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Continuing Education and Training<br />

Silicone hydrogels<br />

Will <strong>the</strong>y displace conventional lenses?<br />

Silicone hydrogel contact lenses first appeared commercially in<br />

Europe in 1999 and in <strong>the</strong> US in 2001. Since <strong>the</strong>n, <strong>the</strong>y have<br />

shown tremendous growth 1 , with approximate worldwide sales<br />

in 2003 of $150m and currently, over one million wearers<br />

worldwide 2 . Initially developed for <strong>the</strong> extended wear market,<br />

practitioners have embraced <strong>the</strong> use of such materials for both<br />

overnight and daily wear use.<br />

This article discusses <strong>the</strong> development of<br />

silicone hydrogels and reviews <strong>the</strong>ir clinical<br />

performance to date. The specific aim is to<br />

determine if <strong>the</strong>se lenses will ultimately<br />

replace conventional hydrogel lens<br />

materials, and become <strong>the</strong> standard lens<br />

type to be fitted for both daily and<br />

overnight wear.<br />

History of overnight wear<br />

Throughout <strong>the</strong> 1970s and early 1980s,<br />

manufacturers released a variety of<br />

materials which were intended for<br />

overnight wear. This was driven largely by<br />

consumer desire for a convenient<br />

correction alternative to <strong>the</strong>ir spectacles.<br />

These early materials were often worn for<br />

up to a month at a time without being<br />

removed and achieved great commercial<br />

success. John de Carle reported success with<br />

over 2,000 patients in <strong>the</strong> early 1970s, and<br />

o<strong>the</strong>r authors reported similarly high levels<br />

of clinical success up to <strong>the</strong> mid 1980s 3-7 .<br />

As a result of such positive data, extended<br />

wear for cosmetic use for up to 30 days was<br />

approved by <strong>the</strong> Food & Drug<br />

Administration (FDA) in 1981, sparking an<br />

explosion in <strong>the</strong> number of patients being<br />

fitted with lenses for overnight wear.<br />

Very soon afterwards, reports of corneal<br />

ulceration with significant vision loss<br />

began appearing in journals 8,9 , and <strong>the</strong><br />

safety of overnight wear was questioned in<br />

both peer-reviewed journals 10 and <strong>the</strong> lay<br />

media. The Contact Lens Institute in <strong>the</strong> US<br />

sponsored studies to investigate <strong>the</strong> relative<br />

risk and incidence of infectious keratitis.<br />

The results from <strong>the</strong>se studies were<br />

published in 1989 11,12 and clearly<br />

demonstrated that overnight wear of lenses<br />

carried with it a significantly increased risk<br />

of corneal infection. As a result, <strong>the</strong> FDA<br />

immediately reduced <strong>the</strong> approved length<br />

of time for overnight wear without removal<br />

from 30 to seven days.<br />

In <strong>the</strong> mid 1980s, it was believed that<br />

<strong>the</strong> corneal infections seen with overnight<br />

wear were probably due to poor hygiene<br />

and compliance, and that <strong>the</strong> principal<br />

factor driving such infection rates was<br />

patients re-inserting poorly disinfected<br />

lenses. It was hypo<strong>the</strong>sised that using<br />

lenses on a disposable or frequent<br />

replacement basis, in which <strong>the</strong> lenses were<br />

inserted once only and <strong>the</strong>n discarded<br />

upon removal, would be likely to have an<br />

impact on <strong>the</strong> infection rates reported.<br />

Such a concept became a clinical reality in<br />

1987, with <strong>the</strong> introduction of disposable<br />

extended wear lenses in <strong>the</strong> US.<br />

The first published large-scale study<br />

appeared to support such a concept 13 , but<br />

fairly rapidly reports of infectious keratitis<br />

started to appear 14 . The final proof that<br />

disposability had no impact on <strong>the</strong> rate of<br />

ulceration with conventional hydrogel<br />

materials worn overnight came with <strong>the</strong><br />

publication of a paper from Holland in<br />

1999 15 , which showed that <strong>the</strong> rate of<br />

ulcerative keratitis was exactly that found<br />

10 years previously in <strong>the</strong> US 12 – before<br />

disposability was commonplace. This<br />

publication clearly showed that overnight<br />

wear with conventional soft lens materials<br />

should be discouraged due to <strong>the</strong> increased<br />

risk that such a modality had on <strong>the</strong><br />

development of sight-threatening keratitis.<br />

Despite this, patients still seek<br />

alternative vision correction methods to<br />

liberate <strong>the</strong>m of spectacles, with refractive<br />

surgery being extremely successful, and <strong>the</strong><br />

most widely undertaken cosmetic<br />

procedure in <strong>the</strong> world today. Patients still<br />

sleep in lenses overnight even when told<br />

not to do so, with an estimated 40% of<br />

patients occasionally or frequently sleeping<br />

in <strong>the</strong>ir lenses 16 . Clearly, patients desire a<br />

lens which can be worn overnight and will<br />

undertake this procedure whe<strong>the</strong>r <strong>the</strong>ir<br />

practitioner sanctions it or not.<br />

Consequently, it became clear that <strong>the</strong><br />

contact lens industry needed to develop<br />

safer materials for overnight lens wear, as<br />

<strong>the</strong>y would swiftly become a commercial<br />

success.<br />

Development of highly<br />

permeable hydrogel materials<br />

In <strong>the</strong> early 1990s, contact lens companies<br />

began to investigate <strong>the</strong> possibility of<br />

developing novel contact lens polymers<br />

which transmitted high levels of oxygen, on<br />

<strong>the</strong> basis that more oxygen may make<br />

overnight wear safer. Traditional hydrogel<br />

materials are polymers which are typically<br />

composed of several monomers joined<br />

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Module 1 Part 2<br />

Modern contact lens practice<br />

About <strong>the</strong> authors<br />

Lyndon Jones is Associate Professor in<br />

<strong>the</strong> School of <strong>Optometry</strong>, and Associate<br />

Director of <strong>the</strong> Centre for Contact Lens<br />

Research (CCLR) at <strong>the</strong> University of<br />

Waterloo, Canada. Kathy Dumbleton is<br />

Senior Researcher in <strong>the</strong> CCLR.<br />

33 | August 20 | 2004 OUT

Continuing Education and Training<br />

Lyndon Jones PhD, FCOptom, DipCLP, DipOrth, FAAO, (DipCL)<br />

and Kathy Dumbleton MSc, MCOptom, FAAO<br />

toge<strong>the</strong>r in chains to form a polymer<br />

network. The oldest of <strong>the</strong>se is <strong>the</strong> first<br />

hydrogel material used for contact lens<br />

wear – poly(2-hydroxyethyl methacrylate)<br />

or polyHEMA – which was developed by<br />

Wichterle in <strong>the</strong> 1960s 17 . This material was<br />

granted FDA approval in 1971, and for<br />

many years was <strong>the</strong> sole hydrogel contact<br />

lens material available; it is still sold in vast<br />

numbers today.<br />

The principal disadvantage of<br />

polyHEMA is that it relies upon water to<br />

transport oxygen through <strong>the</strong> material.<br />

Water has a limited ability to dissolve and<br />

transport oxygen, with an approximate<br />

oxygen permeability (Dk) of around 80<br />

barrers. In conventional lens materials, Dk<br />

is proportional to water content 18 , such that<br />

conventional hydrogel materials have Dks<br />

ranging from about 10 to 35 barrers. In<br />

order to increase <strong>the</strong> Dk of a conventional<br />

hydrogel contact lens material beyond that<br />

of HEMA, it is necessary to incorporate<br />

monomers which will bind more water into<br />

<strong>the</strong> polymer. These higher water content<br />

materials typically use HEMA, or methyl<br />

methacrylate (MMA), in conjunction with<br />

more hydrophilic monomers such as<br />

N-vinyl pyrrolidone (NVP) or methacrylic<br />

acid (MA). In comparison, rigid gas<br />

permeable (RGP) lens materials transmit<br />

<strong>the</strong>ir oxygen through <strong>the</strong> polymer phase,<br />

and use monomers with higher oxygen<br />

permeabilities than those used in<br />

conventional hydrogels 19 .<br />

From a clinical perspective, oxygen<br />

transport to <strong>the</strong> cornea depends upon both<br />

<strong>the</strong> Dk of <strong>the</strong> material and <strong>the</strong> lens<br />

thickness (t). Thinner lenses provide <strong>the</strong><br />

cornea with more oxygen, since <strong>the</strong>re is less<br />

of a barrier for <strong>the</strong> oxygen to diffuse<br />

through. The term ‘Dk/t’ describes <strong>the</strong><br />

oxygen transmissibility of a lens and gives a<br />

quantitative indication of <strong>the</strong> amount of<br />

oxygen that a lens-wearing eye will receive<br />

through <strong>the</strong> lens. It is a more clinically<br />

useful number than Dk, which gives no<br />

Hydrogels<br />

Low water content (38%)<br />

Mid water content (55%)<br />

High water content (70%)<br />

Dk/t<br />

15<br />

27<br />

24<br />

indication of <strong>the</strong> effect of lens thickness or<br />

lens design. Ideally, hydrogel lenses would<br />

have a high Dk (via <strong>the</strong> use of a high water<br />

content) and a thin centre thickness.<br />

However, such lenses are impractical<br />

because <strong>the</strong>y rapidly dehydrate and result<br />

in high levels of corneal staining 20,21 . Such a<br />

process results in lenses with a high water<br />

content having a thickness that is<br />

substantially greater than that seen in low<br />

water content materials, which limits <strong>the</strong><br />

Dk/t clinically obtainable.<br />

Table 1 details typical Dk/t values<br />

available for conventional hydrogel<br />

materials and commonly encountered RGP<br />

materials, at centre thicknesses normally<br />

found for -3.00D lenses. It is clear that<br />

conventional hydrogel materials have a<br />

relatively low Dk/t compared with RGP<br />

materials.<br />

Silicone hydrogel materials<br />

Table 1 shows that RGP materials have<br />

significantly greater oxygen<br />

transmissibilities than hydrogels, due to <strong>the</strong><br />

fact that <strong>the</strong>y all contain fluorine or<br />

silicone, both of which have an incredible<br />

ability to transport oxygen 22 . RGP materials<br />

frequently incorporate into <strong>the</strong>ir<br />

composition a silicon-containing<br />

monomer, commonly called TRIS 23,24 . The<br />

ability of silicone-based lenses to provide<br />

<strong>the</strong> cornea with substantial amounts of<br />

oxygen has been understood for many<br />

years, with silicone-elastomeric lenses being<br />

used clinically for <strong>the</strong>rapeutic and<br />

paediatric applications for over 20 years 25 .<br />

These lenses offer exceptional oxygen<br />

transmission and durability, but a number<br />

RGPs<br />

Silicone acrylates<br />

Fluorosilicone acrylates<br />

Fluoropolymers<br />

Dk/t<br />

27<br />

60<br />

130<br />

Table 1<br />

Typical examples of Dk/t for conventional lens materials<br />

of major limitations are associated with<br />

<strong>the</strong>ir use in clinical practice. Fluid is unable<br />

to flow through <strong>the</strong>se lens materials,<br />

resulting in frequent lens binding to <strong>the</strong><br />

ocular surface 26 , and <strong>the</strong> lens surfaces are<br />

extremely hydrophobic, resulting in<br />

marked lipid deposition 27 .<br />

Ideally, manufacturers would like to<br />

combine <strong>the</strong> hydrophilic properties of<br />

HEMA-based lenses with <strong>the</strong> oxygen<br />

transmission of silicone-elastomers.<br />

Unfortunately, <strong>the</strong> process of combining<br />

conventional hydrogel monomers with<br />

hydrophobic silicone proved to be an<br />

enormous challenge and it has taken over<br />

20 years of considerable intellectual input<br />

and financial resources for <strong>the</strong>se materials<br />

and designs to be created. Indeed, <strong>the</strong><br />

process of combining <strong>the</strong>se monomers has<br />

been likened to efforts of combining oil<br />

with water, while maintaining optical<br />

clarity 24 . Eventually, both CIBA Vision and<br />

Bausch & Lomb were able to overcome<br />

<strong>the</strong>se difficulties and first generation<br />

silicone hydrogel lenses became a<br />

commercial reality at <strong>the</strong> end of <strong>the</strong> 1990s.<br />

Three silicone hydrogel lens materials<br />

are currently commercially available<br />

(Table 2). CIBA Vision’s Focus® Night &<br />

Day® material, lotrafilcon A, employs a<br />

co-continuous biphasic, or two channel,<br />

molecular structure, in which two phases<br />

persist from <strong>the</strong> front to <strong>the</strong> back surface of<br />

<strong>the</strong> lens 29 . The siloxy phase facilitates <strong>the</strong><br />

solubility and transmission of oxygen,<br />

while <strong>the</strong> hydrogel phase transmits water<br />

and oxygen, allowing good lens movement.<br />

The two phases work concurrently, to allow<br />

<strong>the</strong> co-continuous transmission of oxygen<br />

Proprietary name<br />

US adopted name<br />

Manufacturer<br />

Centre thickness<br />

(@ -3.00D) mm<br />

Water content<br />

Oxygen permeability (x 10 –11 )<br />

Oxygen transmissibility (x 10 –9 )<br />

Modulus (psi)*<br />

Surface treatment<br />

FDA group<br />

Principal monomers<br />

PureVision<br />

Balafilcon A<br />

Bausch & Lomb<br />

0.09<br />

36%<br />

99<br />

110<br />

148<br />

Plasma oxidation,<br />

producing glassy<br />

islands<br />

III<br />

NVP + TPVC + NCVE<br />

+ PBVC<br />

Focus Night & Day<br />

Lotrafilcon A<br />

CIBA Vision<br />

0.08<br />

24%<br />

140<br />

175<br />

238<br />

25nm plasma coating<br />

with high refractive<br />

index<br />

I<br />

DMA + TRIS + siloxane<br />

macromer<br />

Table 2<br />

Silicone hydrogel lens materials<br />

Acuvue Advance<br />

Galyfilcon A<br />

Vistakon<br />

0.07<br />

47%<br />

60<br />

86<br />

65<br />

No surface treatment.<br />

Internal wetting agent<br />

(PVP)<br />

I<br />

Unpublished<br />

DMA (N, N-dimethylacrylamide); HEMA (poly-2-hydroxyethylmethacrylate); MA (methacrylic acid);<br />

NVP (N-vinyl pyrrolidone); TPVC (tris-(trimethylsiloxysilyl) propylvinyl carbamate); NCVE (N-carboxyvinyl<br />

ester); PBVC (poly[dimethysiloxy] di [silylbutanol] bis[vinyl carbamate]); PVP (polyvinyl pyrrolidone)<br />

* Modulus data taken from Steffen and McCabe 28<br />

34 | August 20 | 2004 OUT

Continuing Education and Training<br />

Figure 2<br />

Graphical representation of water content, Dk<br />

and modulus of elasticity for <strong>the</strong> three silicone<br />

hydrogel materials and a polyHEMA-based<br />

material (Acuvue 2). The graph shows that <strong>the</strong><br />

silicone-based lens materials with <strong>the</strong> highest<br />

amount of silicone have <strong>the</strong> highest modulus (or<br />

<strong>the</strong> greatest degree of ‘stiffness’) and <strong>the</strong><br />

highest oxygen permeability<br />

and aqueous salts. Lotrafilcon A is<br />

comprised of a fluoroe<strong>the</strong>r macromer copolymerised<br />

with <strong>the</strong> monomer TRIS and<br />

N, N-dimethyl acrylamide (DMA), in <strong>the</strong><br />

presence of a diluent 24,30 . The resultant<br />

silicone hydrogel material has a water<br />

content of 24% and a Dk of 140 barrers 31 .<br />

Bausch and Lomb’s PureVision®<br />

material, balafilcon A, is a homogeneous<br />

combination of <strong>the</strong> silicone-containing<br />

monomer polydimethylsiloxane (a vinyl<br />

carbamate derivative of TRIS) copolymerised<br />

with <strong>the</strong> hydrophilic hydrogel<br />

monomer N-vinyl pyrrolidone (NVP) 30,32-35 .<br />

This silicone hydrogel material has a water<br />

content of 36% and a Dk of 110 barrers.<br />

Vistakon’s Acuvue® Advance material,<br />

galyfilcon A, is <strong>the</strong> newest of <strong>the</strong> three<br />

materials and very little has been published<br />

to date on <strong>the</strong> material’s composition 36 ,<br />

although some deductions can be made<br />

from <strong>the</strong> patent literature dealing with<br />

Vistakon’s HydraClear technology. It has a<br />

higher water content than <strong>the</strong> o<strong>the</strong>r two<br />

materials (47%), and thus <strong>the</strong> lowest Dk<br />

(60 barrers). Whereas both PureVision and<br />

Focus Night & Day are approved for<br />

overnight use, Acuvue Advance is only<br />

approved for daily wear. It is <strong>the</strong> first of <strong>the</strong><br />

so-called ‘second generation’ silicone<br />

hydrogels 37 and is <strong>the</strong> only one available<br />

thus far which has an inversion marker and<br />

a UV blocker, with a reported Class 1 UV<br />

protection, blocking >90% of UVA and<br />

>99% of UVB rays 28,36 .<br />

In addition to increased oxygen<br />

transmission, o<strong>the</strong>r major differences exist<br />

between silicone hydrogel materials and<br />

conventional hydrogels, primarily relating<br />

to <strong>the</strong>ir mechanical and surface properties.<br />

Mechanical properties<br />

Silicone hydrogel lens materials are<br />

significantly ‘stiffer’ than <strong>the</strong>ir conventional<br />

hydrogel counterparts, due to <strong>the</strong><br />

incorporation of silicone. The modulus of<br />

<strong>the</strong> first two silicone hydrogel materials is<br />

some four to six times greater than low<br />

rigidity conventional materials, such as<br />

etafilcon A (which is used in <strong>the</strong> Acuvue<br />

lens). The newest silicone hydrogel<br />

material, Acuvue Advance, has a modulus<br />

which is much closer to conventional<br />

materials, being only 1.5 times more rigid<br />

than etafilcon 28,36 . According to Johnson &<br />

Johnson Vision Care, this reduced stiffness<br />

is due to <strong>the</strong> reduced amount of silicone<br />

present in this new material, along with<br />

benefits derived from <strong>the</strong> internal wetting<br />

agent HydraClear, which is based upon<br />

polyvinyl pyrrolidone (PVP) 28,36 . Figure 1<br />

graphically indicates <strong>the</strong> inverse<br />

relationship between water content of<br />

hydrogel materials and oxygen<br />

permeability and material stiffness. The<br />

materials with <strong>the</strong> highest ratio of silicone<br />

to water are <strong>the</strong> stiffest.<br />

Increased rigidity, or stiffness, has some<br />

advantages, in that <strong>the</strong> lenses handle very<br />

well and so are a perfect choice for people<br />

who exhibit poor handling capabilities.<br />

Increased rigidity might also suggest that<br />

such lenses mask more corneal astigmatism<br />

than traditionally very flexible hydrogel<br />

lenses – but that has not been our<br />

experience clinically or that of o<strong>the</strong>rs 38 . The<br />

mechanical properties of <strong>the</strong>se lenses do<br />

pose some problems, in that <strong>the</strong>y are less<br />

able to conform easily to <strong>the</strong> shape of <strong>the</strong><br />

eye and fitting is critical, with loose lenses<br />

exhibiting poor comfort 39,40 .<br />

Surface properties<br />

Historically, a huge impediment to <strong>the</strong><br />

development of silicone hydrogel lenses<br />

has related to <strong>the</strong> decreased wettability,<br />

increased lipid interaction and accentuated<br />

lens binding inherent in silicone-based<br />

materials, as previously described. In order<br />

to make <strong>the</strong> surfaces of silicone hydrogel<br />

lens materials hydrophilic and more<br />

wettable, techniques incorporating plasma<br />

into <strong>the</strong> surface processing of <strong>the</strong> lens have<br />

been developed 24,29,34,41 . The purpose of <strong>the</strong>se<br />

surface treatments is to mask <strong>the</strong><br />

hydrophobic silicone from <strong>the</strong> tear film,<br />

increasing <strong>the</strong> surface wettability of <strong>the</strong><br />

materials and reducing lipid deposition.<br />

The surfaces of Focus Night & Day<br />

lenses are permanently modified in a gas<br />

plasma reactive chamber to create a<br />

permanent, ultrathin (25nm), high<br />

refractive index, continuous hydrophilic<br />

surface 29,42,43 .<br />

PureVision lenses are surface treated in a<br />

gas plasma reactive chamber, which<br />

transforms <strong>the</strong> silicone components on <strong>the</strong><br />

surface of <strong>the</strong> lenses into hydrophilic<br />

silicate compounds 24,30,34,44 . Glassy,<br />

discontinuous silicate ‘islands’ result 30 , and<br />

<strong>the</strong> hydrophilicity of <strong>the</strong> transformed<br />

surface areas ‘bridges’ over <strong>the</strong> underlying<br />

hydrophobic balafilcon A material. The<br />

flow of oxygen and fluids through <strong>the</strong><br />

lenses is not impeded by <strong>the</strong>se surface<br />

modifications. Both surface treatments<br />

become an integral part of <strong>the</strong> lens, and are<br />

not surface coatings that can be easily<br />

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35 | August 20 | 2004 OUT

Continuing Education and Training<br />

Lyndon Jones PhD, FCOptom, DipCLP, DipOrth, FAAO, (DipCL)<br />

and Kathy Dumbleton MSc, MCOptom, FAAO<br />

‘stripped’ away from <strong>the</strong> base material<br />

during daily handling and cleaning.<br />

The Acuvue Advance lens material is <strong>the</strong><br />

first non-surface treated silicone hydrogel<br />

to become a commercial reality. By<br />

overcoming <strong>the</strong> need for a surface<br />

treatment, <strong>the</strong> cost savings are considerable<br />

and it is likely that future generation<br />

silicone hydrogels will ei<strong>the</strong>r need to avoid<br />

<strong>the</strong> requirement for surface treatment, or<br />

develop cheaper methods to achieve this.<br />

Acuvue Advance uses an internal wetting<br />

agent (HydraClear) based upon PVP, which<br />

is designed to provide a hydrophilic layer at<br />

<strong>the</strong> surface of <strong>the</strong> material which ‘shields’<br />

<strong>the</strong> silicone at <strong>the</strong> material interface,<br />

<strong>the</strong>reby reducing <strong>the</strong> degree of<br />

hydrophobicity typically seen at <strong>the</strong> surface<br />

of silicone hydrogels 28,36 .<br />

Analysis of <strong>the</strong> surfaces of both<br />

PureVision and Focus Night & Day has<br />

shown that <strong>the</strong>se surface treatments have<br />

only been partially effective in masking <strong>the</strong><br />

silicone, with <strong>the</strong> lenses having significantly<br />

more silicon exposed at <strong>the</strong> surface than<br />

conventional lenses 45,46 and a more<br />

hydrophobic surface (as evidenced by <strong>the</strong><br />

presence of higher water contact angles) 47,48 .<br />

Although <strong>the</strong> Acuvue Advance material has<br />

a lower silicone and higher water content,<br />

this does mean that as <strong>the</strong> surface begins to<br />

dehydrate, for example when <strong>the</strong> tear film<br />

breaks up <strong>the</strong>re will still <strong>the</strong>oretically be<br />

exposure of unmasked silicone groups, as<br />

with <strong>the</strong> o<strong>the</strong>r two silicone hydrogels.<br />

Fur<strong>the</strong>r work is required to quantify <strong>the</strong><br />

exact amount of silicon exposed on this<br />

particular surface.<br />

Clinical performance<br />

Since <strong>the</strong> first report on silicone hydrogels<br />

in 1995 49 , over 150 papers have been<br />

published concerning <strong>the</strong>ir performance.<br />

An overview of <strong>the</strong>ir findings is timely to<br />

examine <strong>the</strong>ir success to date.<br />

Hypoxic complications<br />

As outlined above, <strong>the</strong> history of extended<br />

wear is replete with papers describing <strong>the</strong><br />

deleterious effects of hypoxia on <strong>the</strong><br />

cornea. The actual amount of oxygen<br />

required to eliminate oedematous<br />

complications is a matter for some<br />

conjecture 50 , with estimates suggesting that<br />

on a daily wear basis somewhere between<br />

24 51 and 35 Dk/t units 52 are required, and<br />

on an overnight basis, 87 to 125 Dk/t units<br />

are necessary 51-53 . Figure 2 graphically<br />

portrays <strong>the</strong> variation in Dk/t values for<br />

conventional and silicone hydrogel lenses<br />

and shows that on a daily wear basis,<br />

conventional lenses barely meet this daily<br />

wear requirement and fall drastically short<br />

of <strong>the</strong> requirement for overnight use.<br />

Silicone hydrogel lenses provide a<br />

significant degree of safety on daily wear<br />

and for <strong>the</strong> two most permeable materials<br />

(Focus Night & Day and PureVision) <strong>the</strong><br />

desirable overnight values are ei<strong>the</strong>r within<br />

limits or are exceeded.<br />

A number of clinical studies have now<br />

Figure 2<br />

Central oxygen transmissibility (Dk/t) values for silicone hydrogel and conventional lens materials.<br />

It is apparent that <strong>the</strong> three silicone hydrogels have substantially higher Dk/t values than <strong>the</strong><br />

conventional hydrogels<br />

been conducted that allow us to investigate<br />

whe<strong>the</strong>r <strong>the</strong>se <strong>the</strong>oretical Dk/t values have<br />

been confirmed clinically.<br />

Corneal swelling<br />

Corneal swelling is highly patient<br />

dependent and varies with <strong>the</strong> Dk and <strong>the</strong><br />

thickness of <strong>the</strong> lens. Conventional lens<br />

materials typically induce 10-12% corneal<br />

swelling if worn overnight, resulting in <strong>the</strong><br />

appearance of five to 10 corneal striae 54 . The<br />

typical level of oedema recorded following<br />

overnight eye closure with silicone<br />

hydrogels is in <strong>the</strong> order of 2-3% corneal<br />

swelling, which is comparable to that seen<br />

with no lens being worn, suggesting that<br />

silicone hydrogels have a minimal impact<br />

on physiology during overnight wear 49,55,56 .<br />

During daily wear, <strong>the</strong> improved<br />

physiological performance afforded by <strong>the</strong><br />

siloxane-based lenses should translate into<br />

substantial physiological benefits,<br />

particularly for patients who wear thick<br />

lenses due to <strong>the</strong>ir prescription or lens<br />

design (for example, torics or bifocals).<br />

Microcysts<br />

An excellent indicator of chronic hypoxia is<br />

<strong>the</strong> presence of microcysts, which is closely<br />

related to overnight corneal oedema 57 .<br />

Studies have demonstrated that <strong>the</strong><br />

microcyst response seen with continuous<br />

wear of silicone hydrogels is equivalent to<br />

that seen without lens wear 56,58-61 . One factor<br />

to consider at this point is <strong>the</strong> response<br />

seen in patients exhibiting microcysts who<br />

are refitted with silicone hydrogels after<br />

wearing conventional materials. These<br />

patients show a rapid increase in <strong>the</strong><br />

numbers of microcysts as <strong>the</strong> cornea<br />

becomes suddenly re-oxygenated. The<br />

number of microcysts <strong>the</strong>n gradually<br />

reduces over <strong>the</strong> first few weeks of lens<br />

wear, to <strong>the</strong> point where <strong>the</strong>y are<br />

eliminated – which is <strong>the</strong> same response as<br />

that seen in patients who cease lens<br />

wear 58,59 .<br />

Vascular responses<br />

Hyperaemia (both limbal and bulbar) and<br />

neovascularisation with silicone hydrogels<br />

are radically reduced compared with<br />

conventional hydrogels 53,56,62-65 (Figures 3a<br />

and 3b) and are at similar levels to those<br />

reported with non-lens wearers 61 . A small<br />

decrease in <strong>the</strong> degree of neovascularisation<br />

has been reported over a nine-month<br />

period of silicone hydrogel continuous<br />

wear in patients previously demonstrating<br />

moderate levels of vascularisation 63 . These<br />

findings confirm that hypoxia is a major<br />

factor in <strong>the</strong> production of ocular vascular<br />

responses with contact lenses, and lend<br />

considerable credence to <strong>the</strong> concept of<br />

using high oxygen permeable lens materials<br />

for all patients.<br />

Refractive error changes<br />

Studies have demonstrated that following<br />

<strong>the</strong> initial fitting with conventional<br />

extended wear lenses, a myopic prescription<br />

shift of approximately 0.50D occurs in<br />

certain patients, possibly due to a chronic<br />

hypoxic response in <strong>the</strong> cornea 7,56,66-68 . Once<br />

refitted with silicone hydrogel lenses, <strong>the</strong>se<br />

patients show a reversal of this response,<br />

with a hyperopic shift of <strong>the</strong> same<br />

magnitude occurring 56,67,68 . This can be<br />

Figures 3a and 3b<br />

a. Bulbar and limbal hyperaemia subsequent to<br />

daily wear with a conventional hydrogel lens;<br />

b. Reduction in bulbar and limbal hyperaemia in<br />

<strong>the</strong> same eye as Figure 3a when refitted with a<br />

silicone hydrogel lens on a daily wear basis.<br />

This change occurred within a two-week period<br />

a<br />

b<br />

36 | August 20 | 2004 OUT

Continuing Education and Training<br />

clinically significant if a refitting occurs in a<br />

patient who is on <strong>the</strong> verge of presbyopia.<br />

Approximately one month post-refitting, all<br />

patients should be carefully over refracted<br />

as <strong>the</strong> patient may be wearing a lens, which<br />

is over-minused or under-plussed, which<br />

could result in near vision problems.<br />

Mechanical complications<br />

The increased rigidity of <strong>the</strong>se new materials<br />

may be implicated in a variety of mechanical<br />

complications seen with silicone hydrogel<br />

lenses, as described next. It will be<br />

interesting to see if lower modulus silicone<br />

hydrogels such as Acuvue Advance produce<br />

fewer mechanical clinical complications<br />

than <strong>the</strong> first generation materials.<br />

CLAPC<br />

Contact lens-associated papillary<br />

conjunctivitis (CLAPC), or giant papillary<br />

conjunctivitis (GPC), has become a<br />

relatively rare finding in contact lens<br />

patients since <strong>the</strong> introduction of frequent<br />

replacement lenses. It occurs typically in<br />

around 2-3% of patients using <strong>the</strong>ir lenses<br />

on a daily wear, frequent replacement<br />

basis 69,70 , and more frequently in patients<br />

using lenses on an extended wear basis 71 .<br />

Several reports exist of patients developing<br />

CLAPC when wearing silicone hydrogel<br />

lenses, and <strong>the</strong> type of CLAPC found is<br />

often isolated in nature ra<strong>the</strong>r than diffuse,<br />

suggesting a mechanical ra<strong>the</strong>r than allergic<br />

or immunological aetiology 70,72-75 . It is<br />

believed that <strong>the</strong> increased stiffness of <strong>the</strong><br />

lens materials may result in increased<br />

movement of <strong>the</strong> lenses and any potential<br />

marginal fitting characteristics of <strong>the</strong> lens,<br />

resulting in edge lift-off or ‘fluting’<br />

(buckling at <strong>the</strong> lens edge) 76 , will act as a<br />

mechanical irritant and produce a localised<br />

papillary conjunctivitis, similar to that<br />

observed in patients with loose ocular<br />

sutures 77,78 . It is important that lenses are<br />

replaced on a frequent basis and that <strong>the</strong><br />

lens fit is optimised, since <strong>the</strong> development<br />

of CLAPC is a common reason for patients<br />

to cease silicone hydrogel continuous<br />

wear 75 .<br />

SEALs<br />

Superior epi<strong>the</strong>lial arcuate lesions (SEALs)<br />

are whitish, arc-like lesions which stain <strong>the</strong><br />

fluorescein (Figure 4). The lesions are<br />

located on <strong>the</strong> superior cornea in <strong>the</strong> area<br />

normally covered by <strong>the</strong> upper lid, typically<br />

within 1-3mm of <strong>the</strong> superior limbus 70 .<br />

They are not commonly associated with<br />

symptoms and are usually unilateral, but<br />

may be bilateral, in which case <strong>the</strong>y are<br />

typically asymmetric. A number of reports<br />

exist of SEALs occurring in patients wearing<br />

silicone hydrogel lenses 65,73-75,79-84 , suggesting<br />

that <strong>the</strong>y occur more frequently with<br />

silicone hydrogels than with conventional<br />

lens materials. It is believed that <strong>the</strong><br />

increased rigidity of <strong>the</strong> lens materials<br />

results in <strong>the</strong> lens being unable to<br />

conform adequately to <strong>the</strong> limbus, and that<br />

<strong>the</strong> pressure induced by <strong>the</strong> upper lid<br />

results in splitting of <strong>the</strong> superior<br />

epi<strong>the</strong>lium 70,85 .<br />

Deposition<br />

The deposition of contact lenses with<br />

substances derived from <strong>the</strong> tear fluid is a<br />

well-known clinical complication, resulting<br />

in reductions in comfort 86 , vision 87 and an<br />

increase in inflammatory responses 88 . Since<br />

<strong>the</strong>se silicone hydrogel lenses are primarily<br />

intended for overnight use for up to 30<br />

days, it is important that <strong>the</strong>se lenses do<br />

not deposit to <strong>the</strong> degree that silicone<br />

elastomer lenses did, because such<br />

deposition would require frequent lens<br />

removal for cleaning, negating <strong>the</strong> benefits<br />

of overnight lens wear. On a daily wear<br />

basis, it is important that such lenses<br />

remain clean for as long as possible as<br />

<strong>the</strong>re are no currently available daily<br />

disposable options and, given <strong>the</strong> cost of<br />

<strong>the</strong> lenses, this is unlikely to change for<br />

some time.<br />

Protein<br />

To date, <strong>the</strong> degree of in-eye<br />

biocompatibility achieved with silicone<br />

hydrogel materials has received relatively<br />

minimal attention, with <strong>the</strong> published<br />

results indicating that <strong>the</strong> deposition of<br />

protein on <strong>the</strong>se materials is significantly<br />

less than that seen with conventional<br />

materials 68,89-92 . Whilst <strong>the</strong> amount of<br />

protein deposited is minimal, it is<br />

interesting that <strong>the</strong> lysozyme deposited is<br />

largely denatured and inactivated 89,90 . Given<br />

<strong>the</strong> small amount of protein deposited,<br />

enzyme tablets are unlikely to be required<br />

with <strong>the</strong>se materials.<br />

Lipid<br />

As previously described, silicone-based<br />

materials do have a potential to deposit<br />

lipid from <strong>the</strong> tears. Lipid deposition on<br />

silicone hydrogels can be a problem for<br />

certain patients 90 , particularly if <strong>the</strong>y are<br />

refitted from an ionic material such as<br />

etafilcon, which deposits very little lipid.<br />

If patients are seen to be depositing <strong>the</strong>ir<br />

lenses with lipid (Figures 5 and 6), <strong>the</strong>n<br />

moving to non-NVP-containing materials,<br />

such as Proclear® or Acuvue, will reduce<br />

lipid deposition. Fur<strong>the</strong>r options include<br />

adding surfactant cleaners containing<br />

alcohol, such as Miraflow, ensuring that<br />

patients use a physical rubbing process with<br />

<strong>the</strong>ir multipurpose solutions (regardless of<br />

whe<strong>the</strong>r <strong>the</strong>y are approved for use in a<br />

‘no-rub’ format) or moving to more<br />

frequent periods of replacement 93 . To date,<br />

<strong>the</strong> Acuvue Advance material is<br />

recommended for replacement every two<br />

weeks, as compared with typical monthly<br />

replacement for <strong>the</strong> o<strong>the</strong>r two silicone<br />

hydrogels.<br />

Mucin balls<br />

A number of publications have described<br />

<strong>the</strong> presence of ‘mucin balls‘ in wearers of<br />

silicone hydrogel lenses 73,75,94-101 . These are<br />

pearly, translucent, 20-100µm spherical<br />

Figure 4<br />

SEAL subsequent to silicone hydrogel<br />

continuous wear<br />

Figure 5<br />

Lens calculi, which are comprised of<br />

90% lipid, on a PureVision lens<br />

(picture by courtesy of Brian Tompkins)<br />

Figure 6<br />

Rapid lens surface drying due to lipid<br />

contamination of an Acuvue Advance lens<br />

(picture by courtesy of Brian Tompkins)<br />

Figure 7<br />

Fluorescein pooling on <strong>the</strong> corneal surface due<br />

to indentations caused by mucin balls behind a<br />

silicone hydrogel lens<br />

37 | August 20 | 2004 OUT

Continuing Education and Training<br />

Lyndon Jones PhD, FCOptom, DipCLP, DipOrth, FAAO, (DipCL)<br />

and Kathy Dumbleton MSc, MCOptom, FAAO<br />

Figure 8<br />

CLPU in a patient wearing silicone hydrogel<br />

lenses on an overnight basis<br />

debris particles observed between <strong>the</strong> back<br />

surface of a contact lens and <strong>the</strong> cornea 94<br />

and are believed to consist of mucin and<br />

lipid 100 . They are common after overnight<br />

wear of silicone hydrogels, particularly in<br />

patients with steeper than average corneal<br />

curvature 94,98 , and can result in significant<br />

depressions in <strong>the</strong> corneal epi<strong>the</strong>lium<br />

following lens removal 97,101 . These<br />

depressions ‘pool’ fluorescein but do not<br />

result in true staining (Figure 7).<br />

Mucin balls are believed to occur due to<br />

<strong>the</strong> stiff lens materials shearing <strong>the</strong> tear<br />

film and rolling up small balls of tear film<br />

mucin with lipid. They are seen to occur<br />

more frequently with Focus Night & Day<br />

than with PureVision 99 (supporting <strong>the</strong> role<br />

of material stiffness) and we anecdotally<br />

feel <strong>the</strong>y are less likely to occur with <strong>the</strong><br />

Focus Night & Day lens when fitting <strong>the</strong><br />

8.40 base curve than with <strong>the</strong> 8.60<br />

(supporting <strong>the</strong> belief that lens movement<br />

is also a factor).<br />

Comfort and satisfaction<br />

A major problem with contact lenses<br />

continues to be <strong>the</strong>ir reduction in perceived<br />

comfort over <strong>the</strong> wearing period,<br />

particularly as <strong>the</strong> lens surface dehydrates.<br />

Dry eye symptoms are reported by 20-50%<br />

of soft lens wearers 102,103 , with 35% of<br />

patients permanently ceasing lens wear due<br />

to complications associated with<br />

discomfort and dryness 104 . For silicone<br />

hydrogel lenses to be successful, <strong>the</strong>y must<br />

be at least as comfortable as conventional<br />

lens materials.<br />

The sensation of ‘dryness’ and<br />

‘discomfort’ is a complex subject and is<br />

without question related to a variety of<br />

factors. One factor to consider is that of<br />

lens dehydration, since <strong>the</strong> subjective<br />

symptom of dryness appears to occur more<br />

frequently in soft lens wearers whose lenses<br />

undergo greater dehydration during openeye<br />

wear 105 . Material composition influences<br />

dehydration rate and degree 106 . In a clinical<br />

environment, it has been noted that <strong>the</strong><br />

majority of wearers of silicone hydrogel<br />

lenses report that <strong>the</strong>ir lenses feel less ‘dry’<br />

than <strong>the</strong>ir previous conventional lenses,<br />

despite considerably longer wearing<br />

times 54,107 .<br />

Silicone hydrogel materials, which have<br />

lower water contents than currently<br />

available materials, may produce less<br />

subjective dryness symptoms through<br />

reduced in-eye dehydration, enhanced<br />

wettability, reduced hydrophobic<br />

interactions with <strong>the</strong> eyelid, reduced<br />

deposition and/or increased oxygen<br />

performance. Published work to date shows<br />

that silicone hydrogel lens materials<br />

dehydrate at a slower rate and to a lesser<br />

extent than conventional hydrogel<br />

materials 102,109 and may partially help to<br />

explain this reduction in <strong>the</strong> sensation of<br />

dryness. However, despite <strong>the</strong> general<br />

reports of increased comfortable wearing<br />

time with <strong>the</strong>se novel materials, <strong>the</strong> major<br />

symptom reported by patients continues to<br />

be that of dryness 54 . Overall, published<br />

studies to date indicate successful clinical<br />

rates with overnight wear of around<br />

75% 56,65,79,80,99,110 .<br />

No published studies yet exist for<br />

patients wearing <strong>the</strong>se lenses on a daily<br />

wear basis, so clinical success rates for daily<br />

wear are difficult to estimate. Studies in<br />

which silicone hydrogel lenses have been<br />

compared to conventional materials have<br />

confirmed that <strong>the</strong>se lenses are at least as<br />

comfortable as conventional lenses when<br />

worn on a continuous wear basis 54 .<br />

However, in some instances, an<br />

‘adaptation’ period is required, particularly<br />

if a patient is refitted from a thinner, less<br />

mobile, more flexible lens material. This<br />

adaptation period is typically completed a<br />

week or so after <strong>the</strong> initial fitting takes<br />

place, and is usually due to <strong>the</strong> eye getting<br />

used to <strong>the</strong> increased lens movement and<br />

oxygenation afforded by <strong>the</strong> lens material.<br />

Inflammatory and<br />

infective complications<br />

The success of overnight wear with any new<br />

material will ultimately be based upon<br />

<strong>the</strong>ir clinical performance, particularly with<br />

regards to infectious and inflammatory<br />

complications, which are greater when<br />

lenses are worn overnight 70 . Complications<br />

will occur in silicone hydrogel lens wearers,<br />

as <strong>the</strong>y occur with any lens type. Many of<br />

<strong>the</strong> complications that do occur are<br />

patient-driven, being associated with<br />

colonisation of <strong>the</strong> ocular adnexa and/or<br />

contact lens by microorganisms,<br />

predominately bacteria 111 . Adverse events<br />

can <strong>the</strong>n occur as a direct result of<br />

infiltration of tissue by microorganisms, or<br />

secondary to <strong>the</strong> toxins and by-products,<br />

which <strong>the</strong>y produce. In order for a ‘full<br />

blown’ infection such as microbial keratitis<br />

(MK) to occur, <strong>the</strong> bacteria must bind to<br />

<strong>the</strong> corneal epi<strong>the</strong>lium and <strong>the</strong>n be able to<br />

invade <strong>the</strong> cornea to infect <strong>the</strong> corneal<br />

stroma. It is for this reason that<br />

inflammatory complications are more<br />

common than <strong>the</strong>ir more serious,<br />

infectious counterparts. Fortunately, <strong>the</strong> eye<br />

has a range of natural defence systems<br />

which very effectively protect its tissues<br />

from inflammation and infection 111 .<br />

Inflammation<br />

A number of inflammatory complications<br />

may occur with silicone hydrogel lens wear.<br />

These include contact lens peripheral ulcer<br />

(CLPU), infiltrative keratitis (IK) and<br />

contact lens acute red eye (CLARE). While<br />

all of <strong>the</strong>se complications have been<br />

described on many previous occasions, a<br />

number of specific differences are<br />

associated with <strong>the</strong>ir presentation in<br />

silicone hydrogel lens wearers. Probably <strong>the</strong><br />

most distinctive difference is that <strong>the</strong><br />

conditions are generally much less severe in<br />

<strong>the</strong>se wearers. This is most likely due to <strong>the</strong><br />

lack of associated corneal hypoxia and as a<br />

result, a much healthier corneal<br />

epi<strong>the</strong>lium 112 .<br />

A CLPU is an inflammatory response<br />

which results in lesions often termed<br />

‘sterile ulcers’. This is somewhat misleading<br />

since CLPUs do not occur as a result of<br />

infection but ra<strong>the</strong>r as a hypersensitive<br />

reaction to <strong>the</strong> (usually gram positive)<br />

exotoxins released by pathogenic bacteria.<br />

A CLPU is usually a single, peripheral or<br />

mid-peripheral white/grey lesion in <strong>the</strong><br />

anterior stroma and epi<strong>the</strong>lium that is<br />

0.1-2.0mm in diameter, circular in<br />

appearance and stains with fluorescein<br />

(Figure 8). Patients usually complain of<br />

mild to moderate pain (foreign body<br />

sensation), mild lacrimation and mild<br />

photophobia. Approximately 50% of<br />

CLPUs are asymptomatic and present<br />

without symptoms simply as scars at<br />

follow-up appointments 113 . Following <strong>the</strong><br />

acute ‘phase’, <strong>the</strong> epi<strong>the</strong>lium regenerates<br />

within one to two days over <strong>the</strong> lesion.<br />

Diffuse infiltration surrounding <strong>the</strong> lesion<br />

may develop. A very well defined circular<br />

‘scar’ remains, gradually fading with time,<br />

but still present at least six months after <strong>the</strong><br />

event. Differential diagnosis from MK<br />

extremely important 114,115 .<br />

CLARE is a unilateral, acute<br />

inflammatory response to gram negative<br />

organisms which colonise <strong>the</strong> lens and<br />

release endotoxins 116-118 . A higher incidence<br />

of CLARE occurs in patients with upper<br />

respiratory infection, and <strong>the</strong>se cases may<br />

be due to <strong>the</strong> presence of Haemophilus<br />

influenzae 118 . Patients with CLARE are<br />

typically woken in <strong>the</strong> early morning by a<br />

moderately painful (foreign body<br />

sensation) red eye, with associated<br />

epiphora and photophobia. Focal or<br />

diffuse sub-epi<strong>the</strong>lial infiltrates are usually<br />

observed in <strong>the</strong> mid-periphery of <strong>the</strong><br />

cornea close to <strong>the</strong> limbus. The infiltrates<br />

rarely stain and rapidly resolve. There is<br />

associated marked circum-corneal limbal<br />

hyperaemia, but generally no anterior<br />

chamber reaction or lid oedema.<br />

IK is <strong>the</strong> term used to describe all<br />

inflammatory events not categorised as<br />

CLPU, or CLARE. There are many factors<br />

which contribute to <strong>the</strong>se inflammatory<br />

events, including a foreign body trapped<br />

beneath a contact lens, mechanical trauma,<br />

solution preservatives, tight lenses, bacteria<br />

and/or toxins. IK can occur with both daily<br />

38 | August 20 | 2004 OUT

Continuing Education and Training<br />

and overnight wear and may also occur in<br />

non-lens wearers. However, <strong>the</strong> incidence is<br />

higher in lens wearers, presumably as a<br />

response to toxins being concentrated<br />

against <strong>the</strong> cornea by <strong>the</strong> hydrogel lens.<br />

Many IK cases are due to <strong>the</strong> presence of<br />

gram positive exotoxins found on <strong>the</strong> lid<br />

margin 111 .<br />

There is a large degree of variability in<br />

<strong>the</strong> severity of symptoms associated with IK<br />

and in some cases, <strong>the</strong>re are no related<br />

symptoms. In <strong>the</strong>se cases, <strong>the</strong> condition is<br />

termed ‘asymptomatic infiltrative keratitis’<br />

(AIK). Common symptoms reported with<br />

IK include mild to moderate irritation<br />

(often a foreign body discomfort), mild<br />

hyperaemia, lacrimation, photophobia and<br />

occasionally mild discharge. Slit lamp<br />

examination may reveal moderate bulbar<br />

and limbal redness, mild to moderate<br />

diffuse and/or small focal infiltrates<br />

(Figure 9). These infiltrates may be located<br />

anywhere in <strong>the</strong> cornea but are usually<br />

peripherally situated in <strong>the</strong> limbal area.<br />

Most cases of inflammation in contact<br />

lens wearers are self-limiting. The first step<br />

in <strong>the</strong> management of inflammatory<br />

complications in silicone hydrogel wearers<br />

is to temporarily discontinue lens wear<br />

until <strong>the</strong>re is full resolution of signs and<br />

symptoms. As with all inflammatory<br />

adverse events, <strong>the</strong> patient should be<br />

monitored carefully over <strong>the</strong> first 24 hours<br />

to confirm <strong>the</strong> diagnosis. In most cases, no<br />

medication is required, however ocular<br />

lubricants may be dispensed to alleviate<br />

symptoms. Severe cases may benefit from a<br />

prophylactic topical antibiotic to reduce <strong>the</strong><br />

chance of secondary infection. Since <strong>the</strong>re<br />

appears to be a patient predisposition for<br />

inflammatory events 54,113 , <strong>the</strong> introduction<br />

of lid hygiene measures (warm compresses<br />

and lid scrubs) to <strong>the</strong> daily routine for<br />

<strong>the</strong>se patients is strongly recommended.<br />

Patients repeatedly experiencing<br />

inflammatory events with overnight wear<br />

should be advised to wear <strong>the</strong>ir lenses on a<br />

daily wear basis only.<br />

Infection<br />

MK is a much more serious adverse event<br />

which may occur in silicone hydrogel lens<br />

wearers. Fortunately, <strong>the</strong> prevalence of MK<br />

within <strong>the</strong> general population is extremely<br />

low, due in part to <strong>the</strong> exceptional defence<br />

mechanisms that protect <strong>the</strong> ocular surface;<br />

however, MK remains <strong>the</strong> most serious<br />

complication of contact lens wear. Several<br />

case reports of MK with silicone hydrogel<br />

lenses have been published 70,119-123 , and this<br />

is not surprising, given that even non-lens<br />

wearers can develop this condition.<br />

Patients with MK present with marked<br />

increasing pain, intense lacrimation,<br />

hyperaemia and photophobia. A single<br />

paracentral or central irregular lesion may<br />

be observed with focal, and often<br />

significant, diffuse infiltration. The lesion is<br />

characterised by excavation of <strong>the</strong><br />

epi<strong>the</strong>lium, Bowman’s layer and <strong>the</strong><br />

stroma, and usually stains. An anterior<br />

chamber reaction and lid oedema are also<br />

common. The condition is associated with<br />

severe, progressive corneal suppuration.<br />

MK is a true ocular emergency and<br />

lenses must be removed and immediate<br />

medical management sought or instituted,<br />

with referral to an experienced corneal<br />

specialist in severe cases. Treatment usually<br />

consists of high doses of fortified<br />

antibiotics or fluoroquinolone agents (e.g.<br />

Ciloxan – 0.3% ciprofloxacin) every 15 to<br />

30 minutes for <strong>the</strong> first two days, and <strong>the</strong>n<br />

every four hours for a fur<strong>the</strong>r 10 to 12 days.<br />

Prognosis for most patients is good,<br />

particularly if referral occurs early in <strong>the</strong><br />

disease process, but it does depend upon<br />

<strong>the</strong> causative organism. In all cases, a scar<br />

will remain. Most patients are able to<br />

resume daily contact lens wear within a six<br />

to 12-month period, depending upon <strong>the</strong><br />

size and location of <strong>the</strong> scar.<br />

While we anxiously await <strong>the</strong> results of<br />

prospective clinical trials to determine <strong>the</strong><br />

actual rate of infection with silicone<br />

hydrogel lenses 124 , we can use a number of<br />

surrogate measures to predict what <strong>the</strong><br />

likely outcome will be. An epi<strong>the</strong>lium<br />

which is not compromised by lens wear,<br />

and which does not allow significant<br />

binding of bacteria, should alleviate serious<br />

infections 125,126 . Work from Dwight<br />

Cavanagh and colleagues in <strong>the</strong> US has<br />

shown that while Pseudomonas aeruginosa<br />

binds to epi<strong>the</strong>lial cells when <strong>the</strong>y are<br />

exposed to low Dk materials, binding<br />

decreases significantly when high Dk<br />

silicone hydrogel lenses are worn 112,126-129 .<br />

Certainly, from a <strong>the</strong>oretical point of<br />

view, silicone hydrogel lens wear should<br />

reduce <strong>the</strong> risk of MK and prospective data<br />

due within <strong>the</strong> next 12 months will<br />

confirm or deny this premise. However,<br />

given that inflammatory complications are<br />

primarily patient driven, due to lens<br />

contamination, it is likely that until<br />

bacteria-resisting contact lens materials are<br />

developed 130-132 , <strong>the</strong> rate of inflammation<br />

with silicone hydrogel lens materials will<br />

be similar to that encountered with<br />

conventional lens materials.<br />

Future developments<br />

Increasingly, contact lens manufacturers are<br />

looking at developing novel silicone-based<br />

hydrogels and <strong>the</strong> foreseeable future for<br />

this group of lens materials looks<br />

promising. It is likely that <strong>the</strong> next 10 years<br />

Figure 9<br />

Focal infiltrates subsequent to silicone<br />

hydrogel continuous wear<br />

will be dominated by <strong>the</strong> release of<br />

silicone-based hydrogels from all<br />

manufacturers. These materials will<br />

probably have stiffness levels closer to<br />

conventional hydrogels and better surface<br />

treatments, which truly make <strong>the</strong> surfaces<br />

hydrophilic. Ideally, such materials would<br />

support a tear film for longer than <strong>the</strong><br />

typical seven to eight seconds seen with<br />

currently available materials, and consist of<br />

polymers that would resist contamination<br />

with pathogenic organisms 131 . Such<br />

materials should result in increased<br />

comfort and reduced inflammatory<br />

complications compared with currently<br />

available materials, and would have a<br />

significant impact on growing <strong>the</strong> contact<br />

lens market.<br />

Conclusion<br />

To answer <strong>the</strong> title of this article, we have<br />

no doubts that silicone hydrogel materials<br />

will displace conventional lens materials<br />

over time, for both daily and overnight<br />

wear. Their improved physiological<br />

performance, excellent handling<br />

characteristics and improved comfort make<br />

<strong>the</strong>m <strong>the</strong> ideal lens material. We believe<br />

that <strong>the</strong>se lenses should not be reserved for<br />

overnight use only or used as ‘troubleshooting’<br />

lenses, but should become <strong>the</strong><br />

standard lens type to be fitted to all<br />

patients. We predict that, regardless of <strong>the</strong><br />

success of continuous wear, highly oxygen<br />

permeable soft lens polymers will<br />

dominate <strong>the</strong> contact lens market in 10<br />

years’ time.<br />

References<br />

For a full set of references, email<br />

nicky@optometry.co.uk or visit<br />

www.optometry.co.uk/references/<br />

2004.08.04-Jones.doc.<br />

Submit your answers online at www.otcpd.co.uk and be free to join at any<br />

time and take part in any or all of <strong>the</strong> six articles until January 12, 2005<br />

39 | August 20 | 2004 OUT

Continuing Education and Training<br />

Module 1 Part 2 of Modern contact lens practice<br />

Silicone hydrogels – Will <strong>the</strong>y displace conventional lenses?<br />

Please note <strong>the</strong>re is only ONE correct answer<br />

1. How much corneal swelling typically<br />

occurs with conventional hydrogel<br />

lens materials when worn overnight?<br />

a. 0%<br />

b. 5%<br />

c. 10%<br />

d. 20%<br />

2. Which of <strong>the</strong> following statements is<br />

true regarding <strong>the</strong> surface of first<br />

generation silicone hydrogel contact<br />

lenses?<br />

a. The surface is treated with a gas plasma<br />

technique to improve wettability<br />

b. The surface is treated with antibiotics to<br />

resist infection<br />

c. The surface is coated with lipids to resist<br />

protein deposition<br />

d. The surface is very wettable and deposits<br />

lots of proteins<br />

3. Which property does silicone rubber<br />

have that would be most relevant to its<br />

potential use as a contact lens material?<br />

a. Extremely high wettability<br />

b. Extremely high oxygen permeability<br />

c. Extremely high UV blocking properties<br />

d. Extremely high toughness<br />

4. Of <strong>the</strong> following four values, which is<br />

most representative of an approximate<br />

Dk (oxygen permeability) for a typical<br />

silicone hydrogel lens material?<br />

a. Five barrers<br />

b. Twenty-five barrers<br />

c. One hundred barrers<br />

d. Three hundred barrers<br />

5. When a patient with microcysts is<br />

refitted with silicone hydrogel lenses,<br />

what is <strong>the</strong> typical appearance within<br />

<strong>the</strong> cornea over <strong>the</strong> next one to two<br />

weeks?<br />

a. The microcysts worsen for about a week<br />

and <strong>the</strong>n suddenly disappear<br />

b. The microcysts gradually decrease in<br />

number until <strong>the</strong>y disappear<br />

c. The microcysts increase rapidly and<br />

<strong>the</strong>n gradually reduce<br />

d. The microcysts suddenly disappear<br />

after <strong>the</strong> first night of wear of <strong>the</strong> new<br />

lenses<br />

6. Which of <strong>the</strong> following deposits from<br />

<strong>the</strong> tears can be a problem for<br />

some wearers of silicone hydrogel<br />

lenses?<br />

a. Protein<br />

b. Lipid<br />

c. Mucin<br />

d. Fungus<br />

7. Which one of <strong>the</strong> following lenses is<br />

<strong>the</strong> ‘stiffest’, or has <strong>the</strong> highest<br />

modulus of elasticity?<br />

a. Acuvue 2<br />

b. Focus Night & Day<br />

c. Acuvue Advance<br />

d. Proclear<br />

8. What, approximately, is <strong>the</strong> oxygen<br />

transmissibility (Dk/t) required to<br />

minimise oedema for most patients<br />

on a daily wear basis?<br />

a. Ten units<br />

b. Thirty units<br />

c. Sixty units<br />

d. One hundred and fifty units<br />

9. Approximately how much stiffer are<br />

silicone hydrogel lenses than an<br />

Acuvue lens?<br />

a. Three times stiffer<br />

b. Eight times stiffer<br />

c. Fifteen times stiffer<br />

d. Fifty times stiffer<br />

MCQs<br />

10. Which one of <strong>the</strong> following<br />

statements is true?<br />

a. Hypoxia is greater with silicone hydrogel<br />

lenses compared with conventional<br />

lenses during overnight wear<br />

b. Microbial keratitis occurs more<br />

frequently with silicone hydrogel lenses<br />

than conventional lenses on extended<br />

wear<br />

c. Giant papillary conjunctivitis occurs<br />

more often with conventional lenses<br />

than silicone hydrogel lenses<br />

d. Infiltrative events with conventional<br />

lenses on extended wear are similar to<br />

those seen with silicone hydrogel lenses<br />

on extended wear<br />

11. Near vision problems within <strong>the</strong> first<br />

month of refitting with silicone<br />

hydrogel lenses are most likely<br />

attributed to which one of <strong>the</strong><br />

following factors?<br />

a. Lipid deposition<br />

b. Residual unmasked astigmatism<br />

c. A rebound hyperopic shift<br />

d. Mucin ball formation in <strong>the</strong> post lens<br />

tear film<br />

12. The most appropriate management for<br />

<strong>the</strong> majority of cases of infiltrative<br />

keratitis in extended wear silicone<br />

hydrogel patients is which one of <strong>the</strong><br />

following?<br />

a. Lens replacement and daily wear until<br />

resolution<br />

b. Lens replacement and no fur<strong>the</strong>r<br />

extended wear<br />

c. Temporary discontinuation of lens wear<br />

and topical antibiotics<br />

d. Temporary discontinuation of lens wear<br />

and ocular lubricants until resolution,<br />

followed by <strong>the</strong> instigation of lid<br />

hygiene<br />

An answer return form is included in this issue. Paper entries ONLY should be completed and returned by September 22 to:<br />

CPD initiatives (c4938b), OT, Victoria House, 178-180 Fleet Road, Fleet, Hampshire, GU51 4DA.<br />

Please note that model answers for this Pay-As-You-Learn series will not be available until January 14, 2005.<br />

This is so that readers sumbitting answers online can join at any time from now until that date and take part in<br />

any or all of <strong>the</strong> six articles as <strong>the</strong>y are published. Paper entries will be marked on <strong>the</strong> normal monthly basis.<br />

40 | August 20 | 2004 OUT

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