Continuing Education

Author: Lyndon Jones, Lakshman Subbaraman, Ronan Rogers and Kathy Dumbleton

In the fourth in our series on silicone hydrogels, Lyndon Jones, Lakshman Subbaraman, Ronan Rogers and Kathy Dumbleton describe how the various surface and mechanical properties of the currently available silicone hydrogel lenses influence clinical performance (C4643, one contact lens CET point)

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Silicone hydrogel lenses are increasing in popularity, with rapid growth in their usage for daily and overnight wear, in both the UK and all over the world.1,2 Numerous studies have now confirmed that these materials provide oedema-free wear in the majority of wearers, whether worn during the day or overnight.3-5 However, despite their increased oxygen transmission compared with conventional materials (which can be reviewed elsewhere),5,6 patients wearing silicone hydrogels continue to exhibit a number of clinical problems that limit their use.

This article will briefly describe the broad differences between currently available silicone hydrogel contact lenses and then discuss how these differences impact on how they behave in-eye, compared with conventional materials.

SILICONE HYDROGEL MATERIALS: AN OVERVIEW

Five silicone hydrogel lens materials are currently commercially available, with their major features being summarised in Table 1. A new lens (Biofinity) is about to be launched worldwide and the known features of this lens are also listed.

CIBA Vision's Focus Night & Day material, lotrafilcon A, employs a co-continuous biphasic or two channel molecular structure, in which two phases persist from the front to the back surface of the lens.7 CIBA Vision's AirOptix is closely based upon the technology used in Focus Night & Day. Bausch and Lomb's PureVision material, balafilcon A, is a homogeneous combination of the silicone-containing monomer polydimethylsiloxane (a vinyl carbamate derivative of TRIS) co-polymerised with the hydrophilic hydrogel monomer N-vinyl pyrrolidone (NVP).8-11 Vistakon's Acuvue Advance material, galyfilcon A, has the highest water content of all the silicone hydrogel materials (47 per cent) and thus the lowest Dk and is only approved for daily wear. It has an inversion marker and UV blocker, with a reported Class 1 UV protection, blocking >90 per cent of UVA and >99 per cent of UVB rays.12-14 Vistakon's Acuvue OASYS (senofilcon A) is the newest of the commercially available silicone hydrogel lenses. OASYS has a light blue tint, Class 1 UV blocking capabilities and a 1-2-3 inversion marker.14 The latest material, yet to be released, is the Biofinity lens from CooperVision. It is manufactured from comfilcon A and reportedly has a significantly higher oxygen permeability than would be predicted from its water content (see Figure 1), implying that the chemistry upon which it is based is different to that employed in the current silicone hydrogels, which are all based on the highly oxygen permeable monomer TRIS. The inter-relationship between increasing water content and reducing oxygen permeability for the materials shown in Figure 1 is very strong, with a Pearson correlation of -0.85 (p=0.008).

UNIQUE PROPERTIES OF SILICONE HYDROGEL MATERIALS

In addition to their increased oxygen transmission, other major differences exist between silicone hydrogel materials and conventional hydrogels, primarily relating to their mechanical and surface properties and their deposition with substances from the tear film.

Mechanical properties

Silicone hydrogel lens materials are significantly 'stiffer' than their conventional hydrogel counterparts, due to the incorporation of silicone. The modulus of the first two marketed silicone hydrogel materials (PureVision and Focus Night & Day) is significantly greater than low rigidity conventional materials such as etafilcon A (which is used in the Acuvue2 lens). The newer silicone hydrogel materials have reduced degrees of stiffness, due to the fact that they have less silicone within the base material. It is reported that Acuvue Advance, has a modulus that is only 1.5 times greater than etafilcon (see Figure 2).12, 13 According to Johnson & Johnson, this reduced stiffness is not only due to the reduced amount of silicone present in this new material, but is also derived from the internal wetting agent HydraClear.12,13 Analysis of Table 1 and Figure 2 indicates the strong inverse relationship between water content of hydrogel materials and oxygen permeability and material 'stiffness' or modulus, with the materials having the highest ratio of silicone to water being the stiffest (Pearson correlation = -0.92 p=0.001).

Increased modulus has some advantages, as the lenses handle very well and are a perfect choice for people who exhibit poor handling capabilities. The mechanical properties of these lenses do pose some problems, in that they are less able to conform easily to the shape of the eye and fitting is critical, with loose lenses exhibiting reduced comfort.15,16 Initially, this increased rigidity and the limited availability of base curves and designs resulted in reduced initial comfort compared with patients' conventional materials,15 the presence of post-lens debris17 and several mechanical issues such as giant papillary conjunctivitis, superior epithelial splits and occasional epithelial erosions being observed.18 However, as manufacturers have become aware of these issues then greater attention to lens design, the availability of more base-curve options and lens materials with lower moduli have reduced the number of these 'mechanical' complications observed with silicone hydrogel materials (Figure 3).

Surface properties

The incorporation of siloxane groups into hydrogel materials is complex, as silicone is inherently highly hydrophobic and a huge impediment to the development of silicone hydrogel lenses has related to the decreased surface wettability, increased lipid interaction and accentuated lens binding historically seen in silicone-based materials.

In order to make the surfaces of silicone hydrogel lens materials hydrophilic and more wettable, techniques incorporating plasma into the surface processing of the lens have been developed.6, 7, 11, 19 More recent techniques have involved incorporating hydrophilic monomers into the lens material that 'migrate' to the surface of the lens and aid wettability.20-22 The purpose of these surface treatments is to mask the hydrophobic silicone from the tear film, increasing the surface wettability of the materials and reducing lipid deposition.

The surfaces of Focus Night & Day and AirOptix lenses are permanently modified in a gas plasma reactive chamber using a mixture of trimethylsilane oxygen and methane to create a permanent, ultrathin (25nm), high refractive index, continuous hydrophilic surface.7,23,24 PureVision lenses are surface treated in a gas plasma reactive chamber, which transforms the silicone components on the surface of the lenses into hydrophilic silicate compounds.6,9,11,25 Glassy, discontinuous silicate 'islands' result,9, 26 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 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 easily 'stripped' away from the base material during daily handling and cleaning.9

The Acuvue Advance lens material was the first non surface-treated silicone hydrogel to become a commercial reality, closely followed by Acuvue OASYS. The senofilcon A material used to manufacture the Acuvue OASYS lens is based upon similar chemistry to that of the galyfilcon A material in Acuvue Advance. Both materials incorporate a long chain high molecular weight internal wetting agent based on polyvinylpyrolidone (PVP), which is designed to provide a hydrophilic layer at the surface of the material that 'shields' the silicone at the material interface, thereby reducing the degree of hydrophobicity typically seen at the surface of siloxane-hydrogels.12,13 The internal wetting agent for the Advance lens is termed Hydraclear and that used for the OASYS lens is 'HydraClear Plus', implying that more PVP is probably incorporated. The OASYS lens has been well received by the profession, clinically appears to deposit less than Acuvue Advance and has been particularly successful in patients with symptoms of contact lens induced dryness.27

The 'new kid on the block', Biofinity from CooperVision, has very little published information to-date, aside from conference abstracts. It is reportedly highly wettable and has a low degree of deposition for a non-surface treated silicone hydrogel, but no data are yet available that can be included in this review.

By overcoming the need for a surface treatment the cost savings are considerable and it is likely that future generation silicone hydrogels will either need to avoid the requirement for surface treatment or develop less expensive methods to achieve this.

Wettability

Wettability is a highly important property of contact lens materials, governing the interaction between the ocular surface and eyelid with the contact lens surface. Lenses that exhibit poor wettability have a tendency to be uncomfortable, as surface drying between blinks results in hydrophobic areas that irritate the lid as it moves over the lens surface.

Wettability may be assessed in a number of ways, using both in vivo techniques and laboratory-based in vitro techniques, as previously described by French.28 Analysis of the surfaces of both PureVision and Focus Night & Day has shown that these surface treatments have only been partially effective at masking the silicone, with the lenses having significantly more silicon exposed at the surface than conventional lenses29, 30 and a more hydrophobic surface (as evidenced by the presence of higher advancing water contact angles).31-34 The use of surface analytical procedures to examine the degree of silicon exposure on the more recent silicone hydrogels has not yet been published.

Figure 4 graphically portrays advancing sessile drop water contact angles measured using a digital Optical Contact Angle Analyser upon removing various contact lens materials from their packaging and after rinsing for several minutes in saline, to remove the influence of the packaging solutions in which the lenses were stored following manufacture.35 It clearly shows that there are differences in wettability, as determined by advancing contact angle, between the lens materials.

Immediately out of the pack the materials with the lowest contact angles were Acuvue 2 and Acuvue Advance, followed by Biofinity and the two CIBA surface-coated silicone hydrogels (Focus Night & Day and AirOptix), with very high contact angles being recorded for Proclear, Acuvue OASYS and PureVision. Following a saline soak, the angles for most of the materials only changed slightly, with most increasing in contact angle by a few degrees only. However, the contact angles for Acuvue Advance increased substantially, implying that surface active agents within the packaging solution may be rinsed away by the saline rinsing procedure.

The principle reason behind determining contact angle wettability is that lower contact angles should, theoretically, provide enhanced comfort. However, clinical evaluations have shown relatively little difference in reported in-eye comfort between silicone hydrogel products, compared with the large differences in contact angles measured. In addition, in-eye wettability of silicone hydrogels appears similar to that reported with conventional hydrogels, with the tear film typically breaking up after 5-7 seconds.4 This may be due to the tear film components modifying the lens surface, and further work on the clinical relevance of contact angle assessment to in-eye lens performance is required. It may transpire that contact angle analysis following lens removal is more predictive of comfort than angles determined pre-wear.

Deposition

A major factor that is influenced by surface wettability is contact lens deposition, particularly with lipids. Lens surface drying in the inter-blink period produces non-wettable areas on the lens, which encourage the deposition of hydrophobic components of the tear film. These act to further disrupt the tear film and result in further deposition.36, 37 This 'vicious circle' of drying and deposition eventually produces the appearance of either small, discrete deposits (lens calculi Figure 5) or a 'filmy' deposition that interferes with vision and subsequently comfort.

Information concerning the degree of protein and lipid deposition that occurs on silicone hydrogel lens materials is of significant clinical importance, as many of these lenses are intended for in-eye use overnight and previous work has shown that silicone-containing materials have a history of depositing with tear film contaminants, particularly lipids.38 The amount of lysozyme deposited on silicone hydrogel lens materials is significantly less than that seen on group IV traditional contact lens materials. Figure 6 describes the degree of lysozyme deposition encountered on various hydrogel materials after being doped with radiolabelled lysozyme.39 It indicates that silicone hydrogels typically deposit less than 20µg of lysozyme after four weeks, as compared with an etafilcon lens material (Acuvue2), which deposits in the region of 1,000µg of lysozyme after seven days. Other in vivo studies show similar results.39-43

While very little protein deposition clearly occurs, lipids preferentially deposit onto hydrophobic surfaces and data thus far indicate that lipid deposition on silicone hydrogels can be a problem for certain patients.40 Lipid deposition onto conventional lens materials is highly patient dependent44-48 and this fact is similarly observed with silicone hydrogel lens materials. If patients do deposit their silicone hydrogel lenses with excessive lipid then initially getting them to rub their lenses with their multipurpose cleaning regimen on removal may solve the problem. The use of 'no-rub' format care regimens with silicone hydrogels can prove to be a problem. 'No rub' regimens work perfectly well with conventional materials such as etafilcon (Acuvue) or omafilcon (Proclear), that deposit primarily loosely bound proteins. However, a physical rubbing process is required to remove the tenaciously bound lipids and denatured proteins that can be deposited by some patients on certain silicone hydrogel lens materials. These authors frequently recommend to their patients that they should use their regimens in a 'rub' format, regardless of the labelling indication.

If this fails, adding additional surfactant cleaners containing alcohol (such as Miraflow) will aid in deposit removal. Finally, moving to more frequent periods of replacement.46, 49 or reverting back to non-silicone hydrogels that do not contain lipid-attracting monomers such as vinyl pyrrolidone (for example, Proclear or Acuvue) will eliminate this phenomenon. Ultimately, as production costs decrease and daily disposable silicone hydrogels become a commercial reality, and new technologies are developed that overcome the need to coat the material surface, deposition issues will become a thing of the past.

CONCLUSION

Increasingly, contact lens companies are looking at developing novel silicone-based hydrogels and the foreseeable future for this group of lens materials looks very promising. Future materials will likely be uncoated and, ideally, be so wettable that they would support a stable tear film for longer than 15 seconds. Such lenses would exhibit high levels of comfort, deposit only low levels of tear film contaminants and would likely revolutionise the contact lens industry.

References

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◆ Lyndon Jones is Professor, School of Optometry, and associate director, Centre for Contact Lens Research, University of Waterloo where Lakshman Subbaraman is a PhD student, Ronan Rogers is a MSc student and Kathy Dumbleton is a faculty senior research associate

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