Silicone Hydrogels: Has the Promise Been Fulfilled?

by | September 2012

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Silicone hydrogel materials have revolutionized the contact lens market, but have they lived up to the promises with which they were promoted?

Hydrogel contact lenses were first marketed 40 years ago. Those first generation soft lenses were easy to manufacture and far more attractive to patients than the polymethylmethacrylate (PMMA) hard lenses that preceded them. They had drawbacks, however: they were relatively thick and had poor oxygen transmissibility. By contrast, the silicone elastomer lenses of the time, which were used primarily for pediatric applications, had superb oxygen transmission but were difficult to manufacture, extremely hydrophobic, attracted lipid deposits, and had a disturbing tendency to stick to the cornea.

Researchers and contact lens manufacturers acknowledged the advantage of silicone’s effect on oxygen transmission but understood that before it could be widely used in contact lens materials something would have to be done about its hydrophobic surface characteristics. Despite considerable difficulties, the quest to merge silicone and hydrogel monomers was driven by the realization that if hydrogel materials, with their comfort, ease of manufacture, wettability, and deposit resistance, could be given the oxygen-permeability benefits of silicone, the result would be a stunning advance. However, the technical challenges of incorporating hydrophobic silicone into a hydrophilic material—something like mixing oil and water—took decades to achieve.

In the late 1990s, the first generation silicone hydrogel materials were introduced. In the years since, silicone hydrogels have brought many benefits to wearers and have come to dominate the contact lens market. In the following pages we will look at some key ways in which these remarkable materials have met, surpassed, and confounded our expectations.

Extended Wear

The commercial development of silicone hydrogel lenses was motivated, in large part, by the promise of the extended wear market. Patient surveys have repeatedly found that, if offered the alternatives of a disposable lens they could wear every day and remove before bed or an equally safe and comfortable lens they could leave in for a week or a month, the majority of patients would opt for sleeping in their lenses.1 Just like patients who chose refractive surgery, many contact lens patients want good vision without the inconvenience of daily lens insertion, removal, disinfection, and storage—advantages even daily disposable contact lenses do not fully confer.

Soft contact lens extended wear (in hydrogel lenses) was first approved in the US in 1981. By the mid 1980s clinicians had begun to report some negative effects of extended wear, including a frightening rise in the reports of microbial keratitis that seemed to be related to sleeping in lenses.2-5 Finally, in 1989, the conclusions from these case reports were confirmed by a large, prospective study that found that sleeping in hydrogel contact lenses increased the risk of microbial keratitis by a factor of almost 10.6

An important factor in the development of silicone hydrogels was the hope that, with their high oxygen transmissibility, silicone hydrogels would bring down the rate of microbial keratitis and grow the extended wear market. It was predicated on the belief, common at the time, that chronic hypoxia from insufficiently oxygen-permeable contact lens materials was a key factor in contact lens-related microbial keratitis. With the advent of highly oxygen-permeable silicone hydrogel materials, it was hoped that the incidence of microbial keratitis would drop significantly, restoring the potential for extended wear use by the masses.7

Microbial Keratitis

That hope was never fulfilled. By 2005 published studies had begun to make clear that microbial keratitis rates among silicone hydrogel contact lens wearers were not dramatically different from rates in hydrogel lens wearers.8 This was confirmed in later studies and it became clear that overnight wear remained the greatest risk factor for microbial keratitis, regardless of lens material.9,10

That is not to say that silicone hydrogel materials don’t have some benefits with respect to microbial keratitis. While oxygen-permeable materials do not seem to have changed the overall incidence of microbial keratitis, it would appear that there have been meaningful reductions in disease severity, improvements in recovery time, and better visual outcomes.

Because of the increased risk of microbial keratitis, relatively few practitioners today promote extended wear, even with silicone hydrogel lenses. Most practitioners, however, recognize that, for a subset of contact lens wearers, overnight wear is very desirable—indeed, some of these patients are likely to sleep in their lenses regardless of their doctor’s recommendations. For these patients, silicone hydrogel lenses are clearly indicated.

Oxygen Transmission

In traditional hydrogel materials, the level of oxygen delivered to the cornea can be increased by either raising the water content or making the lenses thinner. In practical terms, the only patients for whom the lenses can be made thin enough to ensure hypoxia-free wear are low myopes, and even among these patients, individuals with above-average corneal oxygen requirements may not get enough oxygen to prevent mild hypoxia. For other patients, hydrogel lens wear was seen to induce mild limbal hyperemia, which was often generally thought to be due to mechanical irritation rather than hypoxia, and corneal neovascularization.

Early experience with silicone hydrogels showed a remarkable decrease in these signs—patients wearing silicone hydrogel lenses had strikingly whiter eyes than patients in conventional hydrogels, whether used for daily wear or overnight wear.11-13 We continue to see that hyperemia is markedly reduced in silicone hydrogel lens wearers, and problems like epithelial microcysts, neovascularization, and endothelial changes have nearly been eliminated with the use of these materials.14 Thanks to silicone hydrogel materials, contact lens-related corneal hypoxia is rapidly becoming a phenomenon of interest to historians rather than clinicians.

Comfort

Our understanding of what drives comfort (and discomfort) with soft contact lenses remains incomplete. Irrespective of lens material and/or replacement modality, studies find large numbers of patients complaining of dryness and discomfort at the end of the day. While some studies support the claim that switching patients from hydrogels to silicone hydrogels can increase comfort, many of these studies have not been conducted in such a way as to provide a definitive answer.

What appears to have changed with silicone hydrogels is the length of time for which contact lenses can be comfortably worn. While we still lack well designed, carefully conducted studies to confirm this, I believe that silicone hydrogels have allowed most patients to enjoy comfortable lens wear for a greater number of hours each day. Whether it is their increased oxygen transmission or some other factor that makes this added comfort possible is undetermined.

The same silicone that increases oxygen transmission—which perhaps increases comfort in some wearers—can also have a negative effect on comfort. First, adding silicone tends to increase the modulus of elasticity, making the lens stiffer; and unless the lens is very carefully designed the added stiffness can degrade comfort. Second, silicone is hydrophobic, and the more silicone at the surface of the material, the more difficult it is to keep the lens wettable, deposit-free, and comfortable on the eye.

Sequestering Silicone

Hydrophobicity can be a problem for some patients when transitioning to certain silicone hydrogel lenses. Silicone moieties incorporated into the silicone hydrogel polymer chain are most stable at an air interface (air, like silicone, is hydrophobic). As a result, thermodynamic forces drive the silicone moieties to rotate to the lens surface and stay there. To prevent silicone from creating a hydrophobic surface, biomaterial scientists are challenged to find ways to sequester the silicone—not an easy task in a complex, dynamic material that is only microns thick.

When silicone hydrogels first emerged on the market, the only method of keeping silicone away from the surface was to apply a hydrophilic coating. While plasma treatment was able to create a more wettable surface, it required a second step in the manufacturing process. So for the next generation of silicone hydrogel lenses, designers sought a process that could neutralize silicone’s hydrophobicity while allowing the lenses to be manufactured in a single, continuous operation. Johnson & Johnson did this by incorporating a wetting agent, polyvinylpyrrolidone (PVP), into the silicone hydrogel polymer itself. Other companies, like CooperVision, employed unique chemistry to make the silicone less “visible” at the lens surface.

Novel Approaches

Several new lenses are attempting to obtain a balance of high oxygen permeability with minimal silicone interference. The new DAILIES Total1® lens (Alcon) takes a fresh approach to neutralizing the effect of silicone by creating a lens with a silicone hydrogel core and a gel-like, high water content surface. The silicone-hydrogel core enables a high level of oxygen transmission, while the hydrogel surface maximizes wettability, resulting in a moisture “gradient” through the lens: the core has about 33% water (typical of a highly oxygen permeable silicone hydrogel lens) and the surface, approximately 80% water.

A similarly hydrogel-focused design concept is found in Bausch + Lomb’s new daily disposable, Biotrue™ ONEday lens. Made from a new material, nesofilcon A, the lens is a high water content hydrogel material that contains no silicone at all. It will be interesting to see how the market responds to these technologies, and to see what future role silicone plays in contact lens material development.

Why Are Silicone Hydrogels Dominant?

Silicone hydrogels have by and large delivered on their promises. While silicone hydrogels may not be suitable for every patient, these lenses have been readily adopted by practitioners because, in the majority of cases, eyes are whiter, wearing times are longer, and hypoxic complications are virtually eliminated. Consumer-directed advertising has also contributed to the market share of silicone hydrogels.

Despite their acknowledged shortcomings, the advantages of silicone hydrogels make it difficult to imagine that these materials could be replaced in the market anywhere in the foreseeable future; but new materials suggest that variations in the amount and manner in which silicone is used in contact lenses are likely on the horizon.

THE BOTTOM LINE

Creating contact lens materials with suitable wettability, optical clarity, and oxygen permeability for clear, comfortable vision has been a tremendous achievement. Silicone hydrogels have revolutionized the way we manage patients in contact lenses, resulting in better ocular surface health and all but banishing corneal hypoxia. Improvements in the amount of, and method by which, silicone is incorporated into soft contact lens materials may improve these lenses’ wettability, comfort, and patient acceptance even further.


Lyndon Jones, PhD, FCOptom, FAAO, is director of the Centre for Contact Lens Research and a professor at the School of Optometry at the University of Waterloo, Waterloo, Ontario, Canada. He has received research funding or speaking honoraria from Alcon, Allergan, AMO, Bausch & Lomb, CIBA Vision, CooperVision, Essilor, Inspire, Johnson & Johnson, Menicon, and Visioneering. Refractive Eyecare managing editor Jennifer Zweibel assisted in the preparation of this manuscript.
 
 
 
 
 
 
References

1. Nichols JJ, Mitchell GL, Zadnik K. Daily disposable vs. disposable extended wear: a contact lens clinical trial. Optom Vis Sci. 2000 Dec;77(12):637-47.

2. Galentine PG, Cohen EJ, et al. Corneal ulcers associated with contact lens wear. Arch Ophthalmol. 1984;102;6:891-4.

3. Weissman BA, Mondino BJ, et al. Corneal ulcers associated with extended-wear soft contact lenses. Am J Ophthalmol. 1984;97;4:476-81.

4. Alfonso E, Mandelbaum S, et al. Ulcerative keratitis associated with contact lens wear. Am J Ophthalmol. 1986;101;4:429-33.

5. Baum J, Boruchoff SA. Extended-wear contact lenses and pseudomonal corneal ulcers. Am J Ophthalmol. 1986;101(3):372-3.

6. Schein OD, Glynn RJ, Poggio EC, et al. The relative risk of ulcerative keratitis among users of daily-wear and extended-wear soft contact lenses: A case-control study. New Engl J Med. 1989 Sept;321(12):774-8.

7. Holden BA, Sweeney DF, et al. Microbial keratitis and vision loss with contact lenses. Eye Contact Lens. 2003;29(1) Suppl:S131-4; discussion S143-4, S192-4.

8. Schein OD, McNally JJ, et al. The incidence of microbial keratitis among wearers of a 30-day silicone hydrogel extended-wear contact lens. Ophthalmology. 2005;112(12):2172-9.

9. Dart JK, Radford CF, et al. Risk factors for microbial keratitis with contemporary contact lenses: a case-control study. Ophthalmology. 2008;115(10):1647-54.

10. Stapleton F, Keay L, et al. The incidence of contact lens-related microbial keratitis in Australia. Ophthalmology. 2008;115(10):1655-62.

11. Papas EB, Vajdic CM, et al. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Curr Eye Res. 1997;16(9):942-8.

12. Dumbleton KA, Chalmers RL, et al. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci. 2001;78(3):147-51.

13. Dumbleton K, Keir N, et al. Objective and subjective responses in patients refitted to daily-wear silicone hydrogel contact lenses. Optom Vis Sci. 2006;83(10):758-68.

14. Stapleton F, Stretton S, et al. Silicone hydrogel contact lenses and the ocular surface. Ocul Surf. 2006;4(1):24-43.



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