Editorial

Are silicone hydrogel lenses safer?

www.elsevier.com/locate/clae

Contact Lens & Anterior Eye 28 (2005) 153–155

Having been invited to write a guest editorial to mark my

return to Australia at the end of this year, I have chosen to

address a key issue that has essentially paralleled my 16

years in the UK, which is: ‘‘For extended wear, are silicone

hydrogel contact lenses safer than conventional hydrogel

lenses?’’ Since 1990 – which is by coincidence the year I

arrived in Manchester – the major contact lens companies

have been investing heavily in research and development

with the aim of developing silicone hydrogel materials that

could be used for the manufacture of truly safe contact

lenses for extended wear. In this editorial I shall provide a

brief overview of this developmental work and provide the

answer that my colleagues and I have found to the question

posed above.

The main strategy employed during the 1980s for

extended wear lenses was to use high water content

hydrogel materials. This failed because the oxygen

transmissibility of lenses made from these materials allowed

insufficient levels of oxygen to reach the cornea for normal

metabolic processes to occur [1]. The resultant lens-induced

hypoxia [2] caused acute overnight corneal oedema, as

evidenced by the appearance of striae and folds in the

posterior cornea of those who wore lenses overnight [3].

Other adverse tissue changes observed included epithelial

microcysts and bullae, chronic epithelial and stromal

thinning, and endothelial polymegethism [3].

Templated on top of these adverse physiological changes

was a more serious and potentially sight-threatening

reaction – the development of microbial keratitis. Epide-

miological studies conducted in the late 1980s provided

proof that routine sleeping in contact lenses was associated

with a higher risk if developing microbial keratitis compared

with day time wear only [4]. This finding was widely

publicised at the time in both the professional and lay media

and led to the general conclusion among practitioners and

the public that the risks of overnight lens wear outweighed

the benefits of convenience and freedom of lifestyle.

Despite the failure of extended wear in the 1980s,

consumer surveys keep coming up with the same finding –

that there would be a significant demand for extended wear

lenses if they were safe, as judged primarily by a

demonstrated reduction in the risk of developing microbial

1367-0484/$ – see front matter # 2005 British Contact Lens Association. Publi

doi:10.1016/j.clae.2005.10.004

keratitis. This consumer-driven demand led to the next

attempt at solving extended wear: disposable contact lenses.

The theory here was that by regularly disposing of lenses and

inserting a fresh pair of lenses every week, the risks of

deposit related problems would be reduced and lenses would

be safe to sleep in. However, case reports [5] and

epidemiological studies [6,7] once again showed that there

was still the same increased risk of developing microbial

keratitis when sleeping in disposable lenses.

Although disposable lenses did not make extended wear

safe, they still provided many advantages and within a

decade of their launch, virtually all soft contact lenses were

disposable [8]. Regular lens replacement has essentially

eliminated overt deposit related problems that were common

in the 1980s, such as jelly bumps, heavy protein deposition,

calcium deposits and rust spots [3]. Adverse tissue reactions

such as papillary conjunctivitis and corneal staining were

substantially reduced, and superior performance in terms of

vision and comfort were obtained [9].

So, with all of these advantages, why did overnight wear

of disposable lenses fail to lessen the risk of microbial

keratitis? Research began to emerge that two key problems

needed to be solved to make extended wear truly safe. First,

strong evidence was published linking corneal hypoxia to

the development of microbial keratitis [10]. Bacterial

attachment to the epithelium is a necessary precursor for

corneal infection, and this will only occur if the epithelial

defences are weakened. It was demonstrated that bacteria are

more likely to adhere to the epithelium if levels of corneal

oxygenation are reduced [11]. Therefore, it was necessary to

develop materials with an extremely high oxygen perfor-

mance, so that the cornea could respire in the closed eye

environment as if in the open eye situation, with the risk of

bacterial attachment to the cornea substantially reduced.

Second, lenses needed to be developed to allow significant

tear exchange upon waking so that stagnant tear film debris

at the ocular surface could be quickly flushed away.

Silicone elastomer contact lenses were first tried in the

1970s [12] and it has long been known that silicone rubber

has an extremely high oxygen transmissibility [13].

However, early attempts at producing contact lenses made

from this material failed because silicone is hydrophobic.

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Editorial / Contact Lens & Anterior Eye 28 (2005) 153–155154

This resulted in lenses being extremely uncomfortable and

forming a strong suction on to the eye, making them difficult

to remove [12]. Various strategies were employed in an

attempt to perfect these lenses, such as plasma-treating the

surface to make it hydrophilic, and creating significant edge

lift for better tear exchange and reduced suction. These

attempts failed, so silicone elastomer lenses were never a

commercial success. Nevertheless, it was clear that the only

way of developing lenses of extremely high oxygen

transmissibility was to somehow incorporate silicone into

lens materials.

The approach taken by the contact lens industry in an

attempt to solve this problem was to try and create a hybrid of

hydrogel and silicone materials. This was an almost

insurmountable challenge to chemical engineers, akin to

creating a perfect mix of oil and water. The hydrogel

component would provide the comfort and necessary

mechanical properties, and the silicone component would

provide the required oxygen performance. The balance had to

be just right. Too much silicone would make the lens

uncomfortable and hydrophobic, but too much hydrogel

would result in an insufficient oxygen performance. Follow-

ing a decade of intensive research, the first silicone hydrogel

lenses were released on to the market in 1999. Now, six years

later, there are five such products available, each with a unique

balance of silicone and hydrogel, but all with oxygen

transmissibility values that are far superior to those available

with conventional hydrogels. Indeed, all of these products

allow sufficient oxygen flux to the cornea for normal

metabolism to occur [14] which means that consideration

of other lens characteristics such as material stiffness, visual

performance and comfort are the key parameters when

deciding the best lens to prescribe for a given patient.

Because the absolute incidence of microbial keratitis

associated with contact lens wear is very low [4,6,7] it is

necessary for many tens of thousands of people to be

wearing silicone hydrogel lenses in order to determine their

level of safety with statistical confidence. It is for this reason

that the question of safety of silicone hydrogel lenses could

not be addressed until a few years after their release onto the

market. In addition, any research addressing the issue of

‘relative risk’ would need to be conducted at a time when

significant numbers of people were also still wearing

conventional hydrogel lenses, so that the performance of the

two lens types could be compared.

After weighing up the above considerations, it was

judged that the best time to address this issue was 2003. Dr.

Philip Morgan and I designed and executed an epidemio-

logical study – the Manchester Keratitis Study – with the aim

of determining the relative safety of all forms of contact

lenses currently on the market, including silicone hydrogel

lenses. We realised that, in order to get accurate answers, a

study design that was superior to earlier attempts [4,6,7]

would be required. Previous contact lens epidemiological

studies suffered from many disadvantages. For example,

they all relied on co-operation from literally hundreds of

clinicians in commercial practices and hospital settings in

large geographic areas to record and report all cases of

microbial keratitis over a fixed period [4,6,7]. Despite

constant reminders to practitioners to collect the data, it is

self-evident that such a methodology will result in

significant under-reporting and therefore under-estimations

of the true magnitude of the problem. Also, previous studies

determined lens-wearing characteristics of the population by

conducting telephone surveys, which can be fraught with

difficulties because (a) respondents often tell the researcher

what they think is the right answer, rather than ‘the truth’

[15] and (b) it is necessary to make many thousands of

telephone calls to establish statistical validity, resulting in a

costly and time consuming exercise.

Unlike previous studies, the Manchester Keratitis Study

[16–19] had the advantage of being conducted at a single site –

the Acute Referral Centre of the Royal Eye Hospital,

Manchester. Essentially, all contact lens wearers who

attended this clinic during 2003 were interviewed, and

detailed records were kept on all thosewho presented with any

form of corneal infiltrative event, including microbial keratitis

(which we call ‘severe keratitis’). We adopted a far superior

methodology than that used by previous workers [4,6,7] for

determining patterns of lens wear in the population;

specifically, we accessed sales data from the UK Association

of Contact Lens Manufacturers [20]. This data set contained

accurate details of all lenses sold in the UK during the survey

period. We did not, therefore, have to rely on the ‘educated

guesswork’ involved in interpreting telephone surveys.

The results of the Manchester Keratitis Study have now

been published [16–19] and readers interested in the full

analysis can access these papers. Three interesting findings

from the study are: (1) there is still a greater risk of severe

keratitis when sleeping in any form of contact lenses

(included silicone hydrogel lenses) compared with daily

wear, (2) rigid lenses are safer than soft lenses for daily wear,

and (3) the absolute risk of severe keratitis with any form of

lens wear is low. These latter two points essentially confirm

earlier findings [4,6,7].

The most critical and important finding of our study is as

follows: the annual incidence of severe keratitis among those

who wear conventional hydrogel lenses overnight is 100

cases per 10,000 wearers. With overnight use of silicone

hydrogel lenses, the incidence is 20 cases per 10,000

wearers, a finding which is almost identical to that reported

in a recent study by Stapleton et al. [21]. So, the answer to

the question posed at the outset is clear: for extended wear,

silicone hydrogel lenses are 5� safer than conventional

hydrogel lenses.

So what does this finding mean to you, the practitioner?

One way of looking at the results of the Manchester Keratitis

Study is as follows: whatever your experience was with

managing patients wearing conventional extended wear

lenses last century, it is likely to be 5� better with silicone

hydrogel lenses this century. What does this mean for lens

wearers? Although there is less likelihood of developing

Editorial / Contact Lens & Anterior Eye 28 (2005) 153–155 155

severe keratitis when sleeping in silicone hydrogel lenses, it

is still safer not to sleep in lenses at all. However, it must be

said that the risk/benefit assessment is more finely balance

now, and a growing number of patients will be judging the

benefits of extended wear silicone hydrogel lenses to

outweigh the risks and therefore will be opting for this

modality of wear.

References

[1] Morgan PB, Efron N. The oxygen performance of contemporary

hydrogel contact lenses. Cont Lens Ant Eye 1998;21:3–6.

[2] Efron N, Ang JHB. Corneal hypoxia and hypercapnia during contact

lens wear. Optom Vis Sci 1990;67:512–21.

[3] Efron N. Contact lens complications, 2nd ed, Oxford: Butterworth-

Heinemann, 2004.

[4] Poggio EC, Glynn RJ, Schein OD, et al. The incidence of ulcerative

keratitis among users of daily-wear and extended-wear soft contact

lenses. N Engl J Med 1989;321:779–83.

[5] Efron N, Lowe R, Vallas V, Grusiner E. Clinical efficacy of standing

wave and ultrasound for cleaning and disinfecting contact lenses. Int

Contact Lens Clin 1991;18:24–9.

[6] Cheng KH, Leung SL, Hoekman HW, et al. Incidence of contact-lens-

associated microbial keratitis and its related morbidity. Lancet

1999;354:181–5.

[7] Lam DS, Houang E, Fan DS, Lyon D, Seal D, Wong E. Incidence and

risk factors for microbial keratitis in Hong Kong: comparison with

Europe and North America. Eye 2002;16:608–18.

[8] Morgan PB, Efron N. Trends in UK contact lens prescribing. Optician

2005;229(6004):28–9.

[9] Efron N. Contact lens practice. Oxford: Butterworth-Heinemann,

2002.

[10] Solomon OD, Loff H, Perla B, et al. Testing hypotheses for risk factors

for contact lens-associated infectious keratitis in an animal model.

Contact Lens Assoc Ophthalmol J 1994;20:109–13.

[11] Fleiszig SMJ, Efron N, Pier GB. Extended wear of contact lenses

enhances Pseudomonas aeruginosa adherence to human corneal

epithelium. Invest Ophthalmol Vis Sci 1992;33:2908–16.

[12] Zekman TN, Sarnat LA. Clinical evaluation of the silicone corneal

contact lens. Am J Ophthalmol 1972;74:534–7.

[13] Weissman BA, Fatt I, Phan C. Polarographic oxygen permeability of

silicone elastomer contact lens materials. J Am Optom Assoc

1992;63:187–90.

[14] Brennan NA. Beyond flux: total corneal oxygen consumption as an

index of corneal oxygenation during contact lens wear. Optom Vis Sci

2005;82:467–72.

[15] Aquilino WS. Telephone versus face-to-face interviewing for house-

hold drug use surveys. Int J Addict 1992;27:71–91.

[16] Morgan PB, Efron N, Hill EA, Raynor MK, Whiting MA, Tullo AB.

Incidence of keratitis of varying severity among contact lens wearers.

Br J Ophthalmol 2005;89:430–6.

[17] Efron N, Morgan PB, Hill EA, Raynor MK, Tullo AB. The

size, location and clinical severity of corneal infiltrative events

associated with contact lens wear. Optom Vis Sci 2005;82:

519–27.

[18] Efron N, Morgan PB, Hill EA, Raynor MK, Tullo AB. Incidence

and morbidity of hospital-presenting corneal infiltrative events

associated with contact lens wear. Clin Exp Optom 2005;88:232–

239.

[19] Morgan PB, Efron N, Brennan NA, Hill EA, Raynor MK, Tullo AB.

Risk factors for the development of corneal infiltrative events asso-

ciated with contact lens wear. Invest Ophthalmol Vis Sci

2005;46:3136–43.

[20] Morgan PB. A healthcheck on the UK contact lens market. Optician

2002;223(5854):14–6.

[21] Stapleton F, Edwards K, Keay L, et al. The incidence of contact lens

associated microbial keratitis in Australia. ARVO Abstracts 2005.

46:E-abstract 5025.

Nathan Efron*

Professor of Clinical Optometry

Faculty of Life Sciences,

The University of Manchester,

PO Box 88, Manchester M60 1QD, UK

*Tel.: +44 161 306 3886; fax: +44 870 831 6625

E-mail address: n.efron@manchester.ac.uk