OXYGEN AND LENS WEAR
The Role of Oxygen in Successful Lens Wear
Adequate corneal oxygen is just one of many
factors that contribute to contact lens success.
By Derrick L. Artis, OD, FAAO
Goodlaw
(1946) was one of the first individuals to hypothesize that contact lenses act as
a barrier to the eyes' anterior oxygen supply. Inadequate oxygen, or hypoxia, can
result in the development of a number of contact lens-related complications. This
realization has stimulated a fervent effort to develop contact lens materials capable
of minimizing the adverse effects induced by corneal hypoxia.
Only recently, with the introduction of silicone
hydrogel lenses, has the contact lens industry succeeded in producing materials
able to provide enough corneal oxygen to virtually eliminate hypoxia-related complications,
while at the same time providing a surface that is hydrophilic enough to permit
comfortable wear.
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Figure 1. Relationship of EOP to Dk/t across
a range of oxygen values. Courtesy of William J. Benjamin, OD, PhD.
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Oxygen Isn't Everything
While the introduction of silicone hydrogels has
allowed for the provision of an oxygen-rich environment, it hasn't eliminated complications
or guaranteed successful lens wear. Reports have surfaced of microbial keratitis,
contact lens-related papillary conjunctivitis (CLPC), superior epithelial arcuate
lesions (SEALs) and infiltrative keratitis occurring in patients wearing silicone
hydrogel lenses. Some of these conditions, such as SEALs and CLPC, may result from
mechanical properties of the lens such as the increased modulus of elasticity, or
stiffness, inherent to many silicone hydrogel materials. Even in the absence of
ocular complications, patients may still fail to succeed with lens wear secondary
to discomfort, dryness, poor vision or handling.
With the recent increased focus on the benefits
of delivering increasingly higher amounts of oxygen to the corneal surface, it's
important to not overlook the fact that at some point there's a diminishing return
of this benefit. In fact, with the trade off of higher oxygen resulting in a higher
modulus, non-oxygen related complications might occur more frequently with a higher-modulus
lens than they would in a conventional hydrogel. A perfect hydrogel lens would allow
for adequate oxygen to minimize hypoxic complications, yet it would retain beneficial
conventional hydrogel characteristics such as low modulus to help maximize successful
lens wear.
Quantifying Oxygen Delivery
TABLE 1
Oxygen
Transmissibility Ranking and Equivalent EOP Values* |
|
RANKING |
DK/T
RANGE |
HUMAN EOP RANGE |
|
Low Dk/t |
<12 |
<6% |
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Medium Dk/t |
12-25 |
6-11% |
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High Dk/t |
26-50 |
11-15% |
|
Super Dk/t |
51-80 |
15-18% |
|
Hyper Dk/t |
>80 |
>18% |
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* From Benjamin WJ and Karkkainen TR. Hydrogel hypoxia: where
we've been, where we're going. Contact Lens Spectrum. 1996;11(Suppl):s6-11. |
Dk Understanding how much oxygen
a lens delivers to the cornea, and more importantly how much oxygen the cornea uses,
can be confusing because of the various terminology used. Clinicians are most familiar
with the manufacturers' listed permeability, or Dk values, but these values can
be misleading in a clinical sense because by definition they are a property of the
material and independent of the lens thickness. Therefore, when using Dk values
you can compare only lenses of identical thickness, which is difficult because of
the varying thickness of different designs and powers.
Dk/t
A more useful value for clinicians is the Dk/t, or transmissibility, which takes
into account the lens thickness. The transmissibility value defines the amount of
oxygen that is transmitted through a contact lens in-vitro. Even though this value
is more clinically relevant than the Dk value is, it's still problematic. In many
cases the only published lens thickness is the center thickness, which allows for
information on how much oxygen is delivered only to a small central region of the
cornea and ignores the oxygen performance of the lens over its entire diameter.
EOP Both of the
above mentioned terms are derived from in-vitro techniques and provide little information
on how the cornea actually uses the oxygen provided. To get a better understanding
of in-vivo oxygen usage, we can refer to the equivalent oxygen percentage (EOP)
value for the lens. EOP values represent an indirect estimate of the oxygen concentration
beneath a contact lens.
The first step in determining EOP is
measuring the corneal oxygen uptake rate by placing a membrane-covered Clarke-type
sensor against the corneal surface and determining the rate at which the cornea
consumes oxygen passing through the sensor's membrane. Contact lenses that transmit
little oxygen, such as PMMA lenses, would have very rapid oxygen uptake rates, while
a cornea not wearing a lens would deplete the oxygen in the sensor at a much slower
rate. Researchers can then compare the uptake rates for a particular contact lens
to uptake values obtained with known oxygen concentrations to derive the EOP. Thus
a lens that gives an EOP value of 10 percent would provide the cornea with approximately
half of the available atmospheric oxygen (20.9 percent).
In-vivo EOP values and in-vitro Dk/t
values are related; however the relationship isn't linear throughout the entire
range of oxygen values. Figure 1 presents the relationship between EOP and Dk/t.
Table 1 contains rankings of oxygen transmissibility and their equivalent EOP values,
from very low to hyper transmissible. At first glance it may not make sense why
the Dk/t vs. EOP graph should asymptote, or flatten out, at the higher ranges of
oxygen transmission. After all, if you increase the transmission of oxygen through
the lens, wouldn't you expect a greater amount of oxygen to be available underneath
the lens?
Upon further consideration, the "fall
off" in EOP values with increasing Dk/t does indeed make sense as the EOP values
are derived from corneal oxygen consumption, and as the supply of oxygen closes
in on meeting corneal demand, the uptake will slow and eventually reach a steady
state. The cornea behaves like other biological systems in that there are rate-limiting
steps.
Oxygen Flux Another
term used in quantifying oxygen delivery, oxygen flux also helps to explain the
asymptotic portion of Dk/t vs. EOP graph. Oxygen flux is the volume of oxygen reaching
an area of the corneal surface over a given period of time and is represented by
Fick's Law, which states:
j=(P1–P0)
x Dk/t
where j is the oxygen flux, P1
is the partial pressure of oxygen at the front surface of the lens, P0
is the partial pressure of oxygen at the back surface and Dk/t is the transmissibility.
Oxygen flux relies not only on the transmissibility of oxygen through the lens,
but also on the pressure at the front surface and back surface of the lens as it
rests on the eye. As more oxygen transmits through the lens, there's a resultant
rise in P0 and a subsequent decline in the oxygen flux through the lens.
The oxygen flux vs. Dk/t graph also becomes asymptotic beyond a certain range of
transmissibility in a similar fashion to the EOP vs. Dk/t graph.
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Figure 2. Relationship of oxygen flux and oxygen
permeability for a range of lens thicknesses in open and closed eye states. Reprinted
with permission from Optician.
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Oxygen: How Much is Enough?
Defining critical oxygen tensions depends upon
what physiological indicator you use as your endpoint. Using corneal swelling as
the endpoint, Holden and Mertz determined that the minimum transmissibility values
required to prevent corneal edema were 24 and 87 Dk/t units for the open and closed
eye conditions, respectively. If loss of corneal sensitivity is the physiological
indicator, an epithelial partial pressure of oxygen of at least 55 mmHg is likely
required from a daily wear contact lens to avoid a significant losses of sensitivity.
Table 2 lists critical lens transmissibility values from a number of studies using
different physiological endpoints.
Upon inspection of Table 2, it may seem surprising
to see such a large range of Dk/t values given as critical values to ensure ocular
physiology homeostasis. However this difference is mainly a result of using Dk/t
terms to define these critical values. A recent article (Morgan and Brennan, 2004)
shows that the large difference in critical Dk values published for the various
studies is in reality very small when using oxygen flux values instead of Dk. These
different studies then are in general agreement as to the minimum oxygen requirements
in terms of flux values.
So how much oxygen is enough? The graph
in Figure 2 from Morgan and Brennan shows that for both the open and closed eye
state there's a rapid increase in oxygen flux with increasing Dk. However, as Dk
continues to increase, a gradual leveling off of oxygen flux occurs with just about
all of the conditions becoming asymptotic at a Dk of approximately 30 for the open
eye and 80 for closed. These values are in close agreement with Brennan's (2005)
conclusion that increases in Dk/t beyond the Holden-Mertz criteria of 24 and 87,
for the open and closed eye contact-lens-wearing states respectively, result in
minimal oxygen gains. It would be difficult to recommend a magic cut-off value for
oxygen transmissibility, especially in lieu of individual responses, but from the
above discussion it's evident that once the hypertransmissible area of the curve
is reached, increases in Dk/t beyond this value certainly have less of an impact
in terms of oxygen utilization.
Non-Oxygen Considerations for Successful Contact Lens
Wear
TABLE 2
Critical
Oxygen Values for Various Physiological Indicators |
|
AUTHOR |
INDICATOR |
DK/T |
FLUX |
|
Holden (1984) |
4% corneal edema-closed eye with extended wear |
87 |
5.57 |
|
Harvitt (1999) |
Absence of epithelial
anoxia during extended wear |
89 |
5.58 |
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Papas (1998) |
Absence of induced limbal hyperemia with
extended wear |
125 |
5.73 |
|
Giasson
(1994) |
Absence of epithelial pH change |
300 |
5.94 |
In our rush to cast aside conventional hydrogels
and embrace the hypertransmissibility era, it's wise to not forget desirable attributes
of conventional hydrogels. After all, if oxygen was the only determinant of successful
wear, how could we explain patients who were successful in the PMMA era of lens
wear, where no oxygen passed through the lens? PMMA lenses relied on the constant
exchange of oxygen rich tears with each blink to provide the cornea with a whopping
3 percent EOP environment! Yet, instances of microbial keratitis in these patients
were very low. Oxygen is indeed only one component of successful contact lens wear.
We can determine what lens attributes
are needed for successful lens wear by considering why patients discontinue lens
wear in the first place. In a survey of contact lens dropouts, the number-one reason
cited for discontinuing hydrogel lens wear was discomfort. We can divide comfort
into the initial comfort upon lens application and the comfort that is maintained
until the end of the day.
Initial lens discomfort may result
from a poor fit, lens defect, solution hypersensitivity, lens design or material.
You can readily identify these causes of initial discomfort by slit lamp inspection
or you can empirically diagnose them by switching to a different lens.
Mechanical characteristics of a lens
may also affect initial comfort levels as evidenced by anecdotal reports of lens
awareness when subjects initially switch from conventional hydrogels into silicone
hydrogels having a higher modulus value. The stiffness of these materials may be
responsible for initial lens awareness, which usually subsides with continued wear.
Figure 3 shows the modulus values of some commonly prescribed hydrogel lenses.
It
thus makes sense that a silicone hydrogel lens may provide better initial comfort
if has a low modulus similar to that found in conventional lenses. According to
Figure 3, the only silicone hydrogel lens currently on the market that has a modulus
in the range of conventional hydrogels is Acuvue Advance with Hydraclear. Reduced
modulus also offers another benefit of potentially avoiding adverse mechanical lens
effects such as SEALs and CLPC as discussed earlier.
Numerous factors such as the
lens fit, deposit build-up or reports of end-of-day dryness may affect end-of-day
comfort. Lenses that show no or inadequate movement will tend to become more uncomfortable
throughout the day. A lens that moves freely over the cornea and quickly returns
to its original resting position is optimal in terms of movement. Remember that
lens movement depends on the peripheral design of the lens and to a lesser extent
on base curve; therefore, switching to a flatter base curve may not ensure more
movement, and you may have to try a different lens.
Deposits may also cause contact lens
discomfort. With the adoption of frequent replacement contact lenses, the problem
of deposit-related discomfort has become less of an issue for both patients and
doctors.
End-of-day discomfort and dryness is
a common complaint of many lens wearers, especially those who are diagnosed with
dry eye. Some lenses such as CooperVision's Proclear Compatibles are targeted at
the specific problem of contact lens-related dryness. The lens contains phosphorylcholine
(PC), which is thought to bind water molecules and lead to greater water retention
and less on-eye dehydration. Acuvue Advance is another lens that binds water to
the hydrogel. Hydraclear is a long-chain, high-molecular-weight, hydrophilic molecule
(PVP) that acts as a humectant (moisture loving agent) and lubricant, allowing for
incorporation of silicone into the hydrogel, increasing oxygen transmissibility
while still providing a wettable lens surface and moisture retention without negatively
affecting lens stiffness.
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Figure 3. Modulus values for some commonly
prescribed hydrogels.
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Hyperemia, vision, handling and economics
can also play a role in contact lens dropout. Pritchard, et al (1999) listed red
eyes in addition to discomfort and dryness as primary reasons for lens discontinuation.
Although ocular hyperemia may result from many etiologies, hypoxia has been implicated
in the development of limbal hyperemia. A recent study (Maldonado-Codina et al,
2004) showed that Acuvue Advance did not increase limbal hyperemia from the baseline
non-lens wearing eye in a group of neophyte contact lens patients.
The Right Combination
Although increasing oxygen transmissibility to
meet critical physiological needs is vital, once these needs have been met additional
increases will have a diminishing benefit. Your first duty is to provide the highest
quality of care to your patients and make recommendations based on what you feel
is in their best interest. I believe you can best accomplish this by taking
a holistic approach and weighing the risks and benefits of each treatment that you
offer to your patients.
Although often trivialized, contact lenses
are medical devices and can be associated with sight-threatening complications.
Therefore, you must evaluate the risks and benefits of lenses you recommend for
your patients, with the ultimate goal to provide clear, comfortable vision throughout
the day while minimizing the risk of complications. Keeping this in mind will ensure
patient satisfaction and a successful contact lens practice.
To obtain references for this article,
please visit http://www.clspectrum.com/references.asp and click on document #126.
Dr. Artis is the director of Professional Affairs
for Vistakon, division of Johnson & Johnson Vision Care, Inc.
Contact Lens Spectrum, Issue: May 2006