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CORNEAL OXYGENATION WITH CONTACT LENSES
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COPYRIGHT NOTICE
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
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: [email protected]
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
CONTRIBUTORS
Corneal Oxygenation With Contact Lenses:
Robert Terry, BOptom, MSc
Lewis Williams, AQIT(Optom), MOptom, PhD
TEMPERATURE EFFECTS OF CONTACT LENSES
Open eye:• With SCLs:
–anterior surface 0.5 °C cooler• With GP lenses (lower conductivity):
–anterior surface >0.5 °C coolerClosed eye (cornea warms 3 °C):• No effect (GPs and SCLs)• No differences between surfaces
TEAR FILM EFFECTS OF CONTACT LENSES
• Evaporation rates: SCLs GPs
• CLs reduce BUT
• BUT:
–GPs - 4 to 6 s
–SCLs - 4 to 10 s (H2O content Dk/t)
OXYGEN PERMEABILITY
• Intrinsic material property (resistance to gas flow)
• Permeability P = Dk
where:
D is the diffusion coefficient
k is the solubility coefficient
of oxygen in a given material
OXYGEN PERMEABILITY: D
CLMaterial
O2 INPUT
OUTPUT
OXYGEN PERMEABILITY: k
CLMaterial
O2 k = solubility coefficient
(solubility of oxygen in CL material)
Highlysoluble
Poorlysoluble
Somewhatsoluble
WHAT EVERYBODY WANTS/NEEDS!
Material’s O2
solubility:Extremelyhigh
O2
O2
CLMaterial
O2
O2 passagethroughmaterial:Rapid, direct, &unimpeded O2
OXYGEN PERMEABILITY
• Independent of material thickness
• Dependent on temperature
• Calculated value
PERMEABILITY (Dk)UNIT DERIVATION
Dk = mmHgxmLxs
mLxcmLens
O22
k = mmHgxmL
mLLens
O2
D = scm2
&
Dk = mmHgxmL
mLX
scm
Lens
O22
Dk UNIT
Lens material
1 cm
1 cm
1 cm
after Refojo et al., 1984
1 sec
1 mm H
g
pressure
1 cm
1 cm
1 cm O2
1 cm
1 cm
1 cm O2 Hypothetical
contact lens
(very much largerthan a
real contact lens)
Conditions: STP (0 °C,760 mm Hg) unless stated to be
otherwise, e.g. 21 or 34 °C @ 760 mm Hg
OXYGEN PERMEABILITY
• Contact lens range: 0 - 300 x 10-11
• Units:
–(cm2 x mLO2) / (s x mLLens x mm Hg)
or
–(cm2/s) x (mLO2 / [mLLens x mm Hg])
–commonly, the Lens subscript is omitted
OXYGEN TRANSMISSIBILITY
• Based on material permeability (Dk)
• Related to material thickness (t)
–transmissibility = Dk/t
• Relevant clinically
TRANSMISSIBILITY (Dk/t )UNIT DERIVATION
Dk = mmHgxmLxs
mLxcmLens
O22
Dk/t = cmxmmHgxmLxs
mLxcmLens
O22
Dk/t = mmHgxmLxs
mLxcmLens
O2
OXYGEN TRANSMISSIBILITY
• Contact lens range: 0 - 200 x 10-9
• Units:
–(cm x mLO2) / (s x mLLens x mm Hg)
or
–(cm/s) x (mLO2 / [mLLens x mm Hg])
–commonly, the Lens subscript is omitted
MEASURING OXYGENTRANSMISSIBILITY
In vitro
• Polarographic cell
• Gas-to-gas (volumetric)
• Coulometric
TECHNIQUES
MEASURING OXYGEN TRANSMISSIBILITY
• Typically measured for:
–lens BVP of – 3.00 D
–temperature of 35 °C
• Physical test
(can be controlled and repeated)
MEASURING OXYGEN TRANSMISSIBILITY
• Sensor in measuring cell contains:–anode (+)–cathode (–)–electrolyte
• Contact lens becomes the ‘membrane’
• Controlled humidity and temperature
POLAROGRAPHIC CELL TECHNIQUE
'O' Ring
Oxygen flow
Rigid contact lens sample
Tip of oxygen sensor
Oxygen sensor cathode (–)
'O' Ring
Oxygen-free area
Saline-saturated filter paper
155 mmHgO2
POLAROGRAPHIC CELL
+
POLAROGRAPHIC CELL TECHNIQUE
• Oxygen passes through the lens into the sensor’s
electrolyte
• Sensor current is proportional to the amount of oxygen
available at the cathode
• Oxygen flux j from Fick’s & Henry’s laws:
j = Dk/t x (pO2)
• Permeability calculated from:
–thickness of the lens (t)
–current required to reduce O2 (i)
–partial pressure of O2 (pO2)
–cell constant (C)C x t x i
pO2
Dk =
POLAROGRAPHIC CELL TECHNIQUE
• Utilizes finished contact lens• Potential errors include:
–boundary layers–edge effects–lens thickness–environment–cell integrity–calibration
POLAROGRAPHIC CELL TECHNIQUE
Paths of Oxygen Flow
Anterior Surface
Aqueous Layer ‘Bridge’
Hydrogel Contact
Lens Thickness
Posterior Surface
PolarographicSensor
Cathode (–)
Saline Film
Sensor Aperture
‘Effective’ Sensor
Aperture
EDGE EFFECT
POLAROGRAPHIC CELL TECHNIQUE
• Not suited to highly permeable, non
hydrogel materials
• Over-estimates values for GP lenses
• Variability among investigators
DISADVANTAGES
MEASURING OXGEN TRANSMISSIBILITY
• Two environmental chambers
–pure oxygen
–differential pressures
• Constant temperature (35oC)
• Pressure sensor in each chamber
GAS-TO-GAS
GAS-TO-GAS TECHNIQUE
Anterior Environment
Chamber
Oxygen Flow
Rigid Contact Lens Sample
Posterior Environmental
Chamber
3 ATM Pure
O2
'O' Rings
Gas Pressure Transducer
Gas Pressure Transducer
MEASURING OXYGEN TRANSMISSIBILITY
• Constant pressure in anterior chamber
• Gas flow through the lens alters the
pressure in the posterior chamber
• No boundary layer or edge effects
• Can be used for any gas
GAS-TO-GAS TECHNIQUE
GAS-TO-GAS TECHNIQUE
• Not suitable for hydrogels
–pressure differential too great
–hydrogels too elastic
–hydrogels have low burst strength
DISADVANTAGES
MEASURING OXYGEN TRANSMISSIBILITY
• Two 'environmental chambers'–oxygen–inert gas
• Coulometric sensor • Oxygen flow through lens is measured• Water saturated oxygen or liquid
reservoir required for hydrogels
COULOMETRIC TECHNIQUE
COULOMETRIC TECHNIQUE
Anterior Environment
Chamber
Inflow of Oxygenated Gas
Outflow
Oxygen Flow
Rigid Contact Lens Sample
Posterior Environmental
Chamber
159mm Hg
O2
'O' Rings
Outflow to coulometric oxygen sensor
Inflow of Oxygen-Free Gas
COULOMETRIC OXYGEN SENSORS CHEMISTRY
4 e– + O2 + 2H2O 4OH– Cathode (– ve, Carbon [graphite]):
2 Cd + 4OH– 2Cd(OH)2 + 4e–
Anode (+ ve, Cadmium [nickel-cadmium]):
2Cd + O2 + 2H2O + 4OH– 2Cd(OH)2 + 4OH–
Overview:e = electron
COULOMETRIC TECHNIQUE
• Posterior lens surface is exposed
• Little or no boundary layer effects
• No edge effects
• More accurate than polarographic techniques
with GP contact lenses
ADVANTAGES
COULOMETRIC TECHNIQUE
Requires:
• Specific gas (e.g. oxygen) sensor
• Anterior aqueous reservoir for hydrogels
DISADVANTAGES
TRANSMISSIBILITY CLASSIFICATION
Low < 12
Mod 12 - 25
High > 25
SOFT LENSES
Low < 25
Mod 25 - 50
High > 50
GP LENSES
TRANSMISSIBILITY CLASSIFICATION
THANK YOU
Table of Contents
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See the following slides explaining the symbols, abbreviations, and acronyms used in the IACLE Contact Lens
Course
SYMBOLS
ABBREVIATIONS
ACRONYMS
ACRONYMS
