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HomemieyecareDaily Silicone Hydrogels

Daily Silicone Hydrogels

In some countries silicon hydrogels (SiH) make up 70 per cent of the contact lens market. We are a long way from that in Australia and New Zealand but all signs point to SiH contact lenses dominating the local market over the coming years. With the introduction of the daily SiH last year we look at whether this new generation of eye care products is able to provide the optimum comfort and health for contact lens patients.

The first generation silicone hydrogel contact lenses were designed for monthly continuous wear but although these contact lenses provided high levels of oxygen, the materials tended to be stiffer, not as wettable and attracted more deposits than conventional hydrogel contact lenses.

Since their introduction a decade ago, silicone hydrogel contact lenses have evolved, with the latest generation of materials providing more comfortable contact lens wear. Improvements in materials and manufacturing techniques have resulted in the launch last year of daily disposable modalities of these healthier materials. The objective was to produce a silicone hydrogel contact lens that provided maximum ocular health using a material that is less stiff, less prone to deposits and with a more wettable surface.

This approach would clearly improve wearer comfort by allowing research scientists to include lower levels of silicone monomers in the formulation. The level of silicone needs to be optimised to provide a lens with excellent oxygen performance but keeping it low enough so that it does not negatively impact modulus, wettability, water content and therefore comfort of the lens.

With the increasing popularity of monthly silicone hydrogel lenses the obvious next step has been to develop a daily disposable silicone hydrogel lens

Developing the Daily SiH

With the increasing popularity of monthly silicone hydrogel lenses the obvious next step was to develop a daily disposable silicone hydrogel lens. It is considered that silicone hydrogel contact lenses are likely to dominate the Australian contact lens market in the years ahead.

Figures released last year show that silicone hydrogel lenses make up 47 per cent of the market in Australia whilst New Zealand has seen stronger growth of between 52 to 59 per cent. 1

In some markets, they already account for up to 70 per cent of new spherical (non-daily disposable) contact lens fits. Until early last year, this type of contact lens material has not been available in a daily disposable product.

Exceptional oxygen transmissibility, coupled with the improved hygiene expected with a single use product offers distinct advantages. Major factors in achieving a daily disposable silicone hydrogel lens include the need for a dramatic improvement in yield and of course the development of a high volume process.

Oxygen Performance

For over 30 years the best known methods for measuring oxygen performance of contact lenses have been:

  • Oxygen permeability (Dk) of the lens materials
  • Oxygen transmissibility (Dk/t) of the finished, manufactured lens which takes lens thickness into consideration

However, the important factor to consider is how much oxygen the cornea requires to maintain normal corneal physiology. Holden and Mertz (1984)2 presented the best known value, suggesting that a lens with a Dk/t of 241 would not induce corneal swelling during daily wear. Harvitt and Bonanno (1999)3 subsequently proposed a value of 35 units to avoid hypoxia through the cornea.

Clearly for overnight wear more oxygen is required and Holden and Mertz found that a lens with Dk/t of 87 would limit overnight corneal swelling to four per cent, a value considered equivalent to a non-lens wearer. They also suggested a compromise threshold of 34 units which might induce some additional swelling after overnight wear but would allow the cornea to return to normal thickness soon after waking.

The guru of contact lens oxygen measurement Irving Fatt4 considered that the Dk/t measure failed to inform how much oxygen actually gets to the cornea. Consequently, over the years alternative methods of describing corneal oxygenation have evolved including oxygen flux (oxygen volume reaching cornea during lens wear) and corneal oxygen consumption. Fatt himself advocated using flux as a better measure of corneal oxygenation.

More recently, however, corneal oxygen consumption has been considered as a more important factor by researchers such as Professor Noel Brennan.5 It is considered to be a better index of corneal physiology because it reflects how much oxygen the cornea metabolises.

It can be seen in Figure 1 that there is a marked flattening off of corneal oxygen consumption as Dk/t increases.

In other words, we have a situation of diminishing returns between increasing Dk/t and the amount of oxygen used by the cornea. In turn, this suggests that the difference in percentage in oxygen consumption between an eye wearing a lens with Dk/t of 56 and another wearing a lens with a Dk/t of 300 is surprisingly modest. This relationship does explain why Dk/t was appropriate in the era of low Dk lenses. If we only consider values of Dk/t up to 30 units then the near-linear relationship between oxygenation and Dk/t would mean that doubling transmissibility would account for approximately twice as much oxygen consumed by the cornea.

No such relationship exists for lenses of higher oxygen transmissibility. Although mathematical diffusion models such as those discussed above provide a helpful indicator of the theoretical situation at the cornea during contact lens wear, it is also important to relate this to clinical observations.

A good way to achieve this is to consider the presence or absence of hypoxia related clinical signs across a range of lenses, one example here is limbal hyperemia which is known to be an indicator of corneal hypoxia. In 2004, researchers at Eurolens Research at the University of Manchester6 performed a masked, controlled study of neophyte subjects in which they evaluated changes in limbal redness during daily wear of contact lenses.

Comfort

One of the most important attributes that patients demand from their contact lenses today is comfort.

Surface Wettability

Inclusion of silicone in a lens formulation makes the surface extremely hydrophobic. The first generation silicone hydrogel lenses therefore required some form of surface modification to overcome the hydrophobicity. Certain lenses rely on plasma oxidation to render the surface more hydrophilic. Others also utilise plasma treatment to yield a continuous hydrophilic surface for their lenses or rely on an internal wetting agent, Polyvinyl pyrrolidone (PVP).

The latest lens does not use any form of surface treatment or wetting agents. Instead a patented process is used which produces a unique silicone hydrogel lens material with enhanced oxygen permeability and superior levels of wettability and biocompatibility. This is achieved by controlling how the novel silicone and hydrophilic materials are combined at the molecular level to produce a surface with unparalleled biocompatibility and wettability without the need for surface modification. Meanwhile, excellent clarity and low modulus are retained to achieve the uncompromised visual and comfort standards required by the latest generation of SiH lens products.

Water Content

Patients often cite lens dryness and irritation as reasons for removing lenses earlier in the day and for dropping out of contact lens wear altogether. The water content of silicone hydrogel lenses has tended to increase with the launch of newer materials. This should not be surprising when you consider that a water content of 56 per cent is comparable to the mid water conventional hydrogel lenses that have been popular and preferred for their comfort for many years prior to the advent of silicone hydrogel lenses. This higher water content provides excellent biocompatibility with the ocular surface.

Modulus

Incorporation of silicone makes silicone hydrogel lenses ‘stiffer’ than conventional hydrogel lenses and this rigidity has been implicated in poorer comfort, papillary conjunctivitis and in continuous lens wear superior epithelial arcuate lesions (SEALs).

Higher modulus values can also make switching to these lenses from conventional hydrogels more difficult. We now have one of the lowest modulus values for a silicone hydrogel lens, measuring 0.5 MPa.

Conclusion

With the benefits of exceptional oxygen transmissibility, high levels of patient comfort, added to the improved hygiene expected with a single use product, a daily disposable SiH lens clearly offers distinct advantages over most existing and daily disposable contact lenses available.

Howard Griffiths B.Sc (Hons) M.R. Pharm.S is technical director of Sauflon Pharmaceuticals. With global headquarters in London, UK, Sauflon is a leading contact lens and aftercare solution provider. With satellite companies on five continents, the company manufacture and market contact lenses and aftercare products exclusively to optometrists and other eye-care professionals in over fifty countries. Sauflon products are distributed in Australia and New Zealand by Sauflon Eyecare Pty Ltd, based in Glenelg, South Australia. For further information go to www.sauflon.com.au or phone (AUS) 08 8376 4277.
References
  1. Morgan PB, Woods CA, Tranoudis IG, Efron N, Knajian R, Grupcheve CN, Jones D, Tan K-O, Pesinova A, Ravn O, Santodomingo J, Vodnyanszky E, Montani G, Itoi M, Bendoriene J, van der Worp E, Helland M, Phillips G, Gonzalez-Meijome JM, Radu S, Belousov V, Silih MS, Hsiao JC, Nichols J. International contact lens prescribing in 2008. Contact Lens Spectrum, 2009; 24(2): 28-32.
  2. Holden B, Mertz G. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci, 1984: 25:1161-1167.
  3. Harvitt DM, Bonanno JA. Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia. Optom Vis Sci. 1999: 76: 712-719.
  4. Fatt I. New physiological paradigms to assess the effect of lens oxygen transmissibility on corneal health. CLAO J, 1996; 22: 25-29.
  5. Brennan NA. Beyond flux: total corneal oxygen consumption as an index of corneal oxygenation during contact lens wear. Optom Vis Sci, 2005: 82: 467-472.
  6. Maldonado-Codina C, Morgan PB, Schnider CM et al. Short-term physiologic response in neophyte subjects fitted with hydrogel and silicone hydrogel contact lenses. Optom Vis Sci, 2004: 81: 911-921.
  7. Data on file Sauflon.