DK REALLY MATTERS
The Future of Contact Lenses: Dk Really Matters
Amid discussions of oxygen levels and calculated
flux, the clinical imperative remains to maximize oxygen transmissibility for all
By Brien Holden BAppSc, LOSC, PhD, DSc,
FAAO; Serina Stretton BSc, PhD; Percy Lazon de la Jara, BOptom, PhD, FIACLE; Klaus
Ehrmann, BEng, MSc, PhD; and Donna LaHood, BOptom, MOptom
it's been suggested that the level of oxygen provided by any silicone hydrogel
contact lens is sufficient for corneal health, and that providing the highest possible
oxygen level does not matter to the cornea, the eye or the contact lens wearer.1,2
The implication is that the silicone hydrogel lens with the lowest oxygen transmissibility
(Dk/t) is good enough, and that there's little value in completely eliminating hypoxia.
(oxygen permeability) and Dk/t do matter. The eye has the greatest likelihood
of good health if it receives the highest possible levels of oxygen. Of course,
oxygen availability isn't the only important requisite for successful and safe contact
lens wear. Adequate movement, deposit control, optical and physical design, ocular
compatibility and surface wettability are also essential. And as a corollary, highly
oxygen permeable lenses should be equal or superior to the conventional low-Dk hydrogel
lenses in these and other important aspects of lens performance.
Wearers need the highest Dk
possible for one simple reason: The cornea is designed for normoxia during the day
and the (lower) levels of oxygen provided through the eyelid at night. Any reduction
in oxygen availability requires some adjustment by the cornea, and long-term compromise
will be deleterious in some way. Why compromise the cornea if it is not necessary?
As Efron and Brennan state,3
the real critical oxygen requirement for contact lens wear is 20.9% the concentration
of oxygen in the atmosphere and any lens that delivers less than this concentration
to the cornea ultimately will affect corneal physiology.
Some silicone hydrogel contact lenses
have local oxygen transmissibilities as low as 25 to 30 units
(with a unit measured
as x 10-9 [cm/sec]/[mLO2/mL x mm Hg]), whereas others exceed
100 units across the entire lens. Even for the average wearer of daily wear lenses,
it's unnecessarily challenging to the cornea to provide a lower-than-available oxygen
level, since virtually all wearers nap for some period most days, and substantial
numbers of wearers sleep in their daily-wear lenses, either occasionally or regularly.4
Why take the risk of hypoxic damage to the eyes if it can be avoided?
Effects of Contact Lens-induced
With the possible exception of silicone elastomer
lenses under closed-eye conditions,5 contact lenses impede movement of
oxygen to the anterior cornea, creating lens-induced hypoxia. Since the early days
of contact lenses,6 chronic corneal hypoxia has been a major issue because
it causes obvious corneal edema, variously described as Sattler's veil (haptic lenses),6
central corneal clouding (corneal lenses),7 and striae and folds (soft
lenses).8 In the long term, hypoxia results in corneal exhaustion syndrome
and discontinuation from contact lens wear.9
In recent decades, researchers have found
that contact lenses that don't meet the cornea's oxygen requirements also cause
impaired corneal metabolism and integrity, decreased epithelial thickness, stromal
thinning, increased endothelial polymegethism and limbal redness, and corneal vascularization.10,11
Moreover, laboratory and clinical studies show that hypoxia causes increased bacterial
adhesion to epithelial cells,12–15 and overnight corneal hypoxia
increases the risk of infection.16
However, experience with silicone elastomer
lenses17 and, more recently, silicone hydrogel lenses18 teaches
us that eliminating hypoxia is not sufficient to prevent infection. Although hypoxia
is a risk factor for microbial keratitis, overnight wear (eye closure) and bacterial
contamination can be overwhelming factors.19–23
Estimating and Measuring Oxygen Supply
Researchers first recognized the need for contact
lenses to transmit oxygen to the cornea almost 60 years ago.6 Debate
and controversy continue to this day over the actual level of oxygen required by
the cornea, partly because of the impracticalities of measuring oxygen supply in
the normal clinical situation and partly because the wide variety of physiological
and clinical indicators used in studies have different thresholds.
Variations in the way oxygen flow is measured
or calculated and inconsistent use of terminology has caused great confusion among
practitioners and, through advertising, their patients. Practitioners need reliable
and practical measures of the oxygen performance of a lens. In addition, rather
than rely on spurious and inaccurate statements, such as "all silicone hydrogel
lenses supply essentially the same high levels of oxygen to the cornea," practitioners
need to understand how well lenses of different Dks, powers and thickness profiles
supply oxygen to the cornea.
Figure 1: Lens thickness profiles for a range
of –3.00D silicone hydrogel lenses.
Practitioners have come to rely on
two ways to estimate oxygen supply:
1. In-vitro measurements of lens material
permeability (Dk) and the calculations and models necessary to derive clinically
meaningful numbers from these in-vitro measurements
2. In-vivo clinical assessment and
measurement of the effects on the cornea with such techniques as pachymetry of lenses
of different oxygen transmissibility (Dk/t).
The advantage of an in-vitro measure
such as Dk (D being diffusivity and k, solubility of oxygen) is that it's relatively
simple to standardize techniques and obtain reliable data. Dk is a material property;
Dk/t (where t is thickness and can be central or average) is the local or average
estimate of ease of flow through a lens (the inverse of resistance).
Dk, the material property, is calculated
from laboratory measurements of t/Dk (resistance to flow) and describes the permeability
of a lens material, regardless of material thickness (barrier and edge effects having
been considered). Once the Dk of a lens material is known, then the Dk/t of all
lenses made from the same material can be calculated. Therefore, Dk allows the practitioner
to consider and estimate oxygen supply for lenses of various thicknesses, powers
2: Color-coded views of the oxygen transmissibilities (Dk/t) of –3.00D (top)
and –6.00D (bottom) silicone hydrogel lenses. Plane view shows Dk/t over the
area of each lens. Raised areas in the 3-D view show areas of highest Dk/t for each
To understand part of the "Dk/t – oxygen
flux" story, it's important to know that t/Dk is measured with a lens positioned
between an air-rich and an oxygen-depleted chamber. Dk/t is a measure of the maximum
potential of a lens to deliver oxygen to the eye over a given area when the front
surface of the lens is in contact with air and the back surface is anoxic. This
situation approximates when, for example, wearers just open their eyes after wearing
a very thick, low-Dk/t lens. What Dk and Dk/t allow practitioners to do is estimate
the oxygen performance of lenses across a wide variety of lens shapes, lens powers
and environmental conditions (aphakic and thick-edge lenses in high altitudes, planes,
sleep, and so on).
misleading problem is created for the practitioner in that Dk/t is often quoted
for lenses (or even implied for a lens type), using the central Dk/t, calculated
using the instantaneous center thickness of a -3.00D lens. This Dk/t (at the thinnest
point of a -3.00D lens) has also been used to calculate oxygen flux. Such reporting
oversimplifies oxygen transmissibility and is misleading because it ignores both
central thickness differences between lenses of different powers and differences
across lenses of different power profiles. These differences have a significant
impact on oxygen supply to the cornea and the limbus. After all, people don't wear
lens centers they wear whole lenses that affect the entire cornea, including
the limbus and the limbal conjunctiva. Comparing thickness profiles of some of the
currently available silicone hydrogel lenses (Figure 1) clearly illustrates the
we convert these profiles to Dk/t, we can see that, as expected, the Dk/t of
minus-power lenses is greater in the center than it is in the periphery (Figure
2), and the opposite is true of plus-powered lenses (Figure 3). Contact lenses
made from the same material in different shapes supply different amounts of
oxygen to the cornea.
In an attempt to predict how much oxygen actually
reaches the cornea over a given area and time (oxygen flux) and assess the impact
of corneal oxygen consumption on oxygen flow during lens wear, Hill and Fatt27
modeled the distribution of oxygen across the cornea using Fick's law of diffusion
(Figure 4). Fick's law is used to predict steady-state oxygen flux, and it states
that flux depends on the difference in oxygen tension between the front and the
back surfaces, e.g. of the lens, and the oxygen transmissibility of that lens.
Figure 3: Color-coded views of the oxygen transmissibilities
(Dk/t) of +6.00D silicone hydrogel lenses. Plane view shows Dk/t over the area of
each lens. Raised areas in the 3-D view show areas of highest Dk/t for each lens.
Measurement of the oxygen pressure (P0)
behind a contact lens is difficult. Hamano24 achieved it using a thin-wire
oxygen probe; Bonanno25 used oxygen-sensitive phosphorescent dyes; and
Hill26, 27 measured P0 indirectly using equivalent oxygen
percentage (EOP). EOP is obtained by sliding a lens off an eye, immediately measuring
the oxygen uptake rate and then referring the value obtained to corneal responses
calibrated to known gases.
Brennan1 proposed that total
corneal oxygen consumption should replace Dk/t and oxygen flux as benchmarks for
practitioners comparing lens performance because it better reflects corneal oxygen
metabolism during lens wear. However, difficulties arise from the theoretical nature
of the calculations and their reliance on various assumptions and immeasurable variations
in conditions throughout a wearer's daily cycle and with a variety of lenses.
Brennan's first flux model predicted
that oxygen transmissibilities of 15 units for daily wear and 50 units for extended
wear were sufficient to sustain normal oxygen supply. Clearly, from daily and overnight
edema measurements,28 these levels do not avoid even a crude sign of
poor physiology, such as the onset of visible edema.
One major problem is that flux is a
calculated entity, based on a certain set of assumptions, which in the initial Brennan
model1 included the assumption of fixed corneal oxygen consumption. However,
corneal oxygen consumption varies with the ambient oxygen conditions, corneal pH,
temperature, physical pressure, the layer of cells, the number of cells and their
state of health.
As Brennan points out, his model is
a theoretical exercise only. It does not take into account the dynamic nature of
corneal metabolism or the effects of environmental variations such as acidosis,
as the model by Radke and Chhabra29 has done. Their work confirmed that
125 units should be the minimum to avoid significant impedance to oxygen supply
under closed-eye conditions. These problems should not deter the development of
models, which are useful for theoretical analysis. However, because they rely on
a fixed set of circumstances, they have limited applicability in clinical situations.
In essence, the problems with using
flux to make predictions about overall corneal health are 1) its susceptibility
to the assumed model conditions and 2) the assumption that equal (calculated) fluxes
mean the same consequences for corneal health.
4. Fick's law of diffusion applied to a contact lens.
= Dk/t X (P1 – P0)
J is oxygen flux,
P1 is the oxygen pressure of the atmosphere P0 is the oxygen pressure behind a contact lens
The second issue assuming that
all fluxes are equal can be best understood if we take two different situations
with the same apparent flux. Flux is the mathematical product of Dk/t multiplied
by the partial pressure difference across the lens (Figure 4). It follows, therefore,
that the same flux will be calculated through a lens with a Dk/t of 100 units and
a driving force, P1 – P0, of 10 mm Hg (e.g. 155 mm Hg
to 145 mm Hg), as is obtained with a lens with a Dk/t of 10 units and a driving
force of 100 mm Hg (e.g. 155 mm Hg to 55 mm Hg). Because the calculated fluxes are
the same, the two conditions a high-Dk/t lens and a low driving force (because
of high levels of oxygen behind the lens) and a low-Dk/t lens and a high driving
force (because of poor levels of oxygen behind the lens) are implied to be
equivalent. Clearly, this is misleading, because these are very different circumstances
physiologically. One cornea would be subject to a partial pressure of oxygen of
55 mm Hg (7% oxygen), and the other would have 145 mm Hg (19% oxygen).
Correlating Dk/t and Corneal Effects
Recent evidence from the work of both Ren and
Wilson30 and Cavanagh31 on corneal homeostasis has revealed
some of the reasons for the dramatic and long-lasting effects of lens-induced hypoxia
on ocular physiology. Their work has shown that all contact lens types and wear
modalities influence maintenance and turnover of the corneal epithelium to some
degree, and the impact of lens wear on these processes is mediated in part by lens-induced
hypoxia.14,32–36 Furthermore, the impact of high-Dk silicone hydrogels
on epithelial turnover is less pronounced compared to other lens types, and wearers
show more evidence of adaptive recovery during long-term wear.
The reason for greater epithelial thinning
with lower-Dk/t lenses compared to higher-Dk/t lenses seems to be that oxygen deprivation
creates an imbalance between the production of new cells at the basal epithelium
and the loss of old cells from the corneal surface. A slower rate of cell shedding
signals a lower demand for new cells from the limbus. This reduced demand ultimately
results in fewer cells moving toward the surface, and the central epithelium eventually
thins. The Göteborg Study10 showed that low-Dk/t extended wear disturbed
epithelial metabolism, lowering the eye's oxygen uptake and thinning the epithelium.
Jalbert and colleagues37 have recently shown this effect is minimized
with silicone hydrogels. They found 7% epithelial thinning with high-Dk silicone
hydrogels, compared to 23% for low-Dk hydrogel lenses.
The implications of oxygen deprivation
at the corneal periphery become critical when we consider how the limbus helps the
cornea maintain overall health. The limbus is the sole source of epithelial stem
cells, which provide an unlimited supply of new epithelial cells and ensure rapid
recovery from superficial injury. Any loss or injury to stem cell production can
result in serious sequelae, including recurrent erosion, chronic keratitis and vascularization.38
What Dk/t Is Needed?
The true test of the utility of Dk/t is how well
it aligns with clinical data. If all silicone hydrogel lenses were to deliver essentially
the same levels of oxygen to the cornea, there would be no differences in how these
lenses perform when measured against clinical indicators of hypoxia. Brennan's model39
of diminishing returns predicts that lenses with oxygen transmissibilities greater
than 15 and 50 units will provide no benefit during daily or extended wear, respectively.
But differences in limbal redness and corneal swelling do not support this prediction.
Papas has established a clear relationship
between limbal hyperemia and oxygen deprivation beneath the lens periphery, indicating
that a minimum Dk/t of 125 units is required to eliminate limbal redness with daily
wear.40 If there's no advantage to wearing lenses with oxygen transmissibilities
greater than 15 units for daily wear, then one would expect no differences in the
level of limbal redness observed during daily wear with almost all the conventional
hydrogel lenses and silicone hydrogel lenses. However, this is not the case. Maldonado-Codina's41
comparison of limbal redness with soft-lens daily wear detected significant differences
between lenses with central oxygen transmissibilities of 26 and 86 units.
Corneal swelling is one of the most
recognizable signs of corneal oxygen deprivation used by practitioners and researchers
to assess lens performance. At 4% to 6% swelling, fine structural changes in the
form of striae appear at the posterior stroma; at 8% swelling, endothelial folds
become observable. Moreover, stromal swelling is not uniform across the cornea but
mirrors the variation in oxygen availability in the post-lens tear film. When patients
wear "donut" hydrogel lenses with a large central hole, the cornea swells under
the portion of the cornea covered by the lens and not in the central area42
(Figure 5). Corneal edema correlates well with Dk/t in open and closed eyes.
Figure 5. Mean corneal swelling (%) across
the cornea after 6 hours of wear of a donut lens (n = 10), with respect to the
average lens thickness profile covering the cornea during wear.
The differences in Dk/t between different
silicone hydrogel lenses are reflected in a study by Mueller and colleagues,43
which compared overnight swelling with 140-Dk and 99-Dk silicone hydrogel lens types.
Researchers found using 140-Dk silicone hydrogel lenses that there was no significant
difference in overnight central and peripheral swelling for –1.00D and –6.00D
lenses and no differences in overnight edema with these lenses from no lens wear
among patients. However, patients wearing the 99-Dk silicone hydrogel lens showed
significantly greater corneal swelling in the center and periphery compared to no
lens wear. Moreover, in another study with 99-Dk lenses, 11% of 30 adapted soft
contact lens-wearers experienced greater than 7.7% edema after overnight wear.44
lens Dk/t across the optical zone and lens peripheral Dk/t are two practical benchmarks
that enable practitioners to assess oxygen supply. The former is the basis for the
Holden-Mertz criteria28 for preventing lens-induced hypoxia during open-
and closed-eye wear, and the latter is the Papas criterion for avoiding limbal hypoxia
(and possible hypoxic stem cell effects).40
The Holden-Mertz criteria of
24, 35 and 87 units for avoiding end-of-first-day edema, end-of-seventh-day edema,
and 4% overnight edema respectively were based on average lens thickness.45
Using the Holden-Mertz equations to calculate the Dk/t average to avoid 3.2% overnight
edema (the level of non-lens overnight edema found by La Hood and colleagues46
with more subjects than originally used by Mertz45), we arrive at 125
Dk/t to avoid overnight edema. Harvitt and Bonanno's mathematical model of oxygen
diffusion across the cornea in the closed eye supports the
efficacy of 125 units
as a criterion for overnight wear.47
Coincidentally, the Papas model of
the impact of peripheral lens Dk/t also points to 125 units for avoiding open-eye
limbal hyperemia.40 The Papas model sets the most stringent standard
for daily wear of modern contact lenses. Clearly, any suggestions that oxygen transmissibilities
above 15, 25 or even 80 units do not matter are based on very limited, restrictive
modeling of contact lens requirements.1
Oxygen Transmissibility to Ensure Health
Practitioners should use the highest levels of
Dk/t possible, if for no other reason than to avoid chronic limbal inflammation
and to ensure healthy maintenance and turnover of epithelial and limbal cells during
contact lens wear.
Several long-term clinical trials have compared
the performance of high-Dk silicone hydrogel lenses to both no lens wear48
and hydrogel lens wear.49 The results firmly establish that eyes wearing
silicone hydrogel lenses are clinically indistinguishable from eyes without lenses,
and they exhibit excellent clinical physiology.
Sufficient oxygen flow through a lens
is critical for all patients, but it is particularly important for those who require
high lens powers or lenses that are thicker in the periphery. Some 35% of wearers
are higher myopes, astigmats or hyperopes who require lenses up to 0.35 mm thick
in the center or periphery. Over 23% of the population is presbyopic and requires
thicker lenses, at least in the alternating lens form. Dk is a reliable and practical
method to predict a lens's oxygen performance, providing lens topography and thickness
and the various conditions that wearers experience with both open and closed eyes
We predict that practitioners will
continue the trend toward using more lenses of the highest possible oxygen permeability.
All other performance characteristics being equal, why would practitioners do anything
else? Already, in the United States, the proportion of silicone hydrogel lenses
prescribed for daily wear has increased approximately 8-fold over the last 2 years.50
Our theoretical discussions of oxygen levels and calculated flux should not distract
us from the clinical imperatives.
Dr. Holden is Scientia Professor
at the University of New South Wales; CEO, the Institute for Eye Research (IER);
and deputy CEO, Vision Cooperative Research Centre (CRC), Sydney, Australia.
Dr. Stretton is a project scientist at the IER and
Dr. Lazon de la Jara is project director, clinical
research, at the IER.
Dr. Ehrmann is project director, technology at
the IER and Vision CRC.
Dr. LaHood is a senior project scientist at the IER.
1. Brennan NA. Beyond flux: total corneal
oxygen consumption as an index of corneal oxygenation during contact lens wear.
Optom Vis Sci. 2005;82:467-472.
2. Morgan P, Brennan NA. The decay of Dk?
3. Efron N, Brennan NA. How much oxygen? In
search of the critical oxygen requirement of the cornea. Contax 1987;July:5-18.
4. CIBA Vision market research, Duluth, Ga.
5. Sweeney DF, Holden BA. Silicone elastomer
lens wear induces less overnight corneal edema than sleep without lens wear. Curr
Eye Res. 1987;6:1391-1394.
6. Goodlaw E. Contact lens solutions and their
wearing time. Optom Weekly. 1946;37:1675-1679.
7. Korb D, Exford J. The phenomenon of central
circular clouding. J Am Optom Assoc. 1968;39:223-230.
8. Sarver MD. Striate corneal lines among
patients wearing hydrophilic contact lenses. Am J Optom. 1971;48:762-763.
9. Sweeney DF. Corneal exhaustion syndrome
with long-term wear of contact lenses. Optom Vis Sci. 1992;69:601-608.
10. Holden BA, Sweeney DF, Vannas A, Nilsson
K, Efron N. Effects of long-term extended contact lens wear on the human cornea.
Invest Ophthalmol Vis Sci. 1985;26:1489-1501.
11. Holden BA, Sweeney DF, Swarbick HA, Vannas
A, Nilsson KT, Efron N. The vascular response to long-term extended contact lens
wear. Clin Exp Optom. 1986;69:112-119.
12. Ren DH, Yamamoto K, Ladage PM, Molai M,
Li SL, Petroll WM, Jester JV, Cavanagh HD. Adaptive effects of 30-night wear of
hyper-O2 transmissible contact lenses on bacterial binding and corneal epithelium:
a 1-year clinical trial. Ophthalmol. 2002;109:27-40.
13. Cavanagh HD, Ladage PM, Li SL, Yamamoto
K, Molai M, Ren DH, Petroll WM, Jester JV. Effects of daily and overnight wear of
a novel hyper oxygen-transmissible soft contact lens on bacterial binding and corneal
epithelium: a 13-month clinical trial. Ophthalmol. 2002;109:1957-1969.
14. Ladage PM, Yamamoto K, Ren DH, Li L, Jester
J, Petroll WM, Cavanagh HD. Effects of rigid and soft contact lens daily wear
epithelium, tear lactate dehydrogenase, and bacterial binding to exfoliated epithelial
cells. Ophthalmol. 2001;108:1279-1288.
15. Fleiszig SMJ, Efron N, Pier G. Extended
contact lens wear enhances Pseudomonas aeruginosa adherence to human corneal epithelium. Invest Ophthalmol Vis Sci. 1992;33:2908-2916.
16. Solomon OD, Loff H, Perla B, Kellis A,
Belkin J, Roth A, Zucker J. Testing hypotheses for risk factors for contact lens-associated
infectious keratitis in an animal model. CLAO J 1994;20:109-113.
17. Aasuri M, Venkata N, Preetam P, Rao N.
Management of pediatric aphakia with silsoft contact lenses. CLAO J 1999;25:209-12.
18. Holden BA, Sweeney DF, Sankaridurg PR,
Carnt N, Edwards K, Stretton S, Stapleton F. Microbial keratitis and vision loss
with contact lenses. Eye & Contact Lens: Science & Clinical Practice
19. Morgan P, Efron N, Brennan NA, Hill E,
Raynor M, Tullo A. Risk factors for the development of corneal infiltrative events
associated with contact lens wear. Invest Ophthalmol Vis Sci. 2005;46:3136-3143.
20. Morgan P, Efron N, Hill E, Raynor M, Whiting
M, Tullo A. Incidence of keratitis of varying severity among contact lens wearers.
Br J Ophthalmol. 2005;89:430-436.
21. Radford C, Stapleton F, Minassian D, Dart
J. Risk factors for contact lens related microbial keratitis: Interim analysis of
case control study [ARVO Abstract]. Invest Ophthalmol Vis Sci. 2005;46:Abstract
22. Edwards K, Keay L, Naduvilath T, Brian
G, Stapleton F, Microbial Keratitis Study Group. Risk factors for contact lens related
microbial keratitis in Australia [ARVO Abstract]. Invest Ophthalmol Vis Sci.
2005;46:Abstract nr 926.
23. Stapleton F, Edwards K, Keay L, Naduvilath
T, Brian G, Holden B, Dart J. Incidence of contact lens related microbial keratitis
[ARVO Abstract]. Invest Ophthalmol Vis Sci. 2005;46:Abstract nr. 5025.
24. Ichijima H, Hayashi T, Mitsunaga S, Hamano
H. Determination of oxygen tension on rabbit corneas under contact lenses. CLAO
25. Bonanno J, Stickel T, Nguyen T, Biehl
T, Carter D, Benjamin W, Soni P. Estimation of human corneal oxygen consumption
by noninvasic measurement of tear oxygen tension while wearing hydrogel lenses.
Invest Ophthalmol Vis Sci. 2002;43:371-376.
26. Hill R. Oxygen uptake of the cornea following
contact lens removal. J Am Optom Assoc. 1965;36:913-915.
27. Hill R, Fatt I. Oxygen deprivation of
the cornea by contact lenses and lid closure. Am J Optom. 1964;41:382.
28. Holden BA, Mertz GW. Critical oxygen levels
to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol
Vis Sci. 1984;25:1161-1167.
29. Radke C, Chhabra M. Minimum contact lens
oxygen transmissibility (Dk/L) with monod kinetics for the corneal oxygen consumption
rate [ARVO poster]. In: Association for Research in Vision and Ophthalmology Annual
Meeting; 2005; Fort Lauderdale, Florida USA; 2005.
30. Ren H, Wilson G. Apoptosis in the corneal
epithelium. Invest Ophthalmol Vis Sci. 1996;37:1017-1025.
31. Cavanagh HD. The effects of low- and hyper-Dk
contact lenses on corneal epithelial homoeostasis. Ophthalmol Clin N Am.
32. Ladage PM, Jester JV, Petroll WM, Bergmanson
JP, Cavanagh HD. Vertical movement of epithelial basal cells toward the corneal
surface during use of extended-wear contact lenses. Invest Ophthalmol Vis Sci.
33. Ladage PM, Ren DH, Petroll WM, Jester
JV, Bergmanson JP, Cavanagh HD. Effects of eyelid closure and disposable and silicone
hydrogel extended contact lens wear on rabbit corneal epithelial proliferation.
Invest Ophthalmol Vis Sci. 2003;44:1843-1849.
34. Ladage PM, Yamamoto K, Ren DH, Li L, Jester
JV, Petroll WM, Bergmanson JP, Cavanagh HD. Proliferation rate of rabbit corneal
epithelium during overnight rigid contact lens wear. Invest Ophthalmol Vis Sci.
35. O'Leary DJ, Madgewick R, Wallace J, Ang
J. Size and number of epithelial cells washed from the cornea after contact lens
wear. Optom Vis Sci. 1998;75:692-693.
36. Ren DH, Petroll WM, Jester JV, Ho-Fan
J, Cavanagh HD. The relationship between contact lens oxygen permeability and binding
of Pseudomonas aeruginosa to human corneal epithelial cells after overnight and
extended wear. CLAO J 1999;25:80-100.
37. Jalbert I, Sweeney D, Stapleton F. The
effect of long term wear of soft lenses of low and high oxygen transmissibility
on the corneal epithelium [AAO Abstract]. In: American Academy of Optometry Annual
Meeting; 2005; San Diego, USA; 2005.
38. Stapleton F, Stretton S, Papas E, Skotnitsky
C, Sweeney DF. Silicone hydrogel lenses and the ocular surface. The Ocular Surface.
39. Brennan NA. A model of oxygen flux through
contact lenses. Cornea. 2001;20:104-108.
40. Papas E. On the relationship between soft
contact lens oxygen transmissibility and induced limbal hyperaemia. Exp Eye
41. Maldonado-Codina C, Morgan P, Schnider
C, Efron N. Short-term physiologic response in neophyte subjects fitted with hydrogel
and silicone hydrogel contact lenses. Optom Vis Sci. 2004;81:911-921.
42. Holden BA, McNally JJ, Egan P. Limited
lateral spread of stromal edema in the human cornea fitted with a ('donut') contact
lens with a large central aperture. Curr Eye Res. 1988;7:601-605.
43. Mueller N, Caroline P, Smythe J, Mai-Le
K, Bergenske P. A comparison of overnight swelling response with two high Dk silicone
hydrogels. [AAO Abstract]. Optom Vis Sci. 2001;78:S199 Abstract nr 26.
44. Comstock TL, Robboy MW, Cox IG, Brennan
NA. Overnight clinical performance of a high Dk silicone soft contact hydrogel lens.
In: Silicone hydrogels web site; 1999.
45. Mertz GW. Overnight swelling of the living
human cornea. J Am Optom Assoc. 1980;51:211-214.
46. La Hood D, Sweeney DF, Holden BA. Overnight
corneal edema with hydrogel, rigid gas permeable and silicone elastomer lenses.
Int Contact Lens Clin. 1988;15:149-154.
47. Harvitt DM, Bonanno JA. Re-evaluation
of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed
to avoid corneal anoxia. Optom Vis Sci. 1999;76:712-719.
48. Covey M, Sweeney DF, Terry RL, Sankaridurg
PR, Holden BA. Hypoxic effects on the anterior eye of high Dk soft contact lens
wearers are negligible. Optom Vis Sci. 2001;78:95-99.
49. Brennan NA, Chantal Coles ML, Comstock
TL, Levy B. A 1-year prospective clinical trial of Balafilcon A (PureVision) silicone-hydrogel
contact lenses used on a 30-day continuous wear schedule. Ophthalmol. 2002;109:1172-1177.
50. Morgan, PB. Is daily wear the principal use for silicone
hydrogel materials? December 2005.
Contact Lens Spectrum, Issue: February 2006