1161

Critical Oxygen Levels to Avoid Corneol Edema for
Daily and Extended Wear Contact Lenses
Drien A. Holden and George W. Merrz
The relationship between cornea! edema and hydrogel lens oxygen transmissibility was examined for
both daily and extended contact lens wear by measuring the cornea! swelling response induced by a
variety of contact lenses over a 36-hr wearing period. The relationships derived enable average edema
levels that occur with daily and extended wear in a population of normal young adults to be predicted
to within ±1.0%. The critical lens oxygen transmissibilities required to avoid edema, for the group as
a whole, for daily and extended contact lens wear were obtained from the derived curves. It was found
under daily wear conditions that lenses having an oxygen transmissibility of at least 24.1 ± 2.7 X 10~9
(cm X ml C>2)/(sec X ml X mmHg), an Equivalent Oxygen Percentage (EOP) of 9.9%, did not induce
corneal edema. This level of oxygen transmissibility can be achieved (1) in standard, low water content,
poly-HEMA lenses by using an average lens thickness of 33 n«\ or less, or (2) in a higher water content
material, such as Duragel 75, by using an average thickness of 166 nm or less. The critical hydrogel
lens oxygen transmissibility needed to limit overnight corneal edema to 4% (the level experienced
without a contact lens in place) was found to be 87.0 ± 3.3 X 10~9 (cm X ml O2)/(sec X ml X mmHg)—
an EOP of 17.9%. This ideal level of oxygen transmissibility cannot, at present, be provided with
hydrogel materials. We suggest a compromise criterion for acceptability of hydrogel lenses for extended
wear: zero residual swelling, ie, the level of oxygen transmissibility required to allow the cornea to
return to normal thickness soon after eye opening following sleep with lenses. To meet this criterion,
the oxygen transmissibility needs to be 34.3 ± 5.2 X 10~9 (cm X ml O2)/(sec X ml X mmHg)—an
EOP of 12.1%. The authors suggest this as the minimum desirable oxygen transmissibility for extended
wear as it limits the overnight swelling to approximately 8%, allowing the cornea to recover normal
thickness soon after eye opening. Invest Ophthalmol Vis Sci 25:1161-1167, 1984
on corneal thickness changes in ten subjects during 7
days continuous wear of three types of hydrogel contact
lenses of different back vertex powers. The two main
findings from this study were (1) that current extended
wear lenses cause 10-15% overnight corneal edema
and 2-6% swelling during the day and (2) that when
the lenses were left in place, the cornea showed an
ability to consistently deswell approximately 8% once
the eyes were opened following sleep.
Although the long-term effects of the levels of edema
occurring with current extended wear lenses are not
yet known, the development of contact lenses that do
not produce corneal swelling is intuitively desirable.
We, therefore, set out to determine (1) the relationship
between the amount of contact lens-induced edema
and lens oxygen transmissibility, so that the mean level
of edema could be predicted for any lens of known
oxygen transmissibility, and (2) the critical lens oxygen
transmissibility necessary to avoid corneal edema for
both daily and extended contact lens wear. These aims
were achieved by measuring the corneal edema response over a 36-hr period including a normal wake/
sleep cycle with a variety of lenses having a wide range
of oxygen transmissibilities.
When a hydrogel contact lens is worn, the maintenance of normal corneal function depends primarily
on sufficient oxygen diffusing through the lens material.1"3 The oxygen transmissibility of such a lens
(Dk/L) is directly proportional to the oxygen permeability (Dk) of the material polymer (a function of
water content in hydrogel materials4) and inversely
proportional to the thickness of the lens (L). While
the minimum oxygen level necessary to avoid corneal
swelling has been investigated,5"8 the critical lens oxygen transmissibilities to support both daily and extended wear have not yet been established.
Recently, Holden, Mertz, and McNally9 reported
From the Cornea and Contact Lens Research Unit, School of
Optometry, University of New South Wales, Kensington, Sydney,
N.S.W., Australia.
Supported by Optometric Vision Research Foundation of Australia
grant #6969 and National Health and Medical Research Council
grant no. 820472.
Submitted for publication: August 9, 1983.
Reprint requests: Brien A. Holden, PhD, Associate Professor Cornea and Contact Lens Research Unit, School of Optometry, University of New South Wales, P.O. Box 1, Kensington, Sydney, N.S.W.,
Australia 2033.
1161
Downloaded from iovs.arvojournals.org on 09/14/2020
1162
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Ocrober 1984
Table 1. Ocular parameters summary
Standard
Age (years)
Sph. refr. error (D)
Cyl. refr. error (D)
Horiz. K-reading (D)
Corneal toricity (D)
Mean
deviation
Range
20.4
-1.22
-0.35
42.67
0.61
1.7
1.83
0.25
1.42
0.41
18 to 24
+ 1.00 to -5.00
0 to -0.75
40.75 to 45.50
0.37 against rule
to 1.25 with
rule
Materials and Methods
Subjects
Ten subjects (7 men, 3 women) from whom informed consent had been obtained, participated in the
study. Ocular parameters for the group fell within normal limits and are summarized in Table 1. All subjects
were unadapted to contact lens wear and free of ocular
disease.
Lenses
Lens oxygen transmissibility: The specifications of
the lenses used in this and the previous study9 are
Vol. 25
provided in Table 2. Central and peripheral lens thickness measurements were made for all lenses to an accuracy of ±1 /*m with a device described elsewhere.10
These data were used to calculate average lens thickness
for the central 6 mm of each lens type in a manner
similar to that proposed by Fatt." Using published
polymer oxygen permeability data, 1213 and manufacturers specifications (for Hydrocurve II, Weicon 38
and Weicon 60), lens oxygen transmissibilities were
calculated for each lens. The data for the oxygen permeability (Dk), measured center thickness (Lc) and
calculated average thickness (Lavg), with corresponding
oxygen transmissibilities (Dk/L c and Dk/L avg respectively), are included in Table 2.
Equivalent oxygen percentage: The level of corneal
oxygenation for each lens was also calculated in terms
of the "Equivalent Oxygen Percentage" (EOP)—an
expression used by Hill14"18 to describe the oxygen
level at the anterior corneal surface expressed as a
percentage concentration of oxygen in the atmosphere.
For example, a contact lens that transmits all the oxygen available in the atmosphere would have an EOP
of 20.9%.
Table 2. Lens data
Manufacturer
Lens name
Water
content Diameter
Nominal
center
thickness
35 Mm
Oxygen Nominal
permeability back
vertex
(Dkft
power
(Xl0u)
221 M m
186 Mm
16.0
22.4
15.5
18.4
40
-3.00 D 103 ±
I/O M m
38.8
31.7
12
-1.25 D
63 ± 10 Mm
-9.00 D
41 ± 12 Mm
74 Mm
114 Mm
19.0
29.3
10.5
5 Mm
L\ M m
36.4
38.1
-3.00 D
-1.25 D
99 db 31 Mm
310 ± 18 Mm
111 Mm
310 Mm
8.1
2.6
7.2
2.6
129 ± 21 Mm
137 Mm
5 Mm
Cooper Labs,
Inc. (Palo
Alto, CA)
Permalens
71%
34.3
Duragel 75
75%
13.5 mm Approx.
200 Mm
(var. with
power)
13.8 mm
130 Mm
Soft Lenses Inc.
(San Diego,
CA)
Hydrocurve II
45%
13.5 mm
Hydron
Australia,
(Sydney,
Australia)
Hydron Thin
38.6%
Parallel*
Hydron Mini
38.6%
Hydron Thick •38.6%
Parallel*
120 Mm
300 Mm
4/im||
4 Mm
m
J M
-1.25 D 215 ± 11 Mm
-6.00 D 153 ± 22 M m
14.5 mm
13.0 mm
13.9 mm
32 ±
39 ±
Jo ±
22.2
13.3
10.3
38.6%
20 Mm
SD
Calculated Central Average
avg. lens Dk/Lc
Dk/Lavv
thickness
(X1OV) (X109)
25.0
20.5
21.1
Soflens 04
14.0 mm
mean
Oxygen
transmissibililyf
36 M m
60 Mm
78 Mm
B & L, Inc.
(Rochester,
NY)
60 fitn
Measured
center
thickness (LJ
8.0 X 10
8.0
-1.25 D
-6.00 D
-9.00 D
Piano
22 ±
7 Mm
Dow Corning
Silsoft
Corp.
Silicone
(Midland, MI) Elastomer
0%
12.5 mm
110 Mm
182§
-2.50 D
Titmus Eurocon Weicon 38
(Aschafenburg,
Germany)
Weicon 60
38.7%
13.0 mm
35 M m
7.3
-3.00 D
38 ±
59.0%
13.0 mm 5 5 - 1 0 0 Mm
16.5
-3.00 D
88 ± 12
Mm
16.2
133
133
5 1 Mm
19.2
14.3
98 M m
18.8
16.8
(var. with
power)
* Special order non marketed lenses.
t Values quoted at room temperature.12 Units: (cm2 X ml Oj)/(sec X ml
X mmHg).
$ Oxygen transmissibility. Units: (cm X ml O2)/(sec X ml X mmHg).
Downloaded from iovs.arvojournals.org on 09/14/2020
§ Oxygen permeability of silicone is reported to increase with lens thickness13'
the Dk reported here is representative of the average thickness of the lenses
in the study (137 jim).
II ±1 SD.
No. 10
CRITICAL OXYGEN TRANSMISSIDILITY OF CONTACT LENSES / Hoiden ond Merrz
It is assumed with this technique that a lens that
produces a corneal oxygen uptake rate identical to that
of a gas of known oxygen concentration (balance nitrogen), must be delivering the same level of oxygen
to the cornea. Although this may not be strictly true,
because the goggle situation does not incorporate the
other environmental changes induced by contact lenses
such as raised temperature and decreased tear tonicity,l9>2° the Hill method provides an extremely useful
relative index.
Figure 1 shows EOP as a function of Dk/L for some
of the lenses studied by Hill.15"18 The regression analysis
of this data shows that there is strong correlation between EOP and lens oxygen transmissibility, the empirical relationship derived from the data in Figure 1
is:
1160
20
10
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
g
DK/Lx10 (cmxml OjJ/secxml x mm Hg)
Fig. 1. Equivalent oxygen percentage (EOP) at the anterior corneal surface during contact lens wear versus lens oxygen transmissibility for various lens types studied by Hill during the period
1975-1980.15-18
EOP (%) = 6.195 X In (Dk/L X 109) - 9.778;
r = 0.995,
Ses, = 0.74%.
Procedures
Five of the subjects who had previously participated
in the Hoiden, Mertz, and McNally study9 wore a
silicone lens (Dow Corning Silsoft silicone elastomer)
on one eye and an extremely thin, low water content
hydrogel lens (Hydron special order, 20-^m thick, parallel front and back surface) on the opposite eye for
35 hr. After a month of no lens wear, the same five
subjects wore Titmus Eurocon Weicon 38 lenses on
one eye and Titmus Eurocon Weicon 60 lenses on the
opposite eye for 36 hr. After an additional month of
no lens wear, the same five subjects wore a
CooperVision Duragel 75 lens on one eye for 36 hr
and no lens on the opposite eye.
Corneal swelling response to the lenses was determined by measuring corneal thickness with an electronic digital pachometer as previously described.919'21
Baseline pachometry measurements were taken before
and immediately after lens insertion on the morning
of the first day of wear. Subsequent measurements
with lenses in place were taken after 1, 2, 4, 6, 8, and
12 hr of lens wear. Measurements were taken at the
moment of eye opening following 8 hr of overnight
eye closure (sleep) and at 1,5, and 12 hr after eye
opening. Corneal swelling was calculated as the percentage difference in corneal thickness compared with
the measurements taken immediately after lens insertion.
The corneal swelling responses of the other five subjects were recorded after 2,4, and 6 hr wear of medium
(0.11 mm) and thick (0.31 mm) Hydron lenses. It was
considered inadvisable to allow these subjects to sleep
with these lenses considering the magnitude of their
day 1 swelling responses. These data are included in
this paper, however, to give a greater range with which
Downloaded from iovs.arvojournals.org on 09/14/2020
to derive the relationship between the level of oxygen
transmissibility and maximum corneal swelling with
daily wear.
Published Data
Data from Hoiden, Mertz, and McNally9 that were
collected in an identical manner over the first 36 hr
of lens wear to that reported for the lenses studied in
this paper (Tables 2, 3), are included in the analysis
(Table 4) to provide information from a wider range
of lenses.
Results
The mean corneal swelling results for 36 hr wear of
Dow Corning Silsoft silicone elastomer and Hydron
20 /iin parallel surface lenses are shown in Figure 2.
Similarly, the degree of swelling during 36 hr wear of
Weicon 38 and Weicon 60 lenses is shown in Figure
3 and that for CooperVision Duragel 75 lenses and
the no lens condition in Figure 4. The mean swelling
results during 6 hr wear of thicker Hydron lenses are
shown in Figure 5.
Table 3 summarizes the corneal swelling results obtained in both this and the Hoiden, Mertz, and McNally
study9 for the maximum swelling measured on day 1,
the overnight swelling measured at eye opening, and
the day 2 residual swelling measured 12 hr after eye
opening. These data were submitted to a regression
analysis to derive empirical relationships for corneal
swelling as a function of lnDk/L c and lnDk/L avg . The
results of this analysis are summarized in Table 4. The
highest correlations were achieved using the lens average oxygen transmissibility. Plots of day 1 maximum
swelling, overnight swelling, and day 2 residual swelling
as a function of Dk/L avg are shown in Figures 6-8,
respectively.
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / October 1984
1164
Vol. 25
Table 3. Mean corneal swelling results
Lens type
Dk/L*
(xl0rs)
Power
(D)
Day 1
maximum swelling
First
overnight swelling
Day 2 residual swelling
(36 hr)
Soflens 04f
-1.25
-6.00
-9.00
22.2
13.3
10.3
-0.1%
2.4%
3.6%
10.3%
12.0%
14.9%
0.5%
2.4%
5.8%
Permalensf
-1.25
-6.00
15.5
18.4
2.1%
1.6%
11.5%
9.8%
3.0%
3.0%
Hydrocurve Ilf
-1.25
-9.00
16.2
10.5
2.1%
2.4%
12.1%
12.6%
5.1%
5.5%
Weicon 38
Weicon 60
-3.00
-3.00
14.3
16.8
0.6%
1.6%
11.8%
11.3%
2.1%
2.6%
Duragel 75
-3.00
31.7
0.5%
9.5%
1.6%
Hydron 20 ^m
Hydron 120 pm
Hydron 300 urn
piano
-3.00
-1.25
38.1
7.2
2.6
0.6%
4.8%
8.5%
7.2%
-0.8%
Silsoft
-3.00
1.5%
2.6%
-0.5%
0.7%
4.1%
No lenst
133
—
* Units: (cm X ml O2)/(sec X ml X mmHg).
0.7%
•\ See reference 9.
Discussion
The first objective of this study was to examine the
relationship between lens oxygen transmissibility and
the amount of corneal edema occurring during both
daily and extended wear, so that it would be possible
to predict the amount of corneal edema induced by
any hydrogel lens of known oxygen transmissibility.
The second objective was to derive the critical oxygen
levels that must be transmitted by a contact lens to
avoid corneal edema under daily and extended wear
conditions. These objectives are important both (1) in
evaluating the efficacy of current contact lens designs
and materials and (2) in developing guidelines for the
development of contact lenses that provide less physiologic challenge to the eye. The main deficiencies of
previous attempts to define the oxygen transmissibilities needed to avoid edema in daily and extended
contact lens wear22"26 are that either: (1) center thickness was used to define Dk/L and/or (2) the studies
were not carried out during a normal wake/sleep cycle.
Average versus central oxygen transmissibility: As
Wilson27 has pointed out, using center thickness (L)
of the lens to define oxygen transmissibility falsely
implies that all lenses of a given polymer with the same
center thickness will provide the same amount of oxygen. Indeed, we have shown9 that higher minus-powered, hydrogel lenses cause greater central corneal
swelling than lower minus-powered lenses of the same
polymer and central thickness (due to the greater lens
peripheral thickness). It also is clear from the present
study that oxygen transmissibility based on the average
thickness of the lenses studied is a better predictor of
corneal swelling for day 1 maximum swelling, overnight swelling, and day 2 residual swelling. As can be
seen from Table 4, in all cases the standard error of
the estimate (Sest) for predicting corneal swelling using
lens average thickness is approximately one-half that
of the standard error of the estimate using lens central
thickness.
Normal wake/sleep cycle: Several investigators have
studied corneal swelling caused by various types of
Table 4. Regression analysis—corneal swelling vs lens oxygen transmissibility
Central oxygen transmissibility
Swelling
response
Correlation
coefficient
Day 1
maximum
First
overnight
Day 2
residual
Standard
error of the
estimate
Average oxygen
transmissibility
(SeJ
Regression equations relating corneal
swelling (CS) and average oxygen
transmissibility (Dk/LavJ
-0.96
±0.6%
CS(%) = 12.1 - 3.8 In (Dk/Lavg X 109)
±1.7%
-0.92
±0.8%
CS(%) = 23.9 -4.44 In (Dk/Lavg X 109)
±2.0%
-0.82
±1.2%
CS(%) = 14.5 - 4.10 In (Dk/Lavg X 109)
(SeJ
Correlation
coefficient
-0.89
±1.1%
-0.57
-0.41
Downloaded from iovs.arvojournals.org on 09/14/2020
Standard
error of the
estimate
No. 10
1165
CRITICAL OXYGEN TRANSMISSIDILITY OF CONTACT LENSES / Holden and Merrz
12
»
•
3510
« Silsoft
• Hydron
Parallel
12
»
a? 10
« Control (no lens)
Duragel 75
o
o
Z 8
2 8
Seep
LU
I 2
I 2
O
u
O
U
0
0
-2
- 2
12
•
12
16
20
24
32
28
TIME (HOURS)
Fig. 2. Corneal swelling versus time for five unadapted subjects
wearing a -2.50 D Dow Corning Silsoft silicone elastomer contact
lens on one eye and a special 20-^m thick parallel surface Hydron
lens on the opposite eye continuously for a period of 36 hr.
contact lenses worn during short periods of eye closure
that were preceded by periods of at least 24 hr during
which no lenses were worn.23"26 In both the Holden,
Mertz, and McNally paper 9 and this study, a period
of normal sleep was preceded by a full day's open eye
wear as would be encountered in the normal extended
wear situation.
Predicting Corneal Edema from Lens Oxygen
Transmissibility
The results of the regression analysis on the data
from this study, reported in Table 4 and depicted Figures 6-8, enable the mean level of edema that will
occur in daily and extended wear in young adults to
be predicted from lens average transmissibility to approximately ±1%.
The Derivation of Critical Oxygen Transmissibility
In the model we present here for determining the
critical oxygen requirements for daily and extended
12
»
20
24
28
32
36
Fig. 4. Corneal swelling versus time for five unadapted subjects
wearing a -3.00 D Duragel 75 contact lens continuously for a period
of 36 hr on one eye and no lens on the opposite eye.
wear contact lenses, we assume that corneal swelling
response ranges from some finite maximum level for
lenses with zero oxygen transmissibility, to a base level
beyond which an increase in oxygen transmissibility
does not reduce the swelling response. It has been
shown experimentally, for example, that overnight
swelling ranges from approximately 25% for presurgical
cataract patients wearing very thick aphakic hydrogel
lenses (Dk/L avg = 3 X 10"9 (cm X ml 02)/[sec X ml
X mmHg]) 28 to about 4% for corneas not wearing any
lenses (infinite Dk/Lavg).9i29>3° The regression curves
shown on Figures 6-8 and detailed in Table 4 were
used to derive the critical oxygen transmissions necessary to avoid contact lens-induced edema under both
daily (day 1 maximum swelling) and extended wear
conditions (overnight and day 2 residual swelling). Table 5 summarizes the critical lens Dk/L avg values necessary for no edema on day 1, overnight corneal swelling to be limited to 4%—the level experienced when
no lens is worn—, and the day 2 residual corneal swelling to fall to zero soon after eye opening (zero residual
12
« Weicon 38
i Weicon 6 0
t
« 0.1 1mm Thick Hydron
1
• 0.31mm Thick Hydron
10
gio
o
z
16
TIME (HOURS)
36
8
£
8
o
z
n
6
• Lens wear discontinued!
-2 .
12
16
20
24
28
32
36
TIME (HOURS)
Fig. 3. Corneal swelling versus time for five unadapted subjects
wearing a -3.00 D Weicon 38 contact lens on one eye and -3.00
D Weicon 60 contact lens on the opposite eye continuously for a
period of 36 hr.
Downloaded from iovs.arvojournals.org on 09/14/2020
12
16
28
32
36
TIME (HOURS)
Fig. 5. Corneal swelling versus time for five unadapted subjects
wearing -3.00 D Hydron "Mini" contact lens on one eye and a
special 3l0-/tm parallel surface Hydron contact lens on the other
eye for a period of 6 hr.
1166
INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Ocrober 1984
DAY
DK/L»vofor
DAY 2 RESIDUAL
1 MAXIMUM
10r
(cmxml OgJ/sec x ml x mm Htj)
swelling). These critical Dk/L avg values were converted
to Hill's EOP values using the empirical relationship
described previously. Also recorded in Table 5 are the
critical average lens thicknesses for poly-HEMA and
Duragel 75 materials that would be necessary to meet
the three criteria.
The Critical Oxygen Transmissibility Needed to
Avoid Edema with Daily Wear
Critical lens oxygen transmissibilities for open and
closed eye wear, based on previous estimates of the
oxygen levels needed to avoid edema,5 have been suggested as 5 X 10~9 (cm X ml O2)/(sec X ml X mmHg)
for open eye lens wear and 15 X 10~9 (cm X ml O 2 )/
(sec X ml X mmHg) for closed eye lens wear.26 As can
be seen from Tables 2 and 3, a number of lenses meeting these criteria caused significant corneal swelling
both during day and overnight wear. Our results indicate that the oxygen level needed to avoid edema is
somewhat higher than previously proposed, and are
closer to the levels determined by Holden, Sweeney,
and Sanderson.8
CRITICAL OK/L»,o for ZERO SWELLING. 87*10
E.O.P. lor ZERO SWELLING: 17.9%
10
,30
OK/L.vo XIO (cmxml O 2 )/s
Fig. 7. Overnight corneal swelling versus average lens oxygen
transmissibility for .12 lenses (see Table 3). Critical Dk/Lavg and EOP
values for reducing overnight swelling to the 4% "no lens" level also
are shown.
Downloaded from iovs.arvojournals.org on 09/14/2020
-9
E.O.P. for ZERO SWELLING: 12.1%
r=-0.82
10
Fig. 6. Day I maximum corneal swelling versus average lens oxygen
transmissibility for 13 lenses (see Table 3). Critical Dk/L avg and EOP
values for reducing day 1 swelling to zero are also shown.
CRITICAL
DK/LAVQ for ZERO SWELLING: 34.3x10
ZERO SWELLING:24.1 x i o "
E.O.P. for ZERO SWELLING: 9.9%
DK/L»VG x'O
Vol. 25
15
20
25
30
35
40
45
50
DK/LAVG X 1 0 (cmxml O2)/secxml x mm Hg)
Fig. 8. Day 2 residual corneal swelling versus average lens oxygen
transmissibility for 11 lens types (see Table 3). Critical Dk/Lovg and
EOP values for reducing day 2 residual swelling to zero also are
shown.
The results of this study indicate that, on average,
no corneal swelling will occur under daily wear conditions if lenses have a Dk/L avg of at least 24.1 ± 2.7
X 10~9 (cm X ml O2)/(sec X ml X mmHg)—an EOP
of 9.9%. This result is in close agreement with the
Sarver et al25 figure of 20 X 10"9 (cm X ml O2)/(sec
X ml X mmHg) for the minimum Dk/L value for zero
edema with opened-eye lens wear. With standard, low
water content lenses (poly-HEMA), this can be
achieved by using an average lens thickness of 33 /urn
or less. With a higher water content material, such as
Duragel 75, an average thickness of 166 ixm or less is
required.
The Critical Oxygen Transmissibility Needed to
Avoid Edema with Extended Wear
In considering the critical oxygen requirements for
extended wear lenses, the alternatives we have suggested
are: (1) the lens oxygen transmissibility needed to limit
overnight edema to 4% (the level occurring without a
lens) and (2) the lens oxygen transmissibility needed
to allow the daytime level of edema to fall to zero soon
after eye opening (zero residual swelling).
The Dk/L avg which limits overnight swelling to 4%
is predicted to be 87.0 ± 3.3 X 10"9 (cm X ml O 2 )/
(sec X ml X mmHg)—an EOP of 17.9%. To satisfy
this criterion with hydrogel lenses, the average lens
thickness would have to be reduced well below manufacturing feasibility (See Table 5). The results of this
study suggest that this ideal 4% overnight swelling criteria can only be met at present with silicone lenses.
However, the persistent problem of the adherence of
these lenses to the cornea31 questions their suitability
for extended wear at the present time.
The zero residual swelling criterion for extended
wear allows overnight swelling levels of greater than
4%, as long as the cornea can eliminate the edema
1167
CRITICAL OXYGEN TRANSMISSIDIUTY OF CONTACT LENSES / Holden ond Merrz
No. 10
Table 5. Critical oxygen levels and lens thicknesses for low and high water content lenses
Critical average lens thickness
Critical oxygen levels
Dk/La
Criteria
Zero day one corneal swelling
4% or less overnight corneal swelling
Zero day two residual swelling
24.1 ± 2.7 X 1(T9
87.0 ± 3.3 X 1(T9
34.3 ± 5.2 X 1(T9
* Oxygen transmissibility units: (cm X ml O2)/(sec X ml X mmHg).
t Equivalent Oxygen Percentage: % atmospheric oxygen.
soon after eye opening. From the relationship derived
between lens Dk/L avg and residual swelling shown in
Figure 8 and Table 4, the critical lens Dk/L avg to meet
this zero residual swelling criterion is 34.3 ± 5.2 X 10~9
(cm X ml O2)/(sec X ml X mmHg)—an EOP of 12.1%.
The lens thickness necessary to achieve this appears
feasible from a manufacturing point of view, at least
in higher water content hydrogel materials. We would,
therefore, suggest that this oxygen transmissibility
should be considered the desirable minimum value
for extended wear contact lenses.
Key words: contact lens, oxygen transmissibility, corneal
edema, daily wear, extended wear
Acknowledgments
The authors thank Drs. Nathan Efron and Daniel O'Leary
and Bruce Cook for their helpful comments as well as Stuart
Ingham for assistance with the manuscript preparation.
References
1. Fatt I and St. Helen R: Oxygen tension under an oxygen-permeable contact lens. Am J Optom Arch Am Acad Optom 48:545,
1971.
2. Fatt I and Lin D: Oxygen tension under a soft or hard, gaspermeable contact lens in the presence of tear pumping. Am J
Optom Physiol Opt 53:104, 1976.
3. Poise KA: Tear flow under hydrogel contact lenses. Invest
Ophthalmol Vis Sci 18:409, 1979.
4. Refojo MF and Leong F: Water-dissolved-oxygen permeability
coefficients of hydrogel contact lenses and boundary layer effects.
J Memb Sci 4:415, 1979.
5. Poise KA and Mandell RB: Critical oxygen tension at the corneal
surface. Arch Ophthalmol 84:505, 1970.
6. Carney LG: Studies on the basis of ocular changes during contact
lens wear. PhD thesis, University of Melbourne, 1974.
7. Mandell RB and Farrell R: Corneal swelling at low atmospheric
oxygen pressures. Invest Ophthalmol Vis Sci 19:697, 1980.
8. Holden BA, Sweeney DF, and Sanderson G: The minimum
pre-corneal oxygen tension to avoid corneal edema. Invest
Ophthalmol Vis Sci, 25:476, 1984.
9. Holden BA, Mertz GW, and McNally JJ: Corneal swelling response to contact lenses worn under extended wear conditions.
Invest Ophthalmol Vis Sci 24:218, 1983.
10. Chan T, Payor S, and Holden BA: Corneal thickness profiles
in rabbits using an ultrasonic pachometer. Invest Ophthalmol
Vis Sci 24:1408, 1983.
Downloaded from iovs.arvojournals.org on 09/14/2020
EOPj
9.9%
17.9%
12.1%
Poly-Hema
(Dk = 8X 10-'
33 urn
9 Mm
23 nm
Duragel 75
(Dk = 40 X 10'")%
166
46
117
$ Oxygen permeability units: (cm2 X ml O2)/(sec X ml X mmHg).
11. Fatt I: The definition of thickness for a lens. Am J Optom
Physiol Opt 56:324, 1979.
12. Morris JA and Fatt I: A survey of gas-permeable contact lenses.
The Optician 174(4509):27, 1977.
13. Fitzgerald JK and Jones DP: Oxygen flux can be misleading.
Int Contact Lens Clin 5:134, 1978.
14. Hill RM and Jeppe WH: Hydrogels: is a pump still necessary?
Int Contact Lens Clin 2(4):27, 1975.
15. Hill RM: Hydrogel lens design: the thick and thin of it. In
Proceedings of the Second National Research Symposium on
Soft Contact Lenses, Princeton, Excerpta Medica, 1977, pp. 5 1 56.
16. Benjamin WJ and Hill RM: Ultra-thins: an oxygen update. Contact Lens Forum 4(1):43, 1979.
17. Hill RM: Oxygen permeable contact lenses: how convinced is
the cornea? Int Contact Lens Clin 4(2):34, 1977.
18. Roscoe WR and Hill RM: Corneal oxygen demands: a comparison of the open- and closed-eye environments. Am J Optom
Physiol Opt 57:67, 1980.
19. Holden BA, Poise KA, Fonn D, and Mertz GW: Effects of
cataract surgery on corneal function. Invest Ophthalmol Vis Sci
22:343, 1982.
20. Terry JE and Hill RM: Human tear osmotic pressure: Diurnal
variations and the closed eye. Arch Ophthalmol 96:120, 1978.
21. Holden BA and Payor S: Changes in thickness in the corneal
layers. Am J Optom Physiol Opt 56:821, 1979.
22. Mandell RB and Poise KA: Contact lenses worn during sleep
and rest periods. J Am Optom Assoc 41:937, 1970.
23. Harris MG, Sanders TL, and Zisman F: Napping while wearing
hydrogel contact lenses. Int Contact Lens Clin 2(1 ):84, 1975.
24. Sarver MD and Staroba JE: Corneal edema with contact lenses
under closed-eye conditions. Am J Optom Physiol Opt 55:739,
1978.
25. Sarver MD, Baggett DA, Harris MG, and Louie K: Corneal
edema with hydrogel lenses and eye closure: effect of oxygen
transmissibility. Am J Optom Physiol Opt 58:386, 1981.
26. Poise KA and Decker M: Oxygen tension under a contact lens.
Invest Ophthalmol Vis Sci 18:188, 1979.
27. Wilson G: Hydrogel lens power and oxygen transmissibility. Am
J Optom Physiol Opt 56:430, 1979.
28. Korb DR, Richmond PP, and Herman JP: Physiological response
of the cornea to hvdrogel lenses before and after cataract extraction. J Am Optom Assoc 51:267, 1980.
29. Mandell RB and Fatt I: Thinning of the human cornea on
awakening. Nature 208:292, 1965.
30. Mertz GW: Overnight swelling of the living human cornea. J
Am Optom Assoc 51:211, 1980.
31. Refojo MF and Leong FL: Water pervaporation and adhesion
of silicone rubber contact lenses. ARVO Abstracts. Invest
Ophthalmol Vis Sci 20(Suppl):155, 1981.