lens Educators (IACLE) and is protected, without limitations, by copyright. By accessing this material, you agree to the following terms and conditions: You may only access and use the IACLE Contact Lens Course for personal or educational purposes. Any dissemination or sale of the IACLE Contact Lens Course, either in whole or in part, or use of the materials for other than educational and personal purposes, is strictly prohibited without the express written consent of IACLE. Except as declared below, you may not reproduce, republish, post, transmit, or distribute any material included in the IACLE Contact Lens Course. You may print materials for personal or educational purposes only. All copyright information, including the IACLE logo, must remain on the material. Appropriate reference must be provided to any use of the content of the IACLE Contact Lens Course, including text, images, &/or illustrations. COPYRIGHT NOTICE
- 3. SPONSORS Development and delivery of contact lens education by IACLE is supported through educational grants and in-kind contributions Major In-Kind Supporters Industry Supporters
- 4. Published in Australia by The International Association of Contact Lens Educators First Edition 1997 The International Association of Contact Lens Educators 1996 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission, in writing, of: The International Association of Contact Lens Educators IACLE Secretariat, PO Box 656 Kensington NSW 1465 Australia Email: iacle@iacle.org
- 5. The IACLE Curriculum Project is the result of a desire to raise the general standard of eyecare education, to make contact lens wear safer and more successful, and to develop the contact lens business further by creating the educational infrastructure that will produce the teachers, students, and practitioners of the future. Full acknowledgements, along with the educator’s guide to the IACLE Contact Lens Course (ICLC), can be found on the IACLE website at www.iacle.org ACKNOWLEDGEMENTS
- 6. CONTRIBUTORS Contact Lens Characteristics and Oxygen Transmission: Robert Terry, BOptom, MSc Lewis Williams, AQIT(Optom), MOptom, PhD
- 7. CORNEAL OXYGEN REQUIREMENTS: GENERAL • When no lens worn • Adapted wearers vs neophytes • Daily wear (DW) • Extended wear (EW) • Lens design/material characteristics
- 8. • Critical oxygen levels • Holden and Mertz criteria (Dk/t) • Large individual variation • Abnormal corneas: – surgery – disease CORNEAL OXYGEN REQUIREMENTS: GENERAL
- 9. CORNEAL OXYGEN REQUIREMENTS For zero daytime swelling: • Dk/t = 24.1 ± 2.7 x 10-9 • EOP of 9.9% SCL DAILY WEAR
- 10. OEDEMA vs Dk/t DAILY WEAR -1 0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 CALC Dk/tavg (cm x ml O2 / sec x ml x mm Hg) Corneal Swelling (%) Dk/tavg = 24 x 10-9 r = - 0.96
- 11. CORNEAL OXYGEN REQUIREMENTS For zero residual swelling at eye closure: • Dk/t = 34.3 ± 5.2 x 10-9 • EOP of 12.1% SCL EXTENDED WEAR
- 12. CORNEAL OXYGEN REQUIREMENTS For overnight oedema = 4.0% • Dk/t = 87.0 ± 3.3 x 10-9 • EOP of 17.9% SCL EXTENDED WEAR
- 13. 0 10 20 0 10 20 30 40 Dk/tavg (cm x ml O2) / (sec x ml x mm Hg) Corneal Swelling (%) Dk/tavg = 87 x 10-9 15 5 140 130 // Holden and Mertz, 1984 OVERNIGHT OEDEMA vs Dk/t EXTENDED WEAR
- 14. DYNAMIC OXYGEN SUPPLY • Post-lens tear film exchange rates: –SCL 1% per blink –RGP 15 - 20% per blink • Blinking –rate –quality (completeness) • RGP vs SCL fitting characteristics
- 15. GAS MOVEMENT THROUGH LENSES • Composition of polymer • Temperature effect • Partial pressure of gas at lens surface • Lens thickness • Boundary layer effect
- 16. GAS DIFFUSION Molecules migrate through ‘microvoids’ (intramolecular spaces) within the material’s matrix
- 17. CONCENTRATION GRADIENT O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 PERMEABLE MEMBRANE The Ball and Hill Analogy Concentration (height)
- 19. GAS SOLUBILITY • Absorption process of a gas within the material • Similar to a sponge which absorbs and holds water • Gas is dissolved, or solubilized, into the material
- 20. RGP PERMEABILITY • Siloxane bonds (Si-O-Si) provide molecular sized 'voids' • Size, rotation and flexibility of bonds affect permeability • Concentration gradient
- 21. RGP MATERIALS SILOXANE COMPONENT • Methyl groups small • Methyl groups non-polar, therefore more mobile • Distance between Si atoms is large • Large polymer-free space • Polymer backbone is highly elastic (extensible) Backbone Compressed Relaxed Extended O CH3 Si CH3 O O 130° Extensible backbone CH3 Si CH3 Si Methyl groups can rotate freely about Si atom
- 22. RGP PERMEABILITY Three step process: • Gas dissolves into the anterior lens surface • Diffusion through the lens proper • Gas leaves via the posterior lens surface and dissolves into the post-lens tear film
- 23. RGP PERMEABILITY • Permeability is a function of molecular motion and/or solubility • Permeability increased by: –improved surface characteristics –higher temperatures –decreased cross-linking –greater atmospheric pressure • Polymers involve compromises
- 24. RGP MATERIALS FLUORINE COMPONENT Acrylic (PMMA) Siloxane Acrylate Fluoro-Siloxane Acrylate DISSOLVED OXYGEN O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2 O2
- 25. RGP PERMEABILITY • Siloxane is much more permeable than flourine • Fluorinated polymers have superior surface characteristics • Fluorine increases solubility of oxygen in the material FLUORINATED MATERIALS
- 26. RGP CONTACT LENSES OXYGEN SUPPLY • Material permeability • Effect of deposits is minimal • Lens thickness/BVP • Tear exchange per blink • Tear layer thickness under lens
- 27. SCL PERMEABILITY • Gas flow through the water phase (not polymer) • Range of water contents • Water properties within the polymer –bound (non-freezing water) –free (freezing water)
- 28. WATER CONTENT WATER DIPOLE CH3 H2C = C C CH2 CH2 O H O O Water dipole O H H + + – – + O H H + + – O H H + + – Hydroxyl ‘tail’ HEMA monomer O H – + Water dipoles at the hydroxyl ‘tail’ Hydrogen bond O H H + + – Bound water Free water
- 29. WATER CONTENT POLYMER FACTORS + – – + H C H H C N H2C H2C C = O CH2 + N-vinyl 2 pyrrolidone Pyrrolidone ring Carbonyl group O H H + + – Water dipole Water dipole Polymer-polymer attraction O H CH2 CH2 CH2 CH2 O H + – – + HEMA HEMA
- 30. SCL PERMEABILITY • Only free water is available for O2 transport • Ratio of bound to free water is influential • New polymers (siloxane components) • Effect of tints –opaque –dyes
- 31. NOVEL SCL POLYMER • Biphasic block-copolymer • Siloxane-based polymeric phase • Coupled with a water phase • O2 permeability increases as H2O content decreases
- 32. SCL TRANSMISSIBILITY • Thickness considerations: –average vs centre –BVP • Influences physiological response
- 33. SCL TRANSMISSIBILITY • Increased Dk/t with: –higher water content (fixed thickness) –thinner lens (fixed water content) • Highest Dk/t with thin, mid-water lenses • Very heavy deposits can reduce Dk/t
- 34. CORNEAL SWELLING • HEMA material, 8 hours wear tc (mm) Swelling(%) 0.13 8 0.07 5 0.03 1 SCL DAILY WEAR La Hood, CCLRU Data
- 35. CORNEAL SWELLING • HWC (75%) material, 8 hrs wear tc (mm) Swelling(%) 0. 3 2 0.15 0.5 SCL DAILY WEAR La Hood, CCLRU Data
- 36. CORNEAL SWELLING Material (8 hrs wear) Swelling (%) Low Water 12 Mid Water 10 High Water 11 Novel Polymer 4 Siloxane Elastomer 2.5 SCL OVERNIGHT WEAR La Hood, CCLRU Data
- 37. CORNEAL SWELLING Material (8 hrs wear) Swelling (%) PMMA 6 Low Dk 3 - 4 Mod Dk 1 High Dk 0 RGP DAILY WEAR La Hood, CCLRU Data
- 38. CORNEAL SWELLING Material (8 hrs wear) Swelling (%) Low Dk 10-13 Mod Dk 7-9 High Dk 5-6 RGP OVERNIGHT WEAR La Hood, CCLRU Data
- 39. SILOXANE ELASTOMER • A dimethyl polysiloxane polymer • Inherently hydrophobic • Pure polymer very oxygen permeable • Permeability of the ‘filled’ polymer used is significantly lower
- 40. SILOXANE ELASTOMER • Less overnight swelling than no lens 2.0% vs 3.6% • Possible reasons: –decreased resistance to O2 flow from lid –lens-induced lagophthalmos –altered CO2 level –reduced tonicity change CORNEAL SWELLING
- 41. THANK YOU Table of Contents Feedback on errors, omissions, or suggestions for improvement are invited. Please contact us at: iacle@iacle.org See the following slides explaining the symbols, abbreviations, and acronyms used in the IACLE Contact Lens Course
- 42. SYMBOLS
- 43. ABBREVIATIONS
- 44. ACRONYMS
- 45. ACRONYMS