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Continuing Education and Training
Silicone hydrogels
Will they displace conventional lenses?
Silicone hydrogel contact lenses first appeared commercially in
Europe in 1999 and in the US in 2001. Since then, they have
shown tremendous growth 1 , with approximate worldwide sales
in 2003 of $150m and currently, over one million wearers
worldwide 2 . Initially developed for the extended wear market,
practitioners have embraced the use of such materials for both
overnight and daily wear use.
This article discusses the development of
silicone hydrogels and reviews their clinical
performance to date. The specific aim is to
determine if these lenses will ultimately
replace conventional hydrogel lens
materials, and become the standard lens
type to be fitted for both daily and
overnight wear.
History of overnight wear
Throughout the 1970s and early 1980s,
manufacturers released a variety of
materials which were intended for
overnight wear. This was driven largely by
consumer desire for a convenient
correction alternative to their spectacles.
These early materials were often worn for
up to a month at a time without being
removed and achieved great commercial
success. John de Carle reported success with
over 2,000 patients in the early 1970s, and
other authors reported similarly high levels
of clinical success up to the mid 1980s 3-7 .
As a result of such positive data, extended
wear for cosmetic use for up to 30 days was
approved by the Food & Drug
Administration (FDA) in 1981, sparking an
explosion in the number of patients being
fitted with lenses for overnight wear.
Very soon afterwards, reports of corneal
ulceration with significant vision loss
began appearing in journals 8,9 , and the
safety of overnight wear was questioned in
both peer-reviewed journals 10 and the lay
media. The Contact Lens Institute in the US
sponsored studies to investigate the relative
risk and incidence of infectious keratitis.
The results from these studies were
published in 1989 11,12 and clearly
demonstrated that overnight wear of lenses
carried with it a significantly increased risk
of corneal infection. As a result, the FDA
immediately reduced the approved length
of time for overnight wear without removal
from 30 to seven days.
In the mid 1980s, it was believed that
the corneal infections seen with overnight
wear were probably due to poor hygiene
and compliance, and that the principal
factor driving such infection rates was
patients re-inserting poorly disinfected
lenses. It was hypothesised that using
lenses on a disposable or frequent
replacement basis, in which the lenses were
inserted once only and then discarded
upon removal, would be likely to have an
impact on the infection rates reported.
Such a concept became a clinical reality in
1987, with the introduction of disposable
extended wear lenses in the US.
The first published large-scale study
appeared to support such a concept 13 , but
fairly rapidly reports of infectious keratitis
started to appear 14 . The final proof that
disposability had no impact on the rate of
ulceration with conventional hydrogel
materials worn overnight came with the
publication of a paper from Holland in
1999 15 , which showed that the rate of
ulcerative keratitis was exactly that found
10 years previously in the US 12 – before
disposability was commonplace. This
publication clearly showed that overnight
wear with conventional soft lens materials
should be discouraged due to the increased
risk that such a modality had on the
development of sight-threatening keratitis.
Despite this, patients still seek
alternative vision correction methods to
liberate them of spectacles, with refractive
surgery being extremely successful, and the
most widely undertaken cosmetic
procedure in the world today. Patients still
sleep in lenses overnight even when told
not to do so, with an estimated 40% of
patients occasionally or frequently sleeping
in their lenses 16 . Clearly, patients desire a
lens which can be worn overnight and will
undertake this procedure whether their
practitioner sanctions it or not.
Consequently, it became clear that the
contact lens industry needed to develop
safer materials for overnight lens wear, as
they would swiftly become a commercial
success.
Development of highly
permeable hydrogel materials
In the early 1990s, contact lens companies
began to investigate the possibility of
developing novel contact lens polymers
which transmitted high levels of oxygen, on
the basis that more oxygen may make
overnight wear safer. Traditional hydrogel
materials are polymers which are typically
composed of several monomers joined
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Module 1 Part 2
Modern contact lens practice
About the authors
Lyndon Jones is Associate Professor in
the School of Optometry, and Associate
Director of the Centre for Contact Lens
Research (CCLR) at the University of
Waterloo, Canada. Kathy Dumbleton is
Senior Researcher in the CCLR.
33 | August 20 | 2004 OUT

Continuing Education and Training
Lyndon Jones PhD, FCOptom, DipCLP, DipOrth, FAAO, (DipCL)
and Kathy Dumbleton MSc, MCOptom, FAAO
together in chains to form a polymer
network. The oldest of these is the first
hydrogel material used for contact lens
wear – poly(2-hydroxyethyl methacrylate)
or polyHEMA – which was developed by
Wichterle in the 1960s 17 . This material was
granted FDA approval in 1971, and for
many years was the sole hydrogel contact
lens material available; it is still sold in vast
numbers today.
The principal disadvantage of
polyHEMA is that it relies upon water to
transport oxygen through the material.
Water has a limited ability to dissolve and
transport oxygen, with an approximate
oxygen permeability (Dk) of around 80
barrers. In conventional lens materials, Dk
is proportional to water content 18 , such that
conventional hydrogel materials have Dks
ranging from about 10 to 35 barrers. In
order to increase the Dk of a conventional
hydrogel contact lens material beyond that
of HEMA, it is necessary to incorporate
monomers which will bind more water into
the polymer. These higher water content
materials typically use HEMA, or methyl
methacrylate (MMA), in conjunction with
more hydrophilic monomers such as
N-vinyl pyrrolidone (NVP) or methacrylic
acid (MA). In comparison, rigid gas
permeable (RGP) lens materials transmit
their oxygen through the polymer phase,
and use monomers with higher oxygen
permeabilities than those used in
conventional hydrogels 19 .
From a clinical perspective, oxygen
transport to the cornea depends upon both
the Dk of the material and the lens
thickness (t). Thinner lenses provide the
cornea with more oxygen, since there is less
of a barrier for the oxygen to diffuse
through. The term ‘Dk/t’ describes the
oxygen transmissibility of a lens and gives a
quantitative indication of the amount of
oxygen that a lens-wearing eye will receive
through the lens. It is a more clinically
useful number than Dk, which gives no
Hydrogels
Low water content (38%)
Mid water content (55%)
High water content (70%)
Dk/t
15
27
24
indication of the effect of lens thickness or
lens design. Ideally, hydrogel lenses would
have a high Dk (via the use of a high water
content) and a thin centre thickness.
However, such lenses are impractical
because they rapidly dehydrate and result
in high levels of corneal staining 20,21 . Such a
process results in lenses with a high water
content having a thickness that is
substantially greater than that seen in low
water content materials, which limits the
Dk/t clinically obtainable.
Table 1 details typical Dk/t values
available for conventional hydrogel
materials and commonly encountered RGP
materials, at centre thicknesses normally
found for -3.00D lenses. It is clear that
conventional hydrogel materials have a
relatively low Dk/t compared with RGP
materials.
Silicone hydrogel materials
Table 1 shows that RGP materials have
significantly greater oxygen
transmissibilities than hydrogels, due to the
fact that they all contain fluorine or
silicone, both of which have an incredible
ability to transport oxygen 22 . RGP materials
frequently incorporate into their
composition a silicon-containing
monomer, commonly called TRIS 23,24 . The
ability of silicone-based lenses to provide
the cornea with substantial amounts of
oxygen has been understood for many
years, with silicone-elastomeric lenses being
used clinically for therapeutic and
paediatric applications for over 20 years 25 .
These lenses offer exceptional oxygen
transmission and durability, but a number
RGPs
Silicone acrylates
Fluorosilicone acrylates
Fluoropolymers
Dk/t
27
60
130
Table 1
Typical examples of Dk/t for conventional lens materials
of major limitations are associated with
their use in clinical practice. Fluid is unable
to flow through these lens materials,
resulting in frequent lens binding to the
ocular surface 26 , and the lens surfaces are
extremely hydrophobic, resulting in
marked lipid deposition 27 .
Ideally, manufacturers would like to
combine the hydrophilic properties of
HEMA-based lenses with the oxygen
transmission of silicone-elastomers.
Unfortunately, the process of combining
conventional hydrogel monomers with
hydrophobic silicone proved to be an
enormous challenge and it has taken over
20 years of considerable intellectual input
and financial resources for these materials
and designs to be created. Indeed, the
process of combining these monomers has
been likened to efforts of combining oil
with water, while maintaining optical
clarity 24 . Eventually, both CIBA Vision and
Bausch & Lomb were able to overcome
these difficulties and first generation
silicone hydrogel lenses became a
commercial reality at the end of the 1990s.
Three silicone hydrogel lens materials
are currently commercially available
(Table 2). CIBA Vision’s Focus® Night &
Day® material, lotrafilcon A, employs a
co-continuous biphasic, or two channel,
molecular structure, in which two phases
persist from the front to the back surface of
the lens 29 . The siloxy phase facilitates the
solubility and transmission of oxygen,
while the hydrogel phase transmits water
and oxygen, allowing good lens movement.
The two phases work concurrently, to allow
the co-continuous transmission of oxygen
Proprietary name
US adopted name
Manufacturer
Centre thickness
(@ -3.00D) mm
Water content
Oxygen permeability (x 10 –11 )
Oxygen transmissibility (x 10 –9 )
Modulus (psi)*
Surface treatment
FDA group
Principal monomers
PureVision
Balafilcon A
Bausch & Lomb
0.09
36%
99
110
148
Plasma oxidation,
producing glassy
islands
III
NVP + TPVC + NCVE
+ PBVC
Focus Night & Day
Lotrafilcon A
CIBA Vision
0.08
24%
140
175
238
25nm plasma coating
with high refractive
index
I
DMA + TRIS + siloxane
macromer
Table 2
Silicone hydrogel lens materials
Acuvue Advance
Galyfilcon A
Vistakon
0.07
47%
60
86
65
No surface treatment.
Internal wetting agent
(PVP)
I
Unpublished
DMA (N, N-dimethylacrylamide); HEMA (poly-2-hydroxyethylmethacrylate); MA (methacrylic acid);
NVP (N-vinyl pyrrolidone); TPVC (tris-(trimethylsiloxysilyl) propylvinyl carbamate); NCVE (N-carboxyvinyl
ester); PBVC (poly[dimethysiloxy] di [silylbutanol] bis[vinyl carbamate]); PVP (polyvinyl pyrrolidone)
* Modulus data taken from Steffen and McCabe 28
34 | August 20 | 2004 OUT

Continuing Education and Training
Figure 2
Graphical representation of water content, Dk
and modulus of elasticity for the three silicone
hydrogel materials and a polyHEMA-based
material (Acuvue 2). The graph shows that the
silicone-based lens materials with the highest
amount of silicone have the highest modulus (or
the greatest degree of ‘stiffness’) and the
highest oxygen permeability
and aqueous salts. Lotrafilcon A is
comprised of a fluoroether macromer copolymerised
with the monomer TRIS and
N, N-dimethyl acrylamide (DMA), in the
presence of a diluent 24,30 . The resultant
silicone hydrogel material has a water
content of 24% and a Dk of 140 barrers 31 .
Bausch and Lomb’s PureVision®
material, balafilcon A, is a homogeneous
combination of the silicone-containing
monomer polydimethylsiloxane (a vinyl
carbamate derivative of TRIS) copolymerised
with the hydrophilic hydrogel
monomer N-vinyl pyrrolidone (NVP) 30,32-35 .
This silicone hydrogel material has a water
content of 36% and a Dk of 110 barrers.
Vistakon’s Acuvue® Advance material,
galyfilcon A, is the newest of the three
materials and very little has been published
to date on the material’s composition 36 ,
although some deductions can be made
from the patent literature dealing with
Vistakon’s HydraClear technology. It has a
higher water content than the other two
materials (47%), and thus the lowest Dk
(60 barrers). Whereas both PureVision and
Focus Night & Day are approved for
overnight use, Acuvue Advance is only
approved for daily wear. It is the first of the
so-called ‘second generation’ silicone
hydrogels 37 and is the only one available
thus far which has an inversion marker and
a UV blocker, with a reported Class 1 UV
protection, blocking >90% of UVA and
>99% of UVB rays 28,36 .
In addition to increased oxygen
transmission, other major differences exist
between silicone hydrogel materials and
conventional hydrogels, primarily relating
to their mechanical and surface properties.
Mechanical properties
Silicone hydrogel lens materials are
significantly ‘stiffer’ than their conventional
hydrogel counterparts, due to the
incorporation of silicone. The modulus of
the first two silicone hydrogel materials is
some four to six times greater than low
rigidity conventional materials, such as
etafilcon A (which is used in the Acuvue
lens). The newest silicone hydrogel
material, Acuvue Advance, has a modulus
which is much closer to conventional
materials, being only 1.5 times more rigid
than etafilcon 28,36 . According to Johnson &
Johnson Vision Care, this reduced stiffness
is due to the reduced amount of silicone
present in this new material, along with
benefits derived from the internal wetting
agent HydraClear, which is based upon
polyvinyl pyrrolidone (PVP) 28,36 . Figure 1
graphically indicates the inverse
relationship between water content of
hydrogel materials and oxygen
permeability and material stiffness. The
materials with the highest ratio of silicone
to water are the stiffest.
Increased rigidity, or stiffness, has some
advantages, in that the lenses handle very
well and so are a perfect choice for people
who exhibit poor handling capabilities.
Increased rigidity might also suggest that
such lenses mask more corneal astigmatism
than traditionally very flexible hydrogel
lenses – but that has not been our
experience clinically or that of others 38 . The
mechanical properties of these lenses do
pose some problems, in that they are less
able to conform easily to the shape of the
eye and fitting is critical, with loose lenses
exhibiting poor comfort 39,40 .
Surface properties
Historically, a huge impediment to the
development of silicone hydrogel lenses
has related to the decreased wettability,
increased lipid interaction and accentuated
lens binding inherent in silicone-based
materials, as previously described. In order
to make the surfaces of silicone hydrogel
lens materials hydrophilic and more
wettable, techniques incorporating plasma
into the surface processing of the lens have
been developed 24,29,34,41 . The purpose of these
surface treatments is to mask the
hydrophobic silicone from the tear film,
increasing the surface wettability of the
materials and reducing lipid deposition.
The surfaces of Focus Night & Day
lenses are permanently modified in a gas
plasma reactive chamber to create a
permanent, ultrathin (25nm), high
refractive index, continuous hydrophilic
surface 29,42,43 .
PureVision lenses are surface treated in a
gas plasma reactive chamber, which
transforms the silicone components on the
surface of the lenses into hydrophilic
silicate compounds 24,30,34,44 . Glassy,
discontinuous silicate ‘islands’ result 30 , and
the hydrophilicity of the transformed
surface areas ‘bridges’ over the underlying
hydrophobic balafilcon A material. The
flow of oxygen and fluids through the
lenses is not impeded by these surface
modifications. Both surface treatments
become an integral part of the lens, and are
not surface coatings that can be easily
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35 | August 20 | 2004 OUT

Continuing Education and Training
Lyndon Jones PhD, FCOptom, DipCLP, DipOrth, FAAO, (DipCL)
and Kathy Dumbleton MSc, MCOptom, FAAO
‘stripped’ away from the base material
during daily handling and cleaning.
The Acuvue Advance lens material is the
first non-surface treated silicone hydrogel
to become a commercial reality. By
overcoming the need for a surface
treatment, the cost savings are considerable
and it is likely that future generation
silicone hydrogels will either need to avoid
the requirement for surface treatment, or
develop cheaper methods to achieve this.
Acuvue Advance uses an internal wetting
agent (HydraClear) based upon PVP, which
is designed to provide a hydrophilic layer at
the surface of the material which ‘shields’
the silicone at the material interface,
thereby reducing the degree of
hydrophobicity typically seen at the surface
of silicone hydrogels 28,36 .
Analysis of the surfaces of both
PureVision and Focus Night & Day has
shown that these surface treatments have
only been partially effective in masking the
silicone, with the lenses having significantly
more silicon exposed at the surface than
conventional lenses 45,46 and a more
hydrophobic surface (as evidenced by the
presence of higher water contact angles) 47,48 .
Although the Acuvue Advance material has
a lower silicone and higher water content,
this does mean that as the surface begins to
dehydrate, for example when the tear film
breaks up there will still theoretically be
exposure of unmasked silicone groups, as
with the other two silicone hydrogels.
Further work is required to quantify the
exact amount of silicon exposed on this
particular surface.
Clinical performance
Since the first report on silicone hydrogels
in 1995 49 , over 150 papers have been
published concerning their performance.
An overview of their findings is timely to
examine their success to date.
Hypoxic complications
As outlined above, the history of extended
wear is replete with papers describing the
deleterious effects of hypoxia on the
cornea. The actual amount of oxygen
required to eliminate oedematous
complications is a matter for some
conjecture 50 , with estimates suggesting that
on a daily wear basis somewhere between
24 51 and 35 Dk/t units 52 are required, and
on an overnight basis, 87 to 125 Dk/t units
are necessary 51-53 . Figure 2 graphically
portrays the variation in Dk/t values for
conventional and silicone hydrogel lenses
and shows that on a daily wear basis,
conventional lenses barely meet this daily
wear requirement and fall drastically short
of the requirement for overnight use.
Silicone hydrogel lenses provide a
significant degree of safety on daily wear
and for the two most permeable materials
(Focus Night & Day and PureVision) the
desirable overnight values are either within
limits or are exceeded.
A number of clinical studies have now
Figure 2
Central oxygen transmissibility (Dk/t) values for silicone hydrogel and conventional lens materials.
It is apparent that the three silicone hydrogels have substantially higher Dk/t values than the
conventional hydrogels
been conducted that allow us to investigate
whether these theoretical Dk/t values have
been confirmed clinically.
Corneal swelling
Corneal swelling is highly patient
dependent and varies with the Dk and the
thickness of the lens. Conventional lens
materials typically induce 10-12% corneal
swelling if worn overnight, resulting in the
appearance of five to 10 corneal striae 54 . The
typical level of oedema recorded following
overnight eye closure with silicone
hydrogels is in the order of 2-3% corneal
swelling, which is comparable to that seen
with no lens being worn, suggesting that
silicone hydrogels have a minimal impact
on physiology during overnight wear 49,55,56 .
During daily wear, the improved
physiological performance afforded by the
siloxane-based lenses should translate into
substantial physiological benefits,
particularly for patients who wear thick
lenses due to their prescription or lens
design (for example, torics or bifocals).
Microcysts
An excellent indicator of chronic hypoxia is
the presence of microcysts, which is closely
related to overnight corneal oedema 57 .
Studies have demonstrated that the
microcyst response seen with continuous
wear of silicone hydrogels is equivalent to
that seen without lens wear 56,58-61 . One factor
to consider at this point is the response
seen in patients exhibiting microcysts who
are refitted with silicone hydrogels after
wearing conventional materials. These
patients show a rapid increase in the
numbers of microcysts as the cornea
becomes suddenly re-oxygenated. The
number of microcysts then gradually
reduces over the first few weeks of lens
wear, to the point where they are
eliminated – which is the same response as
that seen in patients who cease lens
wear 58,59 .
Vascular responses
Hyperaemia (both limbal and bulbar) and
neovascularisation with silicone hydrogels
are radically reduced compared with
conventional hydrogels 53,56,62-65 (Figures 3a
and 3b) and are at similar levels to those
reported with non-lens wearers 61 . A small
decrease in the degree of neovascularisation
has been reported over a nine-month
period of silicone hydrogel continuous
wear in patients previously demonstrating
moderate levels of vascularisation 63 . These
findings confirm that hypoxia is a major
factor in the production of ocular vascular
responses with contact lenses, and lend
considerable credence to the concept of
using high oxygen permeable lens materials
for all patients.
Refractive error changes
Studies have demonstrated that following
the initial fitting with conventional
extended wear lenses, a myopic prescription
shift of approximately 0.50D occurs in
certain patients, possibly due to a chronic
hypoxic response in the cornea 7,56,66-68 . Once
refitted with silicone hydrogel lenses, these
patients show a reversal of this response,
with a hyperopic shift of the same
magnitude occurring 56,67,68 . This can be
Figures 3a and 3b
a. Bulbar and limbal hyperaemia subsequent to
daily wear with a conventional hydrogel lens;
b. Reduction in bulbar and limbal hyperaemia in
the same eye as Figure 3a when refitted with a
silicone hydrogel lens on a daily wear basis.
This change occurred within a two-week period
a
b
36 | August 20 | 2004 OUT

Continuing Education and Training
clinically significant if a refitting occurs in a
patient who is on the verge of presbyopia.
Approximately one month post-refitting, all
patients should be carefully over refracted
as the patient may be wearing a lens, which
is over-minused or under-plussed, which
could result in near vision problems.
Mechanical complications
The increased rigidity of these new materials
may be implicated in a variety of mechanical
complications seen with silicone hydrogel
lenses, as described next. It will be
interesting to see if lower modulus silicone
hydrogels such as Acuvue Advance produce
fewer mechanical clinical complications
than the first generation materials.
CLAPC
Contact lens-associated papillary
conjunctivitis (CLAPC), or giant papillary
conjunctivitis (GPC), has become a
relatively rare finding in contact lens
patients since the introduction of frequent
replacement lenses. It occurs typically in
around 2-3% of patients using their lenses
on a daily wear, frequent replacement
basis 69,70 , and more frequently in patients
using lenses on an extended wear basis 71 .
Several reports exist of patients developing
CLAPC when wearing silicone hydrogel
lenses, and the type of CLAPC found is
often isolated in nature rather than diffuse,
suggesting a mechanical rather than allergic
or immunological aetiology 70,72-75 . It is
believed that the increased stiffness of the
lens materials may result in increased
movement of the lenses and any potential
marginal fitting characteristics of the lens,
resulting in edge lift-off or ‘fluting’
(buckling at the lens edge) 76 , will act as a
mechanical irritant and produce a localised
papillary conjunctivitis, similar to that
observed in patients with loose ocular
sutures 77,78 . It is important that lenses are
replaced on a frequent basis and that the
lens fit is optimised, since the development
of CLAPC is a common reason for patients
to cease silicone hydrogel continuous
wear 75 .
SEALs
Superior epithelial arcuate lesions (SEALs)
are whitish, arc-like lesions which stain the
fluorescein (Figure 4). The lesions are
located on the superior cornea in the area
normally covered by the upper lid, typically
within 1-3mm of the superior limbus 70 .
They are not commonly associated with
symptoms and are usually unilateral, but
may be bilateral, in which case they are
typically asymmetric. A number of reports
exist of SEALs occurring in patients wearing
silicone hydrogel lenses 65,73-75,79-84 , suggesting
that they occur more frequently with
silicone hydrogels than with conventional
lens materials. It is believed that the
increased rigidity of the lens materials
results in the lens being unable to
conform adequately to the limbus, and that
the pressure induced by the upper lid
results in splitting of the superior
epithelium 70,85 .
Deposition
The deposition of contact lenses with
substances derived from the tear fluid is a
well-known clinical complication, resulting
in reductions in comfort 86 , vision 87 and an
increase in inflammatory responses 88 . Since
these silicone hydrogel lenses are primarily
intended for overnight use for up to 30
days, it is important that these lenses do
not deposit to the degree that silicone
elastomer lenses did, because such
deposition would require frequent lens
removal for cleaning, negating the benefits
of overnight lens wear. On a daily wear
basis, it is important that such lenses
remain clean for as long as possible as
there are no currently available daily
disposable options and, given the cost of
the lenses, this is unlikely to change for
some time.
Protein
To date, the degree of in-eye
biocompatibility achieved with silicone
hydrogel materials has received relatively
minimal attention, with the published
results indicating that the deposition of
protein on these materials is significantly
less than that seen with conventional
materials 68,89-92 . Whilst the amount of
protein deposited is minimal, it is
interesting that the lysozyme deposited is
largely denatured and inactivated 89,90 . Given
the small amount of protein deposited,
enzyme tablets are unlikely to be required
with these materials.
Lipid
As previously described, silicone-based
materials do have a potential to deposit
lipid from the tears. Lipid deposition on
silicone hydrogels can be a problem for
certain patients 90 , particularly if they are
refitted from an ionic material such as
etafilcon, which deposits very little lipid.
If patients are seen to be depositing their
lenses with lipid (Figures 5 and 6), then
moving to non-NVP-containing materials,
such as Proclear® or Acuvue, will reduce
lipid deposition. Further options include
adding surfactant cleaners containing
alcohol, such as Miraflow, ensuring that
patients use a physical rubbing process with
their multipurpose solutions (regardless of
whether they are approved for use in a
‘no-rub’ format) or moving to more
frequent periods of replacement 93 . To date,
the Acuvue Advance material is
recommended for replacement every two
weeks, as compared with typical monthly
replacement for the other two silicone
hydrogels.
Mucin balls
A number of publications have described
the presence of ‘mucin balls‘ in wearers of
silicone hydrogel lenses 73,75,94-101 . These are
pearly, translucent, 20-100µm spherical
Figure 4
SEAL subsequent to silicone hydrogel
continuous wear
Figure 5
Lens calculi, which are comprised of
90% lipid, on a PureVision lens
(picture by courtesy of Brian Tompkins)
Figure 6
Rapid lens surface drying due to lipid
contamination of an Acuvue Advance lens
(picture by courtesy of Brian Tompkins)
Figure 7
Fluorescein pooling on the corneal surface due
to indentations caused by mucin balls behind a
silicone hydrogel lens
37 | August 20 | 2004 OUT

Continuing Education and Training
Lyndon Jones PhD, FCOptom, DipCLP, DipOrth, FAAO, (DipCL)
and Kathy Dumbleton MSc, MCOptom, FAAO
Figure 8
CLPU in a patient wearing silicone hydrogel
lenses on an overnight basis
debris particles observed between the back
surface of a contact lens and the cornea 94
and are believed to consist of mucin and
lipid 100 . They are common after overnight
wear of silicone hydrogels, particularly in
patients with steeper than average corneal
curvature 94,98 , and can result in significant
depressions in the corneal epithelium
following lens removal 97,101 . These
depressions ‘pool’ fluorescein but do not
result in true staining (Figure 7).
Mucin balls are believed to occur due to
the stiff lens materials shearing the tear
film and rolling up small balls of tear film
mucin with lipid. They are seen to occur
more frequently with Focus Night & Day
than with PureVision 99 (supporting the role
of material stiffness) and we anecdotally
feel they are less likely to occur with the
Focus Night & Day lens when fitting the
8.40 base curve than with the 8.60
(supporting the belief that lens movement
is also a factor).
Comfort and satisfaction
A major problem with contact lenses
continues to be their reduction in perceived
comfort over the wearing period,
particularly as the lens surface dehydrates.
Dry eye symptoms are reported by 20-50%
of soft lens wearers 102,103 , with 35% of
patients permanently ceasing lens wear due
to complications associated with
discomfort and dryness 104 . For silicone
hydrogel lenses to be successful, they must
be at least as comfortable as conventional
lens materials.
The sensation of ‘dryness’ and
‘discomfort’ is a complex subject and is
without question related to a variety of
factors. One factor to consider is that of
lens dehydration, since the subjective
symptom of dryness appears to occur more
frequently in soft lens wearers whose lenses
undergo greater dehydration during openeye
wear 105 . Material composition influences
dehydration rate and degree 106 . In a clinical
environment, it has been noted that the
majority of wearers of silicone hydrogel
lenses report that their lenses feel less ‘dry’
than their previous conventional lenses,
despite considerably longer wearing
times 54,107 .
Silicone hydrogel materials, which have
lower water contents than currently
available materials, may produce less
subjective dryness symptoms through
reduced in-eye dehydration, enhanced
wettability, reduced hydrophobic
interactions with the eyelid, reduced
deposition and/or increased oxygen
performance. Published work to date shows
that silicone hydrogel lens materials
dehydrate at a slower rate and to a lesser
extent than conventional hydrogel
materials 102,109 and may partially help to
explain this reduction in the sensation of
dryness. However, despite the general
reports of increased comfortable wearing
time with these novel materials, the major
symptom reported by patients continues to
be that of dryness 54 . Overall, published
studies to date indicate successful clinical
rates with overnight wear of around
75% 56,65,79,80,99,110 .
No published studies yet exist for
patients wearing these lenses on a daily
wear basis, so clinical success rates for daily
wear are difficult to estimate. Studies in
which silicone hydrogel lenses have been
compared to conventional materials have
confirmed that these lenses are at least as
comfortable as conventional lenses when
worn on a continuous wear basis 54 .
However, in some instances, an
‘adaptation’ period is required, particularly
if a patient is refitted from a thinner, less
mobile, more flexible lens material. This
adaptation period is typically completed a
week or so after the initial fitting takes
place, and is usually due to the eye getting
used to the increased lens movement and
oxygenation afforded by the lens material.
Inflammatory and
infective complications
The success of overnight wear with any new
material will ultimately be based upon
their clinical performance, particularly with
regards to infectious and inflammatory
complications, which are greater when
lenses are worn overnight 70 . Complications
will occur in silicone hydrogel lens wearers,
as they occur with any lens type. Many of
the complications that do occur are
patient-driven, being associated with
colonisation of the ocular adnexa and/or
contact lens by microorganisms,
predominately bacteria 111 . Adverse events
can then occur as a direct result of
infiltration of tissue by microorganisms, or
secondary to the toxins and by-products,
which they produce. In order for a ‘full
blown’ infection such as microbial keratitis
(MK) to occur, the bacteria must bind to
the corneal epithelium and then be able to
invade the cornea to infect the corneal
stroma. It is for this reason that
inflammatory complications are more
common than their more serious,
infectious counterparts. Fortunately, the eye
has a range of natural defence systems
which very effectively protect its tissues
from inflammation and infection 111 .
Inflammation
A number of inflammatory complications
may occur with silicone hydrogel lens wear.
These include contact lens peripheral ulcer
(CLPU), infiltrative keratitis (IK) and
contact lens acute red eye (CLARE). While
all of these complications have been
described on many previous occasions, a
number of specific differences are
associated with their presentation in
silicone hydrogel lens wearers. Probably the
most distinctive difference is that the
conditions are generally much less severe in
these wearers. This is most likely due to the
lack of associated corneal hypoxia and as a
result, a much healthier corneal
epithelium 112 .
A CLPU is an inflammatory response
which results in lesions often termed
‘sterile ulcers’. This is somewhat misleading
since CLPUs do not occur as a result of
infection but rather as a hypersensitive
reaction to the (usually gram positive)
exotoxins released by pathogenic bacteria.
A CLPU is usually a single, peripheral or
mid-peripheral white/grey lesion in the
anterior stroma and epithelium that is
0.1-2.0mm in diameter, circular in
appearance and stains with fluorescein
(Figure 8). Patients usually complain of
mild to moderate pain (foreign body
sensation), mild lacrimation and mild
photophobia. Approximately 50% of
CLPUs are asymptomatic and present
without symptoms simply as scars at
follow-up appointments 113 . Following the
acute ‘phase’, the epithelium regenerates
within one to two days over the lesion.
Diffuse infiltration surrounding the lesion
may develop. A very well defined circular
‘scar’ remains, gradually fading with time,
but still present at least six months after the
event. Differential diagnosis from MK
extremely important 114,115 .
CLARE is a unilateral, acute
inflammatory response to gram negative
organisms which colonise the lens and
release endotoxins 116-118 . A higher incidence
of CLARE occurs in patients with upper
respiratory infection, and these cases may
be due to the presence of Haemophilus
influenzae 118 . Patients with CLARE are
typically woken in the early morning by a
moderately painful (foreign body
sensation) red eye, with associated
epiphora and photophobia. Focal or
diffuse sub-epithelial infiltrates are usually
observed in the mid-periphery of the
cornea close to the limbus. The infiltrates
rarely stain and rapidly resolve. There is
associated marked circum-corneal limbal
hyperaemia, but generally no anterior
chamber reaction or lid oedema.
IK is the term used to describe all
inflammatory events not categorised as
CLPU, or CLARE. There are many factors
which contribute to these inflammatory
events, including a foreign body trapped
beneath a contact lens, mechanical trauma,
solution preservatives, tight lenses, bacteria
and/or toxins. IK can occur with both daily
38 | August 20 | 2004 OUT

Continuing Education and Training
and overnight wear and may also occur in
non-lens wearers. However, the incidence is
higher in lens wearers, presumably as a
response to toxins being concentrated
against the cornea by the hydrogel lens.
Many IK cases are due to the presence of
gram positive exotoxins found on the lid
margin 111 .
There is a large degree of variability in
the severity of symptoms associated with IK
and in some cases, there are no related
symptoms. In these cases, the condition is
termed ‘asymptomatic infiltrative keratitis’
(AIK). Common symptoms reported with
IK include mild to moderate irritation
(often a foreign body discomfort), mild
hyperaemia, lacrimation, photophobia and
occasionally mild discharge. Slit lamp
examination may reveal moderate bulbar
and limbal redness, mild to moderate
diffuse and/or small focal infiltrates
(Figure 9). These infiltrates may be located
anywhere in the cornea but are usually
peripherally situated in the limbal area.
Most cases of inflammation in contact
lens wearers are self-limiting. The first step
in the management of inflammatory
complications in silicone hydrogel wearers
is to temporarily discontinue lens wear
until there is full resolution of signs and
symptoms. As with all inflammatory
adverse events, the patient should be
monitored carefully over the first 24 hours
to confirm the diagnosis. In most cases, no
medication is required, however ocular
lubricants may be dispensed to alleviate
symptoms. Severe cases may benefit from a
prophylactic topical antibiotic to reduce the
chance of secondary infection. Since there
appears to be a patient predisposition for
inflammatory events 54,113 , the introduction
of lid hygiene measures (warm compresses
and lid scrubs) to the daily routine for
these patients is strongly recommended.
Patients repeatedly experiencing
inflammatory events with overnight wear
should be advised to wear their lenses on a
daily wear basis only.
Infection
MK is a much more serious adverse event
which may occur in silicone hydrogel lens
wearers. Fortunately, the prevalence of MK
within the general population is extremely
low, due in part to the exceptional defence
mechanisms that protect the ocular surface;
however, MK remains the most serious
complication of contact lens wear. Several
case reports of MK with silicone hydrogel
lenses have been published 70,119-123 , and this
is not surprising, given that even non-lens
wearers can develop this condition.
Patients with MK present with marked
increasing pain, intense lacrimation,
hyperaemia and photophobia. A single
paracentral or central irregular lesion may
be observed with focal, and often
significant, diffuse infiltration. The lesion is
characterised by excavation of the
epithelium, Bowman’s layer and the
stroma, and usually stains. An anterior
chamber reaction and lid oedema are also
common. The condition is associated with
severe, progressive corneal suppuration.
MK is a true ocular emergency and
lenses must be removed and immediate
medical management sought or instituted,
with referral to an experienced corneal
specialist in severe cases. Treatment usually
consists of high doses of fortified
antibiotics or fluoroquinolone agents (e.g.
Ciloxan – 0.3% ciprofloxacin) every 15 to
30 minutes for the first two days, and then
every four hours for a further 10 to 12 days.
Prognosis for most patients is good,
particularly if referral occurs early in the
disease process, but it does depend upon
the causative organism. In all cases, a scar
will remain. Most patients are able to
resume daily contact lens wear within a six
to 12-month period, depending upon the
size and location of the scar.
While we anxiously await the results of
prospective clinical trials to determine the
actual rate of infection with silicone
hydrogel lenses 124 , we can use a number of
surrogate measures to predict what the
likely outcome will be. An epithelium
which is not compromised by lens wear,
and which does not allow significant
binding of bacteria, should alleviate serious
infections 125,126 . Work from Dwight
Cavanagh and colleagues in the US has
shown that while Pseudomonas aeruginosa
binds to epithelial cells when they are
exposed to low Dk materials, binding
decreases significantly when high Dk
silicone hydrogel lenses are worn 112,126-129 .
Certainly, from a theoretical point of
view, silicone hydrogel lens wear should
reduce the risk of MK and prospective data
due within the next 12 months will
confirm or deny this premise. However,
given that inflammatory complications are
primarily patient driven, due to lens
contamination, it is likely that until
bacteria-resisting contact lens materials are
developed 130-132 , the rate of inflammation
with silicone hydrogel lens materials will
be similar to that encountered with
conventional lens materials.
Future developments
Increasingly, contact lens manufacturers are
looking at developing novel silicone-based
hydrogels and the foreseeable future for
this group of lens materials looks
promising. It is likely that the next 10 years
Figure 9
Focal infiltrates subsequent to silicone
hydrogel continuous wear
will be dominated by the release of
silicone-based hydrogels from all
manufacturers. These materials will
probably have stiffness levels closer to
conventional hydrogels and better surface
treatments, which truly make the surfaces
hydrophilic. Ideally, such materials would
support a tear film for longer than the
typical seven to eight seconds seen with
currently available materials, and consist of
polymers that would resist contamination
with pathogenic organisms 131 . Such
materials should result in increased
comfort and reduced inflammatory
complications compared with currently
available materials, and would have a
significant impact on growing the contact
lens market.
Conclusion
To answer the title of this article, we have
no doubts that silicone hydrogel materials
will displace conventional lens materials
over time, for both daily and overnight
wear. Their improved physiological
performance, excellent handling
characteristics and improved comfort make
them the ideal lens material. We believe
that these lenses should not be reserved for
overnight use only or used as ‘troubleshooting’
lenses, but should become the
standard lens type to be fitted to all
patients. We predict that, regardless of the
success of continuous wear, highly oxygen
permeable soft lens polymers will
dominate the contact lens market in 10
years’ time.
References
For a full set of references, email
nicky@optometry.co.uk or visit
www.optometry.co.uk/references/
2004.08.04-Jones.doc.
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Continuing Education and Training
Module 1 Part 2 of Modern contact lens practice
Silicone hydrogels – Will they displace conventional lenses?
Please note there is only ONE correct answer
1. How much corneal swelling typically
occurs with conventional hydrogel
lens materials when worn overnight?
a. 0%
b. 5%
c. 10%
d. 20%
2. Which of the following statements is
true regarding the surface of first
generation silicone hydrogel contact
lenses?
a. The surface is treated with a gas plasma
technique to improve wettability
b. The surface is treated with antibiotics to
resist infection
c. The surface is coated with lipids to resist
protein deposition
d. The surface is very wettable and deposits
lots of proteins
3. Which property does silicone rubber
have that would be most relevant to its
potential use as a contact lens material?
a. Extremely high wettability
b. Extremely high oxygen permeability
c. Extremely high UV blocking properties
d. Extremely high toughness
4. Of the following four values, which is
most representative of an approximate
Dk (oxygen permeability) for a typical
silicone hydrogel lens material?
a. Five barrers
b. Twenty-five barrers
c. One hundred barrers
d. Three hundred barrers
5. When a patient with microcysts is
refitted with silicone hydrogel lenses,
what is the typical appearance within
the cornea over the next one to two
weeks?
a. The microcysts worsen for about a week
and then suddenly disappear
b. The microcysts gradually decrease in
number until they disappear
c. The microcysts increase rapidly and
then gradually reduce
d. The microcysts suddenly disappear
after the first night of wear of the new
lenses
6. Which of the following deposits from
the tears can be a problem for
some wearers of silicone hydrogel
lenses?
a. Protein
b. Lipid
c. Mucin
d. Fungus
7. Which one of the following lenses is
the ‘stiffest’, or has the highest
modulus of elasticity?
a. Acuvue 2
b. Focus Night & Day
c. Acuvue Advance
d. Proclear
8. What, approximately, is the oxygen
transmissibility (Dk/t) required to
minimise oedema for most patients
on a daily wear basis?
a. Ten units
b. Thirty units
c. Sixty units
d. One hundred and fifty units
9. Approximately how much stiffer are
silicone hydrogel lenses than an
Acuvue lens?
a. Three times stiffer
b. Eight times stiffer
c. Fifteen times stiffer
d. Fifty times stiffer
MCQs
10. Which one of the following
statements is true?
a. Hypoxia is greater with silicone hydrogel
lenses compared with conventional
lenses during overnight wear
b. Microbial keratitis occurs more
frequently with silicone hydrogel lenses
than conventional lenses on extended
wear
c. Giant papillary conjunctivitis occurs
more often with conventional lenses
than silicone hydrogel lenses
d. Infiltrative events with conventional
lenses on extended wear are similar to
those seen with silicone hydrogel lenses
on extended wear
11. Near vision problems within the first
month of refitting with silicone
hydrogel lenses are most likely
attributed to which one of the
following factors?
a. Lipid deposition
b. Residual unmasked astigmatism
c. A rebound hyperopic shift
d. Mucin ball formation in the post lens
tear film
12. The most appropriate management for
the majority of cases of infiltrative
keratitis in extended wear silicone
hydrogel patients is which one of the
following?
a. Lens replacement and daily wear until
resolution
b. Lens replacement and no further
extended wear
c. Temporary discontinuation of lens wear
and topical antibiotics
d. Temporary discontinuation of lens wear
and ocular lubricants until resolution,
followed by the instigation of lid
hygiene
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40 | August 20 | 2004 OUT