A next-generation silicone hydrogel lens for daily wear

Despite advances in contact lens materials, more than half of all soft lens wearers experience discomfort with their lenses at the end of the day. In Part 1 of this two-part article, Robert Steffen and Christina Schnider describe the material properties of Acuvue Advance with Hydraclear, the first in a new generation of silicone hydrogel lenses developed specifically to deliver long-lasting comfort The primary aim of most new contact lens developments is for the lenses to offer improved features, such as better oxygen performance or improved surface properties, rather than to fulfil a real market need. Perhaps a better rationale for developing new lenses is to turn this process around and identify what consumers and practitioners want, then work towards satisfying their needs.
Large-scale surveys of consumer satisfaction with current contact lenses show that by far the greatest unmet need is for lenses that remain comfortable throughout the day.1 The key attribute of soft lenses is the comfort and rapid adaptation they provide in comparison with rigid lenses. But although almost all patients find soft lenses comfortable on insertion, many find that comfort tends to decrease towards the end of the day. Even those who are generally satisfied with their lenses report some reduction in comfort as the day progresses, when questioned closely.
Surveys found that more than half of all soft lens wearers have some degree of discomfort by the end of the day. This represents a significant problem for the contact lens industry and for the practitioner. Discomfort is the most common reason for patients abandoning contact lenses; half of those who discontinue cite this as the principal reason for having given up, and in a majority of cases contact lens failure is product or practitioner related rather than due to patient-specific problems.2
Reduced comfort, either generally or at the end of the day, may be one of the reasons why silicone hydrogel extended wear has not proved as popular as some had expected.3 The suggestion is that patients who experience discomfort in the evening after a day's wear are unlikely to want to keep their lenses in even longer. Rather than sleep with their lenses in, they prefer to take them out.
The first generation of silicone hydrogel lenses for extended wear has now been available in Europe for more than five years, yet only 5 per cent of new soft lens wearers are currently being fitted with this modality.4 While consumer reluctance to embrace extended wear may well be a factor in the lack of uptake, practitioner attitudes to prescribing extended wear and deficiencies in the comfort and clinical performance of first-generation silicone hydrogels have undoubtedly played their part.
Deficiencies in these materials relate principally to material properties other than oxygen permeability, such as modulus or 'stiffness', surface wettability and lipid deposition.5 Although hypoxic problems have been virtually eliminated with silicone hydrogels, inflammation, infection and mechanical complications remain.
The research and development goal in developing the next generation of soft lenses was therefore to provide improvements over current lenses that were noticeable to the patient and to the practitioner, and would encourage consumers to try the lenses and practitioners to recommend them. These new lenses would have the advantages of silicone hydrogels, without the less desirable properties of the current materials, but would maintain the proven fit and comfort of traditional hydrogels.
Launched in the US in January 2004 and introduced to the UK last month, Acuvue Advance with Hydraclear (Johnson & Johnson Vision Care) is the first of this new generation of hydrogels to reach the market and the first silicone hydrogel lens designed specifically for daily wear. The material properties of the lens are summarised in Table 1 and the key features are listed in Table 2.

Structure and transmissibility
Although silicone hydrogels are ideal for increasing oxygen transmissibility to the cornea, the silicone component is hydrophobic and the forms used in current lenses tend to result in a stiff, inflexible material. In creating the Acuvue Advance material, the aim was to achieve the high oxygen transmissibility of silicone hydrogels without sacrificing the soft, supple feel of a traditional hydrogel lens.
The critical component of the polymer structure that makes this combination feasible is a long-chain, high-molecular-weight molecule called Hydraclear. This internal wetting agent, widely used in ophthalmic and other applications, is present throughout the lens matrix to render the material hydrophilic without the need for surface treatment.
The Hydraclear chemical component comes from the PVP (polyvinyl pyrrolidone) family. The molecule used is a very effective humectant, meaning it readily binds to water and retains moisture well. The result is that once the lenses reach equilibrium with the eye's environment, they stay hydrated throughout the period of wear. The lubricant properties of Hydraclear render the lens surface smooth and silky, wettable and resistant to deposits. A further property of the Hydraclear wetting agent is that the molecule is extremely flexible when fully hydrated.
The net result of these properties is a lens with a Dk/t of 86, three times the oxygen transmission of leading traditional hydrogels and well in excess of the minimum criteria for oedema-free and acidosis-free daily wear (Figure 1).6,7 At 47 per cent, the water content of Acuvue Advance has intentionally been made 50-100 per cent higher than that of other currently available silicone hydrogels, which, in addition to the presence of Hydraclear, maintains flexibility and moisture.
Although it is difficult to assign the individual properties of the lens specifically to its oxygen transmission or to the lubricant qualities of the lens surface, one aspect of traditional hydrogels no longer applies. In traditional hydrogels, loss of water means loss of oxygen transmission. In HEMA lenses, water is the medium through which dissolved oxygen reaches the cornea, so when the lens dries out in the course of a day, not only is it losing water it is also losing the ability to carry oxygen to the cornea. In Acuvue Advance, oxygen transmission has been increased by adding silicone, but the relationship of hydration loss to oxygen loss has also been 'decoupled'.
A number of in vitro studies have now been conducted to evaluate various material properties of Acuvue Advance and compare them with those of traditional hydrogels and other silicone hydrogel lenses. The results of these studies are summarised below.

Wettability and deposit resistance
The surface wettability of a contact lens is important for stable vision, comfort and biocompatibility.8 Conventional hydrogels generally have good wettability when fully hydrated but with previous silicone hydrogels the lens surfaces needed to be surface treated to overcome the hydrophobic nature of the silicone element. The PureVision lens (Bausch & Lomb) is treated using plasma oxidation to produce islands of wetting and Focus Night & Day (Ciba Vision) has a plasma coating applied to render the surface more wettable than the underlying material. With Acuvue Advance, the Hydraclear wetting agent is present throughout the lens and no surface treatment is required.
In vitro measurement of the wetting angle (advancing dynamic contact angle) of Acuvue Advance, using ISO standard techniques, has shown that the lens has a low wetting angle and exhibits similar wettability to a traditional mid-water content hydrogel (Acuvue). Assessment of wettability in vivo is more clinically significant and will be discussed in Part 2.
Contact lens surfaces with poor wettability also have a greater tendency to attract deposits.8 Dry spots form areas prone to deposit formation, especially protein, and this in turn further reduces wettability. Laboratory studies using HPLC (high-performance liquid chromatography) assay of lenses collected after exposure to artificial lipid and protein solutions9 have shown Acuvue Advance to be highly resistant to protein deposition; at 9µg/lens, protein uptake was negligible compared to 1,576µg/lens with Acuvue and comparable to Focus Night & Day (12µg/lens). As expected, lipid uptake was greater with the silicone-containing materials than with the traditional hydrogel.
The high wettability and low protein deposition of the Acuvue Advance material are important features in maintaining optimum comfort and vision throughout the wearing period and replacement schedule.

Lubricity
Lubricity is the ability of a hydrated material to resist friction and is expressed as the force required to move a known load across a surface at a given speed.10 In contact lenses, surface lubricity relates most closely to the ability of the eyelid to travel smoothly across the surface of the lens without irritation. Coefficient of friction, a measure of lubricant properties, represents the amount of friction created at the lens surface from a load roughly equal to eyelid forces and oscillated at frequencies to mimic the normal blink. Values are then adjusted for material deformation and the fluid interface due to measuring a wet lens.
Laboratory measurements have shown that Acuvue Advance has a significantly lower dynamic coefficient of friction than that of Acuvue 2, Focus Night & Day and PureVision (Figure 2). Low values indicate a highly lubricious surface which gives the lens a smooth, silky feel that allows the lid to travel over the lens without irritation and enhances comfort.

Modulus
Lens modulus is a material property that has received increasing attention in the literature and is defined as the force per unit area required to compress the material by a given amount.10 The first generation of silicone hydrogels have high modulus values and are significantly stiffer than traditional hydrogels, creating greater lens awareness on the eye and a longer period of adaptation. The stiffness of these lenses also means that the lens flexes less on the eye and a flat fit may give rise to edge fluting.
More significantly, the high modulus of previous silicone hydrogels has been implicated in the increased incidence of mechanical complications such as superior epithelial arcuate lesions (SEALs),11 contact lens papillary conjunctivitis (CLPC)12 and mucin balls,13 as well as in corneal moulding effects.14
Acuvue Advance lenses have modulus values that are similar to traditional hydrogels and four to six times lower that those of the first generation silicone hydrogel lenses (Figure 3). This property aids comfort, and should reduce the mechanical impact of the lens and allows the lens to maintain the fit and feel of a traditional hydrogel.

UV blocking
Because of the increasing emphasis on the hazards of excessive exposure to the sun's harmful rays, some contact lenses, including all Acuvue lenses, now incorporate UV-blocking capabilities. Protection against UV is recommended for all patients and especially those who participate in leisure or work activities that expose them to high levels of UV. Wearing UV-blocking contact lenses in association with other forms of eye protection offers the maximum protection and practitioners should consider prescribing these lenses for all their contact lens patients.15
Acuvue Advance is the first silicone hydrogel lens to incorporate UV blocking and the first soft contact lenses marketed to meet the strictest standards for Class I UV blocking (Figure 4). The lenses block more than 90 per cent of UV-A rays and over 99 per cent of UV-B rays, and offer the highest protection of any soft lens. This superior UV-blocking capability has the benefit of extra reassurance for patients concerned about the hazards of excessive UV.

Summary
Acuvue Advance with Hydraclear is the first in a new generation of silicone hydrogel lenses. Its unique material properties are designed to combine the desirable features of traditional hydrogels with increased oxygen performance but without the deficiencies of current silicone hydrogels. These properties are expected to deliver longer, more comfortable wearing times that satisfy the unmet needs of the consumer and provide compelling reasons for patients to try the lenses and for practitioners to recommend them.
Part 2 of this article will describe the on-eye performance of Acuvue Advance and the clinical benefits that its material properties deliver, as well as providing guidance on positioning the lens in your practice.

Acknowledgements
Thanks to Visioncare Research for help with the preparation of this article.

References
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Robert Steffen is manager materials clinic research and Cristina Schnider is director of academic affairs at Vistakon, Division of Johnson & Johnson Vision Care, Jacksonville, Florida, US