A new silicone hydrogel lens

The development of silicone hydrogel materials and the accelerating interest from practitioners in these lenses is largely being driven by three factors. First, better knowledge of the clinical risks of lenses with low oxygen permeability indicates that there is a need for high oxygen transmissible contact lenses. Second, we have a better understanding of our patients' lifestyles and how they are actually wearing their contact lenses, which is not necessarily what they are telling their practitioner. Third, patients now recognise ocular health as their single most unmet need in contact lens wear.1

The unmet need for oxygen
It has been shown that oxygen is key in providing safe contact lens wear. Silicone hydrogels have a significantly higher oxygen permeability (Dk) and transmissibility (Dk/t) than traditional hydrogel lens materials that we have used for more than 30 years. The effects of oxygen deficiency have been well documented and include limbal2 and bulbar hyperaemia,3 corneal neovascularisation4 and a suppression of central corneal epithelial basal cell proliferation,5 which compromises the cornea's ability to fend off inflammation and infection. These signs and symptoms can be described as corneal oxygen deficiency.16 Silicone hydrogels have virtually eliminated all the clinical hypoxic signs associated with extended wear (EW)3,6-10 and there is documented4 and anecdotal evidence that even daily wear (DW) of low Dk/t hydrogel lenses produces clinical signs of chronic hypoxia.
Figure 1 shows how four silicone hydrogel products, as well as some commonly-used disposable lenses, measure up to the latest Dk/t criteria for daily and overnight wear.11 Oxygen transmissibility is a critical factor to consider when choosing a contact lens, the higher the Dk/t of a lens, the more oxygen it allows to reach the surface of the eye.

Unmet consumer need for healthier contact lens wear
Research tells us that patients may wear their lenses for 11 or more hours at a time and in many cases a lot longer,12 often in polluted or air conditioned atmospheres. Nineteen per cent of patients occasionally sleep in their lenses overnight13 and 84 per cent nap in their lenses.14 As a result, up to 70 per cent of European contact lens wearers report lens awareness, irritation and dryness with their lenses.13 Other studies show 68 per cent suffer some sort of discomfort due to dryness and half of those discontinue wear completely.17
While practitioners observe signs of corneal oxygen deficiency such as limbal redness and neovascularisation, many contact lens patients are unaware of the physiological damage occurring to their cornea as a result of hydrogel lens wear. Particularly when asymptomatic, patients will not always heed their practitioner's advice to reduce wearing times, leaving them at continued risk of the consequences of this problem.
Silicone hydrogels have largely eliminated this situation and as a result address the single most unmet need in contact lens wear, the need for healthier contact lenses.1 Silicone hydrogels give practitioners and patients peace of mind and a safety net if patients do not or cannot change their lifestyles.

O2Optix
UK practitioners now have a choice of four silicone hydrogel soft lenses with the introduction of O2Optix from Ciba Vision in October 2004. This is a monthly lens designed for daily wear with the flexibility for occasional overnight wear. This lens and the Acuvue Advance lens from Johnson & Johnson represent the latest generation of silicone hydrogel lenses following on from the 1999 launch of the PureVision lens from Bausch & Lomb and the Night&Day lens from Ciba Vision.
The original silicone hydrogel products were launched for 30-night continuous wear, both having FDA approval and CE marking for this modality. With a Dk/t of 175, the highest of all soft lenses, the Night&Day lens remains Ciba Vision's recommended choice for patients who regularly sleep in lenses up to 30 nights continuously.
With a Dk/t of 138, O2Optix far exceeds the critical minimum threshold to avoid corneal stress with daily wear. O2Optix offers five times more oxygen than current low Dk/t hydrogel lenses.17 The lens is designed for DW but can allow for long days, napping or occasional overnight wear if necessary. It is CE-marked for DW and up to six nights EW which allows it to be used as a lens which meets the needs and wearing habits of today's contact lens users. It is indicated for wearers who wear their lenses for long days or exhibit signs or symptoms of corneal oxygen deficiency.
The single aspheric base curve of 8.6 and 14.2mm diameter has been shown to optimally or acceptably fit 97 per cent of eyes.18 Currently available in powers from -1.00DS to -6.00DS, a parameter expansion into higher minus, plus and toric is expected shortly. Table 1 shows the product parameters in detail.

Properties Of Silicone Hydrogel materials
Dehydration
A significant benefit of these low water content materials is reduced dehydration. With today's patients using computers in air conditioned environments for long hours, these materials can enable the wearer to achieve longer, more comfortable wearing times. Marginally dry eyed patients may find they can increase their wearing times when refitted from a hydrogel to a silicone hydrogel material. Table 2 shows the differences in water content, Dk, and Dk/t between the four currently available silicone hydrogels.

Surface treatment
Surface modification of the underlying hydrophobicity of silicone is a critical part of silicone hydrogel lens technology and the approach employed varies from lens to lens.
The surfaces of Night & Day and O2Optix lenses are permanently modified in a gas plasma reactive chamber. The treatment creates a permanent, ultrathin (25nm), high refractive index surface which is continuous over the lotrafilcon A and lotrafilcon B bulk materials. PureVision lenses are also surface treated in a gas plasma reactive chamber which transforms the silicone components on the surface into hydrophilic silicate compounds. Glassy discontinuous silicate islands result and the hydrophilicity of the transformed surface areas bridges over the underlying hydrophobic balafilcon A material.
The flow of oxygen and fluids through the lenses above is not impeded by these surface modifications. Both surface treatments become an integral part of the lens and are not surface coatings that can be stripped away from the base material with handling or cleaning.
Acuvue Advance is the only lens which is not surface treated to address decreased wettability and increased lipid interaction inherent in silicone based materials. Instead there is an internal wetting agent (Hydraclear) based on PVP (polyvinylpyrrolidone) which is designed to provide a hydrophilic layer at the surface of the material which shields the silicone at the material interface, thereby reducing the degree of hydrophobicity.
The deposition of protein on surface-treated silicone hydrogels is significantly less than that seen with conventional hydrogel materials, some of which can develop lysozyme deposits very rapidly.19 On average a 100-fold reduction in total lysozyme protein deposition is observed.

Modulus
The higher the relative silicon content in the material, the higher the modulus and the greater the oxygen transmissibility. The first-generation of silicone hydrogels were relatively stiff and, while this can be a handling advantage, did cause some mechanical problems such as superior epithelial arcuate lesions (SEALs). With the introduction of a second basecurve and an aspheric design to the Night&Day lens this problem was reduced.
Some patients report increased lens awareness with silicone hydrogels, particularly if they are used to wearing thin hydrogel lenses. If they are warned about this they often adapt within a few days. Allowing patients to trial lenses with different moduli only highlights the difference in initial comfort. New wearers tend to adapt quickly since they have no previous experience with which to compare. This potential concern has been addressed with the O2Optix and Acuvue Advance lenses which have flexibility and fitting characteristics more similar to hydrogel lenses.

Lens care
When refitting DW patients from conventional hydrogel lenses to silicone hydrogel lenses, we tend to blame any discomfort on the new lens material. However, it is important to give some thought to the lens care regime. Overall it is generally accepted that corneal staining is relatively rare with polyquad-based regimens, and essentially non-existent with hydrogen peroxide-based systems.
When using polyhexamethylene biguanide (PHMB) solutions with silicone hydrogels there is less consistancy in success.20 Regimens with identical concentrations of PHMB can behave differently, depending on solution. Some are more compatible than others and to some extent it is trial and error as a patient's success with a given lens care regimen with conventional hydrogel lenses does not guarantee success of that same system with silicone hydrogel lenses. Figure 2 shows the extent of corneal staining among silicone hydrogel lens users in a study of four lens care systems.

Business opportunity
When asked about barriers to fitting silicone hydrogel lenses some practitioners cite them as being too expensive for their patients. Interestingly, few complain about the increased profitability that these lenses provide. For other practitioners it is the continuous wear modality they are uncomfortable with rather than the cost. Patients who are successful with continuous wear are often happy to pay the extra for the convenience or for simply having the most breathable products available.
The reduced cost of the second-generation materials has reversed these concerns. These lenses cost little more than conventional hydrogels and can greatly improve physiological performance compared to hydrogels and hence end-of-day comfort. Higher profit margins and greater patient satisfaction can only enhance the success of our practices. Work commissioned by the Association of Contact Lens Manufacturers and carried out at the London Business School confirms that silicone hydrogels can be a very profitable part of our businesses.
Keeping patients informed of the latest advances in technology and the full range of options for vision correction are vital to develop strong practitioner and patient relationships and hence loyalty. Simple changes in practitioner attitudes can have a huge effect on the profitability of a practice.

Summary
UK market research shows that for the first time, the EW modality is now at a level approaching that of the US and Australia.24 Silicone hydrogel materials now represent 6 per cent of new fits and 16 per cent of re-fits in the UK and 91 per cent of EW fits are with silicone hydrogel material. With the advent of the second-generation options this figure will surely increase when the 2005 data is published.
It can only be a matter of time before the majority of new fits are in silicone hydrogel material just as gas-permeable material has virtually replaced PMMA. For many practitioners this is already their first-choice material, wishing to offer their patients the most breathable soft lenses. Others, having seen the benefits of more oxygen, are recommending these lenses to troubleshoot problems associated with hypoxia.
Practitioners finally have a real and compelling choice of silicone hydrogel modalities with the addition of the new lenses. Power ranges will extend and a choice of torics is imminent. The race is on to bring a daily lens to the market which may signal the demise of hydrogel soft lenses.

Acknowledgements
Thanks to Dr Vicki Evans, professional affairs manager at Ciba Vision, for her help in preparation of this article.

References
1 Ciba Vision data on file, 2004.
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Jayne Schofield is a contact lens consultant at Ciba Vision and a visiting clinician at City University