CONTACT LENS

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)

18. PERMEABILITY
membrane
membrane
membrane
Permeable
Semi-permeable
Impermeable

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