Biofinity silicone hydrogels

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Silicone hydrogels have had a significant effect on the contact lens market since their launch in the late 1990s. Morgan1 reports that in 2007 silicone hydrogel (SiH) lenses prescribed for use on a daily wear basis accounted for 23 per cent of fits in the UK with a further 10 per cent of fits being with SiH extended wear lenses.

There has been an even more rapid adoption of SiHs in the US with 42 per cent of fits going into daily wear SiH lenses and a further 11 per cent going into SiH extended wear lenses perhaps surprisingly, 21 per cent of US extended wear fits were still with hydrogel lenses in 2007.

The number of SiH lenses sold between 2004 and 2006 quadrupled, driven in part by the introduction of SiH lenses marketed for daily wear and further developments in lens materials. Early SiH lenses were marketed for continuous wear use and offered extremely high levels of oxygen transmission that reduced the level of overnight corneal swelling to approximately 4 per cent2 compared to levels of approximately 10 per cent oedema with hydrogel lenses.3 However, two of the major causes of discontinuation of contact lens wear are still discomfort and dryness,3 factors unrelated to oxygen transmission.

The early SiH lenses were surface treated to try to overcome the inherent hydrophobicity of the materials, more recently wetting agents have been added in an attempt to improve wettability and reduce the incidence of deposits. However, symptoms of dryness, lipid deposits and denatured protein deposits persist.5 Another factor affecting comfort is the stiffness of a lens material. Many wearers found silicone hydrogel lenses - which have a modulus up to three times that of pHEMA lenses - gave reduced initial comfort and higher incidence of conditions such as superior epithelial arcuate lesions (SEAL) and contact lens papilliary conjunctivitis (CLPC).6,7

With corneal oxygen requirements being met by SiH contact lenses, attention has turned to other contact lens properties that can be improved to enhance wearer comfort and improve corneal physiological response: lower modulus, better wettability, deposit resistance and improved back surface and edge design.

Surface and material properties

Overcoming the inherent hydrophobic nature of silicone has been a challenge for contact lens manufacturers. The early SiH materials were surface treated to enhance wettability: lotrafilcon A (Night & Day) and lotrafilcon B (AirOptix) have a 25nm plasma coating surface treatment and balafilcon A (PureVision) has a plasma oxidation surface treatment.

Despite these surface treatments, such materials still have much higher wetting angles than hydrogel materials (Figure 1). The incorporation of poly-vinyl povidone as a wetting agent in galyfilcon A (Acuvue Advance) and senofilcon A (Acuvue Oasys) offer an alternative to surface treatment but still give higher wetting angles than traditional hydrogels. Comfilcon A material (Biofinity) (Table 1), is manufactured using CooperVision's Aquaform technology that produces a naturally wettable SiH material requiring no surface treatment or wetting agent. Aquaform technology creates a naturally hydrophilic contact lens that retains water within the lens, minimising dehydration. This is achieved by the formation of hydrogen bonds with the oxygen component of the water molecules. A highly wettable material has the advantages of better all day comfort and better deposit resistance.

Biofinity has been shown to deposit significantly less protein than PureVision in continuous wear (total protein PureVision 48.8+/-26.7 µg/lens vs total protein Biofinity 7.7+/-3.8 µg/lens)8 demonstrating the surface quality over a month of wear.

Interestingly, the adherence of Pseudomonas aeruginosa (PA) to worn and unworn Biofinity lenses has been shown not to vary significantly, suggesting that the surface characteristics of comfilcon A are maintained during periods of continuous wear. Certain strains of PA were shown to adhere to worn PureVision lenses more than to Biofinity lenses.9

End-of-day comfort clearly results from a number of lens material and lens design-related factors, although surface quality, dehydration and wettability play a large role. Biofinity's end-of-day comfort has been assessed in overnight and daily wear studies part two of this article will focus on the daily wear performance. In overnight wear, Biofinity has been shown to give better comfort and end-of-day comfort when compared to other SiH lenses approved for continuous wear (Night & Day and PureVision).10 In a group of wearers who were switched from older SiH lenses (Night & Day and PureVision) into Biofinity and followed over a six-month period, 66 per cent preferred Biofinity, 26 per cent had no preference and only 8 per cent preferred their old SiH lenses. End-of-day comfort scores were statistically significantly higher with Biofinity than with the other SiH lenses assessed. Biofinity's end-of-day comfort was rated as 8.9 (on a 0-10 scale) compared to an end-of-day comfort score of 8.2 with the older SiH lenses.10 The superior comfort of Biofinity throughout the day, when worn on a continuous wear basis has been confirmed in other studies.11

Modulus

The first SiH materials introduced were stiff with high modulus values: 1.1MPa (balafilcon A) and 1.4MPa (lotrafilcon A)12 when compared to a modulus value for HEMA of 0.5MPa (Figure 2). The high modulus values of these materials relate to the relatively high silicone content of the materials and relatively low water content. A high silicone content was required to achieve a high level of oxygen transmission and this had the unwanted side effect of raising the modulus. Aquaform technology, used in Biofinity, incorporates unique long silicone chains that allow for the efficient transport of oxygen. This efficiency means less silicone needs to be incorporated into the material to provide a high level of oxygen transmission resulting in a lower modulus giving a softer, more flexible lens. Biofinity has a Dk/t of 160 and a modulus of 0.75MPa - a unique combination of high oxygen transmission and softness.

A lower modulus material has the advantage of a lower incidence of SEAL6 and better comfort. High modulus has also been implicated in the development of CLPC7 and conjunctival epithelial flaps (CEF) in daily and overnight wear.13,14

In a review of 323 subjects with an average of 17 months of Biofinity continuous wear 95 subjects with an average of 11 months' PureVision continuous wear and 92 subjects with an average of 16 months' Night & Day continuous wear, the incidence of SEAL was 7.1 per cent of eyes per year for PureVision, 1.7 per cent of eyes per year with Night & Day and 0 per cent of eyes per year with Biofinity.15

CEF have been shown to occur in daily wear as well as overnight wear and the incidence rate related to the modulus of the lens material. In a prospective, masked, double-masked study of 390 eyes, Lin found the incidence of CEF was 6 per cent for AirOptix 1 per cent for Acuvue Advance, 2 per cent for Biofinity and 0 per cent for Proclear.14

Clearly, complications such as SEAL, CLPC and CEF are influenced by a number of factors not just modulus. Factors that also need to be considered include the back surface design, front surface design and edge profile. However, modulus appear to have a major role to play in many of these conditions and moving away from the older SiHs with a modulus over 1MPa may help to minimise the incidence of these conditions.

The comfilcon A material has shown very low levels of solution related corneal staining when tested across a wide range of solution types.16

Design

Biofinity has been developed with the objective of maximising comfort and wettability and minimising the physiological impact to the eye whether the lens is worn as a daily, occasional overnight, or continuous wear lens.

A lower modulus material not only has physiological advantages but has advantages in terms of lens fit as the lens drapes better on the cornea, fitting a wider range of topographies. Back surface lens design is important even with a lower modulus material, as the pressure needs to be distributed across the entire surface for maximum comfort and minimal physiological interaction.

Edge design is also critical both from a comfort and physiological perspective a sharp abrupt edge form can cause conjunctival staining and it has been shown that a rounded edge can give better comfort than a chiselled edge.17

Biofinity incorporates the same aspheric optics found in Biomedics 55 Evolution, which have been shown to reduce the spherical aberrations of the contact lens and the eye while compensating for differing levels of aberrations across the power range.18 Of wearers refitted from Night & Day and PureVision who wore Biofinity for a six-month period on a continuous wear basis, 49 per cent preferred the vision quality with Biofinity 45 per cent had no preference and only 6 per cent preferred their previously worn lenses.10

A new level of performance

While SiH contact lenses continue to evolve with newer materials and designs, Biofinity offers a new approach which can give improvements in terms of comfort and wettability and overall performance relative to some older lens materials and designs. ?

References

1. Morgan P et al. International contact lens prescribing in 2007. Contact Lens Spectrum, 2008 January.
2. Brennan NA Coles M. Do we need a new oxygen criteria? Optom Vis Sci, 2003 80 S245.
3. Comstock TL, Robboy M, Cox I. Overnight clinical performance of a high DK silicone hydrogel contact lens. Contact Lens and Anterior Eye, 1999 22(4) 159.
4. Weed K, Fonn D, Potvin R. Discontinuation of CL wear. Optom Vis Sci, 1993 70, s140.
5. Jones L et al. Eye Contact Lens, 2003 29(1 Suppl):S75-9.
6. Dumbleton K. www.siliconehydrogels.org, 2004.
7. Skotnitsky C et al. Clin Exp Optom, 2002 85(3): 193-197.
8. Jones L et al. Protein Deposition on Continuous Wear Surface Modified (balafilcon A) and Non-surface modified (comfilcon A) Silicone Hydrogel Contact Lens Materials. AAO Poster Presentation, 2007.
9. Lakkis K et al. Pseudomonas aeruginosa. Adherence to silicone hydrogel contact lenses after continuous wear. AAO Poster 2007.
10. Lofstrom T et al. Refitting adapted SiH wearers with a new non-surface treated SiH lens. AAO Poster 2005.
11. Brennan et al. A 12-month prospective clinical trial of comfilcon A silicone hydrogel contact lenses worn on a 30-day continuous wear basis. Contact Lens & Anterior Eye, 2007 30: 108-118.
12. Ross G et al. Silicone hydrogels: trends in products and properties. BCLA Poster 2004.
13. Løfstrøm et al. A conjunctival response to SiH lens wear. Contact Lens Spectrum, Sept 2005.
14. Lin et al. Conjunctival epithelial flaps with SiH lenses for daily wear. AAO Poster 2005.
15. Back et al. The rate of corneal infiltrative events and SEALS in SiH continuous wear studies. AAO Poster 2006.
16. Andrasko http://www.staininggrid.com/
17. Thota S et al. Soft lens edge design and comfort. AAO Poster 2004.
18. Kollbaum P. Aberration correction with soft contact lenses. AAO Poster 2004.

? Guy Whittaker is global category director of silicone hydrogel products at CooperVision